ResearchHighlights of Collaborated Research
Collaborative works with outside researches are carriedout in consideration of the joint research program. Accepted
applications are performed in following each division andfacility of IMR.
1) Research Laboratories
Joint research conducted by out side researchers and
IMR staff members at each research laboratory. Three
categories; “Research in priority areas”, "General Research"and “Exploratory Research for Young Researchers” areprepared.

2) International Research Center for Nuclear Materials
ScienceThis facility is open to university scientists all over Japanto support experiments using Japan Materials Testing
Reactor, JMTR, the fast experimental reactor, JOYO, andthe test reactor, JRR-3, operated by JAEA (Japan Atomic
Energy Agency). The overseas reactor, BR2, located at theBelgian Nuclear Research Center, is also used for irradiation
experiments. This facility acts as a hub for international
collaborations; specimens irradiated in overseas reactors are
accepted here for post-irradiation examinations byparticipating university researchers. Research subjects
covered here include fundamental studies and R&D on fusion
structural materials, high heat-flux materials, and a variety offunctional materials, as well as engineering-oriented studies
for the safety of light water reactors and basic researches
supporting them. Materials studies utilizing radio-isotopes
are also being conducted.

KINKEN Research Highlights 2009 IMR

3) Advanced Research Center of Metallic Glasses (ARCMG)
ARCMG originates from the Laboratory for Advanced
Materials (LAM) established in 1986 for “the investigation
and application of scientific principles of materials.” This
laboratory was composed of two major divisions, scientificresearch on fundamental properties of materials andengineering approach for industry. Since it was recommended
that we should further promote research on bulk metallic
glasses, we decided to set up the ARCMG in April 2006.
Micromotors, pressure sensors, fuel cell separators, soft
magnetic materials and many other devices that use bulk
metallic glasses are about to become practical propositions.

4) High Field Laboratory for Superconducting Materials
This laboratory restarted in 2001, succeeding “High Field
Laboratory for Superconducting Materials” which wasestablished in 1981. The main equipment is a hybrid magnet
which generates steady high magnetic fields up to 31T. In
addition, many cryogen-free superconducting magnets whichhave been developed by our laboratory are installed.
Especially, we succeeded in developing the world’s firstcryogen-free hybrid magnet, which generates 27.5 T. The
laboratory also provides instruments for measuring various
physical properties. These facilities are open to scientists andengineers on superconductors and other materials research.

Highlights of Collaborated Research395) Osaka Center for Industrial Materials research
The Osaka Center was established at Osaka as a special
unit in IMR in April 2006 based on the agreement between
Tohoku University and Osaka Prefecture Government. It is alaboratory dedicated to research and development of nanostructured metallic materials from the application viewpoints
based on the basic research in the material science, chemistry
and physics associated with research organizations in the
Kansai region. Research in the Osaka Center is focused inparticular on the understanding of the fundamental propertiesof nano-structured metallic materials and the rapid realizationof their application to industry, in particular, the small andmedium enterprises in Osaka area.

7) International Collaboration Center (ICC-IMR)
This Center was established in April 2008. The mission isto serve as an international center for materials research andto encourage the international collaboration on researches of
IMR. The International School on advanced materials,
international workshops are coordinated. The Center hosts
the foreign visiting scientists who collaborate on researchwith members of IMR.

6) Center for Computational Materials Science
This center was developed from the Laboratory ofMaterials Information Science which was established in
February 1989. Its main tasks are 1) administration andmaintenance of the supercomputing system in this Institute,2) maintenance of a supercomputing system network, 3)
general support for the usage of the supercomputing system,4) support of materials design by supercomputing simulation
with vectorization and parallelization, 5) construction of
factual database for materials in nonequilibrium phase , and6) support of activity of Nanotechnology group on SINET3
and Asian Consortium for Materials Science, ACCMS-VO.

40

Highlights of Collaborated Research

IMR KINKEN Research Highlights 2009

Research

Effect of Electron-Phonon Interaction on Photoexcited States of
One-Dimensional Mott Insulators

The interplay of strong electron-electron correlation and
EP interaction has recently attracted much attention. One of
the issues is to understand the effect of the EP interaction on
carriers introduced into a Mott insulator. In the ground stateof the Mott insulator, the on-site Coulomb repulsion, U,
makes the charge distribution uniform, and then the EP
interaction is irrelevant. However, the mobile carriers createdin the Mott insulator by chemical and/or photo dopings induce
the charge fluctuation, and thus the EP interaction is expected
to play an important role in various physical quantities.
The charge gap in Mott insulators is a consequence ofstrong electron correlation represented by large U. The
correlation induces novel phenomena in terms of the interplayof charge and spin degrees of freedom. In 1D Mott insulatorssuch as Sr2CuO3, two particles created by photo-excitation,i.e., an unoccupied site and a doubly occupied site ofelectrons can move inside the system without being disturbed
by surrounding spins in the background. This is a manifestationof a separation of the charge and spin degrees of freedom,
called the spin-charge separation inherent in 1D correlated
electron systems. Optical responses in the 1D Mott insulators
are characterized by this phenomenon. However, recently
discovered ultra fast relaxation of photoexcited state in 1D
Mott insulators cannot be understood by the spin-charge
separation. Rather, ordinal EP interaction between charge
and phonon should be important (Fig.1). Therefore, it is
crucial to clarify interplay of electron correlation and EP
interaction in 1D Mott insulators. This is also important for
designing a new type of optical switches utilizing the 1D Mott
insulators.
We apply a dynamical density matrix renormalizationgroup to a Hubbard-Holstein model at half filling and calculate- long-range
Coulomb- excitons
Optical switch
“ON”
“OFF”

g=0
g=0.5tg=0.707t

U=10t

4
2
0

4

5

ω0

6

7

ω/t

8

9

10

Fig. 2 Current-current correlation function in a 20-site chainof one-dimensional Hubbard-Holstein model at half filling. An
excitonic peak at ω=6t for zero EP coupling (g=0), which isinduced by a nearest-neighbor Coulomb interaction V, shifts
to the low-energy side and splits into two peaks separated bya phonon frequency ω0 with increasing g.
current-current correlation function χ(ω) that is proportional
to absorption spectrum [1]. Figure 2 shows the dependenceof χ(ω) on a EP coupling constant g for a 20-site HubbardHolstein chain with U=10t, a nearest-neighbor (NN) Coulombinteraction V=3t, and an Einstein phonon frequency ω0=0.5t,
t being NN hopping interaction. Without g, an excitonic peak
appears at an excitation energy of ω =6t. With increasing g,
the peak shifts toward the low-energy side. For a realistic
value of g for Sr2CuO3 (g=0.707t), the excitonic peak splits
into two peaks whose separation is close to ω0. This isqualitatively consistent with a broaden feature of absorption
spectrum of Sr2CuO3, although a two peak feature has notbeen observed.
We also examine U dependence of χ(ω) in detail [1] andfind that the effect of the EP interaction on an exciton isenhanced by increasing U. This enhancement is in contrast
to the effect of the EP interaction on the ground state, wherein
the Peierls instability is suppressed by U, and thus indicatesa crucial role of phonon degree of freedom in the photoexcitedstates of 1D Mott insulators.
References[1] H. Matsueda, A. Ando, T. Tohyama and S. Maekawa,Phys. Rev. B 77,193112 (2008).

Charge excitations
Strongcoupling

V=3t

6
χ(ω)

Interplay of strong electron correlation and electronphonon (EP) interaction is one of important issues in thephysics of low dimensional correlated electron systems. In
order to understand the interplay, we theoretically examine
the optical properties of the one-dimensional (1D) Mott
insulators, and clarify the effect of the EP interaction on theirphotoexcited states.

Spin-charge
decoupling

Key Words
Mott Insulator, Optical Properties, Electron-Phonon Interaction

SpinContact toTakami Tohyama (Yukawa Institute for Theoretical Physics,
Kyoto University)e-mail: tohyama@yukawa.kyoto-u.ac.jp
Sadamichi Maekawa (Theory of Solid State Physics Division)e-mail: maekawa@imr.tohoku.ac.jp

photonPhononFig. 1 A view of relation among charge, spin and, phonon
degrees of freedom in the process of photoexcitation of 1D
Mott insulators. This relation is crucial for optical switchesutilizing 1D Mott insulators.

KINKEN Research Highlights 2009 IMR

Highlights of Collaborated Research

41

Research

In-plane Conduction and c-axis Polarization in the Misfit-layered
Oxide [Bi2Ca2O4]qCoO2

The different temperature dependence between in-planeand out-of-plane charge transport is observed on a numberof layered materials. Especially in the studies of high-Tc
cuprates, the discrepancy is called the “charge confinement”
in connection with the strong electron correlations or thepseudogap phenomena. In order to gain insight into the originof the anisotropic behavior, it is worthwhile to investigate theevolution of anisotropic charge transport in various types oflayered insulators. The low-frequency dielectric measurement
is appropriate for this purpose because it provides relevant
information about the electrical conduction, the dielectric
polarization and the effects of disorder [1].
A misfit-layered cobaltite [Bi2Ca2O4]qCoO2 is an insulating
end member of Bi-M-Co-O (M=Ca, Sr, Ba) systems. We
succeeded in growing [Bi2+0.52Ca2-0.52O4]0.627CoO2 single
crystals using the flux method. Figure 1(a) shows thetemperature dependence of the in-plane resistivity ρab. The
insulating behavior is observed at all measured temperatures.
In the high-T region, ρab obeys the thermal activation formulawith an activation energy of ~32 meV (Fig. 1(b)). In the low-T
region, the data coincide with Mott variable-range hopping
formula for a noninteracting Fermi glass (Fig. 1(c)), indicating
the existence of the localized states due to some disorders.
The crossover temperature T* is estimated to be 63 K.
Out-of-plane dielectric spectra ε’c are shown in Fig. 2.
With increasing temperature, the rapid ε’c growth is visible in
the lower frequencies (below ~104 Hz), reaching 5×104 at 20
Hz and 250 K. As the high voltage is applied, the ε’c swells
are completely suppressed and turn to negative values [2].
This result implies that the observed polarizability growth isclosely related to an increase in the mobile carrier number,

cm)

105
104

ab (

cm)
ab (

102
10
10

0

10-1
0

1000

104

CoO2

c

103
10

2

b
a

T*

101
101

102

103

104

ω /2π (Hz)

105

250 K
220 K
190 K
160 K
130 K
100 K
70 K
40 K
30 K
20 K
15 K
10 K
8K
7K
6K

106

Fig. 2 Frequency dependence of the dielectric constant ε’c.
A conceptual figure of c-axis polarization is shown by arrows
in the inset.

