Electronic Materials

Neutron Laue Diffraction of Multi-Ferroic MnWO4 in High
Magnetic Fields.

We have succeeded in the time-of-flight neutron Laue diffraction in pulsed high magnetic fields at

the Spallation Neutron Source. The phase diagram of the multiferroic material MnWO4 has been
determined up to 30 T. The control of the field-pulse timing enabled an exploration of magnetic
Bragg scattering through the time dependent of the neutron wavelength. This allowed us to observe
several magnetic Bragg peaks with a single instrument configuration.

Neutron diffraction is an essential probe ofmagnetic structures and phase diagrams ofmagnetic materials. When a magnetic material
has competing interactions, it may result inmacroscopically degenerate ground state.
For such cases, application of a magnetic field
may lift the degeneracy and the interplay
between spin, charge, orbital, and lattice
degrees of freedom lead to rich phase diagram
in magnetic field. A system of current interest ismultiferroic, in which ferroelectric and a noncollinear magnetic structure are in tight
relationship. The determination of a magnetic
field phase diagram of such system is crucial toits understanding, and the neutron diffraction
in high magnetic fields is expected to yield
important insights.

200 field pulses are required at each point andthe scanning in the reciprocal space to explore
unknown Bragg spot is not practical in thismethod.
To overcome this difficulty, we havecombined a pulsed magnetic field with apulsed white neutron beam, employing
wavelength-resolved diffraction based on
neutron time-of-flight method. Here, we present
the first application of neutron Laue diffraction
in pulsed magnetic fields, using a white neutron
beam at the Spallation Neutron Source: SNS of
Oak Ridge National Laboratory as thecollaboration between the magnetism divisionof IMR and SNS[4].
Fig. 1 shows the picture of the experimental
setup. A compact capacitor bank is used as a
power supply and the special magnet insert
and the sample mount developed at IMR areused for the magnetic field generation. The
system is so small and thus is portable as fitting
to a check-in suitcase of air career. This featureenables us to conduct a visiting experiment inabroad easily.

Fig. 1 The picture of the experimental system installed
in SEQUOIA spectrometer at SNS. In the left-handside, a compact capacitor bank is in the rack. The
top of the cryostat insert is in the center.

Conventional neutron diffraction studies
are possible in a steady magnetic field of 17 T,
at the Helmholtz Zentrum Berlin[1]. Recently, a
monochromatic neutron diffraction in a pulsed
magnetic field up to 30 T was carried out using
a reactor-based neutron source [2, 3]. In reactor
experiment,acontinuous
beamof
monochromatic neutrons was used and asingle position in reciprocal space wasinvestigated. An advantage of this method is its
ability to continuously monitor the magnetic
field dependence of intensity. Typically 100 to
43

IMR KINKEN Research Highlights 2011

Fig. 2 The arrangement of the magnet and thedetector. The scattering vector is nearly
perpendicular to the magnetic field.

IMR KINKEN Research Highlights 2011

The cut view of the pulsed magnet and thearrangement of the detectors are shown in theFig. 2. The magnetic field is nearly perpendicular
to the scattering vector in the forward scattering
configuration. When the detector is set in the
backward position, the scattering vector isparallel to the magnetic field. The use of two
different scattering geometries would be useful
to investigate the directions of magnetic
moments.

reciprocal space. It is powerful to determine
the magnetic field dependence of someincommensurate wave vector. In fact, by
repeating a magnetic field scan at different
temperatures and at different timings, we could
determine the wave vector of the HF and theIV phases. It shows the powerfulness of thepresent experimental system.

4x109
50

B (T)

40
30
20
10
0
0

3

4

6

8

10

12

14

15

2

10

1
0
7

2

20

5
8

Fig. 3 The magnetic phase diagram of MnWO4. The
colored area is ferroelectric. The re-entrant phasetransition takes place in the magnetic field scan at
low temperature as shown by the arrow A.

