RESEARCHHIGHLIGHTS 2020
A publication of the WPI Advanced Institute for Materials ResearchTABLE OF CONTENTSMESSAGE FROM THE DIRECTOR
2RESEARCHHIGHLIGHTS 2020
A publication of the WPI Advanced Institute for Materials Research

An ambitious approach to materials science research

RESEARCH HIGHLIGHTS

Advanced Institute for Materials Research
Tohoku University

4

Lithium-ion batteries: Imaging a critical component of batteries

5

Metallic glasses: Nanoparticles with glass-like structure coalesce faster

6

Topological superconductors: Turning a conventional superconductor into a topological one

7

Superconductivity: Crystal geometry affects transition temperatures

8

Lithium–oxygen batteries: Synergism between catalysts boosts efficiency

9

Magnons: Sophisticated amplification process underpinned by simple math

10 Soft-matter physics: Droplets hanging from a liquid surface
11 Solid-state batteries: Record-high lithium-ion conductivity at room temperature
12 Lithium-ion batteries: Making silicon usable

COVER STORY
A three-dimensional
image generated bytopographic scanningtunneling microscopyof strontium vanadate
nanowires formed on an
ultrathin film of strontium
vanadate (see page 13).
COVER IMAGE© 2021 Hirofumi Oka

13 Transition metal oxides: Metal–insulator transition observed in nanowires
14 Topological insulators: Filling in the knowledge gaps
15 Thermoelectric materials: Porous structure captures heat more efficiently

WPI Advanced Institutefor Materials Research
Established in 2007 as part of theJapanese government’s World PremierInternational Research Center Initiative(WPI), the Advanced Institute forMaterials Research (AIMR) was tasked
with pursuing world-class research andpromoting global brain circulation. The
AIMR has since been bringing together
world-class researchers from Japanand abroad to carry out cutting-edge
research in materials science throughinterdisciplinary collaboration among its
four materials-related groups — Materials
Physics, Non-equilibrium Materials, Soft
Materials, Device/System — and theMathematical Science Group.

In 2017, the AIMR became a memberof the WPI Academy, which consists of
WPI centers that have achieved worldpremier status. The institute will continue
to maintain its world-class research
environment and further promote global
brain circulation.
Led by distinguished material scientist
and director Shin-ichi Orimo, the institutepromotes interdisciplinary research acrossthe different groups. It also fosters young
researchers through the Global Intellectual
Incubation and Integration Laboratory(GI3 Lab). This unique program, which iscurrently supported by the WPI Academy,
promotes international joint research
conducted in close cooperation with highprofile researchers invited from countries
around the world.

The AIMR is host to about 100 leading
researchers, around 35 percent of whomcome from abroad, including 26 principal
and junior principal investigators. In
addition to the research hub at Tohoku
University, the AIMR collaborates withresearch centers in China, Germany,
Poland, the UK and the US. Close ties
with other leading overseas institutes aremaintained, going along with the efforts
of foreign principal and junior principal
investigators, as well as adjunct professors
and associate professors.

MESSAGE FROM THE DIRECTOR

An ambitious approach tomaterials science research
The Advanced Institute for Materials Research (AIMR)

molecular control technologies) and meso- and macro-scale

was founded in 2007 with support from the World Premierperspectives (which are related to larger structures and per-International Research Center Initiative (WPI), which wasformances), thereby accelerating material creation and thestarted by Japan’s Ministry of Education, Culture, Sports,

development of a diverse range of unprecedented physicalScience and Technology (MEXT) to set up world-class researchproperties and functionalities.

centers in Japan. Since then, AIMR has advanced the WPI’s

Furthermore, AIMR is promoting the use of the institute’s

four basic objectives: advancing top-level research, creating

angle-resolved photoemission spectrometer (which boasts theinterdisciplinary domains, establishing international researchhighest angular resolution in the world), scanning tunnelingenvironments, and reforming research organizations. As amicroscope, and the advanced measurement technologies

result, it has established a research center for materials scienceavailable at the next-generation synchrotron facility, which isthat attracts outstanding researchers from all over the world.

being built at the Tohoku University’s Aobayama campus.

In 2017, AIMR became a member of the WPI Academy. It has

AIMR aims to adopt an approach that encompasses diverse

upheld world-class research standards as it strives to acceler-advanced-material projects such as electronics, energy andate and expand the global circulation of the world’s best talent.

biomedical technologies by leveraging advanced measure-As an international center for materials science, AIMR has

ment technologies as well as collaboration between math-a key advantage in that it promotes collaboration between

ematics and materials science through the three Advanced

mathematics and materials science. By employing the univer-Target Projects. Its scope will go beyond determining scien-

sal language of mathematics to describe the extremely diverse

tific principles, extending to the creation of materials that will

field of materials science, we are endeavoring to identify thebenefit society. Strengthening collaborations with industry iscommonalities among a wide range of materials and to pursue

thus one of the top priorities moving forward.

new research fields with the goal of realizing novel outcomes.