that is, under the strong field the short-range localized(dielectric) carriers move across potential barriers and havethe screening ability. Note that ε’c appears to rise from thevicinity of T*, above which the thermally activated behavior
becomes dominant within the ab plane. Our observation
indicates that the c-axis polarization is attributed to theactivated carriers, which stay within the ab plane even if theycan excite across the in-plane energy gap. We expect thatthe observed out-of-plane dielectric phenomenon will provide
a clue to the understanding of the mechanisms underlying
the anisotropic charge localization.
References[1] M. Maki, S. Nakao, K. Machida, M. Shiraishi, X.-G. Zheng,
T. Naito and H. Iwasaki, J. Phys. Soc. Jpn. 76, 044711(2007)[2] M. Maki, K. Machida, T. Mori, T. Nishizaki and N.
Kobayashi, Phys. Rev. B 78, 073101 (2008)
Key Words
Localization, Dielectric Property, Misfit-layered Cobaltite

T*

T*
0.01

T > T*

Contact to
Makoto Maki (Department of Physics, Saga University)e-mail: mmaki@cc.saga-u.ac.jp
Norio Kobayashi (Low-Temperature Physics Division)e-mail: koban@imr.tohoku.ac.jp

(c)

1022

103

1

1044 (b)105εc'We have used low-frequency dielectric measurements
to study the evolution of anisotropic charge transport in misfit
layered oxides [Bi2Ca2O4]qCoO2. Above ~60 K, the in-planedc conductivity obeys the Arrhenius law and the dielectric
polarization occurs along the c direction. We point out that
this c-axis polarization is attributed to the activated carrier
which is confined to ab plane.

0.03

1/T (K–1)

0.3

0.4

T –1/4 (K–1/4 )

(a)
100

T (K )

200

300

Fig. 1 (a) Temperature dependence of ρab. The same data
are plotted against 1/T in (b) and T-1/4 in (c).

42Highlights of Collaborated Research

IMR KINKEN Research Highlights 2009

Research

Relationship Between Crystal Structure and Superconductivity
in Iron-based Superconductors
Recently, iron-based superconductors have attracted
great attention as new high-Tc materials. We have clarified
that there is a strong relationship between crystal structureand superconductivity using neutron diffraction technique.
Characteristically, superconducting transition temperaturesattain maximum values when FeAs4-tetrahedrons form aregular shape.
Recent discovery of superconductivity in LaFeAsO1-xFx
at superconducting temperature of Tc=26K has triggered theenergetic study of searching a new superconductor. Soon, it
has been found that fluorine-free LnFeAsO1-y (Ln=lanthanoid)
samples show superconductivity with maximum Tc=55K. As
the Tc is very high, their cooper pairing mechanism could not
be explained by the conventional BCS theory. To elucidate
the mechanism, their crystal structure should bedetermined.
The crystal structure of LnFeAsO is characterized by two
kinds of stacking layers LnO and FeAs (Fig. 1). The Fe atom
is surrounded by four As atoms in the FeAs layer forming a
FeAs4 tetrahedron. Charges are transferred from LnO to
FeAs layers by substitution or introduction of defect of oxygen
atoms. We focus our attention on crystal structure of FeAs
layers, since superconductivity is induced in FeAs layers.
We conducted neutron diffraction measurements using
high-resolution powder diffractometer D2B of the ILL in
Grenoble, France and HERMES of the Institute for Materials
Research, Tohoku University, installed at the JRR-3 reactor
of JAEA at Tokai. The obtained spectra were analyzed by
the Rietveld method. Polycrystalline samples of LnFeAsO1-y(Ln=La, Ce, Pr, Nd, Tb and Dy) were used for themeasurements.
We have clarified the superconducting phase diagram of
LaFeAsO1-y and NdFeAsO1-y by estimating the oxygen
content. Both systems show superconductivity above y~0.06.
But, doping dependence of Tc is different. In LaFeAsO1-y, Tc
attains maximum values at around y=0.12 and decreases

Fig. 2 Tc vs As-Fe-As bond angle α for various pnictide
superconductors. Crystal structure parameters of samples
showing almost maximum Tc in each system are selected.
The vertical dashed line indicates the bond angle of a regulartetrahedron (α = 109.47°).

with increasing y. Whereas in NdFeAsO1-y, Tc increases till
y=0.26. It seems that there is no universal relationshipbetween Tc and carrier concentration.
Figure 2 shows As-Fe-As bond angle as a function of Tc
in various pnictide superconductors [1]. The parameters of
the samples showing almost maximum Tc in each system are
selected to eliminate the effect of carrier doping. The obtained
lanthanoid dependence of crystal structure parameters in
LnFeAsO1-y system shows that FeAs4-tetrahedrons form aregular shape around NdFeAsO1-y. Obviously, Tc becomesmaximum when FeAs4-tetrahedrons form a regular shape,
indicating that there is a strong correlation between crystal
structure and superconductivity.
References[1] C. H. Lee, A. Iyo, H. Eisaki, H. Kito, M. T. Fernandez-Diaz,
T. Ito, K. Kihou, H. Matsuhata, M. Braden, and K. Yamada, J.
Phys. Soc. Jpn. 77, 083704 (2008).
[2] C. H. Lee, A. Iyo, H. Eisaki, H. Kito, M. T. Fernandez-Diaz,
R. Kumai, K. Miyazawa, K. Kihou, H. Matsuhata, M. Braden
and K. Yamada, J. Phys. Soc. Jpn. 77, 44 (2008) Suppl. C.
Key Words
Iron Pnictides Superconductivity, Powder Neutron Diffraction,
Crystal Structure Analysis

Fig. 1 The crystal structure of LnFeAsO. The dashed lines
show unit cells. FeAs4 clusters in the FeAs layers form atetrahedral lattice. Tc reaches maximum when the FeAs4lattices form a regular tetrahedron [2].

KINKEN Research Highlights 2009 IMR

Contact toChul-Ho Lee (Advanced Industrial Science and Technology(AIST))
e-mail: chlee@aist.go.jp
Kazuyoshi Yamada (Metal Physics with Quantum Beam
Spectroscopy Division)e-mail: kyamada@imr.tohoku.ac.jp

Highlights of Collaborated Research

43Research

Effect of Austenite Grain Boundaries on
the Morphology of Lath Martensite
Lath martensite is an important structure for strengthening
and adding toughness to steels. The growth process andkey factors of the morphology, which are important todevelopment of martensitic steels, have not been understood.
Our purpose is to clarify the key factor in the morphology oflath martensite.
Lath martensite in steel is well known to show ahierarchical microstructure consisting of packets, blocks,
sub-blocks, and laths. The prior austenite grain is divided into
packets that consist of blocks. These blocks within thepackets have the same habit plane. The packet is further
divided into plate like blocks, which consist of laths with asimilar crystal orientation. Recently, it has been recognizedthat a block contains components called sub-blocks, each ofwhich corresponds to a single variant characterized by the
Kurdjumov-Sachs orientation relationship (KS OR) withaustenite, as shown in Fig. 1 [1].
The complex microstructure affects the mechanicalproperties, and control of the growth process of the
microstructure is necessary for microstructural control. The
growth process of the microstructure has not been clarified,although controlling the microstructure is important. To
understand the growth process, characterization of the initialstructure of lath martensite, i.e., the rule of the nucleation isimportant because martensite laths and plates appear like
autocatalysis nucleation following the initial martensite laths.
From previous reports [2], the austenite grain boundaries
and annealing twin boundaries are known to be effective
nucleation sites for martensite. However, the nucleation rule
for lath martensite is still unclear. The present work focusedon the characterization of the initial structure of lath martensite,using local crystallographic analyses, and found the rule ofthe initial structure [3].
The specimens used were partially transformed lathmartensite in an Fe-Ni-Mn alloy. The crystallography of thelath martensite was examined by scanning electron
microscopy/electron backscatter diffraction pattern analysis(SEM/EBSD) and transmission electron microscopy/Kikuchi
pattern analysis (TEM/KP).
Figure 2(a) shows an optical micrograph of lath martensite

Fig. 2 (a) Optical micrograph with 3% nital etching and (b)orientation image map measured on the Fig. 2 (a) with SEM/EBSD in the Fe-Ni-Mn alloy. The crystal orientations ofmartensite plates in both austenite grains are similar.
in the Fe-Ni-Mn alloy. The etched and non-etched regions
are lath martensite and austenite regions, respectively. There
are two austenite grains, i.e., γ1 and γ2. The lath martensite
regions (α’) appear around the austenite grain boundary andgrow into both austenite grains. The orientation image mapmeasured with SEM/EBSD in Fig. 2(b), which is the same
region shown in Fig. 2(a), indicates that the crystal orientations
are similar in the martensite plates in both austenite grains.
Other observations showed that lath martensite appears
around the high angle austenite grain boundaries andannealing twin boundaries. From these observations andcrystallographic analyses, the rules of initial lath martensite
were found: 1) Martensite laths held KS OR with austenite
and the other austenite grains; 2) the parallel closed packed
direction of the KS OR held by the martensite lath is nearlyparallel to the austenite grain boundaries; and, 3) thetransformation shear and misfit strain can be accommodated
effectively along the austenite grain boundaries or by slip in
the other austenite grain.
References[1] S. Morito, X. Huang, T. Furuhara, T. Maki and N. Hansen:
Acta Mater. 54, 5323 (2006).
[2] e.g., K. Tsuzaki and T. Maki: J. Jpn. Inst. Metals, 45, 126(1981).
[3] S. Morito, J. Nishikawa, T. Ohba, T. Furuhara and T. Maki:
Proc. of Int. Conf. on Martensitic Transformations 2008,
Santa Fe, NM, USA, 2008.7.1, in press.
Key Words
Lath Martensite, Local Crystallographic Analysis, 3D
Analysis
Contact toShigekazu Morito (Department of Materials Science, Shimane
University)e-mail: mosh@riko.shimane-u.ac.jp
Tadashi Furuhara (Microstructure Design of Structural
Metallic Materials Division)e-mail: furuhara@imr.tohoku.ac.jp

Fig. 1 Schematic illustration of lath martensite [1].

44Highlights of Collaborated Research

IMR KINKEN Research Highlights 2009

Research

Hydrogen Retention Properties on the Divertor Tiles in LHD

C
Fe
Ti

300
200
100
0

(E)
20

40

60

80 100 120 140

Counts [a.u.]Counts [a.u.]