As the first target of the investigation, we
have selected a MnWO4, which shows acomplicatedmagnetic
field-temperaturephase diagram as shown in Fig. 3. There are atleast 6 phases and some of them are expected
to be incommensurate structure withferroelectric states.
Fig. 4 shows the example of a time of flight
spectrum, a pulsed magnetic field and Lauepattern of neutron diffraction. According to
the conventional macroscopic magnetization
measurement, the phase boundary between
the AF2 Anti- ferromagentic phase colored bygreen and the HF phase is located at 12 T. The
determination of the HF phase and the IVphase have not been made for the lack of highmagnetic field neutron diffraction system.
In Fig. 4, the two peaks are found in thetime of flight spectrum and the two Laue spots
are corresponding to these peaks. As clearly
indicated in this example, an advantage ofLaue method is in its ability to map the

9

10

11

12

13

0
14

TOF (msec)

16

T (K)

B (T)

Int./mon. (a.u.)

25

Fig. 4 The waveform of a pulsed magnetic field andthe time of flight spectrum. The time integrated
image of Laue pattern is in the right upper side.
The two spots are C: commensurate and IC:
incommensurate peaks as indicated by the lines.
Two phases can be examined simultaneously byusing the time dependent magnetic field.

To be summarized, we have succeeded in
the high magnetic field neutron Laue diffraction
for the first time. The usefulness of the method
has been proven as the determination ofcomplicated magnetic structure in multi-ferroic
MnWO4. It is notable that the highly positive
remark is made for the present work [5].
This work has been performed as thecollaboration with Kimura laboratory at Osaka
University, the neutron science division of SNS
and McMaster University.
References[1] M. Meissner et al., Neuron News. 12 (2001) 12.
[2] S.Yoshii et al., Phys. Rev. Lett. 103 (2009) 077203.
[3] M. Matsuda et al., Rev. Lett. 104 (2010) 047201.
[4] H. Nojiri et al., Rev. Lett. 106 (2011) 237202.
[5]http://blogs.physicstoday.org/thedayside/2011/07/
whats-wrong-with-the-spallation-neutron-source.html

Keywords: magnetic property
H. Nojiri and S. Yoshii (Magnetism Division)
E-mail:nojiri@imr.tohoku.ac.jp
URL:http://www.hfpm.imr.tohoku.ac.jp

IMR KINKEN Research Highlights 201144

Electronic Materials

Surface Atomic Structure and Vortex Statein Iron-Based Superconductor Ba(Fe0.93Co0.07)2As2

S

canning tunneling microscopy and spectroscopy (STM/STS) have been performed on the lowtemperature cleaved surface of Ba(Fe 0.93Co 0.07)2 As2 single crystals. Two types of atomic surfaces
are identified as the Ba-terminated 2×1 layer and As-terminated √2×√2 layer. The superconducting
gap, 2Δ ≃ 6.9kBTc, is observed only on the 2×1 layer surface; this value indicates the strong paring
interaction. The glassy vortex structure without a long-range hexagonal order and the vortex core
spectra without the zero bias conductance peak suggest a strong vortex pinning effect and aconsiderable degree of disorder.
The
discoveryofiron-pnictide
superconductor [1] prompted extensive
researches on a new class of high-Tc
superconductors(HTSCs).
In
order
to
understand the complex physics in iron-based
HTSCs, the direct observation of thesuperconducting electronic state is important.
Scanning tunneling microscopy/spectroscopy(STM/STS) is a unique technique for detecting
the electronic properties at the atomic scale,because STM is used to observe the atomic
surface with sub-nanometer spatial resolution
and STS provides information on thequasiparticle local density of states (LDOS).
Thus, STM/STS is expected to play a crucial roleinclarifying
theelectronic
structure,mechanism of HTSC, and vortex–matter statein the magnetic field in iron-based HTSCs.
In this Research Highlight, we present
STM/STSoniron-based
superconductor
Ba(Fe0.93Co 0.07)2 As2 single crystals (Tc ≃ 24 K).
The crystal structure of Ba(Fe1-xCox)2 As2 consistsof an alternate stacking of Ba and FeAs layers.
Since the chemical bonding between As andFe ions is strong, the crystal is likely to cleave
either between the Ba and As layers or within
the Ba layer. Fig. 1(a) shows the STM topographic
image of the Ba(Fe 0.93Co 0.07)2 As2 cleaved

dI/dV (nS)

0.8
0.6

81 Å × 81 Å

Ba

Ba(Fe0.93Co0.07)2As2
T = 4.9 K

0.4

As

(d)