AIMR’s approach of promoting collaboration between

Even at the global level, initiatives such as this to foster col-mathematics and materials science has attracted enormouslaboration among mathematicians and materials scientists

interest from industry, particularly through programs suchthroughout an entire research institute are rare; this undertak-as Graduate-level Research in Industrial Projects for Students

ing is therefore illustrative of AIMR’s status as a progressive(g-RIPS). By promoting not just short-term results but also thecenter for materials science.

creation of new materials through fundamental science, AIMR

To further strengthen collaboration between mathematics and materials science, which lies at the core of AIMR’s

is committed to fulfilling its responsibility to be a research hubthat benefits society through advanced materials science.

identity, we launched three new Advanced Target Projects in

I would like to thank all those who have supported

2019, namely local structure control in topological functional

us. AIMR is playing a central role in Tohoku University’smaterials; integrated control of bond variation and its timeefforts to establish an international research environment

evolution; and improvement of self-organization technology

as a Designated National University. At the same time, we

and control of biological response. These ambitious projects

intend to maintain our ongoing efforts to pursue high-quality

seek to transform research into conventional static random-

research as a hub that facilitates the international circulation

ness into investigations of dynamic (or kinetic) randomnessof the world’s best minds and to contribute to cutting-edge

and to elucidate the responsiveness to external fields ofmaterials science and global societal advancement.

hierarchical structures formed through self-ordering. AIMR
researchers are also endeavoring to simultaneously study

Shin-ichi Orimo, Director

microscale perspectives (which are relevant to atomic andAdvanced Institute for Materials Research, Tohoku University

AIMResearch 2020

2

RESEARCH HIGHLIGHTS
The AIMR advances research in materials physics, non-equilibrium materials, soft materials
and devices/systems, and actively promotes collaboration among these divisions toward thedevelopment of ground-breaking technologies that cross the boundaries of conventional
fields of study – bridging the disciplines of materials science, physics, chemistry andprecision, mechanical, electronics and information engineering. The Mathematical Science
Group further complements the AIMR’s research activities.

RESEARCH HIGHLIGHTSLITHIUM-ION BATTERIES

Published online on 27 January 2020

Imaging a critical componentof batteries
A powerful microscopy technique reveals new insights into the least understood part oflithium-ion batteries

AIMResearch 2020

Li-Au alloy

SEI

Au

Electrolyte© 2020 Mingwei Chen

Researchers at the AIMR have observed
how an ultrathin layer on the negativeelectrode (anode) of lithium-ion batteries
forms, grows and fails during battery operation1. The important insights they havegleaned will be useful for developing betterand safer lithium-ion batteries.
Lithium-ion batteries power everythingfrom smart phones to electric vehicles.
First commercialized about 30 years ago,
they are a mature technology that has
been intensively studied and developed.
However, one of the most important but
least understood aspects of lithium-ion
batteries is an ultrathin layer — the solid
electrolyte interphase (SEI) — that grows
on the anode and protects it.
The SEI forms during the first few
charge−discharge cycles and protects
the anode from further decomposition.
Although only a few tens of nanometers
thick, the layer has a major effect on battery operation, notes Mingwei Chen at the
AIMR of Tohoku University. “The SEI is
one of the key factors that determine thesafety, power capability and cycle life oflithium-ion batteries.”
One reason why the SEI is not well understood is because it is difficult to observe
during battery operation.
Now, Chen and his group members haveused a state-of-the-art scanning transmission electron microscope to observe
how the SEI evolves during charging anddischarging in a specially designed liquidcell that closely mimics the conditions
inside a battery.
The researchers were able to image at a
sub-nanometer resolution, which allowed
them to see structures made up of inorganic and organic layers in the SEI (see image).
“We were surprised that our scanning

200nm
An ABF-STEM image showing the formation of SEI films with a bilayer structure on the surface of an Au electrode
undergoing Li−Au alloying reactions.

transmission electron microscope technique allowed us to clearly resolve the bilayer structures of the SEI,” explains Chen.
These structures strongly suggest that the
SEI grows through chemical reduction of
the electrolyte at the boundary between
the anode and the electrolyte. The teamfound that the growth of the SEI is also
assisted by radical species that may form
during the beginning stages of charging
and discharging.
Chen and his co-workers also discovered that the SEI fails when it breaks andthe inorganic layers come into contact withand rapidly dissolve in the electrolyte.
These findings will help researchers to

improve lithium-ion batteries. “The microscopic insights we have obtained haveimportant implications for understanding
the kinetics of the SEI and for developing
high-performance anodes that are protected by robust SEI films,” says Chen.
In the future, Chen’s team intends tofurther investigate the effect of electrolyte
additives on the formation of robust
SEI films.
1. Hou, C., Han, J., Liu, P., Yang, C., Huang, G., Fujita,
T., Hirata, A. & Chen, M. Operando observationsof SEI film evolution by mass-sensitive scanningtransmission electron microscopy. Advanced
Energy Materials 9, 1902675 (2019).

4

RESEARCH HIGHLIGHTSMETALLIC GLASSES

Published online on 25 February 2020

Nanoparticles with glass-likestructure coalesce faster
Metal nanoparticles with a disordered,glass-like structure coalesce faster thanconventional metal nanoparticles thathave a crystalline structure, experimentalists at the AIMR have discovered1. Thisfinding has significant implications fortheir use, since the properties of nanoparticles depend greatly on their size.
Metallic glass nanoparticles are attracting much interest because of theirpotential for use in a wide range ofapplications, including catalysis, biomedical materials and the strengtheningof composite materials. Unlike conventional metal nanoparticles, which havea regular, crystalline structure, metallic
glass nanoparticles have a disorderedarrangement of atoms.
Due to their tiny size, all nanoparticles
are inherently unstable and they often
tend to merge with each other to reduce
their surface energy — in much the same
way that two small bubbles will coalesce
to form a larger, more stable bubble.
Known as coarsening, this mechanism
has been extensively studied in conventional metal nanoparticles because itleads to degrading of their physical andchemical properties. But the coarsening
mechanisms of metallic glass nanoparticles remain largely unknown.
“The coarsening mechanism is important for nanoparticles because it determines their sintering kinetics and theirsubsequent stability,” says Yuan Tian, whois the first author of the study.
Now, by using a state-of-the-art
in situ heating transmission electron
microscope that allowed them to observe
nanoparticles coalesce stably at high temperatures, a team led by Mingwei Chenof the AIMR at Tohoku University has
found that metallic glass nanoparticles

5

Adapted from Ref. 1 and licensed under CC BY 4.0 © 2019 Y. Tian et al.