30
20

(F)

Fe
Ti

10
0

0

Reference position [mm]

20

40

60

80 100 120 140

Reference position [mm]Deposition area
Erosion area(a) (b)

(c)

(d)
1cm

Virgin surface
Private side

(a)

(b)

(c)

(d)

E-L15

D-L20

C-L25

B-L10

Fig. 1 SEM image of the surface (a~d) and distribution of thedeposited elements (E, F).

KINKEN Research Highlights 2009 IMRCounts

300

300

150

(i)
Depth direction

200
100
0
300

(ii)

200
100
0
0

0

100

200

300

Channel number

Top surface of the tile

One of the key issues for the establishment of a steadystate fusion reactor system is impurity control. A continuous,
long-lasting plasma erodes a significant amount of the plasma
facing components (PFCs) on the one hand, and acorresponding amount of deposits is formed on PFCs. In
general, deposited layers are unstable and therefore they are
a possible source of impurity (dust) for the plasma. The
deposited layers and the dusts can retain a huge amount oftritium. Such mobile products as dust with a radiation isotope
are the source of another critical concern in a fusion reactor.
In Large Helical Device (LHD), the first wall panels anddivertor plates are made of stainless steels and isotropic
graphite, respectively, uncontrollable plasma density riseoccurred in long pulse discharges with ion cyclotron heating
due to hydrogen outgas locally induced by heating of thedivertor tile in a discharge [1]. Therefore, quantitative data ofretained hydrogen in the divertor tiles is important for thesteady state plasma operation in LHD as well as for tritium
inventory in the future. In this study, surface modification andhydrogen retention of a LHD divertor tile used for plasma
experiments for 8 years were analyzed.
Fig. 1 shows surface morphology (a~d) and chemicalcomposition (E, F) of the tile surface from the scanningelectron microscope (SEM) and the energy dispersive X-ray
spectroscopy (EDS), respectively. The left side of thephotograph still keeps virgin graphite color, but the right side
has changed to brown. These results indicate that the former
is erosion dominant while the latter is deposition dominantarea. In the deposition area, substantial amount of Fe and Ti
were deposited. It seems that the Fe impurities came fromthe first wall during glow discharge cleaning (GDC) [2], andthe Ti originated from Ti gettering operation which is often
used to reduce oxygen impurities in the vacuum vessel. The
surface at the erosion area (a), and (b), looks porous. This
fact indicates strong sputtering erosion. The surface of thedeposition area is covered by deposits with small hills of
about 1-2μm in size. It seemed that mixed-material composed

Reference depth [nm]450Counts

Surface modification and hydrogen retention of a LHD
divertor tile used for plasma experiments for 8 years wereanalyzed by using the ion beam analytical techniques. On
the deposition dominant area, surfaces were covered mainlywith Fe and Ti, respectively. In this area, the amount ofhydrogen retention was relatively high, while, on the erosion
dominant area, hydrogen retention was much lower thanthat of the deposition area.

400

Fig. 2 ERD spectra of the retained hydrogen atoms from (i)erosion and (ii) deposition areas on the divertor tile.
of C, Fe and Ti of about a few μm thicknesses is formed on
the surface of the (c) and (d).
Depth profile of hydrogen was measured by using anelastic recoil detection (ERD) technique. Fig. 2 shows ERD
spectra from (i) erosion and (ii) deposition areas. A verticalaxis (counts) and horizontal axis (channel number)corresponds to “the amount of hydrogen” and “the analyzed
depth”, respectively. Although in this figure, both axes were
not converted to “the amount of hydrogen” and “the analyzed
depth”, because stopping power of the deposition layer could
not be quantified yet. But if the stopping power of the freshgraphite crystal was assumed to conversion for the channel
number (ch) to the depth (nm), the rate is roughly estimatedto be 1.5nm/1ch. In order to make this figure morecomprehensible, the reference depth estimated by using thisrate was shown in the top of the figure. In the case of (i) erosion
area, high counts of hydrogen concentration are observed at
the depth of around ~50 nm and then, this is gradually
decreased with increasing depth from the surface. However,the shape of the ERD spectrum of (ii) is much different from (i).
A small peak was confirmed at the depth of around ~50 nm,
and very high hydrogen concentration than (i) appeared withincreasing the depth and kept up to the deep range. Judging
from the connection length profile of the magnetic field lines
crossing the divertor plate and probe measurements [3], influxof hydrogen on the deposition area is lower than that of theerosion area. Nevertheless retained hydrogen is much higher
in the deposition side. This fact implies that the retention ofhydrogen does not simply depend on its influx, but change of
surface properties due to impurity deposition is importantfactor in determining hydrogen retention.
References[1] M. Shoji et al., J. Nucl. Mater. 337-339 (2005) 186-190[2] M. Tokitani et al., Nucl. Fusion 45 (2005) 1544-1549[3] S. Masuzaki et al., Nucl. Fusion 42 (2002) 750-758
Key Words
Fusion Energy, Impurity Deposition, Hydrogen Retention
Contact to
Masayuki Tokitani (National Institute for Fusion Science)e-mail: tokitani.masayuki@LHD.nifs.ac.jp
Shinji Nagata (Materials Design Division)e-mail: nagata@imr.tohoku.ac.jp

Highlights of Collaborated Research

45Research

Low Rigidity Titanium Alloy for Orthopedic Implant
Animal Study on Plate Fixation of Tibia Fracture Model in Rabbit
Because there is a big difference of material rigidity
between living bone and conventional implant metals,
orthopedic implant can induce bone atrophy due to decreaseor absence of mechanical stress, so-called stress shielding.
As a solution of the problem, a low rigidity titanium alloy of
Ti-29Nb-13Ta-4.6Zr (TNTZ) was developed where theelastic modulus is 58 GPa and all elements are considered
as non-toxic [1, 2].
In order to investigate bone tissue reaction to the low
rigidity titanium alloy, animal study on plate fixation of rabbit
tibia fracture model was performed [3]. Based on the design ofcommercial fixation device for human finger, Bone plate andscrews were made of TNTZ (E=58 GPa), Ti-6Al-4V (E=108
GPa) and SUS316L (E=161 GPa) where the elastic modulusof human cortical bone is 10 ~ 20 GPa. As experimental
animal, mature New Zealand white rabbits were used.
Under intra-venous anesthesia, the bone plate wasprovisionally placed and fixed by screws, then once removed.
Upper one third of tibia was cut through by using oscillating
saw where the cut position was corresponding to the centerof the bone plate. However, fibula was kept intact. Finally, thebone plate was tightly fixed again by screws. (Fig.1)
Regarding fracture healing, there was no significant
difference among SUS316L, Ti-6Al-4V and TNTZ. As ageneral time course, callus formation was observed at 2
weeks, which became distinct at 3 weeks. Bone union was
obtained at 4 weeks, and the fracture line was hardly
observed around 8 weeks. The trace of the experimental
fracture was completely disappeared at 16 ~ 20 weeks.
(Fig.2)(a) Plate with dry bone .

(b) Plate fixation.

Fig. 1 Plate fixation of rabbit tibia fracture model.

A: Newbone
B: Oldbone(a)Increase of tibia diameter(b) Close-up the junction

Fig. 3 Bone remodeling in TNTZ
However, bone atrophy was observed as porous or poorbone tissue in cortical bone especially under the bone plate,which was different in time course among three materials. In
addition, the diameter of the tibia bone was increased by newbone formation in TNTZ, which seemed to be a result of bone
remodeling. (Fig.3)
As the Wollf’s law of the functional restoration, it is wellknown that the mechanical stress will influence the bone
remodeling. According to this theory, the elastic modulus of
the bone plate will naturally influence the bone tissue reactionin the bone plate fixation. However, since bone formation andremodeling is faster in small animals as compared withhuman and large animals, such remarkable bone atrophy
was probably caused in a short periods of 48 weeks after the
fixation.
From the results, it is suggested that low rigidity titanium
alloy can delay or reduce bone atrophy in terms of retaining
mechanical stress, and also promotes bone remodeling toadapt to new loading condition. Therefore the low rigidity
titanium alloy of TNTZ is promising and applicable for implant
metal, which provides a new option for fracture fixation.
References[1] M. Niinomi, T. Hattori, S. Niwa, BIOMATERIALS IN
ORTHOPEDICS:55-60, Marcel Dekker (2003)[2] M. Niinomi, Materials Science and Engineering A, A243:
231-236 (1998)[3] N. Sumitomo, K. Noritake, T. Hattori, K. Morikawa, S.
Niwa, K. Sato, M. Niinomi, J Mater Sci: Mater Med 19:15811586 (2008)
Key words
Titanium Alloy, Low Rigidity, Orthopedic ImplantContact to
Tomokazu Hattori (Department. of Materials Science andEngineering, Meijo University)e-mail: tomhat@ccmfs.meijo-u.ac.jp
Mitsuo Niinomi (Biomaterials Science Division)e-mail: niinomi@imr.tohoku.ac.jp

0w

2w

4w

7w

48w

Fig. 2 X-ray follow-up for fracture healing and remodeling.
Fig.2. X-ray follow-up for fracture healing and remodeling.
46

Highlights of Collaborated Research

IMR KINKEN Research Highlights 2009

Research

Large Tunnel Magnetoresistance in Magnetic Tunnel Junctions
Using a Co2MnSi Heusler Alloy Electrode and a MgO Barrier

The development of ferromagnetic materials with highspin polarization is the most important subject in the spinelectronics field. An ideal high spin polarization material ishalf-metallic ferromagnets (HMFs) with a band gap at the
Fermi energy level for one spin band. Some full-Heusler
alloys (Co2MnSi, Co2MnGe, etc.) are the most promising
HMFs, because they have been predicted to possess ahalfmetallic electronic structure and have a high Curie
temperature. In addition, experimentally, we have observed
large tunnel magnetoresistance (TMR) ratios of up to 159%
at 2 K in magnetic tunnel junctions (MTJs) with Co2MnSi/
Al-O/CoFe structure; thus proving the halfmetallic property of
Co2MnSi (CMS). Although this TMR effect is pronounced in
MTJs with a conventional amorphous Al-oxide barrier, recent
work on MTJs with a MgO barrier showed better performance(Fig. 1). These results are suggested to be related to thecoherent tunneling process through the MgO barrier. In
addition, theoretically, Miura et al. showed that coherent
tunneling through the MgO barrier could also occur andenhance TMR ratio in a CMS/MgO/CMS MTJ. In this work,
we have succeeded in fabricating high-quality CMS/MgO/
CoFe MTJs.
The MTJs with a structure of MgO (100) substrate/Cr (40)
/Co2MnSi (30) /MgO (2 - 2.5) /Co50Fe50 (5) /IrMn (10) /Ta (5)(unit: nm) were fabricated using an ultrahigh vacuum
magnetron sputtering system [1]. The MgO tunnel barrier
was formed by electron beam evaporation. The pressureduring evaporation was about 2 x 10−6 Pa and typicalevaporation rate was 0.01 nm/s. The MTJs were patterned
into 8 x 8 – 90 x 90 μm2 areas using photolithography and Ar
ion etching. Patterned MTJs were annealed in the range of
300 - 485°C for 1 h in high vacuum by applying a 1 T magnetic
field to improve the crystallinity of the MgO barrier. For
comparison, CoFeB/MgO/CoFeB-MTJ was fabricated in thesame conditions and both the TMR effect and the differential
conductance were investigated.
A cross-sectional high-resolution transmission electron
microscope (HRTEM) lattice image on a sample of Ta=450°C
along the (110) direction of the CMS film is shown in the inset
of Fig. 1. It confirmed that extremely smooth and abrupt
interfaces were formed. Both good crystallinity and high (001)orientation of the MgO barrier can promote a coherent