0.2
0

(c)

surface. The topographic image shows thestep structure between two different atomic
terraces. The bright and dark terraces
correspond to the 2×1 surface (Fig. 1(b)) andthe √2 × √2 surface (Fig. 1(c)), respectively.
According to details of the atomic structure
and a step height (~1.6 Å), it is concluded thatthe 2×1 surface is formed by half of the Baatoms remaining on the As layer, and the √2 ×
√2 surface is the As layer from which Ba atoms
arecompletely
removed[2].
The
superconducting (SC) tunneling spectra withthe energy gap (2Δ ≃ 6.9kBTc) are observed
only on the 2×1 surface (see Fig. 1(d)); this value
is approximately twice that for weak couplings-wave BCS superconductors, indicating astrong pairing interaction similar to thatobserved in cuprate superconductors.
Fig. 2(a) shows the spectroscopic image inthe magnetic field H = 5 T measured
simultaneously with the STM topographic
image in Fig. 1(a). This image maps the intensityof dI/dV at the energy of 7 meV, corresponding
to the coherence peak. The upper left terrace
is the non-SC √2 × √2 surface. Since the SC
coherence peak is well developed at the brightregion on the 2×1 surface, the dark spots
indicated by circles can be identified as vortex

-20

(a)

0

V (mV)

20(b)

81 Å × 81 Å

Fig. 1: (a) Constant-current STM image of cleaved surface of Ba(Fe0.93Co0.07)2As2 single crystal (V = 6.4 mV, I = 15
pA). The fresh surface was prepared by the cleaving method at low temperatures below 20 K in the ultrahighvacuum (UHV, ~10 -10 Torr). The STM image shows the step structure between two different atomic surfaces. (b)Ba-terminated 2×1 surface. (c) As-terminated √2 × √2 surface. (d) Tunneling spectrum taken at the 2×1 surface.

45

IMR KINKEN Research Highlights 2011

IMR KINKEN Research Highlights 2011

H = 5T(a)dI/dV (nS)

0.8

Ba(Fe0.93Co0.07)2As2

0.6

T = 4.9 K
H=5T

0.4
0.2
0
-40

(b)

outside the vortex corevortex core center-200

V (mV)

20

40

Fig. 2: (a) dI/dV image at 7 meV in the magnetic field
5 T obtained simultaneously with the STM image in
1(a). (b) dI/dV spectra at the center of and outside
the vortex core.

cores. The average distance between thevortex cores (a0 ~ 200-300 Å) is in good
agreement with the calculated value (aΔ = 215Å) for the triangle vortex lattice at 5 T. The vortex
core radius rav ≃ 35-45 Å estimated is consistent
with the Ginzburg-Landau coherence lengthξ(0)GL ≃ 34 Å obtained from the upper criticalfield Hc2(0) = φ 0/2πξGL 2 ≃ 28 T [3]. The vortex
lattice shows the disordered structure with ashort-range
hexagonal
order,
butnotranslational or orientational order, indicating
the vortex glass phase. Since there in nocorrelation between the vortex position andthe surface morphology such as the 2×1
structure and surface defects, the vortex
pinning center is not determined by STM/STS,suggesting bulk pinning deep inside the crystal
and three-dimensionality of the vortex system.
The disordered vortex structure due to strong
pinning is supported by the large critical current
density Jc ~ 2×105 A/cm2 [3] and is consistent
with previous STM/STS on Ba(Fe 0.9Co 0.1)2 As2 [4]and small angle neutron scattering (SANS)
experiments on Ba(Fe 0.93Co 0.07)2 As2 [5].