Metallic glass nanoparticles merge with each other faster than crystalline nanoparticles

The disordered arrangement of atoms in metallic glass nanoparticles (bottom) allows two nanoparticles to merge
faster than two conventional metal nanoparticles which have a crystalline structure (top).

coalesce much faster than their crystalline counterparts at temperatures of afew hundred degrees Celsius.
“This special coalescence behavior of
metallic glass nanoparticles came as asurprise to us,” says Tian. “This new aspectof metallic glass nanoparticles is importantfor informing their future study.”
To gain more insight into the coarsening mechanism, the team also performed
computer simulations of the nanoparticles.
They found that the greater mobility ofsurface atoms and the isotropic structureof metallic glass nanoparticles are themain drivers of their faster coalescencecompared to their conventional cousins(see image).
“During coalescence, metallic glass
nanoparticles keep their round shape,whereas crystalline nanoparticles gain
sharp edges,” explains Tian. “The different
morphologies of crystalline and amorphous nanoparticles give their surface
atoms different mobilities, and this, inturn, gives rise to their different coarsening
mechanisms.”
The team intends to investigate thecoarsening process of larger metallic glass
particles. “We plan to extend this study
to larger scales because we anticipate thatthe coarsening mechanism will change
when the particles are larger than about
10 micrometers,” says Tian.
1. Tian, Y., Jiao, W., Liu, P., Song, S., Lu, Z., Hirata,
A. & Chen, M. Fast coalescence of metallic
glass nanoparticles. Nature Communications 10,5249 (2019).

AIMResearch 2020

RESEARCH HIGHLIGHTSTOPOLOGICAL SUPERCONDUCTORS

Published online on 30 March 2020

Turning a conventional superconductor into a topological one
A new way to make topological superconductors — promising materials forrealizing robust quantum computers thatare more tolerant of faults than existing
technologies — has been discovered by
AIMR researchers1. This method shouldmake it possible to convert conventional
superconductors into topological ones,
greatly increasing the number of topological superconducting systems.
Superconductors can conduct electricity without resistance. A special classknown as topological superconductors aresuperconducting everywhere except on
their surfaces, where they conduct electricity with a low resistance, like metals
do. They are exciting interest because they
could host exotic quasiparticles known
as Majorana fermions. Named after the
Italian physicist who predicted their
existence about 80 years ago, Majorana
fermions are attractive for realizing faulttolerant quantum computers. While they
have yet to be definitively observed, there
is mounting experimental evidence for
their existence.
Until now, topological superconductors
have generally been made by bringing
a superconductor into contact with a
topological insulator — a material that is
electrically insulating internally but conductive on its surface. The superconductor
imparts a thin portion of the topologicalinsulator with superconductivity, an effectknown as the superconducting proximity
effect. But it is hard to detect Majorana
fermions in such topological superconductors. “Since the superconducting
proximity effect occurs at an interface far
from the surface, it is generally difficult toprobe Majorana fermions by surface spectroscopy techniques,” explains Takafumi

AIMResearch 2020

Pb

[111]

17ML

ee-

e-

e-

TlBiSe2

Pb
Tl
Se
Bi

Spectroscopic measurements (left) strongly suggest that a superconducting lead film (yellow layer in central
image) on a topological insulator (blue layer) is a topological superconductor.

Sato of the AIMR at Tohoku University.
Sato and co-workers have produced a
topological superconductor by reversing
the influence between the superconductor and the topological insulator — theytransferred topological properties to aconventional superconductor. They did
this by growing a thin film of superconducting lead on a topological insulator,
which proved to be quite challenging.
“Until our study, no-one had ever succeeded in fabricating a lead film on a
topological insulator,” says Sato. “It wasa big challenge and we had to optimize
the growth conditions very carefully andfabricate more than 70 samples until we
got one that was satisfactory.”
The team then performed spectroscopy
measurements that strongly suggested
that they had produced a topologicalsuperconductor.

The team’s method opens up the
possibility of producing topologicalsuperconductors in a much wider
range of systems than before. “Our
concept may be widely applicable toother combinations of superconductors
and topological insulators,” says Sato.
“Exploring new material systems basedon our idea, could accelerate investigations of topological superconductors andthe detection of Majorana fermions andultimately, their possible applications toquantum computation.”
1. Trang, C. X., Shimamura, N., Nakayama, K.,
Souma, S., Sugawara, K., Watanabe, I., Yamauchi,
K., Oguchi, T., Segawa, K., Takahashi, T. et al.
Conversion of a conventional superconductor
into a topological superconductor by topologicalproximity effect. Nature Communications 11,
159 (2020).

6

Adapted from Ref. 1 and licensed under CC BY 4.0 © 2020 C. X. Trang et al.

A new method produces topological superconductors, which are attractive forquantum computing

RESEARCH HIGHLIGHTS

SUPERCONDUCTIVITY

Published online on 27 April 2020

Crystal geometry affects
transition temperatures
The temperature at which superconductivity begins in layered, bismuth-containing
compounds can be increased by altering
the geometry of its crystal structure, an
AIMR-led team has discovered1. This
could open up a new way to manipulate
superconductivity in layered materials andprovide vital hints on the mechanism of
superconductivity.
Some materials made up of alternating
layers are superconductors, the best-known
examples being cuprates and iron-based
compounds. Although these compounds
have been researched intensively, no one
knows exactly how they superconduct.
These materials are usually made tosuperconduct by ‘doping’ them with elements that provide additional electrons.
But a recent study has found that twistinga double layer of graphene can induce superconductivity, suggesting that geometricmanipulation of crystal structure factors
may be important.
Now, Tomoteru Fukumura of the
AIMR at Tohoku University and his
co-workers have found that compounds
consisting of alternating layers of bismuth
and of a rare-earth oxide superconduct.
Furthermore, they discovered that thetemperature for the onset of superconduction increases as the ratio of the distance between the bismuth layers to the
distance between adjacent bismuth atoms
in a layer increases, or, expressed anotherway, as the ratio of the lengths of the twosides of the unit cell increases.
The researchers varied this ratio in two
ways: they made compounds containing
five different rare-earth elements, whichhave different ionic sizes, and they added
excess oxygen atoms to the compounds,
effectively wedging the layers apart7

