RESEARCH HIGHLIGHTS 2015
A publication of the WPI Advanced Institute for Materials Research

Table of contents
MESSAGE FROM THE DIRECTOR
1WPI Advanced Institute forMaterials Research
The Advanced Institute for Materials Research (AIMR)at Tohoku University in Sendai, Japan, is one ofnine World Premier International Research Center
Initiative (WPI) programs established with the supportof the Japanese Ministry of Education, Culture,
Sports, Science and Technology (MEXT). Since its
inauguration in 2007, the AIMR has been bringing
together world-class researchers from Japan andabroad to carry out cutting-edge research in materials
science through interdisciplinary collaboration amongits four groups — Materials Physics, Non-equilibrium
Materials, Soft Materials, Device/System — and theMathematical Science Group.
Led by distinguished mathematician and director
Motoko Kotani, the institute promotes interdisciplinary
research across the different groups while fostering
young researchers through the Global Intellectual
Incubation and Integration Laboratory (GI3 Lab),
where international joint research is carried out inclose cooperation with high-profile researchers invitedfrom countries throughout the world.
The AIMR is host to over 140 leading researchers,around half of whom come from abroad, including
29 principal investigators. In addition to the research
hub at Tohok­­u University, the AIMR collaborates
with research centers in China, France, Germany,
Poland, the United Kingdom and the United States.
Close ties with other leading overseas institutes
are maintained through its Adjunct Professor andAssociate Professor programs.

Fruitful collaboration develops a new frontierof materials science

RESEARCH HIGHLIGHTS
2
3
4

Electrocatalysis: Holey gold boosts activity
Graphene: Co-doping for a hydrogen society
Magnetism: Defects deprive magnetite of good
spintronic properties
5 Silicene: Dirac cones found in band structure
6 Heterojunctions: Superhard interfaces with‘super’ states
7 Solid electrolytes: Sodium-based high performer
8 Iron-based superconductors: Enigmatic phase implicatedin superconductivity
9 Topological insulators: Properties transferred to metal film
10 Lithium batteries: Forming high-quality interfaces
11 Sodium titanate: Nanowires show promise fornuclear decontamination
12 Tissue engineering: The groovy side of hydrogels
13 Supercapacitors: Toward high-energy-density materials
14 Solar energy conversion: Full steam ahead for 3D graphene
15 Iron-based superconductors: Superconductive FeSe
multilayer films
16 Superconductivity: Striking a balance
17 Computer memory: Ultrafast switches reveal a hidden nature
18 Superconductivity: Missing piece of jigsaw found
19 Polymers: Self-folding sheets wrap up droplets
20 Lithium–oxygen batteries: Graphene protects cathode catalyst
21 Metallic glasses: Thermal cycling is the secret to eternal youth
22 Phase transformation: Switching on the next phaseof nanodevices
23 Magnetism: Electric field reveals the ‘hole’ story

IN THE SPOTLIGHT
24 Materials and mathematics collaboration: A new horizon
26 Reaching for the world
28 Ambitious researchers explore the future of materials science
30 Strengthening partnerships across continents
32 Taking spintronics for a spin
34 Bringing together ‘best with best’MESSAGE FROM THE DIRECTOR

Fruitful collaboration develops a
new frontier of materials science
The Advanced Institute for Materials

first large-scale attempt in the world to

Research (AIMR) was established in 2007

introduce a mathematical perspective toto develop a world-class research basematerials science, which gives the AIMR a

in Japan, with the support of the World

leading position in materials science.

Premier International Research Center

In 2015, the institute engaged in

Initiative (WPI) program initiated by the

dynamic collaborations with overseas

Japanese Ministry of Education, Culture,scientists and institutes. The AIMR

Sports, Science and Technology (MEXT).

International Symposium 2015 was held inSince then, the AIMR has unwaveringly

February 2015, and 268 materials scientists

pursued top-level research in specificfrom 14 countries and 36 institutes

areas and has striven to create newparticipated. Among them were 34 invitedmaterials science.

lecturers, including Sir Michael Berry, aThis year, the AIMR has made remark-professor at the University of Bristol inable progress in the field of spin-centered

the United Kingdom and the 1995 Diracscience, addressing all facets of spintronics

medalist. In May, we participated in two

research, from fundamental science tojoint workshops on both sides of the

In December 2015, Springer publishedtechnological applications. This diverse

Atlantic: one with the Center for Integrated

the first in a new series on the interaction

approach was evident in the three-month

Quantum Materials at Harvard University

between mathematics and materials

intensive program “Spintronics: fromin Cambridge, Massachusetts, in thescience, titled SpringerBriefs in the

Mathematics to Devices”, organized by theUnited States, and the other with six

Mathematics of Materials.

AIMR, together with other institutes, at

C’Nano centers in Rennes, France.

the newly established Tohoku Forum for
Creativity from September 2015.

Last year, several organizational reforms

Nine years after being established, the
AIMR is obtaining excellent results inwere initiated at Tohoku University. The

many fields. We would like to thank thoseOrganization for Advanced Studies (OAS)

who have been supporting us. As a global

science, the AIMR has a special mission

was founded to support scientists engaged

hub for talented scientists, we will continue

to connect mathematics and materials

in world-leading research. The OAS hosts

to produce high-level research andscience. We seek to understand extremely

the Research Reception Center as wellcontribute to the worldwide development

wide-ranging phenomena in materialas the AIMR and the Tohoku Forumof materials science.

sciences by using mathematical tools to

for Creativity.

As a global research center for materials

uncover commonalities between different

The collaboration between mathematics

Motoko Kotani

materials and thereby creating new

and materials science is attracting

Director

research themes and results. This is theconsiderable attention around the world.

AIMR

RESEARCH HIGHLIGHTSRESEARCH HIGHLIGHTS
The AIMR advances research in materials physics, non-equilibrium materials, soft materials and device/system, and actively promotes collaboration
among these divisions toward the development of ground-breaking technologies that cross the boundaries of conventional fields ofstudy — bridging the disciplines of materials science, physics, chemistry and precision, mechanical, electronics and information engineering.
The Mathematical Science Group further complements the AIMR’s research activities.

Electrocatalysis

Holey gold boosts activity
AIMR researchers have devised a novel
way to produce high-activity catalytic
films for the electrocatalytic productionof hydrogen, an important future energy
storage medium. Their method potentially has a much broader application, as
it could provide a new way to fabricate
two-dimensional catalysts that possess
both large effective surface areas andhigh catalytic activities.
Hydrogen offers an attractive,
environmentally friendly way of storingenergy as it can be produced by splitting water into its constituent elements
through simply applying an electricalcurrent in the presence of a catalyst — aprocess known as the electrolysis ofwater. The problem with present methodsof electrocatalytic hydrogen productionis that most of them employ platinumbased catalysts, which are prohibitively
expensive for practical applications.
Molybdenum disulfide (MoS2) isemerging as a promising cheaper alternative catalyst. As research has suggested
that atoms located at the edges of MoS2
mainly contribute to the catalysis ofhydrogen production, much effort hasgone into producing catalysts with a highproportion of edges. But such catalysts
can have reduced stability and electricalconductivity.
Now, Mingwei Chen and co-workers
at the AIMR and other institutions inJapan and China have struck upon a
totally different approach that enables
them to obtain a high catalytic activity
from an essentially ‘edge-free’ continuous MoS2 film1.
Their approach involves growing asingle-molecule-thick film of MoS2 ona gold substrate riddled with tiny holesthat are approximately 100 nanometers

2

Modified, with permission, from Ref. 1 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Growing a single-molecule-thick film on nanoporous gold provides a new way toobtain high-activity catalysts for hydrogen production

Diagram depicting a monolayer molybdenum disulfide film (green and yellow spheres) grown on the curved
surface of a nanoporous gold substrate (large gold spheres).

in diameter (see image). To their surprise, they obtained a catalytic efficiency
for hydrogen production that rivals
those of the best MoS2-based catalysts
reported to date.
To explore the cause of this highactivity, the researchers performed theoretical calculations. The results revealed
that the bending of the monolayer MoS2
film induced by the puckered surface ofthe gold substrate altered the chemical
properties of the film. The team considers that the out-of-plane strains inducedby the curved topology are responsible
for the enhanced catalytic activity of the
MoS2 film.
The scientists are excited about thisfinding as it provides new insights
into the effect of strain on the catalytic
properties of two-dimensional catalysts

and potentially could open up a new
way to tailor the catalytic activity of twodimensional catalysts through latticestrain engineering.
“This method allows us to effectively pack two-dimensional materials into three-dimensional devices,”
states Chen, “while keeping the highaccessible specific surface areas of
two-dimensional films.”
The researchers are currently investigating cheaper substrates such as nickel
and graphene as replacements for gold.
1. Tan, Y. W., Liu, P., Chen, L. Y., Cong, W. T., Ito, Y.,
Han, J. H., Guo, X. W., Tang, Z., Fujita, T., Hirata, A.
& Chen, M. W. Monolayer MoS2 films supported
by 3D nanoporous metals for high-efficiency
electrocatalytic hydrogen production. Advanced
Materials 26, 8023–8028 (2014).

AIMResearch 2015

RESEARCH HIGHLIGHTS

Graphene

Co-doping for a hydrogen society

A metal-free catalyst based on graphenethat can be used to produce low-cost,high-efficiency hydrogen fuel cells has
been developed by AIMR researchers1.
This will help realize the goal of a‘hydrogen society’ — one poweredby hydrogen rather than fossil fuel or
nuclear power.
Japan is enthusiastically embracing
the concept of a hydrogen society, with
the Japan Science and Technology
Agency noting the importance ofdevelopinghydrogen-based
fuelcells that could be used to power
everything from vehicles to domestic
residences and commercial activities.
Yoshikazu Ito and Mingwei Chen fromthe AIMR at Tohoku University believethat “the clean energy of hydrogen will
be a central energy target in the twentysecond century.”
Hydrogen fuel cells produce electricity
via two electrochemical reactions — theoxygen reduction reaction and thehydrogen evolution reaction — bothof which require a catalyst. The mosteffective catalysts tend to be noble metals
such as platinum, but their superior performance comes with a prohibitive pricetag. It is thus essential to develop low-cost,
metal-free catalysts that have comparable
catalytic activities to those of metals forboth electrochemical reactions.
A promising contender is graphene —a single layer of carbon atoms arranged
in a honeycomb lattice. However, it ishindered by a relatively low chemical
activity. In a previous study, Ito and colleagues succeeded in significantly boosting the rates of the oxygen reduction
reaction by doping a three-dimensional
interconnected network of graphenesheets with nitrogen2. Enhancing the

AIMResearch 2015

Inside

H2
H20

H2 0
H2

OutsideStructure of nanoporous graphene doped with nitrogen (red) and sulfur (green). The inset depicts theprobable reaction mechanism.

hydrogen evolution reaction, however,has proven more challenging.
Inspired by their finding that sulfur
was critical for catalyzing the hydrogen
evolution reaction for a similar twodimensional material, molybdenum
disulfide3, Ito with other colleagues
at AIMR and collaborators in China
decided to try doping three-dimensional nanoporous graphene with both
nitrogen and sulfur.
Nanoporous graphene generally contains various defects, including missing
carbon atoms and dislocations in its
lattice. Since defects tend to increasegraphene’s chemical activity, theresearchers used unmodified threedimensional nanoporous graphene asa control to determine whether theincreased catalytic activity could besolely explained by lattice defects. They
discovered that the enhanced catalytic
activity for the hydrogen evolution
reaction resulted from the interplay
between all three factors — nitrogen,sulfur and defects.

The researchers are very excitedabout the potential of their catalyst.
“Our metal-free hydrogen evolution
reaction catalyst will contribute to therealization of a hydrogen society andhydrogen stations for fuel cell cars
through enabling on-site hydrogen
evolution,” predicts Ito.
1. Ito, Y., Cong, W., Fujita, T., Tang, Z. & Chen, M. W.
High catalytic activity of nitrogen and sulfur
co-doped nanoporous graphene in thehydrogen evolution reaction. Angewandte
Chemie International Edition 53,
2131–2136 (2014).
2. Ito, Y., Qiu, H.-J., Fujita, T., Tanigaki, K. & Chen, M.
Bicontinuous nanoporous N-doped graphene
for the oxygen reduction reaction. Advanced
Materials 26, 4145–4150 (2014).
3. Tan, Y. W., Liu, P., Chen, L. Y., Cong, W. T.,
Ito, Y., Han, J. H., Guo, X. W., Tang, Z., Fujita, T.,
Hirata, A. & Chen, M. W. Monolayer MoS2
films supported by 3D nanoporous metals
for high-efficiency electrocatalytic hydrogenproduction. Advanced Materials 26,
8023–8028 (2014).

3

Modified, with permission, from Ref. 1 © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

The addition of nitrogen and sulfur to nanoporous graphene in combination withdefects results in a cheap and effective catalyst for hydrogen fuel cells

RESEARCH HIGHLIGHTS

Magnetism

Defects deprive magnetite ofgood spintronic properties
Highly stable ‘antiphase’ defects havefinally been conclusively shown to be
responsible for the poor spintronic
properties of magnetite. While thishas long been suspected to be the case,no one has been able to definitively
demonstrate it until now.
Magnetite (Fe3O4) has been muchstudied, being the oldest known magnetic material and one of the mostabundant iron-containing minerals on
Earth. It is of fundamental interest as
well as being used in diverse applications including catalysis, rechargeable
batteries and magnetic recording.
In theory, it should be an ideal
material for the emerging field of
spintronics, which exploits both the
charge and spin properties of electrons(unlike conventional electronics, which
focuses on their charge properties).
This is because all the spin conduction
electrons in magnetite are predicted to
be spin polarized at room temperature.
However, its experimentally measured
spin polarization has always been much
lower than that predicted by theory.
A long-suspected culprit for this discrepancy has been antiphase defects,
but no one has been able to definitively
confirm this, until now.
Inatheoreticalandexperimental study, Chunlin Chen,
Zhongchang Wang and Yuichi Ikuhara
from the AIMR along with collaborators at the University of York in the
United Kingdom have shown, beyond
any doubt, that antiphase defects are responsible for the low spin polarizationof magnetite1.
They performed first-principles
predictive modeling of the structure of
antiphase defects in magnetite. Using

4

Antiphase defects (shown here in the central row) have been conclusively shown to be responsible for the low
spin polarization of magnetite.

this structural information, they werethen able to infer the electronic andmagnetic properties of the defects.
Finally, they used atomic-resolution
transmission electron microscopy toresolve the three-dimensional structureof the defects — the first time this had
been done.
They obtained excellent agreement
between the model predictions andthe experimental results, confirming
the role of defects in reducing thespin polarization of magnetite. Wang
explains that this finding is valuable
because it shows “that to improve magnetite for spintronic device applications
and achieve 100 per cent spin-polarizedmaterials, we need to remove antiphase
defects.” Chen notes that the defects“may find applications in catalysis,

since defects usually have higher catalytic activity.”
The modeling technique used in the
study is promising for analyzing othersystems. “The agreement between thetheoretical prediction and the experimental images is remarkable,” says Wang.
“The theory came first, demonstrating
its predictive power and utility in materials optimization for applications.”
In the future, the researchers intend
to investigate the atomic structure andproperties of other defects in magnetite,
such as twin boundaries.
1. McKenna, K. P., Hofer, F., Gilks, D., Lazarov, V. K.,
Chen, C., Wang, Z. & Ikuhara, Y. Atomic-scale
structure and properties of highly stableantiphase boundary defects in Fe3O4. Nature
Communications 5, 5740 (2014).

