3FTFBSDI

The Electron Falls Apart
There are two kinds of physics; one is to search forelements in nature and the other is to examine the diversityof nature. The condensed matter physics which our group is
working on is in the latter. An electron is an elementary
particle. However, when many of them interact strongly inmaterials, they give rise to a variety of quantum phenomena.
In addition, electrons cannot often keep their own properties
in such correlated states.

Spin and charge are inseparable traits of an electron, but
in one-dimensional solids, their separation into collectivemodes-as independent excitation quanta (or particles) calledspinons and holons is expected.
We have identified, for the first time, both theoreticallyand experimentally, the evidence of spin-charge separation,i.e., in the angle-resolved photoemission (ARPES) spectrumin SrCuO2 the single quasiparticle peak splits into a spinonholon two peak structure.

Fig.1. (a) The angle-resolved photoemission spectrum in
SrCuO2. (b) The experimental and theoretical (Solid anddashed lines) dispersions of spinon and holon.

References[1] B.J.Kim, H. Koh, E. Rotenberg, S.-J.Oh, H. Eisaki, N.
Motoyama, S. Uchida, T. Tohyama, S. Maekawa, Z.-X.Shen
and C. Kim, Nature Physics 2, 397 (2006).
Contact to
Sadamichi Maekawa (Theory of Solid State Physics Division)e-mail: maekawa@imr.tohoku.ac.jp

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IMR KINKEN Research Highlights 2007

Innovative Electronic Materials

3FTFBSDI

High Magnetic Field X-ray Spectroscopy on YbInCu4-a New Tool of Valence State in IntermetallicsThe world strongest and smallest magnet has beendeveloped for high field X-ray spectroscopy. The firstexperiment is made for the valence state transition of
YbInCu4 and a field variation of the valence is clearly
determined. The system opens a new way to investigate thevalence state of intermetallics in very high magnetic fields.
A synchrotron X-ray is a powerful tool to investigate
materials for the extremely strong intensity and the tunability
in X-ray regime. This can be applied not only for a structure
analysis but also for X-ray spectroscopy such as X-ray
absorption(XAS) and magnetic circular dichroism (MCD).
These spectroscopic techniques have been used for various
systems, however the combination with very strong magnetic
field has been absent. To open this frontier, we havedeveloped a small pulsed-magnet system for X-ray
experiments.
Figure 1 shows the mini magnet used for XAS experiment
up to 50 T. Thanks to the extremely small size, the magnet
and the capacitor bank are portable and thus the combination
with various types of X-ray spectrometer becomes possible.
In fact, the capacitor bank is as small as a small refrigerator
and the storing energy is only 2 kJ. Even such small system,a high magnetic field up to 50 T can be achieved. One ofadvantages of the system is that the magnet can be easily
installed into a small space of the existing spectrometer andthe cryostat system without major change.

Fig. 1 A small pulsed magnet for XAS experiment. A magnetic
field as high as 50 T can be generated.
The first experiment wad made for the YbInCu4, whichshows a valence state transition. Namely, the magnetic
moments disappear at low temperature with a first order likechange of lattice volume and the non-magnetic valencefluctuation state is realized. When a magnetic field is applied,
the metamagnetic like transition is observed. It has been
considered as the recovery of magnetic state associated withthe valence state transition. Since the transition field is above
30 T, no direct evidence of the valence change has beenpresented so far.

Innovative Electronic Materials

Fig. 2 XAS spectra of YbInCu4 at 5.4 K. Yb2+ and Yb3+
components are indicated by arrows with II and III,
respectively.
Figure 2 shows the XAS spectra of YbInCu4 at 0 and 41
T. The latter is taken above the field induced valence statetransition. A distinct change of spectra is observed. Namely,
the Yb2+ component deceases at high magnetic field and,
alternatively, the Yb3+ component increases. By fitting thespectra by a standard method, the field variation of valenceof Yb ion is evaluated. Such direct determination of valence
state is very unique for material investigation. It will be usefulfor the study of spin dependence of electronic state in manyother systems.
More recently, the technique has been applied for X-ray
MCD measurement of antiferromagnets. As is well known,
MCD is a unique element selective method to determine spinand orbital moments. It can be applied only for ferromagnet
and this limits the range of application. By using the presenthigh magnetic field, MCD can be measured many of
antiferromagnet. It would open the new field for X-ray
spectroscopy.
References[1] Y. H. Matsuda, T. Inami, K. Ohwada, Y. Murata, H. Nojiri,
Y. Murakami, H. Ohta, W. Zhang and K. Yoshimura, J. Phys.
Soc. Jpn. 76, 034702 (2007).
Contact toHiroyuki Nojiri (Magnetism Division)e-mail: nojiri@imr.tohoku.ac.jp

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3FTFBSDI

Scanning Tunneling Microscopy/Spectroscopy Studies ofSuperconducting Boron-doped Diamond Films
Scanning tunneling microscopy/spectroscopy experiments
have been performed on (111)-oriented epitaxial films of heavily
boron-doped diamond at T = 0.47 K. The tunneling spectra
show superconducting property and the obtained gap ratio 2Δ/
kBTc = 3.57 is consistent with the weak-coupling BCS theory.

24

IMR KINKEN Research Highlights 2007

ON

5PQMBZFSDBSCPO

B

4FDPOEMBZFSDBSCPO

C

Fig.1 (a) STM topographic image of the (111)-oriented
epitaxial film of heavily boron-doped diamond. (3.2×3.3 nm2,
V = 500 mV, I = 0.1 nA). (b) Structure of C(111)1×1:H surface.