As shown in Fig. 2(b), the dI/dV spectrum
outside the vortex core shows SC characteristics,
similar to the zero-field spectrum (see Fig. 1(d)).
At the vortex core center, however, thecoherence peak is reduced and the spectrum
is nearly flat near the Fermi energy without theZBC peak. The ZBC peak is a signature of thequasiparticle bound state for clean s-wave
superconductors and gradually disappears
with increasing disorder scattering. Thus, thevortex core spectroscopy indicates that
Ba(Fe0.93Co 0.07)2 As2 is not in the clean limit (ξ0 «ℓ); this result is supported by the comparabilityof the parameters between the mean freepath ℓ = ħ(3π 2)1/3/e2 n2/3 ρ 0 ~42 Å and the BCS
coherence length ξ0 ≃ 38-46 Å (i.e., ξ0 ~ ℓ).
Here, residual resistivity ρ 0 = 90 μΩcm [3], Hall
coefficient RH = 1 × 10 -9 m3/C [3], ξGL(0) =0.74ξ0[χ(0.88ξ0/ℓ)]1/2, and the Gor'kov function χ
are used. The absence of the ZBC peak in
Ba(Fe0.93Co 0.07)2 As2 is consistent with previous
STM/STS on Ba(Fe0.9Co 0.1)2 As2, except for the
V-shaped vortex-core spectrum [4], but is in
sharp contrast to the recent experiment onhole-doped (Ba 0.6K0.4)Fe2 As2 [6], which exhibits
an ordered vortex-lattice and a quasiparticle
bound state at a small negative-bias. These
discrepancies can be explained by thedifference in the degree of the disorder strength
between in-plane doped Ba(Fe 0.93Co 0.07)2 As2and out-of-plane doped (Ba 0.6K0.4)Fe2 As2. The
in-plane Co-doping on the Fe site enhances
the disorder potential drastically in the
superconducting FeAs layers. Since all vortices
meet Co atoms (average distance: ~ 11 Å), thevortex structure and vortex-core electronic
state are strongly modified by the disorder, incontrast to (Ba 0.6K0.4)Fe2 As2 [6].
References[1] Y. Kamihara, T. Watanabe, M. Hirano and H. Hosono, J.
Am. Chem. Soc. 130, 3296 (2008).
[2] T. Nishizaki, Y. Nakajima, T. Tamegai, N. Kobayashi, J.
Phys. Soc. Jpn. 80, 014710 (2011).
[3] Y. Nakajima, T. Taen and T. Tamegai, J. Phys. Soc. Jpn.
78, 023702 (2009).
[4] Y. Yin, M. Zech, T. L. Williams, X.F. Wang, G. Wu, X.H. Chen,
and J.E. Hoffman, Phys. Rev. Lett. 102, 097002 (2009).
[5] P. Das, T.O'Brien, MLaver, C. D. Dewhurst, N. Ni, S. L.
Bud'ko, P.C. Canfield and M.R. Eskildsen, Supercond. Sci.
Technol. 23, 054007 (2010).
[6] L. Shan, Y.-L. Wang, B. Shen, B. Zeng, Y. Huang, A.Li, D.
Wang, H. Yang, C. Ren, Q.-H. Wang, S. Pan, and H.-H.
Wen, Nature Phys. 7, 325 (2011).

Keywords: high-Tc iron-based pnictide superconductors, scanning tunneling microscopy
T. Nishizaki and N. Kobayashi (Low Temperature Physics Division)
E-mail: terukazu@imr.tohoku.ac.jp
URL: http://ltp.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201146

Electronic Materials

Anderson Localization Enhanced by Electron Correlations
near Mott Transition

Randomness in the correlated electron system like as the low dimensional organic conductors isessentially important in real materials. The x-ray irradiation to an organic superconductor makes thesuperconducting state to the Anderson-type localization insulating state due to introducing themolecular disorder, while the sister organic compound having the Mott insulating ground statechanges to the similar localization insulating state by the irradiation. These observations indicatethat the stronger electron correlation upon approaching to the Mott transition enhances Andersontype electron localization.