800

LT

700

Co2MnSi//Co2MnSi

100
0

tter

Fig. 2 Temperature dependence of TMR ritio for various MTJs.
tunneling process through the Δ1 band of CMS. We infer thatthe reduction in tunnel resistance by the coherent tunnelingprocess enhances the TMR effect at RT, compared to the
MTJs with an amorphous Al-oxide barrier.
Figure 2 shows the temperature dependence of the TMR
ratio of the MTJ annealed at 475 °C. The MR curves measured
at various temperatures are shown in the inset of Fig. 2. For
reference, the temperature dependences for both CoFeB/
MgO/CoFeB-MTJ and CMS/Al-oxide/CoFe-MTJ are shown
together in Fig. 2. The TMR ratio of the CMS/MgO/CoFeMTJ increased drastically with decreasing temperature, anda huge TMR ratio of 753% was observed at 2 K. The TMR
ratio of 753% at 2 K is the highest in the MTJs using Heusler
alloy electrodes and much larger than those of CoFeB/MgO/
CoFeB-MTJ (400%) and CMS/Al-oxide/CoFe-MTJ (160%).
This result reveals that the half metallicity of the CMS can berealized not only in the MTJ with an amorphous Al-oxidebarrier but in the MTJ with a crystalline MgO barrier. The
larger temperature dependence of the TMR ratio of the CMS/
MgO/CoFe-MTJ [2, 3] than that of the CoFeB/MgO/CoFeBMTJ should be solved for the application to devices.
References[1] S. Tsunegi, Y. Sakuraba, M. Oogane, K. Takanashi, and
Y. Ando, Appl. Phys. Lett. 93, 112506 (2008).
[2] N. D. Telling, P. S. Keatley, G. van der Laan, R. J. Hicken,
E. Arenholz, Y. Sakuraba, M. Oogane, Y. Ando, K. Takanashi,
A. Sakuma, and T. Miyazaki, Phys. Rev. B 78, 184438(2008).
[3] N. D. Telling, P. S. Keatley, L. R. Shelford, E. Arenholz, G.
van der Laan, R. J. Hicken, Y. Sakuraba, S. Tsunegi, M.
Oogane, Y. Ando, K. Takanashi, T. Miyazaki, Appl. Phys.
Lett. 92, 192503 (2008).

1.5 nmCross section of a single crystal
MTJ with a Heusler alloy

Key Words
Heusler Alloy, Tunnel Magnetoresistance, Spintronics

+M
gO

M
gO
eB
oF

300
200

MgO
Co2MnSi

Co2MnSi

Heu
sler

MgO

+

CoFe

CoFe

,C

400

Spu

Crystal structure of Co2MnSi
Heusler alloy
Ta
IrMn

Fe

TMR ratio (%)

500

Co2MnSi//CoFe
MnSi//CoFe

RT

600

EB

A large tunnel magnetoresistance (TMR) ratio of 753%
has been observed at 2 K in a magnetic tunnel junction(MTJ) using a Co2MnSi Heusler alloy electrode and acrystalline MgO tunnel barrier. This TMR ratio is the largest
reported to date in MTJs using a Heusler alloy electrode.

Co2MnSi//CoFe
MnSi//CoFe

Heusler + AlO

Co2MnAl//CoFe
MnAl//CoFe

2003

Co2CrFeAl//CoFe
CrFeAl//CoFe

2004

2005

Year

2006

CMG//
CoFe
MG//CoFe

2007

2008

Fig. 1 Recent trend of TMR ratio of MTJs using a Co2MnSi
Heusler alloy electrode and an MgO (or Al-O) barrier.

KINKEN Research Highlights 2009 IMR

Contact toYasuo Ando (Graduate School of Engineering, Tohoku
University)e-mail: ando@mlab.apph.tohoku.ac.jp
Koki Takanashi (Magnetic Materials Division)e-mail: koki@imr.tohoku.ac.jp

Highlights of Collaborated Research47

Research

Neutron Irradiation Effects in Graphite for Fusion Reactor Applications-Hydrogen Absorption, Diffusion and Annealing EffectsBulk hydrogen retention and absorption kinetics havebeen studied on graphite, which is one of the candidate
materials for plasma facing walls in a fusion reactor. It is
ascertained in this study that two kinds of hydrogen trappingsites exist and be additionally produced during neutron
irradiation. The creation and annealing of these two trappingsites show quite different behavior.
Graphite and carbon fiber/carbon composites (CFCs) are
principal materials for current fusion experimental devices
due to high thermal conductivity and thermal shock resistivity.
However, these materials degrade such various advantagesby irradiation of neutrons originated from the fusion reactionof deuterium and tritium. Moreover, since carbon has achemical affinity to hydrogen, it may cause high tritium
inventory in plasma facing walls and a large amount of fuelgas recycling between fusion plasma and wall materials. The
authors also have found out that neutron irradiation
significantly raised hydrogen retention in graphite in previous
studies. From the viewpoint of plasma density control andreducing the radioactive tritium inventory in plasma facing
components, it is important to clarify the hydrogen behavior
in these materials.
Fig.1 shows a model of hydrogen trapping and transport
in graphite proposed by the authors. In an absorption process,hydrogen molecules penetrate easily through open poresand grain boundaries of graphite filler grains and reach thesurface of the grain. Then, hydrogen will migrate into the filler
grain apparently as molecule (in practice, sequence ofdissociation and recombination) controlled by the diffusion
process with an activation energy of 1.3 eV. The hydrogen
will be trapped at outside edge surface of crystallites by acovalent bond with the adsorption enthalpy of 2.6 eV. This
site has been named as trap 2. If the trap 2 sites are
sufficiently fulfilled with hydrogen atoms, hydrogen canpermeate into the crystallite along the graphite intercalations.
The hydrogen absorption, or atmospheric hydrogen pressure
decrease, in this stage is controlled by first-ordered chemical
reaction, which caused by dissociation at a surface ordetrapping from trap 2. Inside a crystallite, there are interstitialcluster loops or zigzag structures, and they should be theother trapping site for hydrogen (trap 1). The trap binding
energy for hydrogen was estimated to be 4.4 eV. In the caseof unirradiated samples, the number of trap 1 is below 10 %of the total number of trapping sites, and trap 2 is the major

Fig. 2 Change of hydrogen retention and the pre-annealing
effect in neutron-irradiated graphite shown for each trappingsite (absorption temperature: 1273 K, Equilibrium pressure: ~10 kPa).
sites for hydrogen.
Graphite and CFC samples were irradiated in the Japan
Materials Testing Reactor (JMTR) at various fluences up to
5.4 x 1024 n/m2 (>1 MeV), which corresponded to 0.65 dpa,
below 473 K. Hydrogen retention in neutron-irradiatedgraphite determined for each trapping site is shown fordifferent neutron fluences in Fig. 2 (a). The total retention
increases with the irradiation fluence. At lower neutron
fluences below 0.011 dpa, the increase is mainly due to theincrease of trapping in trap 2, while trap 1 retention is clearly
increased at higher fluences above 0.017 dpa. At the highest
fluence of 0.65 dpa, the total retention became 6.3 times aslarge as that for 0.047dpa, and the retention consisted ofnearly the same quantity of trap 1 and trap 2 retention.
The retention obtained for the samples which wereannealed before hydrogen absorption is shown in Fig. 2 (b).
The retention which corresponds to trap 2 tends to decrease
with the increase of pre-annealing temperature. This indicatesthat most of the trap 2 sites induced by neutron irradiation will
be annealed out at high temperatures. However, the trap 2
sites may not be completely annealed out, since the retention
was still larger than the amount obtained for the unirradiatedsamples, which was also pre-annealed at 1873 K. On theother hand, retention in trap 1 tends to remain relatively higheven after the heat treatment at 1873 K. One can conclude
the trap 1 sites should be thermally more stable than trap 2.
Although it is not mentioned in the article, hydrogen diffusion
coefficients and reaction rate constants at trap 2 in neutronirradiated graphite and their pre-annealing effects have been
investigated. These results are described in refs [1] and [2].
References[1] H. Atsumi, A. Muhaimin, T. Tanabe and T. Shikama, J.
Nucl. Mater. 386-388, 379 (2009).
[2] H. Atsumi, T. Tanabe and T. Shikama, J. Nucl. Mater.
390-391, 581 (2009).
Key Words
Fusion Reactor Materials, Graphite, Irradiation Effects

Fig. 1 Schematic illustration of hydrogen trapping sites andhydrogen transport in a graphite material.