© 2020 Hideyuki Kawasoko

Superconductivity can be induced in layered bismuth compounds by varying thegeometry of their crystal structure

An AIMR-led team has found that increasing the ratio of the distance between two bismuth layers and the
distance between adjacent bismuth atoms (purple spheres) in a layer increases the temperature at which
superconduction occurs. They varied this ratio by adding excess oxygen atoms (red spheres; see image on right)
and using different rare-earth atoms (dark-gray spheres).

(see image). Interestingly, the temperature
at which superconductivity starts depended only on the structural ratio — it did notdepend on which rare-earth element waspresent or on the magnetic ordering in therare-earth layer.
“We thought that the compoundcontaining the rare-earth element dysprosium might not be superconducting
because it has magnetic ordering, whichusually destroys superconductivity,”
explains Fukumura. “But even it became
superconducting at about 2 kelvin on
adding excess oxygen, just like nonmagnetic Y2O2Bi.”“Layered compounds such as cuprates
and iron-based compounds have generally been made to superconduct by carrier doping, but our layered compound
becomes superconducting by increasing

the unit cell ratio, which is a purely crystallographic parameter,” says Fukumura.
“It will be very interesting to see if thisparameter induces superconductivity inother layered compounds.”
At this stage, the team is not sure why
the ratio affects the superconducting transition temperature. To get more clues, they
are investigating where the added oxygen
ends up in the crystal structure. They arealso looking for a way to vary the unit cellratio that does not involve adding oxygento the layered compound.
1. Sei, R., Kawasoko, H., Matsumoto, K., Arimitsu,
M., Terakado, K., Oka, D., Fukuda, S., Kimura,N., Kasai, H., Nishibori, E. et al. Tetragonality
induced superconductivity in anti-ThCr2Si2-type
RE2O2Bi (RE = rare earth) with Bi square nets.
Dalton Transactions 49, 3321–3325 (2020).

AIMResearch 2020

RESEARCH HIGHLIGHTSLITHIUM–OXYGEN BATTERIES

Published online on 25 May 2020

Synergism between catalysts
boosts efficiency
Solid and liquid catalysts in a lithium–oxygen rechargeable battery have a synergistic effect that simultaneously boosts thecathode kinetics and energy efficiency ofthe battery, researchers at the AIMR havefound1. This points the way to overcome
two of the main hurdles to the commercialization of these batteries.
“The lithium–oxygen battery is oneof the most promising candidates forthe next generation of electrochemicalenergy-storage devices,” says Mingwei
Chen of the AIMR at Tohoku University.
“But its practical implementation has
been hindered by sluggish kinetics at thecathode and low energy efficiencies.”
Lithium–oxygen batteries consist of
three main components: a lithium metal
anode, a non-aqueous liquid electrolyte
and a porous cathode. During discharging, the battery releases energy throughthe oxidization of lithium to lithium peroxide (Li2O2), and it stores energy during
charging by the reverse reaction in whichlithium peroxide is decomposed intolithium and oxygen. Catalysts are needed
to facilitate both these reactions.
Solid and liquid catalysts have beendeveloped for lithium–oxygen batteries,
but they both have their downsides. “Solidcatalysts suffer from rigid and unstable
catalyst–lithium peroxide interfacial contacts, whereas liquid catalysts have lower
catalytic efficiencies,” explains Chen.
Previous studies had shown the promiseof using both solid and liquid catalysts
for improving the energy efficiency oflithium–oxygen batteries, but until now
the underlying mechanisms and the optimal combinations have not been explored.
The team led by Chen used a state-ofthe-art liquid-cell transmission electron

AIMResearch 2020

© 2020 Mingwei Chen

Using both solid and liquid catalysts enhances the energy efficiency of lithium–oxygen batteries

AIMR researchers used an electron beam (green cones) from a high-angle annular dark-field scanning transmission
electron microscopy to image nanoscale interfacial electrochemical reactions in real space. The two blue arrows
indicate the flow of an electrolyte containing the liquid catalyst tetrathiafulvalene, while the gold plates on theleft are used to apply a potential across two gold electrodes.

microscope to investigate the effect ofusing both kinds of catalysts.
Chen and his team chose rutheniumoxide (RuO2) as the solid catalyst andtetrathiafulvalene (C6H4S4) as the liquid
catalyst. They then imaged the dynamic
nanoscale reactions that took place at
the interfaces between the electrodeand the electrolyte, and between theelectrode and the lithium peroxide(see image).
The researchers discovered a synergistic effect between the two catalysts — inaddition to catalyzing lithium–oxygen
reactions, the solid catalyst enhanced
the catalytic efficiency of the liquid
catalyst during discharging. The
liquid catalyst activated the lithium

peroxide–passivated ruthenium oxide
and accelerated the oxidation of lithium
peroxide during charging. This synergistic effect boosted the energy efficiency of
the rechargeable lithium–oxygen battery.
Chen and his team plan to build uponthis work. “Using the electrochemistry
insights gained in this study, we intend towork toward developing better combinations of solid and liquid catalysts for highperformance lithium–oxygen batteries,”says Chen.
1. Hou, C., Han, J., Liu, P., Huang, G. & Chen, M.
Synergetic effect of liquid and solid catalysts
on the energy efficiency of Li−O2 batteries: cell
performances and operando STEM observations.
Nano Letters 20, 2183–2190 (2020).