Metal-insulator transitions are of considerable
importance for strongly correlated electron
systems. Among the various types of the Metalinsulator transitions, the Mott transition due toelectron-electron interactions is one of themost attractive phenomena [1]. Another wayof the electron localization originates from theinterference of the electron wave functions
due to randomness. This is the Anderson
insulator derived by introducing disorder into
the material [2]. Since the randomness in thecorrelated electron system is essentially
important in real materials, systematic studies
of disorder effects are desired in systems nearby
the Mott transition for understanding their
physical properties.
Organic charge-transfer salts based on adonor
moleculebis(ethylenedithio)tetrathiafulvalene (abbreviated as BEDT-TTF)
have been recognized as one of the highly
correlated electron systems . Among them,κ-(BEDT-TTF)2 A, where A is anion molecule, has
attracted considerable attention as a
bandwidth-controlled Mott transition system.
The crystal structure of κ-(BEDT-TTF)2Cu[N(CN)2]
X (X = Br or Cl) is shown in Fig.1. Its remarkablefeature is that the native quarter filled band is
modified to the effective half filled band by thestrong dimer structure consisting of two BEDTTTF molecules. Thus this family of organic
conductors has been considered as a typicalbandwidth controlled Mott transition systemwith strongly correlated electrons [3]. One cancontrol the strength of the electron correlation
relative to the bandwidth by small pressure or
partial molecule substitution, which leads to a
Mott insulator-metal (superconductor) transition
as shown latter in Fig. 3.
It has been known that x-ray irradiation tothe organic materials causes moleculardisorder and this remains permanently inorganic molecules. Actually the moleculardisorder in the organic superconductor κ-(BEDTTTF)2Cu(NCS)2 causes an increase of theresidual resistivity and then suppresses thesuperconductivity [4]. Recently, Sano et al.
found that the weak molecular disorderintroduced by x-ray irradiation to the organic
superconductor
κ-(BEDT-TTF)2Cu[N(CN)2]Br47

IMR KINKEN Research Highlights 2011

induced the Anderson-type localization
insulating state from the strongly correlated
metallic/superconducting state [5]. The latter
salt having narrower bandwidth W is locatednearby the Mott critical point in the metallic
side in contrast to the former one with wider W(Fig. 3).
Fig. 2 shows the temperature dependenceof the resistivity in the x-ray irradiated organic
Mott insulator κ-(BEDT-TTF)2Cu[N(CN)2]Cl (upperCu[N(CN)2]X
BEDT-TTF

Fig. 1: Crystal structure of the organic superconductor
κ-(BEDT-TTF)2Cu[N(CN)2]X. The BEDT-TTF conducting
layers are sandwiched by the insulating anion
Cu[N(CN)2]X layers. In case of X = Br, it becomes
the superconductor with Tc of 11 K, while the Mott
insulating state appears with X = Cl. In the twodimensional BEDT-TTF layers, the molecules have acharacteristic arrangement. The BEDT-TTF molecule
dimers form the triangular lattice.

IMR KINKEN Research Highlights 2011

panel) andthe organic superconductor
κ-(BEDT-TTF)2Cu[N(CN)2]Br (lower panel). The
samples are irradiated at 300 K using a nonfiltered tungsten target at 40 kV and 20 mA,
which corresponds to approximately 0.5
MGy/h. Introducing the molecular defect by
the x-ray irradiation, the resistivity decreases
and increases from the Mott insulating state
and the superconducting state to the similar
Anderson-type localization insulating state inκ-(BEDT-TTF)2Cu[N(CN)2]X with X = Cl and Br,
respectively. In addition, the hydrostatic

104

0h
50h

103

Mott insulator100hρ (Ωcm)102 150h
225h

101

100 300h

localizationlinsulator ritica
c

1

Mott
insulator
κ-Cl

SC

ity
tiv
c
u
nd
co

metal

κ-Br κ-NCS

N
n
Wc /U

W/U

Fig. 3: Schematic phase diagram for band width W,
carrier number N and disorder. The vertical andhorizontal arrows represent the experimental
processes of the x-ray irradiation for introducing
disorder and then the application of pressure toincrease W, respectively. Localization insulatingstate appears with weaker disorders byapproaching Mott critical point.

400h

10-1

10-1 200h

pressures to both the localization insulatingstates restore the metallic properties due to thebroadening of the bandwidth. Theseobservations indicate that the stronger electron
correlation upon approaching to the Mott
transition enhances Anderson-type electron
localization due to disorder introduced by x-ray
irradiation. Further studies like as the low energy
spectroscopy on the competition and/or
cooperation of the Mott and Andersontransitions close to the Mott critical point areimportant for understanding the criticalbehavior of the electrons in the real materials.

10-2 160h

Referencesinsulator
-2
10

5

10

50 100T (K )

500

103
102 500h101ρ (Ωcm)disorder

100

10

-3

insulator

400h
300h
250h

120h
80h

10-4 40h
10-5

0h

5

10

metal/SC
50 100T (K )

500

Fig. 2: Temperature dependence of the resistivity inthe x-ray irradiated organic Mott insulator κ-(BEDTTTF)2Cu[N(CN)2]Cl (upper panel) and the organic
superconductor κ-(BEDT-TTF)2Cu[N(CN)2]Br (lower
panel). The samples are irradiated at 300 K using a
non-filtered tungsten target at 40 kV and 20 mA,
which corresponds to approximately 0.5 MGy/h.