48

Highlights of Collaborated ResearchContact to
Hisao Atsumi (Department of Electric and ElectricalEngineering, Faculty of Science and Engineering, Kinki
University)e-mail: atsumi@ele.kindai.ac.jp
Tatsuo Shikama (Nuclear Materials Science Division)e-mail: shikama@imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2009

Research

27Al NMR Study of Np-Based Superconductor NpPd5Al2

We have investigated a newly discovered Np-based
heavy fermion superconductor NpPd5Al2 by means of 27Al
NMR. Our NMR data provide evidence for d-wave symmetryof the Cooper pairs. This result strongly suggests that
5f-electron spin fluctuations could play an important role inthe superconductivity of NpPd5Al2 [1].
The actinide compounds with 5f electrons occupy a
special position in research on condensed matter physics.
The reason for this is that 5f electrons in actinides behave as
though they are near the boundary between localized anditinerant character. The electronic properties of theseactinides are often characterized by a so-called heavy
fermion state at low temperatures. A recent discovery of
considerable interest was that of superconductivity in theheavy fermion compound PuCoGa5 in 2002, where thesuperconducting (SC) transition temperature Tc was found to
be 18.5 K. Following that discovery, in 2007 the first Npbased, heavy fermion superconductor NpPd5Al2 was
discovered with the relatively high Tc=4.9 K by D. Aoki et al[2]. NpPd5Al2 has a tetragonal crystal structure as shown in
the inset to Fig. 2. The physical properties of the SC state in
NpPd5Al2 are interesting, e.g. strong Pauli paramagnetic
effect, first-order SC transition, and so on. Recently, we haveinvestigated the superconductivity in NpPd5Al2 using the
NMR technique.
Figure 1 shows a variety of symmetries for Cooper pairs
in a superconductor. S-wave symmetry for the Cooper pairs
is composed of an isotropic orbital form and a singlet spinstate. On the other hand, p- and d-wave symmetries arecharacterized by anisotropic orbital forms with triplet (p) andsinglet (d) spin states, respectively. In the figure, we also
show the possible origin for the attractive force within a
Cooper pair.
NMR is a powerful tool for investigating the symmetry of
Cooper pairs. The orbital form of the Cooper pairs can be
determined by measuring the temperature dependence ofthe nuclear spin-lattice relaxation rate 1/T1 below Tc. 1/T1
reflects the density of states near the Fermi level. On theother hand, the spin state of the Cooper pairs can be
determined by measuring the Knight shift. The Knight shift isassociated with a local field induced by the electron spins at
a nuclear site.
Fig.2 shows the temperature dependence of 1/T1 throughthe SC transition. 1/T1 shows a sharp drop just below Tc
without the coherence peak (dotted line in Fig. 2) characteristicof a conventional s-wave superconductor. The absence ofSymmetry of
the Cooper pairss wavep waved waveー+

Orbital form of
the Cooper pairs

Candidate for the originof the attractive force

B

O
A

Fig. 2 Temperature dependence of 1/T1. Solid line shows
calculation of 1/T1. Black dotted, red solid and red brokenlines show calculated 1/T1 using full, line-node and pointnode gaps, respectively. Inset shows the crystal structure of
NpPd5Al2.
the coherence peak strongly indicates the occurrence of ananisotropic (non s-wave) SC state. The black solid line is aline calculated assuming a line-node in the SC gap. From thebest fit to the experimental data, a relatively large value 2Δ0/
kBTc=6.4 of the SC gap has been obtained, which indicatesstrong coupling superconductivity. On the other hand, fromthe decrease of the Knight shift below Tc (not shown), we
have confirmed a singlet spin state in NpPd5Al2. Therefore,
our 1/T1 and Knight shift results strongly indicate d-wave
superconductivity in NpPd5Al2. This suggests that themagnetic fluctuations of the 5f-electron spins plays animportant role in the superconductivity of NpPd5Al2 [1]. We
are currently investigating the characteristics of the magnetic
fluctuations in the normal state.
References[1] H. Chudo, H. Sakai, Y. Tokunaga, S. Kambe, D. Aoki, Y.
Homma, Y. Shiokawa, Y. Haga, S. Ikeda, T. D. Matsuda, Y.
Onuki, and H. Yasuoka, J. Phys Soc. Jpn. 77, 083702(2008).
[2] D. Aoki, Y. Haga, T. D. Matsuda, N. Tateiwa, S. Ikeda, Y.
Homma, H. Sakai, Y. Shiokawa, E. Yamamoto, A. Nakamura,
R. Settai, and Y. Onuki, J. Phys Soc. Jpn. 76, 063701(2007).

++

ーー

Spin triplet

Spin singlet

Spin state of
the Cooper pairs

C

Spin singlet
PhononFerromagnetic Antiferromagnetic
ﬂuctuations
ﬂuctuations

Fig. 1 Variety of symmetries for the Cooper pairs. The + and– show the sign of the wave function.

KINKEN Research Highlights 2009 IMR

Key Words
Heavy-fermion, Superconductor, Transuranium Compound
Contact toHiroyuki Chudo (Advanced Science Research Center, Japan
Atomic Energy Agency (JAEA))
e-mail: chudo.hiroyuki@jaea.go.jp
Yoshiya Homma (Radiochemistry of Metals Division)e-mail: yhomma@imr.tohoku.ac.jp

Highlights of Collaborated Research49Research

A Novel Structure for Carbon Nanotube Reinforced Alumina
Composites with Improved Mechanical Properties
We report the mechanical properties of multi-walled carbon
nanotube (MWCNT) /alumina composites made with a pristine
MWCNT and an acid-treated version that have nanoscale defects
on their surfaces from an acid treatment. Mechanicalmeasurements reveal that only 0.9 vol.% acid-treated MWCNT
addition results in 27% and 25% simultaneous increases inbending strength and fracture toughness, respectively.
Carbon nanotubes (CNTs) have extremely high tensile strengthand stiffness, good flexibility, and low density. These superior
properties make CNTs attractive for many applications andtechnologies. Engineering ceramics have high stiffness, excellent
thermostability, and relatively low density, but their brittleness
impedes their use as structural materials. Incorporating CNTs intoa ceramic matrix might be expected to produce tough as well as
highly stiff and thermostable ceramic composites. Until now,
however, most results for strengthening and toughening have been
disappointing, and only little or no improvement of strengthening
and toughening has been reported in CNT/ceramic materials. It is
well known that the two problems exist in CNT/ceramic composite
materials research. The first is the inhomogeneous dispersion of
CNTs in the matrix. Pristine CNTs are well known for poor
solubilization, which leads to phase segregation in the composite
owing to the van der Waals attractive force. Severe structural
inhomogeneities result in the degradation of the mechanicalproperties of the CNT composite materials [1]. The second problem
is the poor connectivity between CNTs and the ceramic matrix,
which leads to a limited stress transfer capability from the matrix to
the CNTs. The connectivity with the matrix, and uniform distribution
within the matrix are essential structural requirements for thestronger and tougher CNT/ceramic composites.
Here we report the mechanical properties of multi-walledcarbon nanotube (MWCNT) /alumina composites made with apristine MWCNT and an acid-treated version that have nanoscale
defects on their surfaces from an acid treatment. A novel processing
approach based on the precursor method can diminish the phasesegregation of MWCNTs, and render MWCNT composites that arehighly homogeneous [2]. Combined with a mechanical interlock
induced by the chemically modified MWCNT, this approach leads
to improved mechanical properties. Mechanical measurements oncomposites reveal that only 0.9 vol.% acid-treated MWCNT
addition results in 27% and 25% simultaneous increases in bending
strength and fracture toughness, respectively, indicating enhanced
stress transfer capability from the alumina to the MWCNTs [2].
An interesting geometric structure was observed between

Fig. 2 Fracture surface of the acid-treated MWCNT/alumina
composite. Numerous individual MWCNTs protrude from thefracture surface.
individual MWCNTs and alumina matrix, as shown in Figs. 1(a)and (b). It is revealed that a nanodefect on the acid-treated
MWCNT is filled up with alumina crystal, which may be intruding
into the nanodefect during grain growth. This nanostructure is
novel in that its structure resembles a nanoscale anchor with analumina crystal spiking the surface of the MWCNT. From thefracture surface, the following features can be noted. Numerous
individual MWCNTs protrude from the fracture surface, and thepullout of the MWCNTs can be clearly observed, which had notbeen obtained until now for conventional CNT/ceramic composites(Fig. 2). Some MWCNTs showed that the diameter of pullout
MWCNT drastically slenderized toward their tip[2], quite similar tothat observed in a broken MWCNT under tensile load. This failuremode of the acid-treated MWCNT may be due to the enhancementof effective frictional resistance between individual MWCNTs andthe alumina matrix. In the case of the smaller amount of the acidtreated MWCNTs, no phase segregation was observed, whereas
the composites made with the pristine MWCNT revealed an
inhomogeneous structure even for MWCNT addition as low as 0.9
vol.%. These observations revealed that the high structural
homogeneity and enhanced frictional resistance of the structuralcomponents leads to a simultaneous increase in the strength andtoughness of the acid-treated MWCNT/alumina composites.
Our present work may give a promising future for the applicationof MWCNTs in reinforcing structural ceramic components andother materials systems as well. The acid-treated MWCNT/alumina
composites may have potential applications to tribomaterials suchas joint prosthesis and micro electro mechanical systems (MEMS),
because of the CNTs’ good lubrication properties [3].

References[1] G. Yamamoto, M. Omori, K. Yokomizo and T. Hashida,Diam. Relat. Mat. 17, 1554 (2008).
[2] G. Yamamoto, M. Omori, T. Hashida and H. Kimura,
Nanotechnology 19, 315708 (2008).
[3] G. Yamamoto, T. Hashida, K. Adachi and T. Takagi, J.
Nanosci. Nanotechnol. 8, 2665 (2008).
Key Words:
Carbon Nanotubes, Alumina, Mechanical Properties

Fig. 1 MWCNT morphology in the composites. (a) It isdemonstrated that a nanodefect on the acid-treated MWCNT
is filled up with alumina crystal. (b) Enlarged TEM image,taken from the square area.

50

Highlights of Collaborated ResearchContact to
Go Yamamoto (Institute of Fluid Science, Tohoku University)e-mail: gyamamoto@rift.mech.tohoku.ac.jp
Hisamichi Kimura (Advanced Research Center of Metallic
Glasses)e-mail: hisami@imr.tohoku.ac.jp

IMR KINKEN Research Highlights 2009

Research

Calcification by MC3T3-E1 Cells on RGD Peptide Immobilized on
Titanium Through Electrodeposited PEG
The effect of a cell-adhesive peptide containing Arg-GlyAsp (RGD) immobilized through poly (ethylene glycol) (PEG)on titanium (Ti) on calcification by MC3T3-E1 cells wasinvestigated to develop a new surface modification techniqueusing biofunctional molecules. The hard tissue compatibilityof Ti is improved by immobilizing RGD through functional
molecules which have a long molecular chain.