8

RESEARCH HIGHLIGHTSMAGNONS

Published online on 29 June 2020

Sophisticated amplification process underpinned by simple math
A convenient way to amplify wave-likeprecessions of electron spin in a magnetic
material with a sandwich-like structure
has been found by AIMR researchers1.
This amplification could find use in a
new generation of computers inspired by
the brain.
Conventional, silicon-based computer
technology is rapidly approaching thephysical limits of miniaturization. As
a result, engineers are searching foralternative computing technologies forthe ‘post-silicon era’. One promisingtechnology is neuromorphic computing,which seeks to mimic how the brain
processes data.
Precessions of electron spins known
as magnons, which can exhibit bothparticle-like and wave-like behaviors, areattractive candidates for use as information carriers in neuromorphic computers.
But because magnons cannot sustain
themselves, an energy-efficient, nanoscale
way to amplify them is needed before they
can be used in circuits.
Previously, Shigemi Mizukami of
the AIMR at Tohoku University andco-workers had observed two modes ofmagnons in a synthetic antiferromagnet— a magnetic material that consists of a
non-magnetic layer sandwiched between
two ferromagnetic layers, where the magnetizations of the ferromagnetic layers
point in opposite directions. They saw ahigh-frequency optical mode and a lowfrequency acoustic mode.
Now, by using a technique they developed that employs ultrafast laser pulses,
Mizukami and his colleagues have foundthat the optical magnon mode amplifies
the acoustic mode when they interact(see image). Specifically, they found that9

© 2020 Shigemi Mizukami

An advance that may help to realize a futuristic form of computing can be described by
the same equations as for a child playing on a swing

AIMR researchers have found a way to amplify magnons (blue spheres with arrows indicating spin) in asynthetic antiferromagnet consisting of two ferromagnetic layers (horizontal circles) sandwiched between a
non-magnetic material.

the acoustic mode gets a temporary kickfrom the optical mode when its frequency
is roughly half that of the optical mode.
The researchers anticipate that this effect could be used to produce an energyefficient nanoamplifier of magnons.
“This finding is a first and important step
towards realizing magnonic computing,” says Mizukami. “In particular, it
gives a new perspective for developing
nanoscale spintronic elements that will
form the building blocks for future neuromorphic computers.”
To gain more insights into the effect,
the researchers performed a mathematical analysis of the magnon amplification.
To their surprise, they found that theequations describing the phenomenon

are the same as those for a child propelling a swing by moving their legs. “Themath underpinning this effect is identical to that for a child on a swing,” says
Mizukami. “The math is very simple anddescribes many physical phenomena innature, but we were surprised to discoverthat such fundamental physics is inherent
in a synthetic antiferromagnet.”
The researchers intend to realize new
magnonic computing architectures utilizing this amplification principle in synthetic
antiferromagnets.
1. Kamimaki, A., Iihama, S., Suzuki, K. Z., Yoshinaga,
N. & Mizukami, S. Parametric amplification ofmagnons in synthetic antiferromagnets. PhysicalReview Applied 13, 044036 (2020).

AIMResearch 2020

RESEARCH HIGHLIGHTS

SOFT-MATTER PHYSICS

Published online on 27 July 2020

Droplets hanging from a liquidsurface
A way to suspend droplets of a denser
fluid from the surface of a less dense fluid
has been found by an international teamled by an AIMR researcher1. This unusual
effect could find application in microreactors and microrobots.
If you drop a liquid such as water into aless dense, immiscible liquid such as oil, it
will sink to the bottom of the container. But
a team led by Thomas Russell of the AIMRat Tohoku University noticed somethingcurious while they were investigating theprinting of three-dimensional water structures in water in a previous study—theyfound that some water droplets containinga polycation and dextran (a polysaccharide
made up of many glucose molecules) hungfrom the surface of aqueous solutions of
the polymer poly(ethylene glycol) (PEG)with a polyanion.
What made it more puzzling was
the fact that the two solutions have an
extremely low interfacial tension—the
attractive force between molecules
across the interface of two fluids. Some
insects exploit the high interfacial energy
between their highly water repellent legs
and water to walk on water, but the interfacial energy is very low in the case of
dextran and PEG.
“This effect was unexpected,” says
Russell. “It was very curious how theheavier droplet could hang on the liquidsurface with such a low interfacial tension.”With their curiosity piqued, Russelland his co-workers looked deeper
into the effect. They discovered that apolyelectrolyte complex formed at theinterface between two aqueous solutions
and that this allowed the heavier dextran
droplets to hang from the PEG surface.
“The condition for hanging the dextran droplet from the interface between

AIMResearch 2020

© 2020 Thomas Russell

A curious effect observed during an experiment could find application in microrobots

Dropping a droplet of dextran (top frame) onto the surface of an aqueous solution of poly(ethylene glycol)(PEG) causes it to be suspended from the PEG surface (bottom frame). Although dextran is denser than PEG, apolyelectrolyte complex enables it to hang from the surface.

air and PEG is determined solely by
the force balance between gravitational
stresses and surface stresses on the PEG
side of the droplet,” explains Russell.
“Remarkably, nothing else matters.”
The droplets are highly versatile, making them promising for use in microrobots and microreactors. “By embedding
magnetic microparticles in the hanging
droplets, we could get them to move androtate under the influence of an appliedmagnetic field,” says Russell. “Also, we
found a hanging droplet with an opening
to air could, like a mosquito larva, use air

to perform some reactions and induce a
cascade reaction in a neighboring droplet.”
The team is now creating a bioinspired
meniscus-climbing system, which cantransport and shuttle materials from theland to the solution on the liquid surface.
They also have plans to use the system
to create an artificial cell that can fix
carbon dioxide.
1. Xie, G., Forth, J., Zhu, S., Helms, B. A., Ashby, P.
D., Shum, H. C. & Russell, T. P. Hanging dropletsfrom liquid surfaces. Proceedings of the National
Academy of Sciences USA 117, 8360–8365 (2020).