[1] M. Imada, A. Fujimori and Y. Tokura, Rev. Mod. Phys. 70,1039 (1998) .
[2] P. W. Anderson, Phys. Rev. 109, 1492 (1958).
[3] K. Kanoda, Hyperfine Interact. 104, 235 (1997).
[4] J. G. Analytis, A. Ardavan, S. J. Blundell, R. L. Owen, E. F.
Garman, C. Jeynes, and B. J. Powell, Phys. Rev. Lett. 96,
177002 (2006).
[5] K. Sano, T. Sasaki, N. Yoneyama and N. Kobayashi, Phys.
Rev. Lett. 104, 217003 (2010).

Keywords: organic, superconducting, metal-insulator transition
T. Sasaki (Low temperature condensed state physics)
E-mail: takahiko@imr.tohoku.ac.jp
URL: http://cond-phys.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201148

Electronic Materials

Doping Effects on the Stacking Fault Energy in SiliconIn silicon crystals, the width of a stacking fault ribbon bound by a pair of partial dislocations increases
when n-type dopant atoms (phosphorus, arsenic, and antimony) segregate nearby the ribbon,
while the width is unchanged when p-type dopant atoms (boron and gallium) segregate. The originof the increase for n-type dopant atoms is the reduction of the stacking fault energy due to anelectronic interaction between the ribbon and the dopant atoms segregating at the ribbon, rather
than the reduction of the strain energy around the partial dislocations due to the dopant atoms
segregating nearby the partials. The interaction energy is estimated to be 0.15±0.05 eV.
Grain boundaries and crystal interfaces
including stacking faults are a key issue for highperformance photovoltaic and/or electronic
devices fabricated with silicon (Si) crystals,
since they are frequently introduced in Si
epilayers and miniaturized Si electronic devices,
as well as polycrystalline Si such as cast Si forsolar cells, during crystal growth and device
fabrication. It is theoretically expected that astacking fault interacts with point defects suchas dopant atoms [1], and the interaction leads
to a segregation of the defects (a so-called
Suzuki segregation [2]). The interaction would
modify the dopant distributions and electronic
properties in Si with stacking faults. Also, dopant
atoms segregating at a stacking fault ribbon
bound by a pair of partial dislocations (a socalled dissociated dislocation) would assist thepinning of the dislocation motion [3, 4], as well
as modify the energy levels of the stacking fault
ribbon. In the present work, we have determined
experimentally the segregation of various kindsof dopant atoms nearby stacking fault ribbons
and evaluated the interaction energy [5-8].
Dislocation-free bulk single crystals of
n-type Si doped with phosphorus (P) (with theconcentration of 3x1019 cm−3), arsenic (As)(1x1019 cm−3), or antimony (Sb) atoms (5x1018cm−3), and those of p-type Si doped with boron(B) (2x1020 cm−3) or gallium (Ga) (2x1018 cm−3)
atoms were grown by the Czochralski (CZ)
method. The CZ crystals inevitably contained
oxygen (O) atoms with a concentration about
1018 cm−3. Also, P- (3x1019 cm−3) or B- (8x1018cm−3) doped crystals without O impurities weregrown by the floating zone (FZ) method.
Dissociated dislocations lying on (111), in adensity about 109 cm−2, were introduced in the
crystals by plastic deformation at elevatedtemperature 1173 K for 0.5 h. The deformed
crystals were then annealed at the same
temperature for a period tan up to 110 h without
stress, so that dopant atoms could interact withdislocations via their thermal migration. The
annealed crystals were characterized by
weak-beam dark-field transmission electron
microscopy (TEM).
In heavily-doped crystals annealed for along period, radial strain fields about a few nm
in diameter are observed nearby dissociated49