PEG
Gly

RGD

Ti

Ti

Ti

Ti

RGD/Gly/Ti

RGD/PEG/Ti

RGD/Ti

Ti

Fig. 2 Specimens employed in this study.
One terminal group of twitterions with a -COOH group anda -NH2 group is useful to bond biofunctional molecules such as
RGD. At a certain pH known as the isoelectric point (IEP), the-NH2 group is positively charged and the -COOH group is
negatively charged. On the other hand, the other terminalgroup is required to bind stably with a surface oxide on a metal.
The surface oxide film of a metal is usually covered by hydroxyl
groups. The hydroxyl groups are positively or negatively
charged according to the pH of the surrounding solution. The
surface charge is apparently zero at a certain pH that is known
as the point of zero charge (pzc). Therefore, a zwitterionmediated surface for the immobilization of biofunctional
molecules such as RGD is created with consideration of theinteraction between the charged zwitterion and the charged
metal surface in a solution. A simple immobilization techniquewith consideration of the isoelectric point of molecules IEP and
pzc is necessary.
The IEPs of two zwitterions, glycine and both-terminalterminated PEG (NH2-PEG-COOH) were determined from thetitration curves, and the thickness of zwitterion layers
immobilized on Ti with immersion and electrodeposition at
various pH based on IEPs were evaluated with ellipsometry toinvestigate the effect of pH and the immobilization techniqueon the interactions between the zwitterions and Ti surface.
From the titration curves, pK1, pK2, and the IEP of glycine were
determined as 2.8, 8.9, and 5.9, respectively, and pK1, pK2,
and the IEP of NH2-PEG-COOH were determined as 2.1, 11.7,
and 6.9, respectively (Fig.1). At a certain specific pH, +H3NCH2-COO－ or +H3N-PEG-COO－ were formed by hydrolysisof glycine or NH2-PEG-COOH. In addition, the Ti surface wasnegatively charged at this pH. As a result, for immersion, theelectrostatic reactivity between terminal groups of zwitterion
and hydroxyl groups on Ti surface was the highest and thethickness of the immobilized layer was significantly the largest
PEG

H H H

H H

H 2N C C C

O C C

H H H

n

H H H H H

pK1 pI pK2

10.0

Titer of 0.1 mol L-1 NaOH / mL

H H

H H H H H
O C C C C C COOH

7.5
5.0

+H N-PEG-COOH
3

pH4+H N-PEG-COOH
350+H N-PEG-COO−
3

50%

pI

pH7

+H N-PEG-COO−
3

pK2

pH10

H2N-PEG-COO− 50+H N-PEG-COO−
3

2.5
0

pK1

pH2

NH3+

0

2 34

COO−

6 7 9 10 12

pH12

50

H2N-PEG-COO−+H N-PEG-COO−
3hydrolysis

H2N-PEG-COO− + H2O+H N-PEG-COO− + OH−
3

Fig. 1 Titration curve of PEG (NH2-PEG-COOH) and effectof pH on dissociation of terminals of PEG.

KINKEN Research Highlights 2009 IMR

RGD/Gly/Ti

Ti

RGD/PEG/Ti

RGD/TiControl
Scale Bar: 5 mm

Fig. 3 Pictures of calcified deposits in MC3T3-E1 cells stainedwith a 1% alizarin red S solution after fixation. Calcified
deposits are stained red.
at pH 12. For electrodeposition, glycine, with its smallermolecular weight, was more easily attracted to Ti surface thanNH2-PEG-COOH, which has a larger molecular weight, while
the thickness of the immobilized layer was the largest at pH 12
in both zwitterions [1].
The effect of a cell-adhesive peptide containing RGD
immobilized through PEG on Ti on calcification by MC3T3-E1
cells was investigated. RGD was immobilized on Ti throughPEG. PEG was immobilized on Ti with electrodeposition, and
RGD, with immersion. For comparison, glycine was employedbecause it is the simplest molecule containing both -NH2 and-COOH at its terminals (Fig.2). MC3T3-E1 cells were cultured
and differentiation-induced on each specimen, and the cell
calcification properties were investigated. The level of cell
calcification on RGD/PEG/Ti was the highest (Fig.3) [2].
References[1] Y. Tanaka, H. Saito, Y. Tsutsumi, H. Doi, N. Nomura, H. Imaiand T. Hanawa: J. Colloid Interface Sci. 330, 138 (2009).
[2] K. Oya, Y. Tanaka, H. Saito, K. Kurashima, K. Nogi, H.
Tsutsumi, Y. Tsutsumi, H. Doi, N. Nomura and T. Hanawa,
Biomaterials 30, 1281 (2009).
Key Words
Titanium, Biofunctional Molecule, Electrodeposition
Contact toTakao Hanawa (Institute of Biomaterials and Bioengineering,Tokyo Medical and Dental University)e-mail: hanawa.met@tmd.ac.jp
Hisamichi Kimura (Advanced Research Center of Metallic
Glasses)e-mail: hisami@imr.tohoku.ac.jp

Highlights of Collaborated Research51

Research

Bulk metallic glassy alloys(BMGs) have the strong
interest in material science and technology because they
have the unique characteristics such as very high strength,
high corrosion resistance, high wear resistance as well as
the flexible workability based on viscous super-elasticity
phenomenon of appearing near the glassy transition
temperature Tg of BMG. However, too much deformation
process during this secondarily working for BMG will cause
the nano-microscopic change of amorphous structure as wellas the degradation of BMG’s properties, therefore, newinnovative material processing technique for mass-producing
BMG components is becoming a important subject forcommercial production of BMGs. Recently, an innovative“Rapid-Rotation Centrifugal Casting Process” to thecomponents and commercial production of BMGs wasdeveloped [1]. The schematic figure of this method isschematically shown in Fig. 1.
The melted raw BMG alloys by high frequency induction
heating method were poured into copper mold with changing
the rotation speed over 3000rpm which can cause over 100
times as much centrifugal pressurization force as gravity.
The fabricated BMG component sample is shownin Fig. 2 where the produced artificial tooth model of
Zr55Al10Ni5Cu30b are amorphous state as shown in XRD
pattern.
The morphology is apt to change depending on the alloy
composition and rotating speed of copper dies. Fig.3 shows
the laser micrograph , XRD and picture of [(Fe0.5Co0.5)0.75
B0.2Si0.05]96Nb4 alloy.
In this case, outer part is almost amorphous and a partial
crystallization is observed in inner part probably due to thedifference of cooling capacity from copper mold. According tosynergistic effect of rapid solidification and high pressurization
casting in this manufacturing process, there are technical

Inner mold

Rod
MoltenMetal

Fixation with an upper lid

Coil
CopperMold
Motor

Implant Pipe

φ3.5mm

Zr55Al10Ni5Cu30

30

50

70

90

Diffraction angle 2θ [deg]Fig. 2 Dental implant model and XRD of Zr55Al10Ni5Cu30［（Fe0.5Co0.5）0.75B0.2Si0.05］96Nb4α‐(Fe, Co, Si)γ‐(Fe, Co, Si)(Fe, Co)23(B, Nb)6
(Fe, Co)2B
inner
unknown

Intensity [arb.unit]“Rapid-Rotation Centrifugal Casting Process” wasapplied to manufacturing the amorphous bulk metallic
glassy alloys (BMGs) components. The molten alloys werepoured into copper mold with rotation speed over 3000rpm
which can cause 100 times as much centrifugal pressurization
force as gravity. The samples such as small gear, screw,
biomedical artificial tooth-root model and gripper ofendoscope of Zr or Fe alloy systems were successfully
fabricated within only a few seconds at one process withmore cheap cost.

Intensity [arb.unit]

Metallic Glass Components Produced by Rapid-Rotation
Centrifugal Casting

outer

30

50

70

outer Diffraction angle 2θ [deg]

90

inner

Fig. 3 Laser micrograph, XRD and picture of [(Fe0.5Co0.5)
0.75B0.2Si0.05] 96Nb4 alloy

advantages of expansion of element composition, uniformityof the quality, reduction of internal defects and prevention of
degradation at the time of secondary fabrication process ofBMGs, which seems easy to automation, a large amount ofcost cut and mass production in the very near future.
References[1] K. Hashimoto, T. Kubota, T. Mashiko, T. Okazaki, Y. Furuya
and A. Inoue, J.Japan Inst. Metals. 71-7, 553-558 (2007)
Key words
Metallic Glass, Centrifugal Casting, Amorphous
Contact to
Yasubumi Furuya (Faculty of Science and Technology,
Hirosaki University)e-mail: furuya@cc.hirosaki-u.ac.jp
Hisamichi Kimura (Advanced Research Center of Metallic
Glasses)e-mail: hisami@imr.tohoku.ac.jp

Fixation with a bolt
The foundation of a moldAppearance after combination

The mold for dental implant
The mold for rings
The photograph of the mold for dental implant, and the mold for rings.

Fig. 1 Schematic view of the developed rapid-centrifugal
casting for BMGs

52

Highlights of Collaborated Research

IMR KINKEN Research Highlights 2009

Research

In-Plane Electrical Resistivity in Magnetic Fields and Stripe
Correlations in La2-xSrxCuO4 around x = 0.21

VortexH

Fig. 1 Schematic picture of the formation of the stripe orderaround vortex cores in the CuO2 plane of a high-Tc
superconductor.

KINKEN Research Highlights 2009 IMR

2-x

Ωcm)
ρab (mΩ

0.2

0.1

x

4

H = 27 T
25
23
21
17.5
15
13
11
9
8
6
4
2
1
0.5
0

x = 0.190

27T
0T

0
0.08

La Sr CuO

0.06

x = 0.206

0.04

27T

2-x

x

H = 27 T

4

0.02

25
23
21
17.5
15
13
11
9
8
6
4
2
1
0.5
0

0T

0
0.3

La2-xSrxCuO4
0.2

H = 27 T
25
23
21
17.5
15
13
11
9
8
6
4
2
1
0.5
0

x = 0.215

27T
0.1

0
0

0T
10

20

30

40

T (K)

50

60

70

Fig. 2 Temperature dependence of the in-plane electricalresistivity, ρab, in various magnetic fields along the c-axis up
to 27 T for La2-xSrxCuO4 with x = 0.190 - 0.215.
broadening-like behavior. These results suggest that thesuperconductivity with small SC fluctuations is realized
singularly around x = 0.206 in which Tc is depressed.
Accordingly, it is concluded that the charge stripe order issingularly developed and is stabilized by the application ofmagnetic field in LSCO around x = 0.21.
References[1] T. Adachi, N. Kitajima, T. Manabe, Y. Koike, K. Kudo, T.
Sasaki, and N. Kobayashi, Phys. Rev. B 71, 104516 (2005).
[2] T. Adachi, K. Omori, T. Kawamata, K. Kudo, T. Sasaki, N.
Kobayashi, and Y. Koike, J. Phys.: Conf. Series 51, 259(2006).
[3] N. Kakinuma, Y. Ono, and Y. Koike, Phys. Rev. B 59,
1491 (1999).
[4] I. Watanabe, M. Aoyama, M. Akoshima, T. Kawamata, T.
Adachi, Y. Koike, S. Ohira, W. Higemoto, and K. Nagamine,
Phys. Rev. B 62, R11985 (2000).
Key Words
High-Tc Superconductivity, Stripe Order, Magnetic-Field Effect