1.5

dI/dV (nS)

Recently, Ekimov et al. discovered superconductivity inthe heavily boron-doped diamond. The superconducting
transition temperature Tc was 2.3 ~ 4 K for their polycrystalline
bulk samples with the boron concentration n ~ 4.9×1021 cm-3.
As pointed out by Takano et al., the value of Tc depends onthe concentration of boron, the synthesized process, and theorientation of the film growth; thus it is interesting to study the
relationshipbetweenthesurfacestructureandsuperconducting property.
In order to understand the electronic state of thesuperconductivity in boron-doped diamond, we have
performed low temperature scanning tunneling microscopy/
spectroscopy (STM/STS) experiments on heavily borondoped diamond films at T = 0.47 K under the ultra-highvacuum condition by using 3He-refrigerator based STM [1].
Heavily boron-doped epitaxial diamond films (Tc = 5.4 K)
were grown on the (111)-oriented type Ib diamond substrates
by using the microwave plasma-assisted chemical vapor
deposition (MPCVD) method.
The STM topography on the (111)-oriented film surface
shows a regular atomic arrangement with a sixfold symmetry
as shown in Fig. 1(a). The distance between nearest neighboratoms is estimated to be ~ 0.25 nm and the value agrees withthe distance between the two carbon atoms of the diamond(111) ideal surface. Since the density of the atomic hydrogenis high in the MPCVD process, the monohydride structure,C(111)1×1:H, is most likely the structure in Fig.1(a). Thisatomic structure is realized when each dangling bond of
diamond (111) 1×1 is terminated by one hydrogen atom, as
shown in the structure model of Fig. 1(b). The broad changeof the background electronic state in Fig. 1(a) may result
from the random distribution of the doped boron atom with aconcentration of ~3%.
Figure 2 shows a typical tunneling conductance spectrum
dI/dV (solid circles). The tunneling spectra show thesuperconducting energy gap 2Δ and the coherence peak.
Using the modified BCS expression for the density of states(i.e., Dynes function), N(E,Γ)∝ |Re(E - iΓ)/{(E - iΓ)2-Δ2}1/2|,
parameters Δ = 0.83 meV and Γ = 0.41 meV are obtained
from the best fit to the experimental data (see solid line).
Here, Γ is the pair-breaking parameter. The gap ratio is
obtained to be 2Δ/kBTc = 3.57 and the value is consistent withthe theoretical value (2Δ/kBTc = 3.53) for the weak coupling
BCS superconductor. The dI/dV spectra observed in thisstudy show the broad coherence peak, the high zero bias
conductance and the relatively large value of Γ. Since thesecharacteristic features become remarkable with increasingboron concentration, the introduced disorder by the boron
doping works as the pair breaking creates which enhance thezero bias conductance.

%JBNPOEMBUUJDFTUSVDUVSF

1

0.5

0

B70a(111)
T = 0.47 K-50

V (mV)

5

Fig. 2 Typical dI/dV spectrum (solid circles) at T = 0.47 K. The
solid line shows a modified BCS density of states.
References[1] T. Nishizaki, Y. Takano, M. Nagao, T. Takenouchi, H.
Kawarada, and N. Kobayashi, Science and Technology ofAdvanced Materials 7, S22 (2006).
Contact to
Terukazu Nishizaki (Low Temperature Physics Division)e-mail: terukazu@imr.tohoku.ac.jp

Innovative Electronic Materials

3FTFBSDI

Synthesis and Electronic Phase Diagram of LixZrNCl
Superconductor
Single phase samples of LixZrNCl with controlled doping
levels have been successfully synthesized for the first time, andan electronic phase diagram as a function of carrier concentration
has been established. It has been found that Tc increases rapidly
as the carrier concentration is reduced, and takes a maximum
value on the verge of superconductor-Anderson-insulatortransition point.
β-ZrNCl and β-HfNCl are band insulators with a layered
structure as depicted in Fig.1, and have been known to becomesuperconductors when doped with electrons by means of alkalimetal intercalation. According to recent band calculations, doped
electrons are accommodated into ZrN or HfN double-honeycomb
layers, and form a rather simple, two-dimensional electronic state.
The superconducting transition temperature Tc is about 13 K and
25.5 K for Zr- and Hf-based materials, respectively. One of themost unique features of these materials is that the density of statesat the Fermi level and the electron-phonon coupling constant areboth very small for their Tc values, as clearly evidenced by aspecific heat measurement [1]. Despite such interesting features
and the relatively high Tc values, systematic investigations on thefundamental properties of these materials have thus far been
prevented by the difficulties in synthesizing single phase materials
and in treating the samples that are extremely sensitive to the air.
We have successfully developed a method to obtain single phasesamples of LixZrNCl with controlled doping levels, and clarified theelectronic phase diagram as well as doping-evolution of physicalproperties of the materials [2,3].
The as-intercalated samples of LixZrNCl in the lightly doped
region of x<0.1 tend to be easily phase-separated into Li-doped
and pristine phases, but we found that single phase samples canbe obtained by high temperature annealing at 873 K. For thus
obtained samples, we carefully confirmed the continuous andsteady Li intercalation, namely the formation of solid solution, bymeans of synchrotron X-ray diffraction measurements at BL02B2,
SPring-8 and Raman scattering technique. It should be noted thatsuch solid solution behavior is very rare from a view point ofmaterials chemistry of intercalation compounds. Figure 2 showsthe electronic phase diagram of this system as a function of carrier
concentration [2]. As the doping level is reduced from x=0.4, the Tc
value remains almost constant, but rapidly increases below x=0.12,
taking a maximum value of 15.2 K at x=0.06. At x=0.05, thesuperconducting transition suddenly disappears and the material
turns to an Anderson insulator. Such an increase in Tc on the vergeof the Anderson transition is quite anomalous as the Tc is gradually
reduced and eventually disappears at the transition point in thecase of usual superconductor-Anderson-insulator transition.
In order to see whether the observed increase in Tc can be
accounted for in terms of increased electron-phonon interaction,
we have performed low-temperature Raman scattering experiments
for several samples with a wide doping concentration x [3]. A
detailed analysis of the doping- and temperature-dependent
phonon line width indicated that the electron-phonon coupling
constant rather decreases as the doping concentration is reduced
and the Anderson insulator is approached from the superconducting
side. Therefore, it should be necessary to take some otheringredients into account to explain the observed increase in Tc on
the verge of superconductor-insulator transition.

Innovative Electronic Materials

Fig.1 A crystal structure of LixZrNCl is schematically shown. Red,
light blue, blue, and green spheres represent Li, Zr, N, and Cl
atoms, respectively.

Fig.2 An electronic phase diagram of LixZrNCl system is shown as
a function of carrier concentration.
References[1] Y. Taguchi, M. Hisakabe and Y. Iwasa, Phys. Rev. Lett. 94,
21700 (2005).
[2] Y. Taguchi, A. Kitora and Y. Iwasa, Phys. Rev. Lett. 97, 107001(2006).
[3] A. Kitora, Y. Taguchi, and Y. Iwasa J. Phys. Soc. Jpn. 76,
023706 (2007).