AIMResearch 2015

Reproduced from Ref. 1 and licensed under CC BY 4.0 (creativecommons.org/licences/by/4.0/legalcode) © 2014 K. P. McKenna et al.

A theoretical and experimental study clearly shows that defects reduce the spinpolarization of magnetite

RESEARCH HIGHLIGHTS

Silicene

Dirac cones found inband structure
AIMR researchers have, for the firsttime, unambiguously shown that theelectronic properties of silicene resemble those of its famous carbon cousin— graphene. In particular, they havedemonstrated that the band structure ofsilicene contains a ‘massless Dirac cone’,making the material promising for use
in high-speed electronic devices.
Silicene is a very attractive material on paper but has proven difficult tomake in the lab. It is a single layer ofsilicon atoms arranged in a hexagonal
honeycomb structure, similar to thatof graphene. Unlike graphene, silicene
is not completely flat; rather, it has abuckled structure — something that isboth a blessing and a curse.
Silicene’s buckled structure is predicted to lead to attractive electronic
properties, including a bandgap thatcan be tuned by applying a voltage
perpendicular to the silicene sheet.
Buckling, however, also makes silicene
inherently unstable; consequently, no
one has successfully produced a single
layer of free-standing silicene.
Now, a team led by Takashi Takahashiof the AIMR at Tohoku University has
produced the next best thing — a compound in which silicene layers are sandwiched between flat layers of calcium
atoms (see top of image)1. They theninvestigated the electronic properties of
the compound’s silicene layers using ananalytical technique known as angleresolved photoemission spectroscopy.
When they did so, they obtained conclusive evidence that the silicene layers
have a massless Dirac cone.
The electronic band structure ofgraphene consists of two circular cones
whose tips touch at the origin (see

AIMResearch 2015

Top: A compound in which layers of buckled silicene (blue) are sandwiched between flat layers of calcium atoms(gold) has been produced to measure the electronic band structure of silicene. Bottom: The silicene layer has
been shown to have so-called Dirac-cone electronic states consisting of two circular cones whose tips touch at
the origin.

bottom of image). These so-called Diraccones endow graphene with special
electronic properties that differ fromthose of simple insulators and conductors. In particular, when the Dirac cones
are massless with no bandgap, electrons
on the Dirac cone can move very rapidly in materials. Now, silicene has beenshown to have the same band structure,
confirming theoretical predictions of its
electronic properties.
“Theoretical
calculations
hadpredicted that silicene has a massless Dirac cone, but it had not been
experimentally confirmed whether
there is a Dirac cone in silicene, andif it exists, if it is massless or massive,”
explains Takahashi. “This is because

early studies used ‘silicene’ samples
fabricated on a metal substrate and thus
could not eliminate the interaction withthe substrate.”
The researchers hope to go a stepfurther and “synthesize a genuine
silicene sheet free from extrinsic
components such as a substrate andmeasure the intrinsic electronic structure of silicene,” says Takahashi. They
also plan to explore germanene, thegermanium equivalent of graphene.
1. Noguchi, E., Sugawara, K., Yaokawa, R.,
Hitosugi, T., Nakano, H. & Takahashi, T. Direct
observation of Dirac cone in multilayer silicene
intercalation compound CaSi2. Advanced
Materials 27, 856−860 (2015).

5

Top: Modified, with permission, from Ref. 1 © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Bottom: Reproduced, with permission, from
Pesin, D. & MacDonald, A. H. “Spintronics and pseudospintronics in graphene and topological insulators.” Nature Materials 11, 409–416 (2012).

By making a silicene sandwich, researchers have shown that silicene has similarelectronic properties to its carbon equivalent, graphene

RESEARCH HIGHLIGHTS

Heterojunctions

Superhard interfaces with‘super’ states
Diamond and cubic boron nitride— the two hardest materials in theworld — are vital components in abrasive cutting and polishing tools usedin the manufacturing industry. Now,
AIMR researchers have combined these
two materials to create an interface thathas radically different electronic properties from those of its constituents1.
The interfaces between two different materials are extensively used inapplications as diverse as solar cells andmagnetic recording devices. However, it
is notoriously difficult to form interfaces
between superhard materials because of
the extreme rigidity of their lattices.
Chunlin Chen, Zhongchang Wang andYuichi Ikuhara from the AIMR at Tohoku
University, along with Takashi Taniguchiat the National Institute for Materials
Science and collaborators in Japan, used
the temperature gradient method to growsingle crystals of cubic boron nitride on
diamond seed crystals. They then explored the mechanism that allowed these
two almost incompressible materials withextremely rigid lattices to join.
Lattice mismatch is a critical hurdle
to achieving high-quality layer-bylayer crystal growth. It arises whenparameters such as the bond lengthsof the substrate and growth materials
differ. Such mismatch induces strain
in the crystal lattice, which can resultin poor-quality films. While the difference between the bond length of cubicboron nitride (0.157 nanometers) andthat of diamond (0.154 nanometers) is aminuscule 0.003 nanometers, it is large
enough to cause growth complicationsin these extremely rigid materials.
A frequently used method to overcome mismatch involves relieving

6© 2015 Chunlin Chen

By joining two superhard materials, AIMR researchers show that the resulting
composite is more than the sum of its parts

Bright-field scanning transmission electron micrograph of the interface between diamond and cubic boron
nitride showing misfit dislocations composed of periodic hexagonal loops.

lattice strain by introducing irregularities known as dislocations in the crystal
lattice. Common forms of dislocations
between two layers with different
bond lengths are ‘misfit’ dislocations,where there are missing or dangling
bonds between the two layers. Using
atomic-resolution scanning transmission electron microscopy, Chen andWang observed that the misfit dislocations in their system took the form of
periodically arranged hexagonal loops
connected by a continuous stacking
fault network (see image). They discovered that the misfit accommodation
mechanism differs remarkably from theconventional one.
“By combining transmission electron
microscopy measurements with firstprinciples calculations, we confirmed thatthe carbon in diamond bonds directly
to the boron in cubic boron nitride at
the interface,” explain Chen and Wang,“and also that this bonding electronicallyinduces a two-dimensional electron gas
and a quasi-one-dimensional electricalconductivity, despite both bulk materials
being insulators.”
Chen holds great hope for the newelectronic states observed at the interface. “They could be manipulated for use
in advanced electronic device applications, particularly those involving harshconditions,” he says.
1. Chen, C., Wang, Z., Kato, T., Shibata, N.,
Taniguchi, T. & Ikuhara, Y. Misfit accommodation mechanism at the heterointerface
between diamond and cubic boron nitride.
Nature Communications 6, 6327 (2015).

AIMResearch 2015

RESEARCH HIGHLIGHTS

Solid electrolytes

Sodium-based high performer
A new sodium-based material with an excellent ion conductivity is a promisingcheap alternative to lithium-based electrolytes

AIMResearch 2015

Superionic conductivityNa© 2015 Shin-ichi Orimo

A sodium-based material highly
promising for use as a solid electrolyte
in rechargeable batteries has been
discovered by an international team ofresearchers1. The material is inexpensive
since it consists of common elements,
and it exhibits an exceptionally highconductivity for sodium ions at temperatures above 110 degrees Celsius.
Solid electrolytes are superior to theirliquid counterparts for use in rechargeable batteries because they do not leakor explode. Lithium-based solid electrolytes are currently the best performers,
but the relative scarcity of lithium meansthat their price fluctuates with global
availability. Consequently, researchers
are searching for alternative materials
made from more abundant elements.
Now,
Atsushi
UnemotoandShin-ichi Orimo at the AIMR and
Motoaki Matsuo of the Institute forMaterials Research, along with otherresearchers at Tohoku University andoverseas collaborators, have discovered apotential rival to lithium-based electrolytes — a complex hydride containing themetals sodium and boron (Na2B10H10).
The material is inexpensive as it consistsof three abundant elements: hydrogen,sodium and boron. Most importantly, it
can rapidly ferry sodium ions between
the electrodes of a battery, making it
attractive for high-power applications.
When the researchers heated thematerial from room temperature, thesodium-ion conductivity initially increased considerably, but it suddenly
leapt by almost a hundredfold when
the temperature reached about 110 degrees Celsius. This dramatic increase in
conductivity was due to a change in thematerial’s structure from a tightly packed

B10H10
Rapid reorientational jump
Both the wide corridors between the B10H10 anions and the reorientational motion of these anions arethought to contribute to the exceptionally high sodium-ion conductivity of Na2B10H10.

structure to one containing wide, open
corridors through which charge-carrying sodium ions could readily travel.
The resulting sodium-ion conductivity
is over ten times higher than those ofpreviously investigated sodium-based
complex hydrides.
The researchers strongly suspect,however, that another mechanism also
contributes to the excellent sodium conductivity of the material. They believethat the ‘reorientational motion’ of theanion columns in the structure in some
way assists the sodium ions as they travel
through the corridors (see image).
“We anticipated that the material
would exhibit a high ionic conductivity,” explains Matsuo, “because we hadfound a strong correlation between
the reorientational motion of complex

anions and the mobility of cations incomplex hydrides in previous studies.”
The researchers are keen to explore
the potential of this material. “In the
future, we hope to reduce the onset
temperature for sodium-ion conductionin Na2B10H10 from its present 110 degrees
Celsius to close to room temperature,”says Matsuo. “In the longer term, we
aim to construct all-solid-state sodium
rechargeable batteries by using the material as the electrolyte.”
1. Udovic, T. J., Matsuo, M., Tang, W. S.,
Wu, H., Stavila, V., Soloninin, A. V., Skoryunov,
R. V., Babanova, O. A., Skripov, A. V., Rush, J. J.,
Unemoto, A., Takamura, H. & Orimo, S.-i. Exceptional superionic conductivity in disorderedsodium decahydro-closo-decaborate. Advanced
Materials 26, 7622−7626 (2014).

7

RESEARCH HIGHLIGHTS

Iron-based superconductors

Enigmatic phase implicated insuperconductivity
An unusual phase known as the nematic phase appears to be responsible for thesuperconductivity of iron selenide

8

Fe
Fe
Se

© 2015 Takashi Takahashi

AIMR researchers have taken a vitalstep toward understanding the mechanism of superconductivity in ironselenide (FeSe) by analyzing the material using photoemission spectroscopy.
Their findings support the conjecturethat the material’s superconductivity
is linked to an unconventional state
known as the nematic phase, which
is characterized by a roughly parallelalignment of atoms.
The discovery of iron-based superconductors in 2006 came as a complete
surprise because iron’s ferromagnetism
had been expected to prevent superconductivity. Since the discovery,
scientists have been scrambling to
explain the mechanism that gives
rise to the superconductivity of theseiron-based materials.
Iron selenide has been attracting
special interest because it has thesimplest crystal structure of iron-based
superconductors (see left side of image)and also because one-atom-thick filmsof FeSe have been found to be superconductive up to temperatures close
to the boiling point of liquid nitrogenat 77 kelvin. However, its electronic
structure has not been studied in detailbecause of the difficulty of producing
high-quality crystals.
Taking advantage of recent advancesin FeSe crystal growth, researchers at Tohoku University led by
Takashi Takahashi of the AIMR have
now used angle-resolved photoemission spectroscopy to explore theelectrical structure of single crystalsof FeSe as a function of temperature1.
Their findings suggest that the nematic
phase plays a critical role in the superconductivity of FeSe.

(Left) Crystal structure of iron selenide (FeSe). (Right) Top view of the plane of Fe atoms indicated by the grayshaded region in the top left panel. Green shading indicates the elongated electronic states derived from the
Fe orbitals in the nematic phase.

The measurements revealed a sizeable splitting in the electronic band
structure of the material that is present at low temperatures and persists
beyond 90 kelvin — the temperature
at which the structure of FeSe changes.
Since bulk FeSe lacks long-range magnetic order, this result demonstrates
that long-range magnetic order is notessential for inducing large band splitting. Instead, the researchers strongly
suspect that the splitting is caused byelectronically driven nematic states — aform of electronic order that breaks therotational symmetry of the lattice whileleaving its translational symmetry
intact (see right side of image).
“The present finding provides a
key for understanding the unconventional superconductivity in FeSe,” says
Takahashi, “and also for elucidating

the origin of nematicity in iron-based
superconductors.”
The researchers intend to explore
the material further. “Our future goals
are to elucidate how superconductivity evolves from the bulk material toa film,” explains Takahashi. “We will
also try to increase the superconducting transition temperature — hopefully, to much higher than the boilingpoint of liquid nitrogen — by tuning
the electronic states by adjusting thestructures of devices made from multiple materials.”
1. Nakayama, K., Miyata, Y., Phan, G. N.,
Sato, T., Tanabe, Y., Urata, T., Tanigaki, K. &Takahashi, T. Reconstruction of band structure
induced by electronic nematicity in an FeSe
superconductor. Physical Review Letters 113,
237001 (2014).

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RESEARCH HIGHLIGHTS

Topological insulators

Properties transferred tometal film

AIMResearch 2015

Bi
film

VacuumTlBiSe2bulk

AIMR researchers have shown for the
first time that the unique conduction
properties found on the surface of materials known as topological insulators
are transferred to ultrathin metal films
in contact with them1.
Topological insulators are exciting
new materials that are electricallyinsulating in their interior but simultaneously support the flow of electrons
on their surfaces. They are highly
promising for use in the emerging
field of spintronics because, unlike inconventional conductors, the spins ofelectrons traveling on the surface of a
topological insulator are unaffected by
defects or non-magnetic impurities.
The combination of a topologicalinsulator and a conventional insulator,
such as air, has been widely studied as aresult of surface conduction that occurs
when the two types of materials are incontact. In contrast, the interaction
between a topological insulator anda metal has not been investigated inmuch detail, despite the importance of
this system for practical devices.
Now, Seigo Souma, Akari Takayama
and Takashi Takahashi at the AIMR in
Tohoku University, together with otherresearchers from Tohoku University
and collaborators at Osaka University,
have explored the interface between
a topological insulator and a bilayer
metal film. In particular, they usedspin- and angle-resolved photoemission spectroscopy to probe a bilayer
film of bismuth on the topologicalinsulator TlBiSe2.
Their measurements revealed that
intriguing electronic states on thesurface of the topological insulator migrate to the bismuth film (see image).

Electronic states that behave as Dirac fermions transfer from a topological insulator (TlBiSe2) to an ultrathin
layer of bismuth metal (red spheres).