CuO2
plane
Stripeorder

La Sr CuO

Ω cm)
ρab (mΩThe application of magnetic field to the high-Tc
superconductors leads to the creation of vortices in the
superconducting (SC) state. In the vortices, scanningtunneling-spectroscopy and neutron-scattering experiments
have revealed the formation of a checkerboard-like chargeorder and a charge-spin stripe order as shown in Fig. 1,
respectively. However, whether or not these are competingordered states with superconductivity has not yet beenclarified.
In order to investigate magnetic-field effects on the
charge-spin stripe order from the viewpoint of transport
properties, we previously measured the in-plane electricalresistivity, ρab, in magnetic fields parallel to the c-axis forLa2-xBaxCuO4 and La2-xSrxCuO4 (LSCO) around x = 1/8 [1,2].
It has been found for samples in which the charge stripe
order is fairly stabilized that the SC transition curve sifts tothe low-temperature side in parallel (parallel shift) withincreasing magnetic field. These results suggest that thesuperconductivity with small SC fluctuations due to the shortcoherence length is realized in the stripe-ordered state.
As for the overdoped regime of LSCO around x = 0.21,
significant anomalies have been observed so far; a slight
depression of Tc [3], a development of the Cu-spin correlation
at low temperatures [4] and an enhancement of the lattice
instability by the application of magnetic field. These aresuggestive of a possible development of the stripe correlations
around x = 0.21 as well as around x = 1/8. Therefore, wehave investigated magnetic-field effects on ρab in LSCO
around x = 0.21.
Single crystals used for the measurements were grownby the traveling-solvent floating-zone method. The ρabmeasurements were performed in magnetic fields up to 27 T
by the standard dc four-probe method, using a SC magnet
below 17.5 T and a hybrid magnet above 17.5 T at the High
Field Laboratory for Superconducting Materials (HFLSM),
IMR, Tohoku University.
Figure 2 shows the temperature dependence of ρab in
LSCO with x = 0.190 - 0.215. With increasing magnetic field,
the SC transition curve tends to show a parallel shift for x =0.206 in which Tc is singularly depressed. For x = 0.190 and
0.215 in which no singular depression of Tc is observed, on
the other hand, the SC transition curve shows rather

0.3

Ωcm)
ρab (mΩIn recent years, magnetic-field effects on the so-called
dynamical charge-spin stripe correlations, which might bein the heart of the mechanism of the high-Tc superconductivity,
have been an issue of note. In contrast to the underdoped
regime, the validity of the magnetic-field-induced stripes
has been under debate in the overdoped regime due to lackof studies.

Contact to
Tadashi Adachi (Department of Applied Physics, Tohoku
University)e-mail: adachi@teion.apph.tohoku.ac.jp
Norio Kobayashi (Low Temperature Physics Division)e-mail: koban@imr.tohoku.ac.jp

Highlights of Collaborated Research

53

Research

Development of High-Field Scanning Tunneling Microscopefor 18 T Cryocooled Superconducting Magnet
We have developed the high-field scanning tunnelingmicroscope (STM) to study the nanoscale electronic orderand the vortex state in high-Tc superconductors. Using thenonmagnetic vibration-isolation table, we have succeeded inobserving STM atomic images in the magnetic field appliedby the cryocooled superconducting magnet up to 18 T.
Scanning tunneling microscopy/spectroscopy (STM/STS)studies provide important information on the nanoscale spatial
variation of the electronic state. In high-Tc superconductors(HTSC), the self-organized electronic patterns observed in
the local density of states tend to appear when the
superconductivity is suppressed, the magnetic field strong
enough can be an important parameter to study the electronic
state. To study the field dependence of nanoscale electronic
states and the vortex state in HTSC in the high field region,
we have developed a new STM system for the 18 T cryocooled
superconducting magnet (18T-CSM) [1].
Figure 1 shows schematic illustration of the high-field
STM system installed on the 18T-CSM at High Field Laboratory
for Superconducting Materials, IMR, Tohoku University. The
18T-CSM is a large-scale magnet with a total weight of 5.8
tons, and the superconducting coils and a radiation shield arecooled by Gifford-McMahon-Joule-Thomson (GM-JT)
cryocooler and two GM cryocoolers, respectively. It is
expected that the mechanical vibration and/or soundgenerated from three cryocoolers become crucial noise for
STM/STS measurements. To make a stable tunneling contact
with atomic-scale stability, we have developed the compact
vibration-isolation table which consists of the diaphragm typeair springs. The vibration-isolation table should be completely
nonmagnetic without any electric circuits because the strayfield near the top flange becomes ~ 1 T when H = 18 T. As
shown in the left photograph in Fig. 1, we have designed thesmall and rigid STM head (20mm in diameter × 55mm inlength) to enhance the resonant frequency.
To evaluate the vibration noise of the 18T-CSM and theperformance of the vibration-isolation table, we havemeasured the vibration acceleration signal by using a

piezoelectric accelerometer and the FFT analyzer [1]. Whenthe vibration-isolation table is not active, the STM atomic
resolution is strongly hindered by the strong vibration noisefrom cryocoolers. However, the vibration noise is effectively
reduced by the vibration-isolation table and the STM image
shows atomic resolution. Figure 2 demonstrates the STM
image of highly oriented pyrolytic graphite (HOPG) at H = 18
T. The STM image shows a clear atomic arrangement with alattice constant of 2.5 Å for HOPG. The results indicate thatthe performance of STM does not degrade in high fields andthe stray field (~ 0.2 T at the top part of the STM insert) does
not affect very much on the preamplifier for the tunnelingcurrent signal. In this study, we have succeeded in expanding
the field-range for STM measurements beyond the maximum
field (H ~ 10 T) of previous STM studies.
We also performed continuous STM measurements in thefield-sweep condition with a sweep rate of 18T/60min, in order
to examine the stability of the STM operation in the magnetic
field. In this condition, the deformation of the STM image isvery small and the atomic arrangement is clearly resolved.
These results indicate that the STM head developed in thisstudy has a good stability in the scanning (x,y) and also
feedback (z) directions even in the varying magnetic field.
Since the STM system developed in this study is compatible
with the 30 T hybrid magnet, we are planning to extend thepossible field-range for STM/STS up to 30 T in the future.
References[1] T. Nishizaki and N. Kobayashi, Journal of Physics:
Conference Series 150, 012031 (2009).
Key Words
Scanning Tunneling Microscopy, High Field, Cryocooled
Superconducting Magnet
Contact to
Terukazu Nishizaki (Low Temperature Physics Division)e-mail: terukazu@imr.tohoku.ac.jp

Vibration-isolation table

STM head

STM insert

Tube scanner•X Y scan•F eedback Z
Signalwire

position B
position A
z
y

590mm
590mm

GM cryocooler

GM-JT
cryocooler

1230 mm

Tip holder
PtIr tip
SampleholderBias
wire

Shear piezo(Coarse Z)

Leaf
spring

18 T

11mm

18T-CSM2342 mmFig. 1 Schematic illustration of the high-field STM installed on the 18T-CSM. Main parts
for the STM head are shown in the left photograph. The nonmagnetic vibration-isolationtable (upper right photograph) is set up between the 18T-CSM and the STM insert.

54

Highlights of Collaborated Research

Fig. 2 STM image of HOPG (I =0.1nA, V = 200mV) at H = 18 T.
35.4 Å × 35.4 Å.

IMR KINKEN Research Highlights 2009

Research

Development of Hydroxyapatite Modified with Additives for High
Performance Bioceramics

Hydroxyapatite (HAP) is a major inorganic component of bone
and teeth, as shown in Figure 1. It possesses high biocompatibility
as a biomaterial for implant part, bone filler, and bone-cement, etc.
However, the achievement of the higher bioactivity for HAP is still
demanded as a high performance bioceramic. In our group, thesyntheses of HAP substituted with other metals have been
attempted by various synthetic process, such as hydrothermal
treatment, ion-exchange treatment, and normal heat-treatment athigh temperature to enhance the bioactivity of HAP [1, 2, 3]. Thus,
the syntheses of modified HAPs have been attempted by various
processes, since the substitution with other metals in HAP is
expected to improve the ability of bioactivity of HAP. In this study,
the Si incorporated HAP were successfully synthesized by thenormal heat-treatment and the effect of Si addition on micro- andlocal structure of HAP was clarified.
Commercial monolithic HAP powder was used as a starting
material. Commercial SiO2 powder (3, 10, and 30wt%) was added
into HAP powder and mixed in ethanol with ball milling for 24h.
Mixed HAP/SiO2 powder was compacted with stainless mold andheat-treated at various temperatures in air atmosphere. The
component of samples was identified with XRD. The microstructureof samples was observed by SEM and TEM. The local structure of
Ca-K edge for samples heat-treated at 600 to 1200°C in air
atmosphere was examined with XAFS (X-ray absorption finestructure) measurement at SPring8 in Kouto (Hyogo, Japan) andalso at Photon Factory in Tsukuba (Ibaragi, Japan).
From XRD results of HAP samples containing with 3 to 30wt%
SiO2, monolithic HAP samples heat-treated at 600 to 1200°C werecomposed of hydroxyapatite phase without another calcium
phosphate phase. HAP/SiO2 samples heat-treated at 1200°C werecomposed of mainly α-tricalcium phosphate and in parthydroxyapatite, although the component of samples was composedof monolithic hydroxyapatite heat-treated below 1200°C. Thus, it
was found that SiO2 addition into HAP enhanced the formation of
α-tricalcium phosphate above1200°C. According to FT-IR spectraof HAP/SiO2 samples heat-treated at 1000°C with various SiO2
content, HAP/SiO2 samples with higher SiO2 content indicated thestronger peak from SiO44- with SiO2 content. However, the peakform phosphate (PO43-) for HAP/SiO2 samples decreased withincrease of SiO2 content. In addition, the peak from OH- for HAP/

6.88

0wt

3wt%

10wt%

30wt%

Normalized Intensity/a.u.

The hydroxyapatites modified with Si were synthesized by thenormal heat-treatment in air atmosphere. The microstructures ofthese modified hydroxyapatites were characterized. XAFS study
for these modified hydroxyapatites indicated the effect of Si on
Ca-local structure of hydroxyapatite.