IMR KINKEN Research Highlights 2011

Fig. 1: A dissociated dislocation in CZ-Si crystals;
doped with P (tan = (a) 0.5 and (b) 10.5 h) or (c)non-doped (tan = 10.5 h). The left, center, and rightimages in each figure are, respectively, taken withthe reflection g = (022), (220), and (202). Doublearrowheads in (b) indicate P agglomerates. The
structural nature of each dislocation in (a–c) is
schematically shown in (d). Segregation of (e) P(tan = 110.5 h) and (f) Ga (tan = 10.5 h) atoms nearbya dislocation.

dislocations (Fig. 1). Since the strain fields
increase in size with increasing tan and they are
not observed in lightly-doped crystals annealed
for a short period, dopant atoms would
segregate at dissociated dislocations during
annealing, via their thermal migration, andlattice planes nearby the dislocations are
distorted due to the agglomerates of thedopant atoms [5, 6]. The number density andthe size of radial strain fields are similar in CZand FZ-crystals doped with P atoms in the same
concentration, and this suggests that the
segregation of O atoms is negligible in thepresent experimental condition [5].
The width of a stacking fault ribbon boundby a pair of partial dislocations is estimated with
the intensity profile of the partials in TEM images
taken into account the diffraction condition,

IMR KINKEN Research Highlights 2011

and the apparent stacking fault energy γ isevaluated with the estimated width with ananisotropic elasticity theory [8]. The estimated γ
are summarized as a function of the number of
dopant atoms that can agglomerate nearby
partials via their thermal migration N; N is
defined as N=π[(Dαtan)0.5]2cα, in which Dα andcα are, respectively, the diffusion constant andthe concentration of α dopant atoms (Fig. 2).
For n-type crystals, γ decreases with increasing
N in the range up to about 5x102 nm -1, and it isunchanged in the range above 5x102 nm -1,
irrespective of the growth method and theatomic number of dopant atoms [6, 7]. On theother hand, γ is unchanged with increasing N
for p-type crystals [5].

Fig. 2: The apparent stacking fault energy γ vs thenumber of dopant atoms that can agglomerate
at dissociated dislocations N, for non-doped,
p-type, and n-type crystals. Open and closed
marks indicate the data for FZ- and CZ-crystals,
respectively.

In heavily-doped crystals, the width of a
stacking fault ribbon bound by a pair of partial
dislocations would be different from the widthin non-doped crystals, and it depends on the
charge state and strain around the partials, aswell as the stacking fault energy. The first effect,
due to the ionization of partials via the Patel
effect [9], is ignored in our heavily doped
crystals since the Debye screening length israther short (less than about 5 nm). Also, thesecond effect, due to the reduction of strainaround partials via the segregation of dopant
atoms, is negligible since the separation of apair of partial dislocations is unchanged evenwhen a number of Ga atoms segregate nearby

the partials. The decrease of γ in n-type crystals
is, therefore, due to an electronic interaction
between a stacking fault ribbon and n-type
dopant atoms resulting in the reduction of thestacking fault energy. Actually, ab-initio
calculations indicate that P atoms would
segregate at a stacking fault under anelectronic interaction and the segregation
results in the reduction of the stacking fault
energy, while B atoms do not interact withstacking faults [5]. Also, 3D atom probe
measurements reveal that [10], P and As atoms
segregate but B atoms do not segregate atinterfaces in poly-crystals.
As a simple model for the segregation ofn-type dopant atoms at a stacking fault, γ
could be written as (1-p)γSi+pγSiX, in which p
indicates the occupation probability of X atoms(X: P, As, or Sb) at a stacking fault in thermal
equilibrium, and γSi and γSiX are, respectively,
the stacking fault energy in pure Si crystals andthat in SiX crystals. The segregation probability
p could be written as p=1/[1+(1/C0 *)exp(-E/kT)],in which E is the interaction energy of an X atomwith a stacking fault and C0 * is the concentrationof X atoms nearby the stacking fault, on theanalogy of the segregation at a dislocation [4].
Considering that the probability p would reachthe maximum at N~5x102 nm -1 (Fig. 2), theinteraction energy E is experimentally estimatedto be 0.15±0.05 eV (the broken curve in Fig. 2)[5], and the estimated value is consistent withthe theoretical result of 0.1 eV [1].
references[1] J. F. Justo, A. Antonelli, T. M. Schmidt, & A. Fazzio, Physica
B 273-274 (1999) 473.
[2] H. Suzuki, J. Phys. Soc. Jpn. 17 (1962) 322.
[3] I. Yonenaga, Mater. Sci. Eng. B 124–125 (2005) 293 .
[4] K. Sumino & I. Yonenaga, Solid State Phenom. 85/86(2002) 145.
[5] Y. Ohno, Y. Tokumoto, H. Taneichi, K. Togase, S. R.
Nishitani, & I. Yonenaga, Physica B (2011) in press.
[6] Y. Ohno, Y. Tokumoto, & I. Yonenaga, Thin Solid Films(2011) in press.
[7] Y. Ohno, T. Taishi, Y. Tokumoto, & I. Yonenaga, J. Appl.
Phys. 108 (2010) 073514.
[8] Y. Ohno, T. Shirakawa, T. Taishi, & I. Yonenaga, Appl.
Phys. Lett. 95 (2009) 091915.
[9] J. R. Patel, L. R. Testardi, & P. E. Freeland, Phys. Rev. B 13
(1976) 3548.
[10] K. Inoue, F. Yano, A. Nishida, H. Takamizawa, T.
Tsunomura, Y. Nagai, & M. Hasegawa, Appl. Phys. Lett. 95(2009) 043502.