Since these measured states have halfinteger spins and conform to the Diracequation, they can be regarded as Diracfermions. Theoretical calculations suggest that the migration results fromthe strong spin-dependent mixing ofelectronic wave functions that occurs
at the interface.
“We found that, in sharp contrast
with conventional wisdom, when a
topological insulator is interfacedwith a metal, a large portion of the
Dirac fermions in the topologicalinsulator are transferred to the metal,”
explains Souma.
This demonstration points to a new
avenue for manipulating the topological properties of materials. “For example, it may be possible to enhance theperformance of a metallic spintronicmaterial by imparting it with topological protection,” says Souma.
The researchers intend to investigate
the effect further. “We will fabricate metalthin films with various thicknesses on topological insulators and then characterizethem using spin-resolved angle-resolved
photoemission
spectroscopy,”
says
Souma. “We are also interested in forming
various exotic metals, such as ferromagnets and superconductors, on topologicalinsulators to see how the electronic stateof Dirac fermions evolves when coupledwith specific orders in materials.”
1. Shoman, T., Takayama, A., Sato, T., Souma, S.,
Takahashi, T., Oguchi, T., Segawa, K. & Ando, Y.
Topological proximity effect in a topologicalinsulator hybrid. Nature Communications 6,6547 (2015).

9

Adapted by permission from Macmillan Publishers Ltd: Nature Communications (Ref. 1), copyright (2015)

An ultrathin metal film on a topological insulator is found to adopt the conduction
properties of the topological insulator

RESEARCH HIGHLIGHTS

Lithium batteries

Forming high-quality interfaces

Lithium-ion batteries are the clear leaders in the rechargeable battery market,
being used to power everything frommobile phones to Mars rovers. Since
batteries with liquid electrolytes cansuffer from leakage or explosions, there
has been a concerted effort to improve
the safety of lithium-ion batteries byusing solid electrolytes, but often at
the expense of battery performance.
Now, researchers from the AIMR at
Tohoku University have produced
lithium-based solid interfaces that havevery low resistances and thus promise
to deliver both safety and excellent
performance for high-energy-density
applications1.
For a battery to achieve high energy
densities, all of its components musthave high ionic conductivities. However,in batteries with solid electrolytes, theformation of negative space-charge layers at electrode–electrolyte interfaces
gives rise to a poor ionic conductivity.
“The major drawback of all-solid-statebatteries is the low ionic conductivity at electrode–electrolyte interfaces,”
explains Susumu Shiraki, one of theresearchers involved in the study.
Most solid electrolytes are powder
based, which results in ill-definedinterfaces. To overcome this problem,
Shiraki and his colleagues produced
electrolytes by growing high-quality
thin films with sharp, well-definedinterfaces.
“Many researchers face difficulties infabricating all-solid-state thin-film batteries, which mainly stem from internal
short circuits between the cathode andanode and from the weak adhesionand high interface resistivity between
stacked films,” notes Shiraki.

10© 2015 Susumu Shiraki

Solid–solid lithium-based interfaces are promising for next-generation
high-energy-density batteries

Photograph showing high-quality thin films being fabricated by pulsed laser deposition.

The researchers avoided these difficulties by drawing on their extensive
experience in growing high-quality thinfilms. They used magnetron sputteringin a vacuum to grow 100-nanometerthick films of lithium phosphorus
oxynitride directly on electrodes oflithium cobalt oxide.
The interface boasted a remarkably
low resistance — one that was over ten
times smaller than those reported forother all-solid-state batteries and evenlower than those of batteries with liquid
electrolytes. The result indicates that
this electrode–electrolyte combination
has a negligible negative space-charge
layer at the interfaces.
Shiraki attributes his team’s successto stringent quality control during
fabrication: “The keys to obtaining a
low interface resistance are minimizing sputtering damage when depositing electrolyte films and avoiding

contamination at the interfaces.”“Our findings suggest that it is possible to develop bulk-type all-solid-statebatteries with a low interface resistanceby preparing clean electrode–electrolyte interfaces,” says Shiraki, notingthat this will require making theprocess compatible with industrial
fabrication conditions.
The team intends to investigate otherelectrode–electrolyte combinations. For
example, “we plan to employ crystalline
electrolytes to fabricate all-solid-statebatteries by stacking epitaxial films
of cathode, anode and electrolytes,”says Shiraki.
1. Haruta, M., Shiraki, S., Suzuki, T., Kumatani,A., Ohsawa, T., Takagi, Y., Shimizu, R. &Hitosugi, T. Negligible “negative space-charge
layer effects” at oxide-electrolyte/electrodeinterfaces of thin-film batteries. Nano Letters
15, 1498–1502 (2015).

AIMResearch 2015

RESEARCH HIGHLIGHTS

Sodium titanate

Nanowires show promise fornuclear decontamination
A new way of producing sodium titanate
nanowires for decontaminating radiation-tainted water has been developed
by AIMR researchers1.
The meltdown of three reactors at
the Fukushima Daiichi nuclear power
plant in Japan following the tsunami
in 2011 generated vast amounts of radioactively contaminated water — over
600,000 metric tons according to a 2014
estimate by the Japanese government. To
prevent this water from contaminating
the environment, it is vital to develop
materials capable of both trapping radioactive species and storing them safely.
One-dimensional sodium titanate
nanostructures are attractive for thispurpose because their sodium ions canbe readily swapped for radioactive ionssuch as strontium.
Various methods for fabricating onedimensional titanium oxide nanostructures have been developed. “However,most methods involve heating, which
limits the properties of the nanostructures, including their crystallinity andshape,” explains Naoki Asao, who led thestudy. “In contrast, our method proceeds
under non-thermal conditions, allowing
ultrafine structures to form.”
Asao,
Koji
Nakayama
andtheir colleagues at the AIMR at
Tohoku University took a method usually used to produce nanoporous gold
and applied it to ribbons of a titanium–
aluminum alloy: they treated the ribbons
at room temperature in aqueous sodiumhydroxide and then centrifuged or
decanted the resulting solution to obtain
sodium titanate nanowires (see image).
The nanowires were produced by thesimultaneous leaching of aluminumfrom the alloy and oxidation of titanium.

AIMResearch 2015

Adapted with permission from Ref. 1. Copyright 2015 American Chemical Society.

Ultrafine sodium titanate nanowires that exhibit remarkable exchange of radioactiveions have been produced by a heat-free method

5 nm

A transmission electron micrograph showing several ultrafine sodium titanate nanowires.

This is the first time that ultrafine sodium
titanate nanowires have been producedby this process.
X-ray diffraction and transmission
electron microscopy revealed thatthe nanowires had a layered atomic
structure in which layers of sodium
ions were sandwiched between layers of
TiO6 octahedrons.
“Because such layered structures cangive rise to effective adsorbents, thisresult strongly encouraged us to study
the nanowire’s ion-exchange properties with a view to decontamination at
Fukushima,” says Asao.
The researchers discovered that the
nanowires exhibit a remarkably highstrontium ion exchange capacity and avery rapid uptake rate.
Furthermore,thenanowiresselectively absorbed strontium ions.

“The radiation-tainted water contains
not only radioactive ions but also
various nontoxic ions,” notes Asao.
“This necessitates selective ion exchange.
Our material is promising because it
selectively captures strontium ions evenwhen there are high concentrations of
sodium ions.”
While the ultrafine nanowires show
great potential, further research isnecessary before they can be deployed
in decontamination efforts, both at
Fukushima and other clean-up sites. The
team intends to improve the fabrication
method to boost the selective capturing
ability of the nanowires.
1. Ishikawa, Y., Tsukimoto, S., Nakayama, K. S. &Asao, N. Ultrafine sodium titanate nanowires
with extraordinary Sr ion-exchange properties.
Nano Letters 15, 2980–2984 (2015).

11

RESEARCH HIGHLIGHTS

Tissue engineering

The groovy side of hydrogels

Using microfluidic spinning, AIMR
researchers have grown hydrogel fibersthat induce the hierarchical cellularorganization commonly found in skeletal
muscles and blood vessels1.
Tissue engineering is the cornerstoneof regenerative medicine. Developing
suitable scaffolds on which to growanything from bone cells to heart cells is
critical for growing artificial organs thatthe body will accept. But this is not easy.
“Many tissues in the body vary greatly incomposition, cell type and organization,”
explains Serge Ostrovidov of the AIMRat Tohoku University. “Consequently,
it is very challenging to build a material
with varying properties able to support
different cell types with their specificneeds and organization.”
Recognizing that conventional methods
for fabricating fibers were inadequate formaking the grooved structures needed tosupport complex cellular environments,
Ostrovidov and his AIMR colleagues,
together with overseas collaborators,
turned to microfluidic spinning.
Microfluidic spinning affords considerable control over the macro- andmicroscale characteristics of fibers andcan produce fibers that are centimeters
to meters in length and micrometers indiameter. Ostrovidov notes that the easeand versatility of the technique enabled
his team to concentrate on research without worrying about lengthy setting-up
and waiting times.
The choice of fiber material was critical:
instead of using alginate, a common microfluidic spinning material that strongly
repels cells, Ostrovidov and colleagues
modified the natural hydrogel gelatin withmethacrylic groups to create the polymer
gelatin methacryloyl (GelMA).

12© 2015 Serge Ostrovidov

Fibers capable of inducing complex cellular organization have been fabricated using
microfluidic spinning

Scanning electron micrograph of a fiber made of gelatin methacryloyl showing its 20-micrometer-wide
grooves and ridges.

To support complex cellular organizations, such as those in muscles or bloodvessels, fibers need to induce cells toalign as well as promote cell adhesion
and encapsulation.
The researchers tested smooth andgrooved GelMA fibers (see image) for cellalignment using myoblasts — the building blocks of muscle engineering — andfound that the grooved fibers induced
greater myoblast alignment than thesmooth fibers. They then tested grooved
GelMA and alginate fibers for cell adhesion and encapsulation using myoblasts
and bone-synthesizing cells called
osteoblasts. In both cases, the GelMA
fibers exhibited superior cell adhesion,
encapsulation and viability. “The grooved
microfeatures on the fiber both improvethe cell–material interaction and induce

cell alignment by topographical constraint,” explains Ostrovidov.
The scientists also demonstrated thatgrooved GelMA fibers could be used to‘co-culture’ two different cell types — they
encapsulated endothelial cells that typically line blood vessels within the fibers andseeded myoblasts on the fiber surfaces.
“These results really open the door tofabricating a hierarchical biological tissue
with several layers, different cell types andtissue organization,” remarks Ostrovidov.
“We will pursue the development of hierarchical tissues in the future.”
1. Shi, X., Ostrovidov, S., Zhao, Y., Liang, X., Kasuya,M., Kurihara, K., Nakajima, K., Bae, H., Wu, H. &Khademhosseini, A. Microfluidic spinning ofcell-responsive grooved microfibers. Advanced
Functional Materials 25, 2250–2259 (2015).

AIMResearch 2015

RESEARCH HIGHLIGHTS

Supercapacitors

Toward high-energy-density
materials
A supercapacitor material that has a highcharge storage capacity (capacitance) per
unit mass and operates over a very widevoltage range has been developed by
AIMR researchers1. The combination ofthese two properties makes it a promising material for high-energy-density
supercapacitors that could complement
or even replace conventional batteries.
Supercapacitors are attractive for powering devices ranging from portable electronic devices to hybrid electric vehicles
as they can harvest and deliver charge
much faster than batteries and so can berecharged in seconds rather than hours.
They can also be recharged significantly
more times than batteries. However, theytend to be very bulky — about ten times
larger than batteries — to compensate
for the fact that they store energy at
relatively low densities. Consequently,
supercapacitor materials capable ofachieving high energy densities arehighly sought after.
Now, a team led by Mingwei Chenat the AIMR, Tohoku University, has
succeeded in producing just such amaterial. Importantly, it simultaneously has a very high capacitance perunit mass (specific capacitance) andoperates over a very wide voltage range(large working potential window). While
many new supercapacitor materials havebeen developed that have one of thesetwo properties, very few exhibit both.
But since the energy density increases
with increasing specific capacitance andworking potential window, maximizing both is essential for realizing highenergy densities.
Previously, the team had found that
two-component oxyhydroxides offer
high specific capacitances but have

AIMResearch 2015

© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A material with a remarkable supercapacitor performance holds promise as abattery replacement

A nanoporous supercapacitor material consisting of an oxyhydroxide supported by interconnected metal
skeletons is a promising material for replacing conventional batteries.

narrow working potential windows
because they are unstable at high potentials. To overcome this limitation, they
produced a three-component system bydoping a nickel hydroxide with copperand manganese. They discovered that
this co-doping extended the working
potential window considerably beyond
an important bottleneck.
“The most important finding is thatthe working potential window of theoxyhydroxide exceeds the thermodynamic potential window of waterelectrolysis,” explains Chen. “Most otheractive electrode materials are limited bythis potential.”
The researchers fabricated the material
by creating nanopores in a nickel–manganese–copper alloy by leaching someof the manganese and then oxidizing
the alloy by treating it in a solution ofpotassium hydroxide to produce a nanoporous metal oxyhydroxide (see image).
The material is already attracting theinterest of industry. “We are currently
working with a couple of companies
to use this material in uninterruptible
power supplies and other large-scaleenergy storage devices,” says Chen.
The researchers also have plans tofurther optimize the structure and composition of the material.
1. Kang, J., Hirata, A., Chen, L., Zhu, S., Fujita, T. &Chen, M. Extraordinary supercapacitor performance of a multicomponent and mixed-valence
oxyhydroxide. Angewandte Chemie International
Edition 54, 8100–8104 (2015).

13

RESEARCH HIGHLIGHTS

Solar energy conversion

Full steam ahead for 3D graphene

An eco-friendly steam generator powered
only by sunlight that can purify brackish
or polluted water has been developed
by AIMR researchers1. This technology is based on a three-dimensional (3D)
nanoporous graphene material that both
captures light and moves water into local
hot zones — a combination that converts
sunlight into steam with a remarkably
high efficiency of 80 per cent.
One of the simplest ways to harvest
solar energy is to condense sunlightheated water in a solar still — a transparent plastic sheet stretched over a pit in
the ground, for example. To improveon primitive designs, researchers areinvestigating ‘generator’ nanomaterials
made from graphite powders. These
optically active materials absorb sunlightwhile floating on water and then release
this energy as heat at interfaces between
air and water where evaporation occurs.
They also have large thermal insulating
capabilities that minimize heat loss.
Yoshikazu Ito and Mingwei Chen
with their colleagues from the AIMR at
Tohoku University considered that evenhigher conversion efficiencies could beobtained by using graphene sheets in solar
stills, as these two-dimensional, extremely
lightweight supermaterials have recordbreaking optical absorption and heat-retention properties. Unfortunately, the flat,hydrophobic nature of graphene sheets
prevents them from operating as the solecomponent of solar harvesting devices.
The researchers overcame this problem
by using a protocol they recently developed
for turning flat graphene sheets into 3D
frameworks. They deposited atomicallythick carbon and nitrogen dopants onto
smooth, nanoporous nickel templates andthen dissolved the templates with acid.

14© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A renewable energy device uses sunlight to distill water into steam with an extrahigh conversion efficiency thanks to the unique properties of nanoporous graphene

An innovative 3D nanoporous graphene material uses capillary action to transport water to sunlight-powered
heating zones for solar distillation.

This strategy produced nitrogen-doped
3D nanoporous graphene structures,
35 micrometers thick and several centimeters wide, whose low densities enable
them to float on water.
When the team tested this 3D nanoporous graphene in a solar still, they
saw a 24 per cent jump in efficiency
compared to that of carbon powders.
High-resolution microscopy and thermaltransport measurements revealed thata combination of factors caused thisenhancement. The graphene units acted
as a heater by absorbing scattered light,
while the nanoporous structure localized
the solar heat radiation and continuously
pumped water to the air–water interface

via capillary action. Furthermore, thenitrogen dopants helped water droplets
stick closer to the carbon heat source,
raising the evaporation rate (see image).
“Conventional carbon heaters havea powder-like morphology that is goodat heat insulating, but poor at confining
heat and pumping water,” explains Ito.
“Our 3D nanoporous graphene improveson these weak points for a completely
clean renewable energy technology — itrequires no electricity or fossil fuel.”
1. Ito, Y., Tanabe, Y., Han, J., Fujita, T., Tanigaki, K. &Chen, M. Multifunctional porous graphene forhigh-efficiency steam generation by heat localization. Advanced Materials 27, 4302–4307 (2015).

AIMResearch 2015

RESEARCH HIGHLIGHTS

Iron-based superconductors

Superconductive FeSe
multilayer films
A new way to explore
the superconductivity ofiron selenide (FeSe) thinfilms that involves coatingthem with a layer of potassium has been developedbyAIMR
researchers.
The method has provided
insights into what causes
superconductivity in FeSe.
FeSe is an intriguing
superconductor. In its bulk
state, it superconducts
at temperatures below
8 kelvin. This onset temperature for superconductivDepositing potassium (K) atoms (orange spheres) on top of a multilayer (in this case, bilayer) film of iron selenide (FeSe) on a
ity — known as the criticalstrontium titanate (SrTiO3) substrate results in superconductivity due to doping of electrons (yellow spheres) in the film.
temperature (Tc) — rockets
up to about 65 kelvin whena single atomic layer of FeSe
is placed on a substrate of strontium(see image)1. Using this method, they
Furthermore, it is expected to lead totitanate (SrTiO3). However, when one
were able to realize superconducting
practical applications. “Demonstratingor two layers of FeSe are added to themultilayer FeSe films for the first time.
high-Tc superconductivity in atomically
FeSe monolayer, all traces of superconThe result demonstrates that superthin films represents an important step
ductivity appear to vanish. Scientists
conductivity had not been previously
toward developing next-generation
are anxious to discover the cause of thisobserved because insufficient carriers
nanoscale superconducting devices,”
variation because it may provide themwere doped from the substrate.
says Takahashi.
with vital clues about how to realize“Surprisingly, this simple method
The research team plans to investiroom-temperature
superconductors
had not been tried previously,” exgate the material further. “We suspect— the ultimate goal of researchers inplains Takahashi. “Consequently, priorthat the interface between the FeSe
this field.
studies had erroneously concluded
film and the substrate plays a critical
Researchers have long had a hunchthat multilayer FeSe films are notrole in generating superconductivity,”that charge-carrier doping of FeSe
superconducting.”
explains Takahashi, “and so we intend
thin films plays a critical role in their
The method provides a powerful
to fabricate FeSe thin films on varisuperconductivity. But the only way
way for enhancing Tc in ultrathin films
ous different substrates and observe
they could test this was to dope a filmof iron-based superconductors. The
the change in electronic structure as
via the substrate, which supplies only afinding also indicates that the origin ofwell as Tc.”
limited number of carriers.
superconductivity in FeSe monolayers
The team of Tohoku University
is probably solely electronic rather than
1. Miyata, Y., Nakayama, K., Sugawara, K., Sato, T.
researchers led by Takashi Takahashi ofdue to interactions between electrons& Takahashi, T. High-temperature superconthe AIMR struck on a new way to introand vibrations of the crystal lattice ofductivity in potassium-coated multilayer FeSe
duce carriers into FeSe films — depositthe SrTiO3 substrate, as had previously
thin films. Nature Materials 14,
ing a potassium layer on top of the films
been suggested.
775–779 (2015).

AIMResearch 201515

© 2015 Takashi Takahashi

Potassium coating allows researchers to realize superconductive FeSe multilayer
films for the first time

RESEARCH HIGHLIGHTS

Superconductivity

Striking a balanceSuperconductors conduct electricity
without resistance and hence do not dissipate energy as heat. As it is exceedingly
difficult to make superconductors thatwork at relatively high temperatures,increasing the critical temperature atwhich this intriguing phenomenon
occurs is a very active research area.
Unlike typical superconductors,
which consist of regular arrangements
of atoms, molecular superconductors arecharacterized by periodic arrangements
of molecules. Molecular superconductors that have an ordered lattice offullerene molecules (C60, also known
as ‘buckyballs’), and alkali-metal atoms(see image) currently boast the highest
critical temperature (38 kelvin) of all
known molecular superconductors.
An international team led by
Kosmas Prassides of the AIMR at
Tohoku University has investigated one
such molecular superconductor, cesium
fulleride (Cs3C60)1. By replacing some ofits cesium atoms with smaller rubidium
atoms the researchers were able to vary
the distance between adjacent fullerene
molecules within the periodic structure.
This substitution of smaller atoms mimics the effect of increasing the hydrostaticpressure, because it forces the fulleridemolecules to pack more closely together.
The researchers found that the criticaltemperature has a dome-like variation
with the density of fulleride molecules
and that the peak of this dome occurs
precisely at the point where the molecularand extended lattice features of the electronic structure are optimally balanced.
This material exhibits a wide rangeof phases: it is an insulator at ambientpressure but becomes superconducting
under hydrostatic or chemical pressure;16© 2015 Kosmas Prassides

A new metallic state is discovered in an unconventional superconductor basedon buckyballs

A schematic depiction of the lattice structure of alkali-metal fulleride superconducting materials.

in addition, it has metallic and magnetic
phases. The scientists have now identified
a new metallic phase, which they term ‘aJahn–Teller metal’ because delocalized,
metallic electrons coexist with electrons
localized on the fullerene molecules.
“We have shown that this new state,
which gives access to the highest criticaltemperature, has its origins in the electronic structure of the C60 molecule,”
says Prassides. “This study will allow
theorists to pinpoint how the competing
insulating and superconducting groundstates are connected, and experimentalists to modify the materials to control
the transition and perform detailed
measurements to further elucidate how
the electronic ground states are related.”
These molecular materials are
exciting because by tailoring thesynthetic method and the starting materials, chemists will be able to control
the chemical and electronic structureof the molecular components. Hightemperature superconductivity could beachievable by optimizing their design.
“This research direction is not possible
in the atom-based analogs that dominate
most known families of superconducting materials,” notes Prassides. “It could
eventually make superconductors viable
for widespread use and hence, greatly
increase electrical efficiency.”
1. Zadik, R. H., Takabayashi, Y., Klupp, G.,
Colman, R. H., Ganin, A. Y., Potočnik, A., Jeglič, P.,
Arčon, D., Matus, P., Kamarás, K. et al. Optimized
unconventional superconductivity in amolecular Jahn–Teller metal. Science Advances
1, e1500059 (2015).

AIMResearch 2015

RESEARCH HIGHLIGHTS

Computer memory

Ultrafast switches reveal ahidden nature
By probing crystallization rates in a special class of glasses known as chalcogenides, AIMR researchers have uncovereda ‘crossover’ in atomic mobility that may
help optimize these glasses for nextgeneration computer memory cells1.
Chalcogenides such as germanium–
antimony–tellurium (GST) and silver–indium–antimony–tellurium (AIST)
switch between glassy and crystalline
states on the application of an electrical pulse. This switching, which occurs
on nanosecond time scales, makes
chalcogenides attractive for writing anderasing digital bits since phase-change
memory devices based on thin films
can store data some 100,000 times
faster than conventional magnetic
hard drives.
One fundamental challenge withphase-change memory is the need tomake switching rapid but not so easythat data storage becomes unstable. To
get around this, designers exploit thestrong effect of temperature on atomic
mobility. For example, normally slow
transformations to crystalline states canbe hastened by heating to temperatures
just below the melting point.
The most common description ofchemical reaction rates, known as
the Arrhenius law, breaks down over
the wide temperature range used byphase-change memory devices. Instead,researchers base their analyses on chalcogenide ‘fragility’, a measure of how
viscous liquids deviate from Arrhenius
kinetics at different temperatures.
Lindsay Greer and Jiri Orava fromthe AIMR at Tohoku University andcollaborators in the United Kingdom
applied ultrafast calorimetry — a technique that employs heating rates of over

AIMResearch 2015

© 2015 Jiri Orava

The discovery of an unexpected temperature complexity in high-speed
phase-change memory devices will help resolve performance issues

A stage of an ultrafast calorimeter on which samples of silver–indium–antimony–tellurium (AIST)
chalcogenide were mounted and subjected to rapid heating rates.

10,000 kelvin per second — to measureheat flow during crystallization with sufficient precision to characterize kinetics
over a wide, practical temperature span.
The team had previously found that GST
displays fragile liquid behavior. In contrast, their latest measurements of AIST
chalcogenide crystal growth revealed
that its temperature dependence appears
to follow the Arrhenius law.
Careful calorimetry analysis of ‘supercooled’ liquid AIST samples (see image)suggested that this apparently simple
temperature dependence results froma gradual crossover from the expectedfragile behavior to a contrasting andmore Arrhenius-like ‘strong’ behavior
on cooling. This type of kinetic crossover
parallels the behavior of water, explains

Greer, but has never been explored forphase-changing chalcogenides.
“This study shows that the idea of
selecting chalcogenides based on theirfragility was too simple,” says Greer.
“This shifts our attention from the degreeof fragility to the crossover temperature
itself, and there are already clues about
how to tailor this parameter.”“We cannot promise a revolution
in device operation, but we expect
this finding will lead to a much better
understanding of performance limits,”
he adds.
1. Orava, J., Hewak, D. W. & Greer, A. L. Fragile-tostrong crossover in supercooled liquid Ag-InSb-Te studied by ultrafast calorimetry. Advanced
Functional Materials 25, 4851–4858 (2015).

17

RESEARCH HIGHLIGHTS

Superconductivity

Missing piece of jigsaw found

Superconductivity in the compound
BaC6 has been observed for the firsttime by AIMR researchers1. While its
low critical temperature of 65 millikelvinmeans that BaC6 is unlikely to find muchapplication as a superconductor, the discovery is of deep significance for gaininga refined understanding of the superconducting mechanism of ‘conventional’
superconductors.
BaC6 belongs to a group of materials
known as graphite intercalation compounds (GICs), so-called because they
consist of two-dimensional graphite
sheets with metal atoms sandwiched,
or intercalated, between them. Many
GICs are considered to be conventional
superconductors — that is, they conduct
electricity without resistance below acertain critical temperature because theirelectrons form Cooper pairs, which can
travel through the crystal lattice without
being scattered by it.
Interest in conventional superconductors has been revived by the
recent discovery of high-temperature
conventional superconductivity inpressurized hydrogen sulfide. GICs arevaluable materials for studying conventional superconductivity because of
their layered structure and because thesuperconducting critical temperature
varies greatly with the distance between
their layers. But one important observation for this family had been missing formany years — that of BaC6 (see image).
Indeed, there had been a long-standing
debate about whether it was superconducting or not — theory predicted itshould be, but experiments had failed toconfirm these predictions.
Satoshi Heguri and Katsumi Tanigakiof the AIMR at Tohoku University and18© 2015 Satoshi Heguri

A long-standing debate has been resolved with the observation of superconductivity
in BaC6, shedding light on its superconducting mechanism

The crystal structure of BaC6, where the small black spheres represent the carbon atoms of the twodimensional graphite layers and the large red spheres represent barium atoms, which are intercalated
between the graphite layers.

collaborators at the University of Hyogo
have now succeeded in observing superconductivity in BaC6.
“This finding is important because
it provides a complete picture of GIC
superconductors, which is crucial forunderstanding the mechanism of theirsuperconductivity,” says Heguri. In particular, the measurement finally allows
full descriptions of how the critical temperature varies with the distance between
adjacent layers, thus how superconductivity is controlled.
“Historically, the superconducting
mechanism of GIC superconductors
has been understood in the frameworkof the conventional electron-pairing
mechanism,” explains Heguri. “However,
our results suggest that some other factors should be considered for a complete
description. We anticipate that thisfinding will advance our understandingof two-dimensional superconductivity.”
The researchers had tried for abouta year to observe superconductivity in
BaC6. Heguri attributes their success to“improved sample quality, a speciallydesigned measurement cell and the performance of our dilution refrigerator.”
The team intends to continue investigating the parameters that demonstrably
affect superconductivity in GICs. In addition, they also want to investigate anothermaterial about which there has been muchdebate regarding whether it is superconducting — metal-decorated graphene.
1. Heguri, S., Kawade, N., Fujisawa, T., Yamaguchi, A.,
Sumiyama, A., Tanigaki, K. & Kobayashi, M. Superconductivity in the graphite intercalation
compound BaC6. Physical Review Letters 114,
247201 (2015).