Contact toYoshihiro Iwasa (Low Temperature Condensed State Physics
Division)e-mail: iwasa@imr.tohoku.ac.jp

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3FTFBSDI

Anomalous Lattice Softening in Cu-oxide Superconductors Discovered by aComplementary Use of Quantum Beams of Neutron and Synchrotron X-ray
Lattice vibrations of Cu oxide superconductors with ahigh transition temperature (high-Tc) have been systematicallystudied by using both neutron and X-ray scattering. We
newly discovered an anomalous softening in the Cu-O
parallel bond-stretching mode which occurs in the
superconducting phase of the Cu-oxide system. Thisobservation have triggered a new direction of research on
the mechanism of high-Tc superconductors.

doping dependence by only changing the position of incoming
X-ray beam spot onto the quasi-single crystals.
As shown in Fig.2, we newly found two anomalous aspectsof the softening, first we found a specific doping dependence
and second a softening at a specific wave number (see thefigure caption for detail). Such anomalous softening strongly
suggests the coupling between the specific phonons and the
doped carriers. Particularly, the specific wave number
corresponds to that we observed in the spin density modulationcommonly seen in the superconducting phase.
Our recent study on the lattice dynamics has triggered
studies which focus on the interplay between spin and latticein this system. In near future, we can study charge responseof this system directly by using X-ray inelastic scattering andcan discuss interplay between spin, lattice and charge
fluctuations in such strongly correlated system.

Fig.1: Crystal structure (left) and phase diagram (right) of Srdoped La2CuO4. Superconductivity appears upon hole
doping with 0.06 < x < 0.27.
Twenty years have passed since the discovery of the firstsuperconducting Cu-oxide, but no one can answer to asimple question why superconductivity can survive at suchhigh temperatures beyond the boiling temperature of liquid
nitrogen. Figure 1 shows the crystal structure and phase
diagram of Sr-doped La2CuO4 (LSCO) which is one of thetypical superconducting Cu-oxides. Superconductivity
appears when charge carriers are doped into insulating
La2CuO4 with substitution of La sites by Sr ions.
There have been two main streams of research on themechanism of high- Tc superconductivity. One is the researchon magnetic properties of Cu-oxides which focuses on thefacts that 1) the insulating mother compounds before carrierdoping show a strong magnetic interaction between theneighboring Cu spins, and 2) well-defined dynamical spin
fluctuations exist even in the superconducting phase.
The other stream is the research on the collective lattice
vibrations or phonons, particularly the phonons related with
the Cu-O bonding which are expected to strongly couple with
doped carriers. Before the present study it is well-known that
almost all the transition metal oxides commonly exhibit asoftening upon carrier-doping irrespective of the existence ofsuperconductivity. The specific phonon mode which shows
the softening is a Cu-O bond-stretching wave which
propagates in the direction parallel to the Cu-O bonding(parallel bond-stretching mode). We studied this mode very
carefully by using both neutron and X-ray beams.
For this study we grew many large single crystals of
LSCO with different Sr concentrations. Particularly, in the Xray experiment we utilized a quasi-single crystals with acontrolled gradient of Sr concentration. Since the beam spot
size is of the order of 0.1mm, we can effectively study the26

IMR KINKEN Research Highlights 2007

Fig.2: Dispersion curves of the Cu-O parallel bond-stretchingmode for LSCO with different Sr concentrations (pink, blue,
green and black lines in the left figure) and YBa2Cu3O6+y (redand purple lines in the left). All the compounds exhibit asoftening of the phonon mode, but the shape of the dispersion
curve, particularly the shape near the wave vector
q=0.25~0.30 depends on the compounds. Amplitude of thephonon softening (difference in the phonon frequencies
between the frequencies with the wave number q=0 and
q=0.5) as a function of Sr concentration (right). Only parallelbond-stretching mode(labeled by (π,0)) shows an extra
softening in the superconducting concentration region.
Another bond-stretching mode (labeled by (π,π)), which
propagates in the direction diagonal to the Cu-O bonding,does not show such extra softening.
References[1] D. Reznik, L. Pintschovius, M. Ito, S. Iikubo, M. Sato, H.
Goka, M. Fuijta, K.Yamada, G.D. Gu and J.M. Tranquada,Nature 440 1170 (2006).
[2] K. Ikeuch, T. Chuurei, K. Izawa, K. Yamada
Tatsuo Fukuda, Jun-ichiro , Mizuki, A.Q.R. Baron, and S.
Tsutsui, J. Appl. Phys. 45, 1594 (2006).
Contact toKazuyoshi Yamada (Metal Physics with Quantum Beam
Spectroscopy Division)e-mail: kyamada@imr.tohoku.ac.jp