10

RESEARCH HIGHLIGHTS

SOLID-STATE BATTERIES

Published online on 31 August 2020

Record-high lithium-ion
conductivity at room temperature
Researchers at the AIMR have hit upon
a way to get polyanions to rotate at much
lower temperatures than normal, opening
the possibility of realizing solid-state batteries with short charging times1.
Most commercial batteries use liquid
electrolytes to convey charges between
their electrodes, but if solid electrolytes
could be used instead, batteries would be
both safer and able to store more energy at
the same volume.
Solid-state batteries require solid electrolytes that have high conductivities forions. One promising class of material for
realizing this is ionic conductors that contain polyanions — ions that have multiple
negatively charged groups. The polyanions
rotate at high temperatures, and this motion assists the conduction of positive ionsthrough the material. But this rotation,and hence the high ion conductivity, does
not occur at room temperature, making it
impractical for many applications.
Now, four researchers at Tohoku
University, including two from the AIMR,
have realized a room-temperature lithiumion conductivity in a hydride that is morethan three times higher than the highest
value obtained to date.
The results exceeded the team’s expectations. “We were surprised since the activation energy for rotation was lower than
we had expected,” says Shigeyuki Takagiof the Institute for Materials Research at
Tohoku University. “We anticipate that
our finding will promote the developmentof rechargeable batteries with very shortcharging times.”
Previous studies had used bulky anions
as the rotating elements, which requiresignificant activation energies to get themto rotate. In contrast, Takagi and his team11Reprinted from Ref. 1, with the permission of AIP Publishing.

Lightweight rotating structures in hydride complexes are promising for developing
solid-state electrolytes

Typical rotatable polyanions, such as B12H2−12 (left), are bulky and thus require a large activation energy to getthem to rotate. In contrast, the pseudorotation of MoH3−9 (right) only involves the rotation of hydrogen atoms(blue spheres) and hence the polyanion can rotate even at room temperature.

achieved the high lithium-ion conductivity at room temperature by using the transition metal molybdenum surrounded bynine hydrogen atoms. Because the rotating components are lightweight hydrogenatoms, the rotation occurs even at room
temperature (see image).
The rotation of the hydrogen atoms is actually not a true rotation, but a pseudorotation involving just small displacements of
the hydrogen atoms. This further promotes
the lowering of the activation energy.
“The pseudorotation enhances the
dynamic disorder of the hydrogen atoms,
thereby increasing the entropy, which
thermodynamically stabilizes the pseudorotating high-temperature phase at low
temperature,” says Takagi.

The researchers anticipate that thesame strategy should be applicable toother polyanionic materials. “Because
the mechanism is quite general, we thinkthat it should be useful for developing
all-solid-state secondary batteries withdifferent carriers, such as sodium andmagnesium ions,” notes Takagi.
The team is now planning to experimentally demonstrate room-temperature
superionic conduction, which has been
predicted theoretically.
1. Takagi, S., Ikeshoji, T. Sato, T. & Orimo, S.
Pseudorotating hydride complexes with highhydrogen coordination: A class of rotatablepolyanions in solid matter. Applied Physics Letters
116, 173901 (2020).

AIMResearch 2020

RESEARCH HIGHLIGHTSLITHIUM-ION BATTERIES

Published online on 28 September 2020

Making silicon usable
Anodes with specially designed nanoarchitectures beat the limitations of silicon anodes


Adapted, with permission, from Ref. 1. Copyright (2020) American Chemical Society.



  

The structure of the nanoporous silicon anode produced by AIMR researchers. One side of the silicon is coated with hybrid silicate, while the other side is coated withnitrogen-doped graphene.

Hybrid silicon anodes for lithium-ion
batteries that overcome problems that had
previously beset silicon anodes have beencreated by AIMR researchers1. This advance will help increase the energy storageof next-generation lithium-ion batteries.
Lithium-ion batteries are ubiquitous
today, being used to power everything fromsmart phones to electric vehicles. However,
the graphite-based anodes used in today’s
batteries are approaching the limit of theirdevelopment, and a new anode material
is needed to realize lithium-ion batteries
with even higher energy capacities.
Silicon is a promising alternative anodematerial because it is cheap and abundantand has a large reversible capacity and agood working potential. “Exploiting theseadvantages of silicon will result in a further
breakthrough in the energy density oflithium-ion batteries,” predicts Mingwei
Chen of the AIMR at Tohoku University.
But silicon suffers from three significant problems. One is that it expands

AIMResearch 2020

and contracts a lot during charging anddischarging, changing its volume by morethan a factor of three. This degrades theelectrodes and can result in loss of electrical contact inside the battery. Anotherproblem with silicon is its low conductivity, which reduces the charging anddischarging rate. A third problem is thatthe protective film that forms between
silicon and the electrolyte during the firstfew charging cycles is unstable.
Now, the team led by Chen has overcome all these problems by designingand producing a nanoporous silicon
anode that has a sandwich-like structure.
In this anode, the silicon is coated on one
surface by nitrogen-doped graphene andon the other surface by a hybrid silicate(see image).
Both surfaces play important roles
in overcoming the above-mentioned
limitations of silicon. “The nanoporousgraphene in the anode acts as a flexible and conductive backbone thatsimultaneously accommodates silicon’s
volume change and improves its electrical conductivity,” explains Chen. “On theother hand, the hybrid silicate coating
acts as a protective outer shell, enhancing the robustness and suppleness of theelectrode and facilitating the formationof stable films on the anode surface.”
In tests involving half and full cells, theanode exceeded the team’s expectations,realizing both an excellent cycling performance and rate capability. “We were reallysurprised by the ultrahigh stability of thishybrid silicon anode,” says Chen. “It was
stable over 10,000 cycles and had a highcapacity and ultrahigh charging rate.”
The team now plans to stack several
free-standing graphene−silicon sheets toboost the energy storage capacity.
1. Huang, G., Han, J., Lu, Z., Wei, D., Kashani, H.,
Watanabe, K. & Chen, M. Ultrastable silicon
anode by three-dimensional nanoarchitecture
design. ACS Nano 14, 4374−4382 (2020).