AIMResearch 2015

RESEARCH HIGHLIGHTS

Polymers

Self-folding sheets wrap up
droplets
By using polymer sheets about 2,000
times thinner than a human hair, a teamfrom the AIMR and the University of
Massachusetts has developed a method
for wrapping liquid droplets into folded,
origami shapes that resemble stuffedfoods such as samosas or dumplings1.
A technique known as capillary
origami can be used to sequester toxic or
corrosive liquids from their surroundings. When a liquid droplet comes into
contact with a small rubbery plastic
sheet, surface tension effects cause thecorners of the sheet to spontaneously curl
up and fold, thereby trapping the droplet.
Depending on the initial polymer sheet
design, this approach produces a multitude of origami shapes, such as flower
petals and pyramids. However, the force
needed to bend the elastic sheets limits
the volume of enclosed liquid.
Now, Thomas Russell from the AIMRat Tohoku University and colleagues
have investigated how capillary origami
techniques perform when the energy
costs of polymer bending are effectively
removed. They used spin coating to produce an incredibly flexible polystyrene
sheet approximately 100 nanometers
thick. The researchers then carefully
manipulated the ultrathin sheet until it
rested on a water droplet immersed insilicone oil. To initiate the origami process, the droplet’s volume was gradually
reduced using a small straw.
The ultrathin sheet was nearly flat at
the beginning of the experiment, butremoving water caused the polymer
to wrinkle, crumple and fold aroundthe droplet (see image). This surprised
the researchers as they had anticipatedthat the bendable film would simply
conform to the droplet shape. “Trying to

AIMResearch 2015

© 2015 Joseph Paulsen

New geometric rules ensure that ultrathin polymer sheets encapsulate liquid
droplets with optimal efficiency

A circular polystyrene sheet, only 39 nanometers thick, partially encloses a water droplet by wrinkling andfolding. When placed on a smaller droplet, the sheet forms a shape resembling a stuffed dumpling.

analyze these wrapped shapes appeareddaunting at first,” notes Joseph Paulsen,
a co-author of the study. “However, wehave shown that for very thin sheets you
can simply ignore these complicatedsmall-scale features and still predict
the overall three-dimensional shape of
the wrapping.”
The team constructed a general geometric model that revealed the ultrathin
sheets automatically optimize theirwrapping shapes based on the volumeof the liquid droplet. “The wrapper tries
to enclose the maximum liquid volume
in a fixed area of sheet, much like how
stuffed foods try to enclose the largest

amount of filling in a circle of dough,”explains Paulsen.
The researchers found that these
efficient wrappings also lower the pressure inside the droplets. The enclosed
liquids are therefore protected fromleaking into the surrounding fluid overlong periods of time. “These sheets spontaneously select the shape that offers thebest protection from outer fluids,” points
out co-author Vincent Démery.
1. Paulsen, J. D., Démery, V., Santangelo, C. D.,
Russell, T. P., Davidovitch, B. & Menon, N. Optimal
wrapping of liquid droplets with ultrathin sheets.
Nature Materials 14, 1206–1209 (2015).

19

RESEARCH HIGHLIGHTS

Lithium–oxygen batteries

Graphene protects
cathode catalyst
A graphene electrode loaded withcatalytic nanoparticles significantly
improves the performance of rechargeable lithium–oxygen batteries, AIMR
researchers have found.
Lithium–oxygen
batteries
offer
ten times higher energy densities thanconventional lithium batteries. They arealso much lighter because one of theircrucial components — oxygen — comesfrom the air around them, rather thanbeing stored in the cathode.
When a lithium–oxygen battery
discharges, its lithium metal anode produces a current of electrons by freeing
positive lithium ions. These ions migrate
through a liquid electrolyte and enter aporous cathode, where they react withoxygen to form lithium peroxide (Li2O2).
This chemical reaction is reversed during charging.
But conventional porous carbon
cathodes often break down during thesereactions, generating other lithiumcompounds that reduce the battery’s
efficiency. Cathodes made from oneatom-thick carbon flakes, known asgraphene, offer greater stability andconductivity, but require high charging
voltages of 4.5 volts or more. Catalysts
such as ruthenium oxide (RuO2)
nanoparticles can reduce this voltage
by breaking down stored Li2O2, but thenanoparticles agglomerate into inactive
clumps after a few charging cycles.
Now, Xianwei Guo, Mingwei Chen
and colleagues from the AIMR at
Tohoku University have dramatically improved the energy efficiency and lifetimeof a lithium–oxygen battery by encapsulating RuO2 nanoparticles with graphene1.
The researchers created a porous
nickel framework as a template for their20© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Nanoparticles of ruthenium oxide trapped between atom-thin layers of carbon
boost the performance of lithium–oxygen batteries

Ruthenium oxide nanoparticles (red) trapped between two layers of graphene improve the performance oflightweight lithium–oxygen batteries.

cathode and then added the chemical
pyridine at a temperature of 800 degrees
Celsius. As the pyridine decomposed,
it coated the insides of the templatewith a form of graphene that containedsome nitrogen atoms. After loading thisnitrogen-doped graphene with RuO2
nanoparticles roughly 3–5 nanometers
in diameter, the researchers added asecond layer of nitrogen-doped graphene. Etching away the nickel left aporous cathode that contained the RuO2
nanoparticles sandwiched between two
graphene layers (see image). Lithium
ions penetrated deep into these poresand combined with oxygen at nitrogenbinding sites.
A lithium–oxygen battery containing
this cathode required an average charging voltage of just 3.7 volts, and sustained

a high energy capacity for more than
100 charge–discharge cycles. The teamconfirmed that the RuO2 nanoparticles
remained safely encapsulated between
the graphene sheets and that very few
unwanted lithium by-products formed.
The researchers hope to improvetheir cathode by using more economical materials. “We are trying to develop
low-cost cathodes with cheap catalysts
for higher efficiency, longer lifetimes andlower charging voltages to realize thecommercialization of lithium–oxygen
batteries,” says Guo.
1. Guo, X., Liu, P., Han, J., Ito, Y., Hirata, A., Fujita, T.
& Chen, M. 3D nanoporous nitrogen-doped
graphene with encapsulated RuO2 nanoparticles
for Li–O2 batteries. Advanced Materials 27,
6137–6143 (2015).

AIMResearch 2015

RESEARCH HIGHLIGHTS

Metallic glasses

Thermal cycling is the secret to
eternal youth
AIMR researchers have discovered that
simply cycling the temperature of ametallic glass ‘rejuvenates’ it, making it
less brittle1. This finding has important
implications for the nature of metallic
glasses as well as their applications,which include components in electronic
devices, biomedical implants and sporting equipment.
Metallic glasses are so called because
they are composed of metals but are
simultaneously glasses, so that their
atomic arrangement is closer to thatof liquids than of conventional alloys,
which have crystalline structures. This
structure imparts metallic glasses withhigh strength and toughness.
However, metallic glasses generally become more brittle over time,and hence increasingly susceptible tocracking. This ‘aging’ can occur at room
temperature, but is accelerated at hightemperatures, which is unfortunate because high-temperature processing canenhance their magnetic properties.
Now, an international team led
by Lindsay Greer of the AIMR at
Tohoku University has found an unexpectedly simple way to reverse this aging
at any point during their use — thermal
cycling between room temperature andliquid-nitrogen temperature (77 kelvin).
This discovery sheds light on the
structure of metallic glasses. If metallic
glasses had truly featureless structures,such thermal cycling would have noeffect. “A uniform material would
expand and contract uniformly onchanging its temperature,” explains
Greer. “The uniform expansion or contraction would be completely reversible
and there would no changes in thematerial properties.”

AIMResearch 2015

In a metallic glass, the thermal expansion coefficient (here depicted by ellipsoids) varies with location due tonanoscale structural variations. Consequently, thermal cycling sets up internal stresses (red and blue regions
indicating regions of compression and tension, respectively) in the glass.

But to the researchers’ surprise, they
discovered that relatively gentle thermal
cycling had very significant effects,
equivalent to those of much more extreme
processing. “The extra energy stored in a
metallic glass sample after ten cycles is of
the same magnitude as that expected if
a sheet of the same glass had been rolled(at room temperature) down to about
only 20 per cent of its original thickness,”says Greer.
Thus, metallic glasses cannot be completely homogeneous. “We are forced toconclude that a metallic glass is, in someway, non-uniform,” says Greer.
The thermal cycling at low temperatures
is thought to exploit tiny nanoscale variations in the structure of metallic glasses.
These variations mean that when the temperature is cycled, different regions will
experience different thermal expansions(see image), inducing internal stresses
that irreversibly alter the atomic arrangement. Such rearrangements enhance theplasticity of the metallic glass, effectively
rejuvenating it.
This finding opens up many possibilities,
Greer notes. “Rejuvenation may improveother properties of metallic glasses. Also,
the same technique may be applicable toother types of glasses, including conventional oxide glasses and polymer glasses.”
1. Ketov, S. V., Sun, Y. H., Nachum, S., Lu, Z., Checchi,
A., Beraldin, A. R., Bai, H. Y., Wang, W. H.,
Louzguine-Luzgin, D. V., Carpenter, M. A. &Greer, A. L. Rejuvenation of metallic glasses by
non-affine thermal strain. Nature 524,
200–203 (2015).
2. Hufnagel, T. C. Metallic glasses: Cryogenic
rejuvenation. Nature Materials 14,
867–868 (2015).

21

Reprinted by permission from Macmillan Publishers Ltd: Nature Materials (Ref. 2), copyright 2015

The brittleness of metallic glasses can be readily alleviated by subjecting them tothermal cycling

RESEARCH HIGHLIGHTS

Phase transformation

Switching on the next phaseof nanodevices
A way to switch the atomic packingof an inorganic crystal between two
solid phases on a nanoscale has beendeveloped by AIMR researchers led by
Yuichi Ikuhara1. This technique, whichuses a scanning transmission electron
microscope (STEM) to produce patterns
on a scale of mere nanometers, is promising for boosting the memory storage
densities of computing devices.
Phase transformations in metallic
crystals can be induced by applying amechanical force or altering the temperature or pressure. These changes to theatomic structure often modify the properties of materials in beneficial ways, such as‘shape-memory’ alloys that revert to their
original forms on heating. However, triggering phase transformations on nanoscaledimensions to realize patterned devices
is challenging because thermodynamic
fluctuations make it difficult to precisely
control atoms at such scales.
Recent advances in STEM imaging — particularly aberration-correctedlenses that produce subnanometer electron beams capable of resolving single
atoms — have created new opportunities
for controlling phase transformations onvery small scales.
Now, Ikuhara, Chunlin Chen
and colleagues from the AIMR at
Tohoku University, in collaboration
with Johannes Georg Bednorz at IBM
Research and researchers at ETH Zurich,have used a state-of-the-art STEM to
study intriguing ceramics known asstrontium niobates (SrNbOx). Slightly
adjusting the oxygen content inside thismaterial induces a transformation fromlayered atoms with insulating properties
to a more compact ‘perovskite’ crystal
packing with a high conductivity.

22

Top: A schematic showing a scanning electron transmission electron microscope (STEM) tip (yellow) producing
nanopillars (purple) with smaller dimensions than achievable by conventional lithography. Bottom: A high-angle
annular dark-field STEM image showing four such nanopillars, whose size and spacing are less than 15 nanometers.

The team grew single crystals of SrNbOx
and thinned them to microscale dimensions for STEM imaging. Initially, a layered
structure appeared consisting of slabs ofstraight atomic chains alternating withslabs containing zigzag arrangements ofatoms. Then, by increasing the STEM beam
current for several minutes, the researcherssqueezed oxygen atoms out of the crystal
structure with an atom-by-atom accuracy.
Images captured the phase transformationin action, showing the chain-like slabs zipping together to form the perovskite phase.
“It was exciting to see these images,”
notes Chen, “because we realized thisphase transformation could be used tofabricate nanodevices.”
To test this possibility, the teamused the scanning STEM beam to create ‘nanopillars’: three-dimensional,

elongated islands of the perovskite phasein a layered SrNbOx matrix (see image).
The atomically sharp island boundaries
and 5-nanometer interspacings they produced have the potential to realize higher
storage densities in chips than modern
memory devices, in which 20-nanometer
separations are the norm.
The next challenge for the researchers
is to fabricate the SrNbOx material in athin-film device suitable for prototyping.
They anticipate STEM-based phasetransformations will have applications
in material processing as well as nextgeneration nanodevices.
1. Chen, C., Wang, Z., Lichtenberg, F., Ikuhara, Y. &Bednorz, J. G. Patterning oxide nanopillars at
the atomic scale by phase transformation. Nano
Letters 15, 6469–6474 (2015).

AIMResearch 2015

Adapted with permission from Ref. 1. Copyright 2015 American Chemical Society.

Atomically precise phase transformations generate ‘nanopillar’ structures withsmaller device features than conventional lithography

RESEARCH HIGHLIGHTS

Magnetism

Electric field reveals the‘hole’ story
An electric field can fine-tune a crucial
property of a ferromagnetic material,
AIMR researchers have found. Theirapproach could be useful in applications such as spintronics, a promising form of low-energy computingthat aims to encode data using the spinof electrons.
Spin causes an electron to behave likea miniature bar magnet. Ferromagnetic
materials typically contain unpaired
electrons whose spins all point in
the same direction, so that the wholematerial becomes magnetic — or more
specifically ferromagnetic.
An external magnetic field canalter the orientation of these spins,with the size of the effect depending partly on the Gilbert damping
constant, an intrinsic property of thematerial. But the factors that determine the damping constant itself arepoorly understood.
Fumihiro Matsukura, Lin Chen and
Hideo Ohno of the AIMR at Tohoku
University have now used an electricfield to control the Gilbert damping constant of a semiconductor called gallium
manganese arsenide, or (Ga,Mn)As1.
Whereas metals carry electrical current as a flow of electrons, current in(Ga,Mn)As is mostly carried by ‘holes’— positively charged points in thematerial where an electron is missing.
These holes interact with unpaired electrons on the material’s manganese ions
to make the material ferromagnetic.
The researchers fabricated several
devices that contained a 4-nanometerthick film of (Ga,Mn)As on a gallium
arsenide substrate, covered with alayer of insulating alumina and agold–chromium electrode (see image).

AIMResearch 2015

© 2015 Fumihiro Matsukura

Applying a voltage across a ferromagnetic semiconductor can control its magnetic
properties, offering a way to create better spintronic devices

The magnetic properties of the gallium manganese arsenide film in this device can be controlled byan electric field.

The devices contained different proportions of gallium and manganese in the
ferromagnetic layer.
Subjecting the devices to external magnetic fields at various angles, and underpositive and negative voltages, allowed
the team to measure how the Gilbert
damping constant changed. These ferromagnetic resonance experiments showedthat the key factor in determining thedamping constant was the hole concentration in the (Ga,Mn)As film.
Areas of (Ga,Mn)As that have a lot ofholes are fully ferromagnetic, whereas
regions with relatively few holes aremuch less magnetic. Applying a voltage changes the distribution of holes in(Ga,Mn)As, which affects the balance
between these two regions.

“This indicates that the damping
constant of (Ga,Mn)As can be modulated electrically,” says Matsukura. Thisapproach could also help to reveal
the mechanism that determines thedamping constants in other materials,
he adds.
“Electric-field effects on magnetism
are interesting from a fundamental
physics point of view, as well as toexplore new functionality in spintronics
devices,” he adds. “We hope to use theseeffects to realize practical devices withlow power consumption.”
1. Chen, L., Matsukura, F. & Ohno, H. Electric-field
modulation of damping constant in a ferromagnetic semiconductor (Ga,Mn)As. PhysicalReview Letters 115, 057204 (2015).

23

IN THE SPOTLIGHT

IN THE SPOTLIGHT
The AIMR has grown rapidly since its inauguration in 2007. It now boasts over 140 leading researchers from all over the world, including
29 internationally renowned principal investigators who are charged with pioneering new and innovative breakthroughs in materials science. The
institute is also active in developing young, promising researchers with a focus on strong cross-disciplinary collaboration and creativity. AIMResearch
spotlights these talented researchers of the present and future, detailing their daily research activities and scientific ambitions.