Innovative Electronic Materials

3FTFBSDI

Dislocations in GaN: Elucidation of Dynamics and Luminescence
Defects cause spatial variations in electrical and optical
functions and also their degradation of semiconductor
devices. Thus, a great deal of efforts is being made in
elucidating defect properties in order to control them forachieving ultimate efficiency of devices.
Gallium-nitride (GaN) is one of the most attractive
semiconductors for future opto- and electronic devices as
high-power/high frequency devices, blue and UV lightemitting devices, photo-detectors, chemically stable
substrate, and so forth. Since dislocations are induced during
hetero-epitaxial growth with a foreign substrate, fundamental
knowledge on kinetics and properties of defects is crucial for
addressing their stable and efficient functionalities. Key
topics that should be clarified on GaN are: 1) how easily or
hardly are dislocations activated into GaN and 2) what are
the intrinsic opto-electric properties of dislocations? For
example, there remaines a controversy whether broad
luminescence at around 2.2 eV, termed the yellow-band(YB), originates from recombination at dislocations generated
during crystal growth. Generally, grown-in dislocations
become contaminated with native point defects or impurities
during the growth process. Hence, it is necessary toinvestigate the intrinsic properties of fresh dislocations. In
these viewpoints, GaN single crystals were plastically
deformed at elevated temperatures and induced fresh
dislocations were evaluated in photoluminescence studies.
High quality GaN bulk single crystals can be compressively
deformed at temperatures higher than 900˚C. Figure 1 shows
the yield stress of various semiconductors plotted against
reciprocal temperature for deformation. The yield stress of
GaN is around 100–200 MPa, even at 1000˚C, more than
two orders of magnitude higher than that of Si, Ge, GaAs,
and the other compound semiconductors, and is slightly
higher than that of SiC with basal slip. These results indicatethat dislocations are hard to move in GaN. Thus, it is clarified
that the main origin of dislocations is the lattice-mismatchwith foreign substrate during epitaxial growth. From thetemperature dependence of the yield stress an activationenergy for dislocation motion in the GaN is estimated to be
2–2.7 eV.
By the plastic deformation a high density of fresh edge
dislocations with a Burgers vector (a/3)[1120] on the (1100)
prismatic plane are found selectively introduced into GaN.
Figure 2 shows the PL spectra of a GaN crystal deformed at
950˚C and a GaN crystal subsequently annealed at 950˚Cafter deformation together with that of an as-grown GaN
crystal. It is found that plastic deformation effectively leads tonumerous non-radiative recombination centers that drasticallyreduce both near-band edge and yellow-band luminescence.
That is, plastic deformation is quite effective in carrier
passivation. Indeed, The deformed GaN crystals losetransparency. Fresh dislocations lead to the development ofseveral luminescence bands centered at 1.79, 1.92, and 2.4
eV in the deep center region due to the introduction ofradiative recombination centers. From the PL features
dependent on deformation and annealing, it is clarified that
fresh dislocations do not directly contribute to the yellow
luminescence. The yellow luminescence is destroyed during
the plastic deformation and recovered by subsequentannealing, which may suggest the development of a Ga
vacancy-oxygen complex (VGa-ON) due to the segregation/
reaction of O impurities with dislocations being induced
during the process of deformation.

Fig. 2 Photoluminescence spectra of as-grown GaN crystal
and GaN crystal deformed to a shear strain of 30% at 950˚C,
and GaN crystal subsequently annealed at 950˚C for 2 h,measured at 11 K. PL spectra in the 1.45- to 3.1-eV photon
energy range are enlarged 200 times.
References[1] I. Yonenaga, H. Makino, S. Itoh, T. Goto, and T. Yao, J.
Electron. Mater. 35, 717 (2006).
[2] I. Yonenaga, Mater. Trans. 46, 1979 (2005).

Fig. 1 Yield stresses of various semiconductors plottedagainst reciprocal temperature for deformation under a strain
rate of 2x10-4 s-1.

Innovative Electronic Materials

Contact toIchiro Yonenaga (Physics of Crystal Defects Division)e-mail: yonenaga@imr.tohoku.ac.jp

KINKEN Research Highlights 2007 IMR

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3FTFBSDI

N-Polarity GaN on Sapphire Substrate Grown by MOVPE
GaN with N-polarity opposite to Ga-polarity used in blue LEDs

investigated by TEM. The density of dislocation in this region can be

was successfully grown on (0001) sapphire by metalorganic vapor-counted to be 7×108/cm2. This value for GaN grown by using only aphase epitaxy (MOVPE). This GaN had a mirror-like surface, andGaN buffer layer is considerably small. The photoluminescence ofwas superior to usual Ga-polar GaN in the viewpoints of X-ray

undoped GaN was measured at room temperature. The strong

diffraction measurements and the density of threading dislocations.

near-band edge emission is observed without the yellow-band

The p-type conduction was also realized.

emission. Magnesium was doped into this GaN. Mg-doped GaN
was annealed at 700°C for 10 minutes in nitrogen atmosphere so as

The progress of nitride semiconductors has been remarkable.

to activate magnesium by diffusing out hydrogen. The carrier density

Blue LEDs and ultraviolet LDs have been developed. Their devices

was measured by Van der Pauw’s method at room temperature.

are fabricated using materials with Ga-polarity (+c polarity). Usually,

The density and mobility of holes was 1~3×1017/cm3 and 3~13 cm2/

GaN with Ga-polarity are grown on (0001)-plane sapphire substrates

Vs. This density is a little smaller than the reported value of GaN withby MOVPE which is suitable for mass-production. The growth ofGa-polarity, because the conditions of doping and so on have not

GaN with N-polarity (-c polarity) has not been reported. It has been

been optimized yet.

reported that N-polar GaN tends to be pyramidal and is said to

These results shatter that the N-polar GaN grown by MOVPE

cause the rough surface. Even if the smooth surface was obtained,

has a rough surface. The success of N-polar GaN means thep-type conduction has not been realized. We report the first successpossibility of high quality InN, which is the hottest topic in nitridein MOVPE growth of N-polarity GaN with high quality.

semiconductors [2] and the most difficult material in nitride

GaN was grown using the two-step MOVPE growth techniquesemiconductor because of extremely high equilibrium vapor

with low-temperature buffer layer on a (0001) sapphire substrate.

pressure of nitrogen. In the growth of N-polarity, a nitrogen atom is

Just before growth, the surface was nitrided. This nitridation wascaught with three gallium atoms. Therefore, by N-polar growth, theone key point for N-polarity. The 20-nm-thick GaN-buffer-layer wascharacteristics of InN can be expected to be improved. In neargrown at 550°C using triethylgallium (TEG) and ammonia. TEG isfuture, the properties of InN, which is not clear at present, will be

decomposed at lower temperature than trimethylgallium (TMG) and

precisely measured. Moreover, the device design becomes flexible

carbon was hardly incorporated into the buffer layer. The annealing

because we can control the direction of spontaneous polarization.

at 1030°C for the crystallization of a buffer layer was proceeded. In
this process, it is important to control the density of nuclei. The

References

1~2μm-thick GaN film was successively grown at 1020°C using TMG.

[1] T. Matsuoka, T. Mitate, H. Takahata, S. Mizuno, Y. Uchiyama, A.

In this process, it is important to promote lateral growth.

Sasaki, M. Yoshimoto, T. Ohnishi and M. Sumiya, phys. stat. sol. (b)The GaN polarity, which is the main subject in this paper, was243, 1446 (2006).

carefully investigated by the coaxial impact-collision ion spectroscopy[2] T. Matsuoka, H. Okamoto, M. Nakao, H. Harima and E. Kurimoto,(CAICISS) and the convergent-beam electron diffraction (CBED).