12

RESEARCH HIGHLIGHTS

TRANSITION METAL OXIDES

Published online on 26 October 2020

Metal–insulator transition
observed in nanowires
The electronic states of nanowires of atransition metal oxide go from being
metallic to insulating on reducing the
nanowire width, a team of AIMR researchers has shown1. In the future, sucha metal–insulator transition could findapplication in phase-change memory
devices and sensors.
Physicists find transition metal oxides
fascinating systems to experiment withbecause their electrons interact strongly
with each other, giving rise to a rangeof exotic effects such as high-temperature superconductivity and colossal
magnetoresistance.
One-dimensional nanowires of transition metal oxides are expected to harboreven more interesting phenomena because
their electronic states are subject to strong
quantum fluctuations. Three-dimensional
crystals and two-dimensional ultrathin
films of transition metal oxides have been
extensively studied, but it is difficult tofabricate one-dimensional nanowires
using conventional nanofabrication techniques because of their relatively complex
crystal structures.
Now, Hirofumi Oka of the AIMR at
Tohoku University and his co-workers
have succeeded in spontaneously forming nanowires of the transition metaloxide strontium vanadate (SrVO3) for
the first time.
The team found that the electronic
states of the nanowires depended strongly
on the nanowire width, going from metallic to insulating as the nanowires got
narrower. This finding mirrors what isobserved in ultrathin films of strontium
vanadate, which undergo a metal–insulator transition with decreasing thickness.
While the mechanism has yet to be13

© 2020 Hirofumi Oka

Nanowires of a transition metal oxide switch from metallic to insulating withdecreasing widthA three-dimensional image generated from a topographic scanning tunneling microscope image of strontium
vanadate nanowires formed on an ultrathin film of strontium vanadate.

definitively determined, the researchers
speculate that the electronic states in the
nanowire may be quantized due to thequantum confinement caused by the low
dimensionality of the nanowires.
The team used pulsed laser deposition
to form the nanowires on the surface
of a strontium vanadate ultrathin film
(see image). This was unexpected, and themechanism is currently being explored.
“The spontaneous formation of one-dimensional strontium vanadate nanowires
was very surprising,” notes Oka. “Despite
the simple cubic perovskite structure ofstrontium vanadate, the crystal grows in
only one direction.”
The state-of-the-art equipment at the
AIMR was crucial for producing the
nanowires. “The fabrication of the strontium vanadate nanowires was due in largepart to our world-class low-temperaturescanning tunneling microscope combinedwith pulsed laser deposition,” notes Oka.
“This system allowed us to characterize
the atomic structure in situ.”
In the future, the team intends to see
if they can induce superconductivity
in the nanowires. “It will be interesting
to see if insulating strontium vanadate
nanowires become superconducting on
carrier doping because one-dimensional
superconductors are expected to exhibit
high-temperature superconductivity due
to strong quantum fluctuations,” says Oka.
1. Oka, H., Okada, Y., Kaminaga, K., Oka, D.,
Hitosugi, T. & Fukumura, T. Width-induced
metal–insulator transition in SrVO3 lateralnanowires spontaneously formed on the ultrathinfilm. Applied Physics Letters 117, 051603 (2020).

AIMResearch 2020

RESEARCH HIGHLIGHTSTOPOLOGICAL INSULATORS

Published online on 30 November 2020

Filling in the knowledge gaps
A new approach succeeds in inducing a Dirac gap in a topological insulator, bringing
the potential for exotic quantum effects

AIMResearch 2020

Bi2Se3

Quintuple
Layer (QL)
Modified from Ref. 1 and licensed under CC-BY © 2020 T. Kato et al.

Growing a material known as a topological insulator on a ferromagnet could
open the way to create various exotic
quantum effects in the topological insulator, AIMR researchers have shown1. This
structure could be used in spintronic
applications for converting between spinand charge currents.
Topological insulators have generated
a huge amount of interest since their discovery in 2007. Their most well-known
property is their ability to conduct electricity along their surfaces while being
electrically insulating inside.
Although
topologicalinsulators
are interesting in their own right andpromising for applications in the area of
spintronics, even more exotic quantum
effects could be induced in them if werepossible to create an energy gap known
as the Dirac gap. One way to do this is
to introduce magnetic impurities into
a topological insulator, but this creates
defects in the material and can be used
only at low temperatures.
Now, Takafumi Sato of the AIMR at
Tohoku University and his co-workers
have demonstrated a new strategy to create a Dirac gap in a topological insulator.
They grew ultrathin films of a topologicalinsulator on a ferromagnet and observeda Dirac gap when the topological insulator was about 2 nanometers thick(see image).
This Dirac gap was induced by an effectknown as interfacial coupling, whereby
the ferromagnetism of the magnetic
layer influences the electron states on thesurface of the topological insulator. “Ourresult suggests that interfacial coupling,which had not been considered much inprevious studies, is crucial for controlling Dirac fermions in heterostructure
systems involving a topological insulator,” says Sato. “This will help toward