Published online on 30 March 2015

INTERNATIONAL SYMPOSIUM

Materials and mathematics
collaboration: A new horizon
Leading scientists attended AMIS2015 to share the scientific benefits of fusion researchProminent physicists, mathematicians and materials scientists
converged on Sendai, Japan, toparticipate in the AIMR International
Symposium (AMIS) 2015, held on
17–19 February. Under the theme of“A new horizon for materials science withmathematics collaboration,” the conference
showcased many examples of interactions
between mathematics and materials science, including the theoretical discovery ofpreviously unknown topologically insulating materials and optical singularities ofanisotropic crystals.
Top-quality science
Susumu Satomi, president of Tohoku
University, made the opening remarks byextending a warm welcome to the participants. In his address, he recognized theexcellent research and strong international
partnerships initiated at the AIMR under
the leadership of Director Motoko Kotani.
“AIMR’s experiences have become a driving
force for promoting system reform and internationalization of the whole university,”he said. In July 2014, Tohoku University
established the Organization for Advanced
Studies (OAS), which is modeled on the
AIMR and which incorporates the AIMR
as the first research institute of the OAS.
Representing the WPI, Deputy Program
Director Akira Ukawa shared the results ofa 2014 review of five WPI centers, including
the AIMR. The goals of the WPI program
are to produce top-quality science and initiate internationalization, fusion studies andresearch systems reform. Ukawa remarked
that all five centers had achieved World
Premier Status by fully meeting these ambitious goals. “The WPI Program Committee
was impressed by the extremely high levels

24

268 researchers representing 36 research institutes across 14 countries, including 34 invited speakers, attended
the AIMR International Symposium 2015 in Sendai on 17–19 February.

of science being advanced by the five WPIcenters,” said Ukawa, who cited, among
other examples, their significant contributions to the top one per cent of most highly
cited papers published between 2007 and2013, according to an analysis by Thomson
Reuters. “All five centers achieved World
Premier Status.” Furthermore, Ukawa notedthat the AIMR had made remarkable progress in mathematics–materials science collaboration under the leadership of Kotani
and had realized a highly international
research environment by implementing a
number of system reforms.
Speaking next, Kotani offered a personal
introduction to the AIMR and its researchgoals. “In 2011, we made our identity clear,which is to create a basis for new materials
science that predicts functions and designs
structures through mathematics and materials science collaboration,” she said. “Suchinstitution-level collaboration is unprecedented and has invited significant interest
from both communities.” She encouraged
participants to attend the poster session,
where close to 100 displays offered tangible
proof of the merits in AIMR’s approach.
Condensed-matter physics and theYear of Light
The first scientific presentation of the
day was by Duncan Haldane, professorof physics at Princeton University and2012 Dirac medalist for advancing ourunderstanding of curious materials known
as topological insulators, which conduct
electricity only on their surfaces. Haldane
shared the story of how these materials
were theoretically predicted before they
were experimentally observed, eventually
becoming the hottest topic in condensedmatter physics. “Topologically insulating

AIMResearch 2015

IN THE SPOTLIGHT

materials existed on shelves in labs under
the noses of materials scientists,” he said.
“But no one told them that they should be
looking for something interesting there.”
Variants of the topological insulator
state that exhibit unique quantum phenomena have since been observed in two- andthree-dimensional materials, bringing withthem the promise of spintronic devices
and practical quantum computers. In one
example, researchers have discovered materials that physically separate electrons traveling in different directions to enable theone-way movement of information — “likein a divided highway,” Haldane explained.
Speaking next was Sir Michael Berry,
professor of physics at the Universityof Bristol in the United Kingdom andrecipient of the 1995 Dirac medal forhis discovery of a widespread effect inmicroscopic physics known as the Berryphase. Berry took the occasion to celebrate
the 2015 International Year of Light fromthe perspective of geometry. Light with anelectric field oscillating in a single direction, known as polarized light, is often used
to identify crystal properties. “Every mineralogist knows this old technique,” said
Berry. He decided to instead use crystals to
identify the polarization structure of light.
“Crystal optics is a nineteenth-century subject, but a lot is hidden in it that hasn’t been
extracted,” said Berry. When polarized lightwaves cross paths, they form distinct points
known as singularities. Berry identified
several types of singularities using crystal

Condensed-matter physicist Duncan Haldane
shared the story of how topologically insulating
materials were theoretically predicted before being
experimentally discovered.

AIMResearch 2015

models. “These singularities are fundamental to light.”
Returning to the subject of condensedmatter physics, Erio Tosatti, a professor
at the International School for Advanced
Studies in Italy, discussed the merits ofdeeper collaboration between two theoretical camps in the branch of physics that
explores the properties of matter. On one
side are inventors of simplistic models,
which Tosatti likened to the wooden puppet
Pinocchio. And on the other side are those
who perform realistic calculations andsimulations to “transform the puppet into areal boy,” said Tosatti. “Both approaches areclearly essential.” He offered three examplesof his group’s attempts to bring the two
closer together in the areas of strongly correlated superconductivity, current transport
through nanocontacts and friction. “It is forthe present generation to find formulations
that join models and realistic calculations
together,” he said.
Hydrogen embrittlement, solid liquids
and quantum brains
Researchers who discussed specific materials at the meeting included Petros Sofronis,
director of another WPI institute called theInternational Institute for Carbon-Neutral
Energy Research (I2CNER), and Thomas
Russell, a principal investigator at the AIMR.
I2CNER is currently pursuing researchthat will bring us closer to an economy
based on hydrogen, from production
through to transport, storage and ultimate
consumption in fuel-cell vehicles. “In allthese applications, materials are exposed
to hydrogen, and we need to worry about
their interaction,” said Sofronis, who offereda tentative diagnosis for a specific type ofmaterial failure caused by hydrogen that ispoorly understood — hydrogen embrittlement. “We need to develop predictive models on how to tame the problem of hydrogen
embrittlement,” he said.
Russell introduced an entirely new classof materials that lie on the border between
a liquid and a solid. By mixing two repellent
liquids, such as water and oil, with functionalized nanoparticles and polymers, his
team was able to create liquid droplets that
had arbitrary solid shapes by applying an
external field, like elongated teardrops, forexample. Even after removing the external

On the occasion of the 2015 International Year of
Light, mathematical physicist Michael Berry presented
research on the different types of polarization
singularities that can occur in crystals.

field, these structured liquids retained thedeformed shape and properties of the two
original liquids, a peculiarity that could
be exploited to produce liquid circuits andall-liquid batteries, among other applications. “We have taken one plus one to getsomething other than two,” said Russell.
The final speaker of the opening session was Kazuyuki Aihara, professor at the
University of Tokyo in the field of mathematical engineering. Aihara shared his
work on developing mathematical models ofneural networks that could contribute to an
artificial quantum brain. He demonstrated
an example of a two-million-dimensional
chaotic neural system that can retrieve
stored images successively, intermittently
and chaotically. “Just as human beings invented airplanes in the likeness of birds,
we can make a very strong information
processing machine inspired by the brain,”he said.
“I am really proud to have been able toinvite researchers who are leaders in theirfield in the contemporary and historicalsense,” said Kotani of AMIS2015. Attending
the conference were 268 researchers
representing 36 research institutes across
14 countries, including 34 invited speakers.
The opening session was followed by threedays of plenary and parallel sessions. “I
promise to continue our efforts to act as aworld leader in our research area and contribute to society.”

25

IN THE SPOTLIGHT

DIRECTOR’S INTERVIEW

Published online on 25 May 2015

Reaching for the world
The AIMR celebrates its newly awarded ‘World Premier Status’ and joins forces with research centersaround the worldA

fter an eventful year in which the
AIMR was evaluated as having
met the rigorous conditions for‘World Premier Status’, Director Kotani
reflects on the institute’s strengthened
international research network and its
strategy for the year ahead.
AIMResearch: In 2014, the World PremierInternational Research Center Initiative(WPI) program committee conducted amajor evaluation of five WPI centers andconcluded that the AIMR had achieved
World Premier Status. What is the significance of this assessment?
I am very gratified that the AIMR has
been recognized as a globally visible,
international research hub for worldclass materials science. Achieving World
Premier Status represents a very special
milestone for us, as it recognizes theeffectiveness of our approach in realizing
the mission of the WPI.
A key objective of the WPI program isto produce top-quality science, which has
always been an extremely high priority at
the AIMR. Researchers at the AIMR havepublished many highly cited scientificpapers in prestigious journals such as
Science, Nature and Nature-branded
sister journals, in addition to receiving
internationally acclaimed awards. For
example, three AIMR researchers werelisted in Thomson Reuters’ Highly Cited
Researchers 2014. Also, Mingwei Chen
won the Materials Today ConferenceAward for his outstanding contributions to the field of materials science,
while Shin-ichi Orimo was awarded
the Science of Hydrogen & EnergyAward 2015.

26

Motoko Kotani, director of the AIMR.

The WPI program committee also
recognized that the AIMR has achieved
a high level of internationalization.
We have made a great effort to recruit
outstanding researchers from aroundthe world, and now almost half of AIMR
researchers come from abroad. Moreover,
we promote international joint research
projects through AIMR Joint ResearchCenters in China, the United Kingdom
and the United States, together with a network of 15 overseas partner institutions.
The WPI program actively seeks tobreak down the boundaries between
traditional disciplines by engaging incross-disciplinary research. At the AIMR,
we emphasize the use of mathematics
in materials science research and have

AIMResearch 2015

IN THE SPOTLIGHT

initiated three Target Projects to ensure
closer mathematics–materials sciencecollaboration. Mathematics is a very
powerful tool for guiding materials science research. The committee recognized
our remarkable progress in establishing aunique and convincing identity within ashort time.
Our proactive efforts to introduce
system reforms at the AIMR were also
crucial to achieving World Premier
Status. Among other initiatives, we haveintroduced a top-down approach to decision making, established a merit-basedsalary system and adopted English as ourofficial language. In this, we have received
strong backing from our host institute,
Tohoku University.
AIMResearch: What are some waysthat the AIMR has been promoting
collaboration with overseas researchers
and institutions?
Collaboration is not just about establishingfriendships but also about engaging moredeeply in joint research. International collaboration occurs on various levels at the
AIMR. We encourage our researchers toinitiate conversations and identify mutualareas of interest with other researchers intheir field. Many of our principal investigators are jointly affiliated with an overseas institution. At the institutional level,
we try to be selective with our alliances,setting up key long-term partnerships
with distinguished research institutes
that will bring us closer to achieving ourresearch goals. Last April, we signed anagreement with the University of Chicago
to establish a Joint Research Center, which
will be a valuable addition to our existing partnerships with the University ofCambridge, the Institute of Chemistry at
the Chinese Academy of Sciences and the
University of California, Santa Barbara.
AIMResearch: Are there any upcoming
events that you are excited about?
In May, we will enter into a formal agreement with Harvard University as wellas participate in a joint workshop on theemerging field of quantum materials
and devices.

AIMResearch 2015

The coming year will also see major
developments in spin-centered science,
which has been an important area of
research for the AIMR. Prominent
condensed-matter physicists such as
Sir Michael Berry — whose discoveries
have been groundbreaking for the realization of spintronic devices — were invited
to speak at our annual AIMR International
Symposium in February 2015. And the
Tohoku Forum for Creativity is organizing
a forum on spintronics from September
to December 2015, in which leading
scientists will be invited to share anddiscuss their latest findings in spin-related
phenomena. The program committee ismade up of researchers from the AIMR
and Tohoku University.
Tohoku University is the only Japanese
member of SpinNet, a tight-knit international network that seeks to develop
energy-efficient spin-based technologies
and support the education of students
and researchers in the field. Establishedin 2013, the network is sustained by a
one million euro commitment from theGerman government over four years. In
February 2015, Tohoku signed an agreement with a partner institute of SpinNet— Johannes Gutenberg University ofMainz in Germany — to launch a jointly
supervised PhD program in the naturalsciences and engineering, with an emphasis on spintronics.
AIMResearch: What do you intend to
focus on this year?
This year, I want to put more emphasison the realization of our unique approach. I am more interested in adopting
a potentially revolutionary approachto materials science than in studying
specific materials. The sheer complexityof materials systems has meant that, in thepast, most research in materials sciencehas progressed on the basis of unguided
trial and error. But now that the relevant
areas of mathematics have matured, we
can use mathematics to conduct researchon materials in a more directed manner.
This has the potential to save a lot oftime in the laboratory. Tohoku University
is very strong in the field of materials
science and its researchers have generated

Kotani is passionate about using mathematics toinform materials science research.

vast amounts of experimental data. But
this data cannot be used effectively without appropriate descriptions. The AIMR
seeks to provide materials scientists with
the necessary vocabulary for systematic
discovery and design of materials.
Mathematics is a kind of treasure box
containing tools known only to mathematicians. I would like to open this treasure
box to materials scientists and help themdiscover new applications. Take, forexample, the emerging field of persistenthomology, which is a new subject even formathematicians. Unlike the situation intheoretical physics, in materials science it
is not possible to create ideal conditions
or environments. Real systems containmany impurities and defects, whichmeans that researchers have to abstract
the robust properties of a system. As afield concerned with robust topology,
persistent homology offers a powerfultool for materials scientists.
This year, I would like to focus onproving that materials–mathematics
fusion research is more effective than the
traditionally split approach. Of course,
this is a very difficult undertaking,but I am a mathematician so I have toprove things.