Appl. Phys. Lett. 81, 1246 (2002).

The spectra of CAICISS are shown in Fig. 1. Fig. 1 shows thedependence on polar angle in the azimuth of [1100]. GaN spectra
are compared with single crystal ZnO because the polarity of ZnO
can be determined with chemical etching. In Fig. 1, the shape ofspectra for GaN is almost the same as one for ZnO -C. Therefore,

Contact toTakashi Matsuoka (Advanced Electronic Materials Research
Division)e-mail: matsuoka@imr.tohoku.ac.jp

GaN can be determined to have -C polarity, that is, N-polarity. In
addition, the discrimination by CBED also showed
the same N-polarity as by CAICISS.
The issue of surface morphology in N-polar
GaN is important, because still now the N-polar
GaN film has been reported to have a roughsurface with many hexagons. The GaN films
grown here had mirror-like smooth surfaces to thenaked eye. Their surface morphologies wereprecisely observed by atomic force microscopy(AFM) as shown in Fig. 2. There is no pyramid, butmany parallel steps are observed. From thisobservation, the surface is found to be atomicallysmooth, although pits are also observed. This film
was crystallographically evaluated using four
crystal X-ray diffractometer. The full width at half
maximum of the spectra in ω-2θ and ω scans is 19
and

28

35

arcseconds,

respectively.

To

our

knowledge, the value in ω scan is the smallest in

Fig. 1 CAICISS spectra of GaN and ZnO

the reported ones. The dislocation density wasdependent on polar angle in [1100] azimuth.

IMR KINKEN Research Highlights 2007

Fig. 2 AFM image.

Innovative Electronic Materials

3FTFBSDI

The First Observation of Quantum Hall Effect in a Transparent
Transition Metal Oxide
Transparent electronics is an emerging technology
aiming at producing “invisible” electronic circuits anddevices. The existence of transparent objects is not asapparent to people, so the objects can be applied in various
ways in the ubiquitous network society. Such applications
include security sensors, invisible electronic tags, andtransparent displays. Here we have observed quantum Hall
effect in a transparent oxide semiconductor, making aremark that the world’s first transparent device with acapacity close to that of a silicon semiconductor device is
now realized.
Our division has been working on thin-film synthesis withvarious transition metal oxides and developed new devices
using them. We have focused especially on ZnO, which istransparent and has electrical properties that can be varied
from insulators to semiconductors and metals, also exhibiting
high-efficiency ultraviolet luminescence, when optically
excited. In this way, we have continued a number of attempts
to improve the crystalline quality of thin-film since 1997, when
we succeeded in observing ultraviolet laser emission at room
temperature. In 2004, we were able to synthesize p-type ZnO
for the first time in the world [1]. Meanwhile, a blue-violet
light-emitting diode has been achieved.
We have recently developed technology to further
enhance the quality of ZnO film, and successfully produced adevice exhibiting high-electron mobility that has properties
equivalent to the silicon metal-oxide-semiconductor transistor
used in an integrated circuit [2]. The developed device
consists of multilayers of ZnO and solid solutions of ZnO andmagnesium oxide (MgZnO), with diluted two-dimensional
electron gas formed between the two transparent layers (seeFig. 1). The performance of the developed device has been
confirmed through the observation of the quantum Hall effect,
which emerges only in exceptionally high-quality
semiconductors [3].

Fig. 1 Left, optical microscope image of a quantum Halldevice made of ZnO; right, optical microscope image of a
micropatterned Hall-bar array (film mounted on a chip
carrier).
The quantum Hall effect is a well-known phenomenon for
typical semiconductors such as silicon and gallium arsenide.
Even though the quantum Hall effect has been applied widelyas the international standard for electrical resistance, apractical application has not yet been developed. However,

Innovative Electronic Materials

the quantum Hall effect has been studied for a long time as
an important research topic in the physics of condensed
matter. No doubt this is because this profound physicalphenomenon is of exceptional interest to researchers, inwhich electrical resistance coincides with the fundamental
physical constant (the ratio between the charge on anelectron and Planck’s constant) with a very high degree ofprecision, regardless of materials or nature of the elements,
and it is true in our device too (see Fig. 2).

Fig. 2 Quantum Hall effect measured at 45 mK
Perhaps the significance of the experiments we haveconducted lies in ushering in the field of oxide electronics,
which includes transparent electronics, through thedevelopment of oxide semiconductors of practical grade. The
semiconductor devices developed to date consist of
semiconductors made of typical elements from the III, IV, andV families in the periodic table of elements, and many of them
contain toxic elements that could pollute the environment. On
the other hand, transparent semiconductors are chemicallystable and biologically harmless, and are attractive materials
that occur in abundant quantities in nature. Our successwould facilitate further progress in the area, and there is no
doubt that the history of improvement in crystal growthtechnology and the consequent development of high-quality
devices will be repeated also with ionic crystals other thanoxides and organic semiconductors.
References[1] A. Tsukazaki, A. Ohtomo, T. Onuma, M. Ohtani, T. Makino,
M. Sumiya, K. Ohtani, S. F. Chichibu, S. Fuke, Y. Segawa, H.
Ohno, H. Koinuma, and M. Kawasaki, Nature Mater. 4, 42(2005).
[2] A. Tsukazaki, A. Ohtomo, and M. Kawasaki, Appl. Phys.
Lett. 88, 152106 (2006).
[3] A. Tsukazaki, A. Ohtomo, T. Kita, Y. Ohno, H. Ohno, and
M. Kawasaki, Science 315, 1388 (2007).
Contact to
Akira Ohtomo (Superstructured Thin Film Chemistry Division)e-mail: aohtomo@imr.tohoku.ac.jp