4020

4040
4060
Energy/eV

4080

4100

Fig. 2 XANES of Ca-edge for HAP samples with 3 to 30 wt%-SiO2
heat-treated at 1200°C
SiO2 samples decreased with high SiO2 content of 30%. Since
XRD results indicated no formation of calcium silicate in this study,
it is speculated that this decrease in the peak from OH- resulted
from the formation of calcium silicate caused by the reaction HAPand SiO2 content. Therefore, FT-IR results indicate that SiO44-,
below the optimum SiO2 contents, was substituted for PO43- site inhydroxyapatite structure during the heat-treatment at 1000°C.
Figure 2 shows the spectra of XANES of Ca-K edge for HAP
samples with 3 to 30 wt%-SiO2 heat-treated at 1200°C.
HAP/3wt%SiO2 samples heat-treated at 1200°C indicate thedifferent spectra from monolithic HAP and HAP/10~30wt%SiO2
samples. This result indicated that local structure around Ca-K
edge was significantly different from monolithic HAP and also
those from HAP containing with large amount of SiO2 (10 to 30wt%)
heat-treated at 1200°C. These XANES results also suggest that
SiO44- was substituted for PO43- in hydroxyapatite structure. Thus,
XANES results are consistent with the results of FT-IR. In special,
the results of derivative from the normalized spectra for
HAP/3wt%SiO2 samples were clearly varied form others. The
detailed examinations of Si-K and P-K are still attempted at anothersynchrotron facility. Thus, both XANES and FT-IR results indicatedthe noticeable structural changes accompany the substitution of
SiO44- into PO43- site in hydroxyapatite structure. In addition, thebioactivity for these HAP modified with Si was excellent. As aresult, it was found that these HAP modified with Si possess higher
bioactivity, compared to monolithic HAP.
References[1] A. Nakahira, S. Nakamura, and M. Horimoto, IEEE Transactions
on Magnetics, 43[6], 2465-67 (2007).
[2] A. Nakahira, M. Horimoto, S. Nakamura, S. Ihshihara, H. Nagata,
T. Kubo, and C. Karatani, J. Ion Exchange,18, 306-309 (2007).
[3] A. Nakahira, T. Okajima, T. Honma, S. Yoshioka, and I. Tanaka,
Chemistry Letters, 35, 856-857 (2006).
Key Words
Bioceramics, Hydroxyapatite, Microstructure

9.43
Fig. 1 Schematic drawing of hydroxyapatite (HAP) structure

KINKEN Research Highlights 2009 IMR

Contact toAtsushi Nakahira (Graduate School of Engineering, Osaka
Prefecture University)e-mail: nakahira@mtr.osakafu-u.ac.jp
Takashi Goto (Multi-Functional Materials Science Division)e-mail: goto@imr.tohoku.ac.jp

Highlights of Collaborated Research55

Research

New Metallic Carbon K4 Crystal
It has been successfully shown theoretically that thethird structure of carbon crystals, K4, after diamond andgraphite should exist by the CREST team in Tohoku
University headed by Prof. Kotani with IMR members using
our supercomputer SR11000 at the Center for Computational
Materials Sciences. This K4 carbon crystal has an atomic
structure in 3 dimensional space composed with sp2
bondings between all carbon atoms, different from 2
dimensional structure of graphite. Diamond has also
completely different which composed of sp3 bondings. ThisK4 crystal is metallic in nature, and it is expected that by
doping we can create stable superconductor, macroscopic
conducting line, etc. which have been difficult to be realizedby pure carbon nanostructures..
Recently, Prof. Sunada indicated by mathematicalanalysis[1] the sp2 bonded systems with an attractive 3D
structure, K4 crystal, which corresponds to the third crystal
structure after diamond and graphite of carbon. An ab initio
computer simulation has been conducted to apply this finding
to a real material; carbon crystal. [2]Figure 1 shows the K4 crystal structure, which iscomposed of 10 membered rings of carbons with all sp2
bondings.
A possibility of pressure-induced structural phasetransition between different crystal structures was investigatedto confirm whether the K4 crystal is produced or not, by using
a thermodynamic phase transition theory. According to thistheory, negative slope of the tangent line between the energy
vs volume curves of two different crystals indicates the
positive transition pressure Pt which allows pressure induced
structural phase transition between them. To the K4 crystal,
it is apparent that Pt takes positive value only from graphite
crystal with pressure induction parallel to the a axis as shownin Fig. 2.
According to the phonon calculation for the pure K4

Fig. 2 Binding energy vs. volume curves of diamond (blue),graphite (grey, and black), and K4 (red) crystal structures
composed of carbon atoms based on (a) LDA, and (b) GGA
developed by Ceparley and Alder (CA), and Perdew andWang (PW91) ; The curves are fits of the Murnaghan’sequation of state to the calculated points. Tangential line
between the curves of K4-graphite crystals; XRD patterns (inthe inset of (a)).
carbon crystal, this crystal structure is indicated to haveimaginary frequencies and should be unstable. We are now
trying to expand the unit cell and find out not exactly K4 but
more complex crystal structure around K4 which is morestable than K4 carbon crystal. Since K4 carbon crystal ismetallic, there are many possibilities of K4 crystal for
applications. Especially doping of atoms makes it possible tostabilize the system and to add more functionality such as
superconductivity.
References[1] T. Sunada, Not. Amer. Math. Soc., 55, 208 (2008).
[2] Masahiro Itoh, Motoko Kotani, Hisashi Naito, Toshikazu
Sunada, Yoshiyuki Kawazoe and Tadafumi Adschiri, Phys.
Rev. Lett., 102[5] (2009) pp.0557031-0557034
Key Words
Carbon Nanostructure, K4 Symmetry, Metallic Carbon

Fig. 1 K4 carbon crystal structure, which is composed of 10
member rings with all sp2 bondings.

56

Highlights of Collaborated ResearchContact toYoshiyuki Kawazoe (Materials
Simulation Division)e-mail: kawazoe@imr.edu

DesignbyComputerIMR KINKEN Research Highlights 2009

Research

Optical Conductivity in the t-J Holstein Model
Using recently developed numerical method we computeoptical conductivity of the t-J model coupled to opticalphonons. Near the crossover to the strong electron-phonon
coupling regime, optical conductivity shows a two-peakstructure. This finding suggests that in the framework of thet-J Holstein model the two peak structure seen in recent
optical measurements is due to magnetic excitations coupled
to lattice degrees of freedom via doped charge carriers [1].
The enhanced interest in correlated models, coupled tolattice degrees of freedom is primarily fuelled by experimental
evidence given in part by angular resolved photoemission data
demonstrating that strong electron-phonon (EP) interaction
plays an important role in low-energy physics of high-Tc
materials. Using a numerical method based on the exact
diagonalization within the limited functional space (EDLFS)[2, 3] we investigate optical properties of the t-J Holstein modelfor the case of a single hole in the antiferromagnetic
background.
Figure 1 shows the influence of increasing EP coupling λon optical properties of the t-J Holstein model in the adiabatic
regime. Increasing λ leads to three main effects: a) the spectra
progressively shift towards higher frequencies while the total
spectral weight decreases; b) magnetic excitations that form aband in the mid-infrared (MIR) regime broaden and diminish
with increasing λ; c) a large gap opens in the SC regime.
We find quantitative agreement at λ~0.24 withmeasurements on (Eu{1-x}Cax)Ba2Cu3O6 in the low hole-doping
regime. In our calculation λ~0.24 represents the maximum EP
coupling constant where the low-ω peak, located at ωI, is justbarely visible. This peak is due to the local magnetic excitation
and remains separated from the continuum forming the rest of
the MIR band. The higher-ω peak at ωII corresponds to MIR
band, broadened and renormalized by phonon excitations.
Our explanation of the experimental results relies on thea)λ=0.35b)c)

λ=0.35σxx

ω0/t=0.1

λ=0.30λ=0.30λ=0.25λ=0.25

1.56J/tλ=0.22

J/t=0.3
λ=0.10λ=0.10λ=0.21

ω0/t=0.1

ω0/t=0.3

4

λ=0.20λ=0.00λ=0.002

1.4 λ=0.24

λ=0.23λ=0.20λ=0.200

λ=0.25

0

2

4
ω/t

0

1

2

3

Fig. 1 Optical conductivity σxx for ω0/t=0.3 in (a), ω0/t=0.1 in(b) and (c) at J/t=0.3 and at k=(π/2,π/2). Up to 56 phonon
quanta were used to obtain accurate results for λ>0.2. Arrowsin (c) indicate ωI~1.56J~187 meV (choosing t=400 meV and
J/t=0.3) and ωII ~1.4t~560 meV.

KINKEN Research Highlights 2009 IMRa) λ=0.1, ω0/t=0.1 b) λ=0.2, ω0/t=0.1 c) λ=0.0

σxx

x3

x3

0.8
0.7
0.6
0.5
0.4
0.3
0.2
J/t=0.1

0

1

2

0

1 2
ω/t

0

1

2

3

Fig. 2 σxx for ω0/t=0.1 and λ=0.1 in (a), λ=0.2 in (b) and λ=0.0(c) at k=(π/2,π/2) and different values of the exchangeinteraction J/t. Dashed lines, multiplied by a factor of 3 forbetter visibility, in (a) and (b) represent results of the Holsteinmodel with ω0/t =0.1 calculated at corresponding values of λand k=(0,0). Arrows in (a) and (b) indicate the position of thelowest-ω peak for the Holstein model.
conjecture that lightly doped (Eu{1-x}Cax)Ba2Cu3O6 compoundlies in the crossover from weak to strong coupling electronphonon regime where physical properties (quasiparticle
weight, charge stiffness and dynamic properties) are extremely
sensitive to small changes of λ.
The lack of low-frequency peak above ω=ω0 associatedwith phonon excitations in the limit of weak EP coupling isinvestigated in Figure 2. We have applied EDLFS method tosolve the Holstein model with a single electron using essentially
identical Hilbert space describing lattice degrees of freedom
as the one used for the t-J Holstein model. σxx(ω) is for thiscase presented with dashed lines. We present optical spectra
for various values of magnetic exchange interactions J/t of thet-J Holstein model. By increasing J/t magnetic peaks shift
towards higher frequencies that should separate magnetic andlattice degrees of freedom. Nevertheless, we observe nosignature of peaks above ω=ω0 as they would appear in thecorresponding Holstein model.
Authors acknowledge the hospitality of IMR, Tohoku
University, where this work has been initiated.
References[1] L. Vidmar, J. Bonča and S. Maekawa, Phys. Rev. B 79,
125120 (2009).
[2] J. Bonča, S. Maekawa and T. Tohyama, Phys. Rev. B 76,
035121 (2007).
[3] J. Bonča, S. Maekawa, T. Tohyama and P. Prelovšek,
Phys. Rev. B 77, 054519 (2008).
Key Words
Optical Conductivity, t-J Holstein Model, Strong Correlations
Contact to
Sadamichi Maekawa (Materials Property Division)e-mail: maekawa@imr.tohoku.ac.jp

Highlights of Collaborated Research57