27

IN THE SPOTLIGHT

ROUNDTABLE INTERVIEW

Published online on 27 July 2015

Ambitious researchers explore thefuture of materials science
With prestigious ERATO grants, two researchers at the AIMR plan to usher in a post-nanocarbon era and create a
new science of spin beyond spintronics

O

rganic chemist Hiroyuki Isobe
and condensed matter physicist
Eiji Saitoh at the AIMR haveboth received grants for projects under
the prestigious research fund Exploratory
Research for Advanced Technology(ERATO). Isobe’s laboratory, establishedin 2013, is creating new organic materials containing covalent bonds known as
π bonds, which are commonly found inaromatic compounds such as benzene
and DNA nucleotides. His team has
begun incorporating these solid materials into organic devices with surprising
results. Saitoh’s project was approved by
the ERATO program in 2014. He aims
to exploit the constant and unidirectional nature of spinning electrons to
produce power sources for nanoscale
mechanical devices.
AIMResearch spoke to both researchers about their ambitious goals and theimportance of collaborative research.
AIMResearch: Professor Isobe, youhave said that we are entering a ‘postnanocarbon era’. Could you elaborate on
what you mean by this?
Isobe: My undergraduate research involved isolating carbon-based structures
known as fullerenes. We were very excitedabout these new spherical nanocarbon
structures. Then, in the late 1990s, wewere introduced to another type of nanocarbon called carbon nanotubes, also withunique properties. While fullerenes aremolecular entities, the larger nanotubes

28

Hiroyuki Isobe (left), an organic chemist, and Eiji Saitoh (right), a condensed-matter physicist, agree that one of thebest things about working at the AIMR is the rich interaction between researchers working in very different fields.

are a mixture of various structures. One
question remained in my mind — werethese fullerenes and nanotubes reallynecessar y to discover unique andextreme properties? Could such uniqueproperties be developed using simpler
aromatic systems? This is what I call the
post-nanocarbon era, where chemists
are inspired by nanocarbon structures tocreate discrete molecular versions of themthrough exploratory research.
AIMResearch: How is your Degenerate
π-Integration Project under the ERATO
program bringing this goal closer
to reality?
Isobe: Our recent breakthrough camewhen we succeeded in synthesizing finite

versions of carbon nanotubes using aromatic hydrocarbons. This led us to make ahuge jump: we were able to explain the
curved π-systems of nanocarbons and theorigin of their uniqueness by using the welldeveloped language of chemistry. But this isjust the beginning. We found that we could
encapsulate spherical fullerenes in these carbon nanotubes with the highest association
force ever observed in a supramolecularcomplex. Despite this strong force, the bearing is still free to rotate. This is a very exciting second step in our exploration of new
phenomena or properties. We are currently
looking for clues to take the third step.
AIMResearch: Professor Saitoh, youhave described your Spin Quantum
Rectification Project under the ERATO

AIMResearch 2015

IN THE SPOTLIGHT

program as ‘constructing a new paradigmof physics and matter’, in which you plan
to expand concepts in quantum mechanics to the field of mechanics. Could you
explain what you mean by this?
Saitoh: The field of spintronics emergedin the early 2000s as a new paradigm ofelectronics that uses electron spin in thesame way that conventional electronic
devices use electron charge, sometimes
using both spin and charge in combination. Spintronics basically uses spinningelectrons as small magnets, but it tends tooverlook the fact that they actually rotate.
I want to create the seeds for a new science of spin beyond spintronics.
Quantum mechanics says that
an electron has spin and will rotate
forever. We cannot stop this rotation.
Thus, electrons can be thought of as verysmall motors — perpetually self-rotating
motors — and can therefore be used toredirect fluctuations in matter to flow only
in one direction, such as for the conversion of waste energy into useful energy.
Take the famous Watt steam engine, forexample. It consists of a water boiler anda rotating pump that redirects the random
fluctuations of water molecules to produce
mechanical motion. By embedding thisnatural rectification property of electron
spin into nanoscale devices, we canprovide a ubiquitous source of power for
actuators or even create energy converters.
AIMResearch: What are the benefits ofworking at the AIMR and how do they add
to your work?
Saitoh: The degree of freedom. We have very
few constraints here.

between researchers in different disciplines
for your research?
Saitoh: My team consists of researchers
from a wide range of backgrounds, including physicists, materials scientists, mathematicians and even cosmologists. Before
joining the AIMR, I focused on physics— applying mathematics only to rationalize
or formulate physical phenomena. But after
joining the AIMR, I realized that as a wayof thinking, mathematics has a lot to offer.
Mathematicians want to know the structureor essence of a thing. Using mathematics,
we have been able to discover similarities
between two apparently different physicalphenomena.
In quantum mechanics, there are only
two objects for which the direction of timecannot be reversed — a black hole anda spinning electron. Just as electron spin
can rotate in only one direction, a blackhole can only expand, it cannot shrink.
Mathematically, these concepts are very
similar. Basically, we have to employ thesame symmetry concepts as general relativity to fully understand why the angularmomentum of an electron and the body
hosting the electron remains constant— this is a very new idea that no-one hasthought about before. Mathematics allows
us to discuss independent phenomena on
an equal footing and gives us new insights
into nature.
To implement these concepts, we needmaterials science and engineering. Ourstarting point has been the cutting-edge
technology of very small machines known
as microelectromechanical systems(MEMS). They lie at the intersection of

Isobe: The best thing about working at the
AIMR is meeting brilliant guys like him! It’s
really stimulating to hear them speak about
concepts that I’m not familiar with.
Saitoh: I feel the same. The two of us occasionally exchange ideas and have evenrecognized some similarities in our research
directions. His fullerene-and-carbonnanotube bearing is a molecular rotatorand mine — the spinning electron — is aquantum rotator. It might be a coincidence,but it smells interesting!
AIMResearch: We hear a lot these days
about the importance of cross-disciplinary
research. How significant is collaboration

AIMResearch 2015

Isobe is excited about the prospect of entering a postnanocarbon era in materials research.

Saitoh sees spinning electrons as quantum rotatorsthat can provide a ubiquitous source of power forelectromechanical systems.

theoretical physics and nanotechnology, butwhile MEMS are very popular among engineers, physicists don’t know much about
them. We have already begun to exchange
information to test whether our theoretically
predicted phenomena can be deployed inpractice using MEMS.
Isobe: Collaborative research is not about
shouting at each other from opposite banks
of a river — it’s about stepping into thewater to communicate more freely. Thiskind of collaboration has really benefited
our group as well and has led to new molecular designs.
When researchers at Kodak first developed organic light-emitting diodes (OLEDs)
in the early 1980s, they separated organic
semiconductor materials into two types
based on their understanding of conventional light-emitting diodes and inorganic
materials — a ‘p-type’ hole transporter andan ‘n-type’ electron transporter. Commercial
OLEDs are therefore made of five to six
layers of different materials, each layer
fulfilling a separate function to produce
luminescence.
But it turns out that certain hydrocarbons can act as both electron and holetransporters. This discovery came out ofan exchange between a materials scientist
and an organic chemist in my group. The
chemist asked: Do we really need an n- andp-type, since there is no such thing as ‘n’ and‘p’ for organic molecules? The chemist wasbrave enough to question assumptions taken
for granted in the other field. As a result, wehave been able to develop an OLED device
made of a single layer of one molecule.

29

IN THE SPOTLIGHT

INTERNATIONAL WORKSHOP

Published online on 28 September 2015

Strengthening partnerships
across continents
Researchers from the AIMR traveled to the United States and France to share new ideas with old friends

A

IMR researchers shared their latest
innovations at several international
workshops held in the United Statesand France during May. Speakers moved
with ease through the many dimensions ofmaterials science and mathematics, fromproposing candidate spintronic devices
for rapid and reliable information storage,
to observing atoms, assembling molecularunits and discussing abstract theoreticalconcepts. The global tour also strengthened
relations between the AIMR and its key
partners, including Harvard University, theC’Nano French centers of competence inthe nanosciences and members of the wider
WPI community in Japan.
Reuniting at Harvard University
Over the past five years, the AIMR
has maintained a strong partnershipwith Harvard University founded onshared research interests and close
collaboration between two physics
professors: Robert Westervelt, directorof the Center for Integrated Quantum
Materials at Harvard University, andHideo Ohno, a principal investigator at the
AIMR. In 2013, delegates from Harvard
attended a joint workshop hosted by
Tohoku University; this time, researchers
from Japan visited the Massachusetts
campus of Harvard University to attend
two workshops held consecutively on May
21 and 22. This occasion was marked by
the extension of the academic co-operationagreement between Tohoku and Harvard
for an additional five years.
On the first day of the workshop,
researchers from countries as far flung as
China, Switzerland and the Netherlands
spoke on the theme of “Frontiers in
Quantum Materials and Devices”. Among

30

Participants at the “Frontiers in Quantum Materials and Devices” workshop and the “Tohoku/Harvard Workshop”
at Harvard University in the United States.

them, Ohno presented the pros and consof different spintronic nanodevices forrealizing nonvolatile very large-scale integration (VLSI), in which intelligent systems
are integrated on a single chip. Specifically,
he compared the data capacity, read–write
speed and reliability of device structures
based on magnetic tunnel junctions withthose based on current-induced magnetization reversal. “The performance of eachdevice is rapidly improving and practicalnonvolatile VLSI will soon become areality,” he said.
The oral presentations were followed bya poster session at which young researchers shared their work. Among them werefour young researchers from Tohoku
University, including an assistant professor
from the AIMR, Koji Sato, who is taking
a theoretical approach to the study ofquantum materials.
Topological insulators have attracted
considerable excitement because of their
exceptional properties, which originate
from their spatial configuration, or

topology: they are insulators in the bulk,
but their surfaces are conductors. These
topological properties are described by
topological indices, which can be obtained
from their bulk properties or from theiredge properties. Topological indicesobtained in these two ways have beenshown to coincide — a principle referred
to as bulk–edge correspondence. Sato has
provided a way to formulate the bulk–edgecorrespondence of topological insulators
through the mathematical framework of
K-theory. “Collaboration with mathematicians at the AIMR has been an eye-opening
experience for me,” he said. “It has exposed
me to interesting ideas and different angles.”The workshop on the second day
was exclusively for Tohoku and Harvard
researchers to present research on a broad
range of subjects. The day began with theMemorandum of Understanding Signing
Ceremony between Tohoku University
President Susumu Satomi and Richard
McCullough, vice provost for research at
Harvard University. This followed with

AIMResearch 2015

IN THE SPOTLIGHT

Koji Sato, an assistant professor at the AIMR, spoke to young researchers from Harvard University and
Massachusetts Institute of Technology about the results of his theoretical study carried out in collaboration
with mathematicians.

introductions by Toshiya Ueki, executive
vice president of Tohoku University,
Motoko Kotani, director of the AIMR,
William Wilson, executive director of theHarvard University Center for Nanoscale
Systems, and Westervelt.
Among the first speakers was
Hiroyuki Isobe, a principal investigator at
the AIMR, whose laboratory recently succeeded in synthesizing a molecular bearing
composed of freely rotating fullerene
molecules embedded in finite single-walledcarbon nanotube molecules. The bearing,said Isobe, could help realize nanoscalemachines with minute friction at the interface — a prediction renowned physicist
Richard Feynman made in his seminal 1959
lecture There’s plenty of room at the bottom.
Isobe suggested that single-molecule

Shigemi Mizukami, a principal investigator at the
AIMR, presented his research on spin-based devices
known as magnetic tunnel junctions at a workshop in
Rennes, France.

AIMResearch 2015

chemical synthesis offered the most powerful technology to realize, for instance, tinymolecular motors.
The success of both workshops in the
United States shows promise for the continued collaboration between the AIMR andHarvard University.
France–Japan connection
Flying across continents, AIMR researchers
made their way to northwestern France
to attend NanoMat 2015 in Rennes — asister city of Sendai. The event was coorganized by six French C’Nano centersand four WPI institutes (MANA, iCeMS,
I2CNER and the AIMR) and was spreadover four days, 27–30 May 2015. Five
AIMR researchers were invited to givetalks and four young researchers presented
posters at the gathering. The visit included
a reception at Rennes City Hall, where
Director Kotani gave a speech on behalf of
the Japanese participants.
On the first day of scientific presentations, Shigemi Mizukami, a principal
investigator at the AIMR, shared resultsfrom his experiments in which he useddifferent materials to make magnetic tunnel junctions. These spin-based devices
are already a core component of many
spintronic technologies, notably magnetic
read–write heads in hard disk drives andmagnetic random access memory, butthey have a huge unrealized potential for
medical and technical applications, suchas detecting magnetic nanoparticles andmeasuring magnetic fields radiating fromthe brain. Researchers, however, havestruggled to develop magnetic tunnel junctions sensitive enough to measure weakmagnetic fields. Mizukami introduced thejunction devices recently developed byhis collaborators, which could achieve theperformances required to detect magnetic
fields as weak as a picotesla, as demonstrated in his laboratory. Furthermore,
Mizukami’s team has made headway indeveloping manganese-based magnetic
tunnel junctions for application in highdensity memory and telecommunications,as well as organic magnetic tunnel junctions, which could preserve spin information over extreme lengths.
Taro Hitosugi, a junior principal
investigator at the AIMR, revealed thenanoscale surfaces and interfaces of an
intriguing class of compounds called
transition metal oxides, which are known
for their versatile electronic, chemical,
mechanical and thermoelectric properties.
He presented clear images of atoms on thesurfaces of thin films of oxide materials,
as well as the electronic structure of thesmall area corresponding to each atom,
observed using scanning tunnelingmicroscopy and spectroscopy. Hitosugi
further impressed the audience with athree-dimensional model of the surface
atoms, which was constructed based onscanning tunneling microscopy images
using a three-dimensional printer. The
insights obtained from scanning tunnelingmicroscopy and spectroscopy could also
be used to obtain clues on how to improve
lithium-ion battery performance. For
example, the atomic-scale investigation of
the electrode–electrolyte interface could
enable higher lithium-ion conduction
across the interface.
Participants gathered at the poster
session, where Natsuhiko Yoshinaga,
an assistant professor at the AIMR’s
Mathematical Science Group, shared
his theoretical research. Yoshinaga useda mathematical model to describe theformation of defects in a crystal — a poorly
understood dynamic phenomenon.
The workshop organizing committee agreed to reunite next year for
another France–Japan workshop at Kyushu
University, in the southern island of Japan.
“The workshops in the United Statesand France are integral components of the
AIMR’s strategy to promote international
collaboration,” said Masaru Tsukada,administrative director at the AIMR
who coordinated the Japan delegation
attending the workshops. “The reputationof AIMR has been steadily increasing
through such initiatives.”