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Switching of a Nanomagnet Utilizing a FePt
Perpendicular Spin Injector
In the field of “Spin-electronics”, which takes advantage of thefreedom of spins and provides electronic devices with multifunctionality, it has been discovered that the magnetization of ananomagnet is switched by spin-polarized electric current, which iscalled current-induced magnetization reversal. This new switching
technique has attracted considerable attention from fundamental
and practical points of view.
When electric current flows in a magnetic nanostructure consistingof two ferromagnetic layers separated by a nonmagnetic layer, aconduction electron spin-polarized by one ferromagnetic layer (spininjector) interacts with the local magnetic moment of the otherferromagnetic layer (free layer) through the transfer of spin angularmomentum. The transferred angular momentum acts as a torque tochange the magnetization direction of the free layer, which is called
spin-transfer torque. Thus, the magnetization direction can be
manipulated without external magnetic field. Current-induced
magnetization reversal is observed in current-perpendicular-to-plane
giant magnetoresistance (CPP-GMR) pillars or magnetic tunnel
junctions (MTJs) with the size smaller than a few hundreds nanometer,and conventional ferromagnetic materials such as Co and NiFe haveusually been used for the spin injector and the free layer to date.
Our group has paid attention to L10-FePt alloy as a material for
future spin-electronics. L10-FePt shows excellent thermal stability ofmagnetization in a nanometer scale due to the large uniaxial magnetic
anisotropy (7 x 107 erg/cm3: two orders of magnitude larger thanconventional ferromagnetic materials), whith may lead to the largescale integration of spin-electronic devices. In addition, the easymagnetization axis of the L10-FePt thin film is controlled by changing
the growth condition of the thin film [1]. In other words, FePt shows
either in-plane magnetization or perpendicular magnetization, whichenables us to demonstrate the current-induced magnetization reversal
in a variety of magnetization configurations [2,3].
Nanometer-sized CPP-GMR pillars with FePt layers areschematically illustrated in Fig. 1. One is a “conventional configuration”
with only in-plane magnetized layers, and the other one is a “90°configuration” consisting of two in-plane magnetized layers and one
perpendicularly magnetized layer, where the free layer is the bottom(middle) FePt layer in the conventional configuration (90°configuration). Electric current flows perpendicularly to the film plane.
The change of the magnetization direction of the free layer is detected

as the change of device resistance (R) depending on the relative
angle between two in-plane magnetized layers, which is a well-known
GMR effect.
Figure 2 shows R as a function of current pulse (Ip) measured at
77K. After applying Ip, R was measured using a small sensing current.
Both configurations show clear hysteretic transitions of R withsweeping Ip, where high and low resistance values correspond to theantiparallel and parallel alignments, respectively, of the two in-planemagnetized layer. These resistance changes indicate magnetization
reversal is induced by electric current for both configurations. A
striking feature is that the current for the magnetization reversal in the
90° configuration is obviously smaller than that in the conventional
configuration. The reduction of switching current in the 90°
configuration indicates that spin-transfer torque successfully acts on
the magnetization of the in-plane magnetized layer from theperpendicularly magnetized layer. The FePt perpendicularly
magnetized layer serves as a perpendicular spin injector to reducethe current density for magnetization reversal [2], which is an important
finding for practical applications such as magnetoresistive random
access memory (MRAM) requiring the low switching current. Ourgroup also demonstrated the current-induced magnetization reversal
in a perpendicular configuration, where both free layer and injector
show perpendicular magnetization [3]. This perpendicular configuration
has attracted much attention because of the high thermal stability ofmagnetization and the low switching current due to the negative
shape anisotropy.

Fig. 2 Resistance R versus the amplitude of current pulse Ip for theconventional configuration (black marks) and the 90° configuration(red marks). The measurement temperature is 77K.
References[1] T. Seki, T. Shima, K. Takanashi, Y. Takahashi, E. Matsubara, andK. Hono, IEEE Trans. Magn. 40, 2522 (2004).
[2] T. Seki, S. Mitani, K. Yakushiji, and K. Takanashi, Appl. Phys. Lett.
89, 172504 (2006).
[3] T. Seki, S. Mitani, K. Yakushiji, and K. Takanashi, Appl. Phys. Lett.
88, 172504 (2006).
Fig. 1 Schematic illustrations of nano-sized CPP-GMR pillars for(a) a conventional configuration and (b) a 90° configuration. White
arrows represent the magnetization directions. Electric current

Contact toKoki Takanashi (Magnetic Materials Division)e-mail: koki@imr.tohoku.ac.jp

flows perpendicularly to the film plane.

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First Heavy-fermion Superconductivity in the Neptunium Compound
Heavy fermion superconductivity is one of the mostinteresting topics in the condensed matter physics. In thepast, several rare earth and uranium compounds were foundto become superconductivity. Recently we found theneptunium based heavy fermion superconductor for the firsttime.
The elements, which is located at the neighbor of uranium
element on the right in the periodic table, are called
transuranium elements, such as neptunium, plutonium. These
compounds attract much attention not only for the nuclear fuelbut also for the condensed matter physics, although there are
not many institutes which can treat transuranium compounds
in the world, because of the strong radioactivity.
In general it is believed that neptunium compounds have
5f-itinerant features like uranium compounds. Our recent
studies on de Haas-van Alphen (dHvA) experiments in NpGe3,
NpIn3, NpNiGa5, NpCoGa5 and NpRhGa5 clearly demonstratethat their Fermi surfaces can be well explained by the bandcalculation based on the 5f-itinerant band model[1,2]. On theother hand, the complicated magnetic properties are explainedby the 5f-localized model including the orbital degree of
freedom. The dual nature of 5f-electron is one of thecharacteristic feature in the neptunium compounds.
Recently we succeeded in growing a new neptuniumcompound NpPd5Al2 with the tetragonal crystal structure.
Surprisingly, NpPd5Al2 is found to be the first heavy fermion
superconductor in the neptunium compounds[3]. Figure 1
shows the photograph of the single crystal NpPd5Al2 and the
crystal structure. It is characteristic that the c-value of lattice
parameter is extremely larger than the a-value, meaning thatthe Brillouin zone becomes flat along the [001] direction.
Figure 2(a) shows the temperature dependence of theelectronic specific heat in the form of C/T. The superconducting
transition occurs at Tc=5K, which is relatively high compared tothe other heavy femion superconductors. The large specificheat jump of ΔC/γTc is much larger than the weak-coupling
BCS value, indicating that NpPd5Al2 is a strong-couplingsuperconductor. The specific heat C at low temperaturesapproximately follows the T3 dependence. This indicates thatthe existence of anisotropic superconducting gap with point
nodes.