Bi
Se

Cr
Si
Te

Cr2Si2Te6
AIMR researchers have induced a Dirac gap in a topological insulator by growing an ultrathin film of the
topological insulator Bi2Se3 on the ferromagnet Cr2Si2Te6.

realizing exotic topological phenomena
and device applications of topologicalinsulators.”
The team hadn’t expected the effect to be so strong. “Before doing theexperiment, we thought that interfacial
coupling would modulate the Dirac cone
only by a very small amount,” says Sato.
“However, to our surprise, the interfacial
coupling turned out to alter the overallband dispersion of the Dirac-cone surface states when the topological insulator
film was thin enough.”Fabricating high-quality topologicalinsulator thin films on the ferromagnet
was challenging, but the team succeeded
thanks to state-of-the-art techniques andhelpful collaborators. “Critical to our
successful film growth was a sophisticated molecular-beam-epitaxy techniquedeveloped in our laboratory and highquality single crystals provided by Eiji
Saitoh’s team,” says Sato.
The team now intends to explore
different combinations of topologicalinsulators and ferromagnets in order toenhance the ferromagnetic proximity.
1. Kato, T., Sugawara, K., Ito, N., Yamauchi, K., Sato,
T., Oguchi, T., Takahashi, T., Shiomi, Y., Saitoh,
E. & Sato, T. Modulation of Dirac electrons inepitaxial Bi2Se3 ultrathin films on van der Waals
ferromagnet Cr2Si2Te6. Physical Review Materials
4, 084202 (2020).

14

RESEARCH HIGHLIGHTSTHERMOELECTRIC MATERIALS

Published online on 21 December 2020

Porous structure captures heatmore efficiently
An ultralight material developed by AIMR
researchers that can absorb waste heat andconvert it directly into electricity could significantly enhance the energy efficiencies
of vehicles and power plants1.
When one side of a thermoelectricmaterial is heated, the resulting temperature gradient generates a voltage acrossthe material. In principle, these materials
should be ideal for recapturing the large
amounts of energy lost as waste heat bymany machines and devices. “The exhaust
heat emitted from engines of automobiles
and turbines of power plants is huge, andas much as 60% of it is lost to the environment,” says Hiroshi Yabu of the AIMR at
Tohoku University.
However, although many thermoelectricmaterials are known, few operate with highefficiency in the 125–625 degrees Celsius
temperature range that corresponds tothe flow of exhaust heat from vehicles, gas
turbines and other industrial processes.
Magnesium silicide is an exception.
“Magnesium silicide is one of the few thermoelectric materials composed of Earthabundant elements and that exhibits an
optimum temperature near 425 degrees
Celsius, which almost perfectly matches
the temperature of industrial waste heat,”says Yabu.
A good thermoelectric material musthave a high electrical conductivity, but itshould also have a low thermal conductivity to ensure that its cold side stays cold andthe temperature gradient is maintained.
Since magnesium silicide’s thermal
conductivity is too high for practical use,
Yabu and his colleagues have been developing ways to slow heat transfer throughthe material. “The introduction of aporous structure is an effective approachto reduce the thermal conductivity, butconventional techniques to fabricate
porous magnesium silicide also reduce

15

Adapted, with permission, from Ref. 1. Copyright (2020) American Chemical Society.

Honeycomb-like material hits the sweet spot for harvesting waste heat

Treatment with magnesium vapor at 725 degrees Celsius converts a porous silica structure (bottom left, where thegreen spheres are oxygen atoms and the blue spheres are silicon atoms) into a high-performance thermoelectricmaterial (top right, where the orange spheres are magnesium atoms and the blue spheres are silicon atoms).

its electrical conductivity due to defects
forming,” he says.
Yabu’s team has now come up with amethod to produce magnesium silicidewith a honeycomb-like structure, introducing pores into the material but not defects.
The researchers had previously developed away to make a polymer honeycomb structure — in which the pores in the polymer
were templated by water droplets — and tocoat this structure with silica. They havenow shown that silica can be converted
into magnesium silicide honeycomb bytreating silica-coated honeycomb withmagnesium vapor at temperatures above
725 degrees Celsius (see image).
The resulting material was so lightthat it floated on water — an advantage

for weight-critical applications such asheat harvesting on vehicles. Critically, its
thermal conductivity was 11% lower thanthat of bulk magnesium silicide, while its
electrical conductivity did not change.
“This result means that magnesium silicide honeycomb is a near-ideal thermoelectric material — it is ultralight, is madefrom abundant materials, and has a highthermoelectric conversion efficiency,”
Yabu says.
1. Yabu, H., Matsuo, Y., Yamada, T., Maeda, H. &Matsui, J. Highly porous magnesium silicide
honeycombs prepared by magnesium vapor annealing of silica-coated polymer honeycomb films
toward ultralightweight thermoelectric materials.
Chemistry of Materials 32, 10176–10183 (2020).

AIMResearch 2020

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 wealth of 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 and importance of the original
research article. Published monthly on the AIMResearch website in both English and Japanese, AIMResearchresearch highlights bring the very best of AIMR research to a global audience of specialists and nonspecialists
alike. 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.
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wpi-aimr.tohoku.ac.jp/en/aimresearch/
AIMResearch is published by the Advanced Institute for Materials Research(AIMR) at Tohoku University in collaboration with Nature Research Custom
Media, a part of Springer Nature.
© 2021 AIMR, Tohoku University. This publication may be reproduced in its
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prior permission from the copyright holder is prohibited.

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

Shin-ichi Orimo

Managing Editor

Takashi Takahashi

Editorial AssistantEtsuko Hagita

ISSN 1884-491X

Advanced Institute for Materials Research
Tohoku University