31

IN THE SPOTLIGHT

INTERNATIONAL WORKSHOP

Published online on 30 November 2015

Taking spintronics for a spin
A new center established by the AIMR and Tohoku University plays host to leading researchers
in the field of spintronics

S

pintronics, the study of electron
spin, underpins the high storage
capacities of today’s disk drives andcould lead to smaller, faster, more powerful
and more efficient memory and computer
processing technologies. Research into theelusive quantum mechanical property isaccelerating, with new avenues for spinbased electronics suggested every year.
From September through to
December, leading researchers, includinga Nobel laureate, participated in a seriesof events at Tohoku University aimed
at plotting the course of spintronics
research. The gathering was the resultof years of advocacy by AIMR Director
Motoko Kotani to create a space dedicatedto entertaining visiting researchers andorganizing interactive events.
“This style of institute is common inmathematics and theoretical physics andworks very well for locating important
emerging problems,” says Kotani, referring to institutes in the US and Europe.
“We set up the first such institutein Japan.”
The Tohoku Forum for Creativity wasestablished in 2013 with support from theJapanese government and semiconductor manufacturer Tokyo Electron. The
spintronics workshop series follows ahost of other events on black holes andbrain science.
Mathematics explained
Kotani organized the first workshop of theseries on October 5–9, which examined
potential applications of a field of mathematics called topology for spin science.
Topological insulators are novel materials

32

Spintronics researchers attended a series of workshops at the TOKYO ELECTRON House of Creativity, a new centerestablished at Tohoku University after years of advocacy by AIMR Director Motoko Kotani.

that conduct electricity on their surfaces
due to unique spin phenomena. Thesematerials have topologically protectedstates, which are theoretically predictedby applying advanced mathematics. New
discoveries in physics and materials science often inspire new mathematics, andvice versa. Kotani wanted to organize aworkshop that would facilitate such happy
encounters — an unusual new experiencefor some of the estimated 60 participants.
“As a mathematician, I was surprised
to be invited,” says Spyridon Michalakis
f rom t he Inst itute for Q uantum
Information and Matter at Caltech. But
Michalakis quickly realized just how
much he had to offer. “It seems that wehave come to a place in mathematics
where we can start giving good hints tothe materials scientists about the kindsof structures to look for, or to create,” hesays. “We can even offer advice on how
to probe these materials to get exotic

properties that could be used for quantum
computing, among other applications.”
Mathematician Bruno Nachtergaelefrom the University of California, Davis,
was also impressed by the design of theTOKYO ELECTRON House of Creativity,in which the forum is housed. The layoutof the building encourages researchers to exchange ideas on wall-to-wall
blackboards while enjoying a coffee ortea. “If you want mathematics explained
to you then it is always good to have a
blackboard around to jot down a coupleof formulas and go through it in conversation,” he adds. “Otherwise, it can be very
difficult to penetrate a field that is not
yours just by reading specialized papers.”
Quantum nanostructures
The theme of the second workshop(October 19–21) was the emerging fieldof quantum nanostructures, focusing on
the interaction between the electron and

AIMResearch 2015

IN THE SPOTLIGHT

nuclear spin in such systems. Nuclear
spin has been widely exploited innuclear magnetic resonance (NMR)
and magnetic resonance imaging (MRI)
devices, but less so at the quantum level.
“Understanding the physics of electronand nuclear-spin related phenomena isvital for improving our understanding of
the physics of quantum systems,” explains
Yoshiro Hirayama of Tohoku University,
who organized the well-attended workshop with approximately 80 participants.
“Potential applications of the technology include spin- and nuclear-spin-based
quantum computing, which promises tobe the next revolution in computing.”
The highlight of the workshop was afascinating, and at times humorous, talkby Klaus von Klitzing, winner of the 1985
Nobel Prize in Physics for his experimental discovery of the quantum Hall effectin quantum nanostructures. While there
had been some theoretical studies prior
to the discovery, von Klitzing’s experiments revealed an unanticipated finding— quantization with an extraordinarily
high precision.
In his lecture, von Klitzing noted one
surprising outcome of this field: the overhaul of the definitions of the international
system of units. Instead of using objects
or physical systems as the standards forphysical units, units such as the kilogram,

Nobel laureate Klaus von Klitzing gave a lecture onelectron spin in gallium-arsenide–aluminum-galliumarsenide heterostructures to a packed lecture hallfitted with blackboards and folding desks.

AIMResearch 2015

The Tohoku Forum for Creativity was established with generous support from the government of Japan and thesemiconductor manufacturer Tokyo Electron.

ampere and kelvin will be based on
fundamental physical constants, such as
Planck’s constant, the elementary charge
and the Boltzmann constant.
Hirayama invited von Klitzing basedon a personal connection with him that
extends back 25 years, but the new center
also played a big part. “Thanks to theestablishment of the Tohoku Forum for
Creativity, it is now easier to invite NobelPrize-class researchers — somethingthat has until now been very difficult,”
says Hirayama.
Many types of magnetism
Researchers attending the third workshop on November 16–17 discussed
a lesser known entrant to the worldof spintronics: antiferromagnetism.
Unlike ferromagnets in which nearby
spins tend to align in the same direction, antiferromagnets contain parallelbut opposing spins. “This was the firstinternational conference dedicated tothe subject,” say workshop organizers
Oleg Tretiakov, a researcher at Tohoku’s
Institute for Materials Research (IMR),
and Gerrit Bauer, who is affiliated withboth the IMR and the AIMR.
“Antiferromagnets have an edge overferromagnets,” explain Tretiakov andBauer, because they have zero net magnetization, and therefore do not produce
unwanted stray magnetic fields. “But forthe same reason, it is difficult to control
them using external magnetic fields.”
Recently, however, researchers have founda way to manipulate antiferromagnets
using an electric current, which makesthese materials potentially interestingbuilding blocks for spintronic memories
and logic devices.
Since 2006, Tohoku’s ResearchInstitute of Electrical Communication(RIEC), headed by Hideo Ohno, who isalso a principal investigator at the AIMR,
has been organizing an annual international workshop on spintronics. The fifth
workshop in the Tohoku Forum’s series
coincided with the 13th RIEC workshop(November 18–20).
Spintronics researchers at the AIMR
and Tohoku are also using their expertiseto pack thousands of transistors onto asingle chip, known as very large-scale
integration (VLSI) — the subject of asixth workshop (November 20–21).
The cost of shrinking transistors while
continuing to enhance chip performanceis high power consumption and signaltransfer delays, says Ohno. Spintronics
devices, however, could be used to make
VLSI less power hungry while maintaining high performance.
“As current electronics based on
semiconductors reaches the limits of
its performance, a new paradigm isrequired,” says Koki Takanashi, directorof IMR, who is organizing the final
workshop (3–4 December) on the interplay between spin, heat, electricity andmotion. “Spintronics, which uses semiconductor and magnetic materials, offers
great promise.”

33

IN THE SPOTLIGHT

INTERNATIONAL PARTNERSHIPS

Published online on 25 January 2016

Bringing together ‘best with best’Researchers at the AIMR Joint Research Center at Cambridge are making ground-breaking discoveries in materials
needed for faster data storage and solar-powered hydrogen generationS

everal years of collaboration
between the Advanced Institute forMaterials Research (AIMR) andthe University of Cambridge have yieldedsome intriguing insights into exotic
materials, including chalcogenides, metallic glasses and hydrogen materials. These
insights could bring us closer to efficientcomputer memories that retain information without a power source, and an
economy fueled by sunlight and hydrogen.
“Cambridge seeks only ‘best with best’collaborations, and that is what makes the
AIMR such an obvious partner in materials science,” says Alan Lindsay Greer,head of the School of Physical Sciences at
Cambridge and a principal investigator at
the AIMR who has led the collaborative efforts. In 2012, the AIMR set up the AIMR
Joint Research Center (AJC) at Cambridge
to formalize a long-standing partnership
between the two institutes. This center has
acted as a model for two more AJCs: one at
the University of California, Santa Barbara,
and the other at the Institute of Chemistry,
Chinese Academy of Sciences.
Initially focusing on metallurgy, theAJC at Cambridge now covers a wider
range of subjects in materials science,chemistry and mathematics. It has hired
three full-time research associates andorganizes annual workshops to deepen theexchange. “For research institutions to behealthy, the range of research conductedmust be wide,” says Greer. “Many of themost exciting developments are at theinterfaces between traditional disciplines.”
Rejuvenating metallic glasses
Greer first visited Japan as a postdoctoral
fellow in 1981 to attend a conference at

34

In November 2015, AIMR Director Motoko Kotani (left) and Alan Lindsay Greer, head of the School of PhysicalSciences at the University of Cambridge, signed an agreement to extend the term of the AIMR Joint ResearchCenter at Cambridge.

Tohoku University. “The visit was more
eventful than I had planned,” Greer
reminisces. His train journey from Tokyo
to Sendai was disrupted by a typhoon andGreer fell ill en route, which forced him
to spend three weeks in a Sendai hospital— “among other things, giving me time tolearn the Japanese syllabaries hiragana andkatakana and to acquire a taste for nattō[gooey fermented soybeans].”
The visit also sowed the seeds for collaboration in the area of metallic glasses,
materials that exhibit record-breaking
properties in strength and flexibility.
“Metallic glasses have been studied forover half a century, yet still provide
surprises,” says Greer.
Most recently, Greer has worked
closely with AIMR principal investigator
Dmitri Louzguine, exploring changes to thestructure and properties of metallic glasses
exposed to thermomechanical treatments.
One such treatment known as thermal
annealing takes metallic glasses to ‘aged’

states of lower energy and higher density,
which often causes them to become brittle.
“Just as for people, it would be desirable
to reverse the effects of aging,” says Greer.
“Fortunately for metallic glasses, ‘rejuvenation’ is more achievable than for people.”
In August 2015, the joint AIMR–
Cambridge team, together with co-workers in China, published a paper in Naturein which they describe an easy route to
rejuvenating metallic glasses simply bycycling the materials between room temperature and liquid-nitrogen temperature.
This thermal cycling results in glasses thathave higher energies and are less relaxed.
“The work would simply not have happened without the collaboration,” says
Greer. “It demonstrates the advantages ofbringing together complementary skills,
techniques and instrumentation.”
Glass and not glass
Glasses are formed by cooling liquids insuch a way as to prevent the formation

AIMResearch 2015

IN THE SPOTLIGHT

of ordered structures known as crystals,
which are periodic arrangements of atoms
and molecules. Most researchers working
on metallic glasses try to find glassforming liquids that have high resistances
to crystallization (or good glass-forming
ability) because they can be used to produce glasses in larger dimensions.
Researchers at the AJC, on the other
hand, are exploring liquids that have a low
resistance to crystallization, such as puremetals and a class of elements known aschalcogenides. “We generally require theworst glass formers,” says Jiri Orava, aresearch associate who joined the AJC in
November 2012 and works closely withGreer, Louzguine and Mingwei Chen,
another AIMR principal investigator.
Some chalcogenides maintain theirglassy state up to 150 degrees Celsius, but
rapidly crystallize at higher temperatures.
And this fast glass-to-crystal transition isreversible — a phase-changing property
that makes these materials attractive forspeedier electronic data storage devices.
“Effectively, these glasses show extreme
rejuvenation,” adds Greer.
More specifically, Orava’s work focuses
on studying chalcogenides and their potential for improving the existing range
of non-volatile memories, which retain
stored information even without a powersource. For non-volatile memories tocompete with volatile, power-dependent,
memories, crystallization needs tohappen over nanosecond time scales.
Chalcogenides could potentially achievethese crystal growth rates at elevatedJiri Orava, a research associate at the AIMR Joint
Research Center (AJC), is studying a class of elements
known as chalcogenides, which are attractive for use
in non-volatile memories.

AIMResearch 2015

temperatures, but researchers did not havethe means to measure and understandthe process.
In 2012, Orava and Greer were thefirst to quantify the growth rate in achalcogenide liquid over the operationaltemperature range; their results were
published in Nature Materials. And in
July 2015, they characterized the crystal
growth behavior of another chalcogenide
glass, which responds differently to temperature changes.
Interestingly, the glass-to-crystal transition in chalcogenides can occur in stages,
with up to 16 intermediate steps identified
so far. “This could allow us to extend thebinary recording system of zeros and ones
to a hexadecimal system,” says Orava.
It could also be exploited in computer
systems designed to mimic the behavior ofneurons in the human brain.
“The real benefit of being at the AJC
has been the freedom I get to do myresearch,” says Orava. “My ideas can beeasily realized by accessing the uniquefacilities available to me at both organizations.” Over the years, he has been able
to gain the trust and friendship of hiscolleagues. “This might not sound likemuch, but trusting each other’s research isimportant when collaborating.”
Trapping solar power in hydrogen
From data to energ y storage,
Katherine Orchard, another research associate at the AJC, and Erwin Reisner, aprincipal investigator at the Department ofChemistry in Cambridge, have discovereda more direct route to trap solar energy inthe form of hydrogen fuel.
For renewable energy sources like solar
and wind to take a more prominent position in the energy infrastructure, scientists
need to find a cheap and easy way ofstoring energy for later use. One way is toconvert it into chemical bonds. Hydrogenis a strong storage contender because it isclean, energy dense, and can be produced
from water and sunlight. Semiconductor
nanoparticles can facilitate this process ofabsorbing light to split a water molecule
into hydrogen and oxygen. Orchard andReisner have found a way to improve
the activity of these systems by up to
200 times.

Katherine Orchard, another research associate at the
AJC, has discovered a more direct route to storing
solar energy in hydrogen fuel cells.

By bringing together Reisner’s knowledge of photocatalysis with the nanomaterials expertise of Tadafumi Adschiri, aprincipal investigator at the AIMR, andthe molecular synthesis capabilities of
Naoki Asao, a professor at the AIMR,
Orchard is trying to immobilize thesenanoparticle systems onto electrodes toform light-activated electrodes that cangenerate hydrogen.
In 2015, Orchard, Reisner and Adschiri,
together with researchers at the Universityof Leeds, created a biohybrid photoelectrode made of titanium oxide nanoparticles assembled onto a protein film.
“This work is important as it uses nature’s
strategies for transporting electrical charge(conductive proteins) to improve synthetic,
fuel-making devices,” says Orchard.
“Having an open line of communication between our labs allows new materials
developed in each lab to be explored for
new applications fairly rapidly,” she adds.
“For example, novel nanomaterials developed for environmental clean-up in Asao’s
lab are currently under investigation as
catalytic materials in Reisner’s lab.”“It is clear that the broad range of
research being conducted at the AIMR
and the University of Cambridge provides
an excellent basis for diversifying collaborations between the two institutes
more generally,” says AIMR Director
Motoko Kotani.

35

AIMResearch
AIMResearch is an online and print publication that highlights the scientific achievements and activities of
the AIMR. First published in June 2009, AIMResearch selects the most important papers from the wealthof research produced by AIMR scientists throughout the year, distilling the essence of the achievements
into timely, concise and accessible research highlights that are easy to digest, but retain all of the impact andimportance of the original research article. Published monthly on the AIMResearch website in both English
and Japanese, AIMResearch research highlights bring the very best of AIMR research to a global audience ofspecialists and nonspecialists alike. AIMResearch also publishes a range of feature articles introducing otheractivities of the AIMR’s research groups. Visitors to the website can register for monthly email alerts in either
English or Japanese to keep abreast of the latest developments and discoveries made at the AIMR.
research.wpi-aimr.tohoku.ac.jp

research.wpi-aimr.tohoku.ac.jp
AIMResearch is a publication of the Advanced Institute for Materials Research(AIMR), a Tohoku University institute funded by the Ministry of Education,
Culture, Sports, Science and Technology of Japan.
© 2016 AIMR, Tohoku University. This publication may be reproduced in its
original form for personal use only. Modification or commercial use without priorpermission from the copyright holder is prohibited.

Editorial
AIMR, Tohoku University
2-1-1 Katahira, Aoba-ku, Sendai,
Miyagi 980-8577, Japan
Editor-in-Chief

Motoko Kotani

Managing Editor

Takashi Takahashi

Editorial AssistantEtsuko Hagita

ISSN 1884-491X

Tohoku University’s Advanced Institute for Materials Research (AIMR)
promotes fusion research in materials science across disciplinary barriers

The AIMR is leading the world in facilitating collaboration between materials science and mathematics. In
an environment where more than half of the researchers come from abroad, scientists at the AIMR
— young and international included — contribute to discussions and joint research projects, enabling the
AIMR to achieve world-leading research results.
AIMResearch’s website introduces cutting-edge research from the AIMR through research highlights,
spotlights, news and job vacancies. Access AIMResearch online now!

research.wpi-aimr.tohoku.ac.jp
Latest research highlights• Magnetism: Electric field reveals the ‘hole’ story• Phase transformation: Switching on the next phase of nanodevices• Metallic glasses: Thermal cycling is the secret to eternal youth• Polymers: Self-folding sheets wrap up droplets• Lithium–oxygen batteries: Graphene protects cathode catalyst

Advanced Institute for Materials Research
Tohoku University