Fig. 2 (a) Temperature dependence of the electronic specificheat in NpPd5Al2. The large jump at 5K corresponds to thesuperconducting transition. (b)The temperature dependenceof the upper critical field Hc2 for H // [100] and [001].
Figure 2(b) shows the temperature dependence of theupper critical field Hc2, where the superconductivity is destroyed
under magnetic fields. The initial slope at Tc=5K is very large
for both field directions, indicating that the superconductivity isgoverned by the heavy quasi-particles. That is, the 5f electronwith heavy effective mass becomes itinerant and contributes
to the conductivity. The upper critical field is strongly
suppressed with decreasing temperature, suggesting the large
Pauli paramagnetic effect. The d-wave superconductivity withpoint nodes is most likely realized in this compound.
This work has been done in collaboration with Japan
Atomic Energy Agency and Osaka University.
References[1] D. Aoki, Y. Homma, H. Sakai, S. Ikeda, Y. Shiokawa, E.
Yamamoto, A. Nakamura, Y. Haga, R. Settai and Y. Onuki, J.
Phys. Soc. Jpn. 74, 084710 (2006)[2] D. Aoki, H. Yamagami, Y. Homma, Y. Shiokawa, E.
Yamamoto, A. Nakamura, Y. Haga, R. Settai and Y. Onuki J.
Phys.: Condens. Matter 17, L169 (2005).
[3] D. Aoki, Y. Haga, T. D. Matsuda, N. Tateiwa, S. Ieda, Y.
Homma, H. Sakai, Y. Shiokawa, E. Yamamoto, A. Nakamura,
R. Settai and Y. Onuki, J. Phys. Soc. Jpn. 76, 063701 (2007).
Contact toYoshiya Homma (Radiochemistry of Metals Division)e-mail: yhomma@imr.tohoku.ac.jp

Fig. 1 (a) Photograph of NpPd5Al2 single crystal and (b) its
crystal structure.

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Observation of Field-Induced Reverse Transformation in
Magnetic Shape Memory Alloy Ni50Mn36Sn14
The structural and magnetic properties of ferromagnetic
shape memory alloy Ni50Mn36Sn14 were studied bymagnetization and high field X-ray powder diffraction
measurements. A field induced reverse martensitic
transformation was observed. In addition, it found that thelarge magnetoresistance effect of 50% occurs accompaniedby the transformation.
Ferromagnetic Heuslar alloys Ni50Mn50-ySny (X = In, Sn,
and Sb) show martensitic transformation from the cubic L21type (C) structure to an orthorhombic four-layered (4O). The
alloys will become to be new candidate as actuator materials
controlled by magnetic field. It is very important to reveal themagnetic, structural and thermal properties in magnetic field
for development of the new magnetic materials, Ni50Mn50yXy. In this study, in order to clarify the basic properties of
Ni50Mn36Sn14 in magnetic fields, we have performed
magnetization and electrical resistivity measurements infields B up to 17 T, and High-Field X-ray diffraction (HF-XRD)
in fields up to 5 T and in the temperature ranging from 4.2 K
to 320 K at the High Field Laboratory for Superconducting
Materials, Institute for Materials Research, Tohoku
University.
Figure 1 shows the typical results of the HF-XRD profile
of Ni50Mn36Sn14 in fields up to 5 T at 250 K (a), 240 K (b) and
220 K (c) [1]. Here, all field dependence profiles are measured
after zero-field heating from 100 K. As shown in Figs. 1(a)and 1(c), the profiles do not drastically change by applying
magnetic fields up to 5 T in the L21 and 4O structures,
respectively. In contrast, we clearly see the field-induced
reverse martensitic transformation from the 4O to L21 typestructures at 240 K, as shown in Fig. 1(b). It is first time thatthe field-induced reverse martensitic transformation wasobserved by using XRD.
Figure 2 shows the magnetoresistance (ρ-T) in magnetic
fields up to 17 T at 4.2, 150 and 220 K. In a zero field, ρ at 4.2(4O) and 220 K (L21) are 320 and 180 mΩcm, respectively,
and they decreases linearly with increasing magnetic field.
On the other hand, we observed large negative
magnetoresistance effect in Ni50Mn36Sn14 at the vicinity ofthe martensitic transformation temperature. The data at 150
K are shown in Fig.2 (red) as a typical result. The change ofthe negative magnetoresistance is approximately 50% in
magnetic fields up to 17 T. Our recent results indicate thatthe structural transformation and negative magnetoresistancewill be controlled in a lower magnetic field.
Results of magnetization measurements show that themagnetic moment of the L21 phase is larger than that of the
4O phase [1,2]. The observed field-induced reverse
transformation from the 4O to L21 phase probably occurs todecrease the magnetic energy because of the addition of the
Zeeman energy.
References[1] K. Koyama, K. Watanabe, T. Kanomata, R. Kainuma, K.
Oikawa, and K. Ishida, Appl. Phys. Lett. 88, 132505 (2006).

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Fig. 1. XRD profiles of Ni50Mn36Sn14 in magnetic fields up to
5 T at 250 K (a), 240 K (b) and 220 K (c). hklc and hklo denotethe Miller for the cubic L21-type and four-layered orthorhombic(4O) structures, respectively.

Fig.2. Magnetoresistance of Ni50Mn36Sn14 at 4.2 K (blue),
150 K (red) and 220 K (green).
[2] K. Koyama, H. Okada, K. Watanabe, T. Kanomata, R.
Kainuma, W. Ito, K. Oikawa, K. Ishida, Appl. Phys. Lett. 89,
182510 (2006).
Contact to
Keiichi Koyama (High Field Laboratory for Superconducting
Materials)
e-mail: kkoyama@imr.tohoku.ac.jp

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Unusual Generation and Break Down of Massive Quasi-Particles
in the Strongly Correlated Electrons System CexLa1-xB6

CeB6 is well known as a typical cubic heavy fermion
system. In this compound, massive quasi-particles are
generated by the strong correlation between the electrons.
The ground electric crystal field state of CeB6 is the Γ8 which
carries magnetic dipole, electric quadrupole and othermultipole moments.
In the diluted CexLa1-xB6, regardless of the random
distribution of magnetic Ce ions, the de Haas-van Alphen
oscillations originated from massive quasi-particles areobserved [1]. This indicates that periodicity of the magnetic
ions is not a necessary condition of the generation of themassive quasi-particle in the system.
The quasi-particles break down with changing the Ceconcentration or magnetic field. Figure 1 displays thetemperature dependence of the electrical resistivity of
CexLa1-xB6 [2]. When the Ce concentration (x) becomes low,temperature dependence of the resistivity deviates from
theT2-law predicted by the Landau Fermi liquid theory. Itshows TD (D<2) dependence in the low x region.

colors indicate non-Fermi liquid state where the massive
quasi-particle picture is not applicable.
10

CexLa1-x B6
H // [001]

8

H (T)

We have investigated the electronic ground state in thestrongly correlated electrons system CexLa1-xB6 where local
4f electron carries plural degrees of freedom by means of
the measurements of the electrical resistivity and de Haasvan Alphen effect at very low temperatures. Unusual
generation and break down of the massive quasi-particles
are found.

D

2.0
1.5

6

1.0

II (AFQ)

4
III (AFM)

I (P)

2

IV (?)

0
0.0

0.2

0.4

0.6

Ce-concentration x

0.8

1.0

Fig. 2. Magnetic phase diagram of CexLa1-xB6 for H || [001]as a function of Ce concentration (x) and field (H) at low
temperatures [2]. The color denotes the value of exponent D
in the temperature dependence of resistivity ρ(T)=ρ(0)+ATD.
References[1] M. Endo, S. Nakamura, T. Isshiki, N. Kimura, T. Nojima,
H. Aoki, H. Harima and S. Kunii, J. Phy. Soc. Jpn., 75, 114704(2006).
[2] S. Nakamura, M. Endo, H. Yamamoto, T. Isshiki, N.
Kimura, H. Aoki, T. Nojima, S. Otani and S. Kunii, Phy. Rev.
Lett. 97, 237204 (2006).
Contact toShintaro Nakamura (Laboratory of Low Temperature
Materials Science)e-mail: nakamura@imr.tohoku.ac.jp

Fig. 1. (a) Temperature dependence of the electrical resistivityof CexLa1-xB6 at zero field as a function of T2 in the orderedphases III (antiferromagnetic state) and IV (underinvestigation) and (b) as a function of TD in the paramagnetic
phase (phase I) [2].
Figure 2 shows the Ce concentration and magnetic fielddependence of the exponent D. Blue color indicates Fermi
liquid state formed by the massive quasi-particles. Other

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First Principles Approach: Single Molecular Transistor
Modulated by Transverse Field
We use a self-consistent method to study the current of
the single molecular transistor modulated by the transverse
field in the level of the density functional theory (DFT) andthe nonequilibrium Green's function method. The numericalresults show that both the polyacene-dithiol molecules andthe fused-ring thiophene molecules are the potential highfrequency molecular transistors controlled by the transverse
field.
Recently, the search for new active molecular devices
becomes a worldwide effort as these devices represent theultimate size limit of functional devices. The current-voltage
I-V characteristics of the molecular devices show profoundpotential for applications, including the negative differential
resistance and switches, etc. Traditionally, electronictransport phenomena are studied in the context of bulk
semiconductor devices, the theoretical description of whichis largely built on two premises, the effective-mass
approximation and the Boltzmann transport equation. At themolecular scale, the effective-mass approximation breaks
down and the electronic structure of the system must be
taken explicitly into account. In a word, transport in moleculardevices is different from that in semiconductor devices in two
aspects, 1) the effect of the electronic structure and 2) the
effect of contact.
At present, people have realized two major approaches
to control quantum transport. One is through theconformational change in the molecule, and the other isthrough the external transverse field to switch the moleculefrom the “on” state to the “off” state. Now, the attention istransferred to the latter one due to its high operation
frequency. The organic semiconductors can be employed as
active layers in the field effect transistors FETs. At present,
some theoretical works have shed light on the organic
molecular transistors controlled by the transverse field. In
this study, we use the self-consistent calculation of themolecular-transistor-electrode system controlled by thetransverse-field effect (TFE) on the basis of DFT andnonequilibrium Green's function method.
The calculated molecular device has the sulfur atom
attached by the gold electrode from both sides and thesystem is set under both the longitudinal and transverse
fields. The potential zero point is set at the coordinate origin,
the middle of the line connecting two sulfur atoms, and thewhole molecule is not symmetric to the Au-S bond, which will
be in favor of TFE.
Figure 2 shows the gate bias successfully controls the I-V
characteristics of the pentathienoacene (PTA). At the Vsd >-2.25 V, due to the contribution from HOMO, the positive gatebias achieves the bigger molecular current and at the Vsd <-2.25 V, as LUMO enters the Vsd window and contributes tothe molecular conduction, the inverse order of current
appears. The inset illustrates the electron with spin-up
number deviation from the equilibrium state as a function ofVsd. At Vsd > -2.25 V, for gate bias Vg = -3.90 V, the electron
is responsible for the conduction (N>N0) and electron number

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IMR KINKEN Research Highlights 2007

rises slowly, while for Vg > -1.95 V, the hole is responsible forthe conduction (N<N0), so the electron number descends theelectron flows out of the molecule to the lead. At Vsd < -2.25
V, the molecule enters the electron and hole hybrid conduction
region, accompanying the rapid rise of electron number,which means that the electron contribution dominates the
conduction. Both the TFE and charge effect have contributions
to the I-V curves.
Moreover, our self-consistent results show that the longlength organic molecule can achieve better transportcharacteristics. Our investigation proves that both thepolyacene-dithiol molecules and fused-ring oligothiophenemolecules can be made as the high-frequency moleculartransistors controlled by the transverse field.

Fig. 1. Comparison between macroscale and nanoscale for
conductance calculation.

Fig. 2. Gate modulation of the I-V characteristics for PTA.
The Vsd = -2.25 V is the crossover point for the current
controlled by the gate bias. For Vsd > -2.25 V, either electron(Vg = -3.90 V) or hole (Vg > -1.95 V) is responsible for thecurrent. For -3.4 V < Vsd < -2.25 V, the molecule enters theelectron and hole hybrid conduction region, and the positive
biased current is smaller than the negative ones.
References
F. Jiang, Y. X. Zhou, H. Chen, R. Note, H. Mizuseki, and Y.
Kawazoe, J. Chem. Phys. 125, 084710 (2006).
Contact to
Ryunosuke Note (Center for Computational Materials Science)e-mail: erasure@imr.edu

Innovative Electronic Materials