Keywords: grain boundaries, dopant, transmission electron microscopy (TEM)
Yutaka Ohno (Physics of Crystal Defects)
E-mail: yutakaohno@imr.tohoku.ac.jp
URL:http://lab-defects.imr.tohoku.ac.jp

IMR KINKEN Research Highlights 201150

Electronic Materials

TOhoku Mixed Basis Orbitals ab initio Program (TOMBO)A

mong many first-principles methods, all-electron mixed basis approach using both plane waves
and numerical atomic orbitals confined inside non-overlapping atomic spheres has a distinct
advantage to reduce the computational cost of all-electron electronic structure calculations ofvarious kinds of systems, localized and extended systems, isolated and bulk systems, etc. with anenough high accuracy. This is our original method, which has been implemented in a program
called TOMBO.

51

IMR KINKEN Research Highlights 2011

1.18

Bond length of C-H,

1.16

(a)

AOs of CH4:
1s
1s, 2s1s, 2s, 2p(b)

AOs of CH4:
1s
1s, 2s1s, 2s, 2p

1.14
1.12
1.10
1.08

-1082
-1084

Total energy, eV

TOhoku Mixed Basis Orbitals ab initio
program (TOMBO) is a tool for first principles
calculations [1], which has been developed by
Kawazoe Laboratory over 15 years. It is basedon our original all-electron mixed basis
approach [2-4], in which one electron wave
functions are expressed by using both planewaves (PWs) and atomic orbitals (AOs). Since
AOs are numerically defined inside the nonoverlapping
atomic spheres in radial(logarithmic) mesh, all-electron calculations
with PWs can be performed very accurately
with a modest computational cost. The numberof PWs required in this method is significantly
fewer than that required in standard
pseudopotential or PAW methods. Moreover,
one can avoid the problems such as the basis
set superposition error (BSSE) appearing instandard
LCAO
methods
andtheovercompleteness problem appearing instandard mixed-basis methods. TOMBO can
describe extended PW-like states as well as well
localized core states with modest number ofbasis functions. It is applicable to various kindsof systems including atoms, molecules, clusters,
surfaces, and crystals. Therefore, it has anapparent advantage compared to many
preexisting first-principles methods. Using
TOMBO, we can study, for example, withenough high accuracy, the properties affectedby core levels such as the hyperfine structures[5].
To demonstrate an advantage of TOMBO,
we show in Fig. 1 the dependence of (a) theC-H bond length (Å) and (b) the total energy(eV) of a CH4 molecule on the cutoff energy(eV) of PWs used in the calculations. The curves
correspond to the three different cases, where
the following AOs are included in thecalculations: 1s only (-○-), 1s and 2s (-□-), and1s, 2s and 2p (-△-) for carbon. (For hydrogen, 1s
AO is always included.) It is obvious from thisfigure that one can reduce the cutoff energyof PWs by introducing 1s, 2s, and 2p AOs forcarbon; in this case (-△-), 300 eV of the cutoff
energy is enough to obtain good convergence.
This statement is general for any systems
including hydrogen and carbon atoms, forwhich usually very high cutoff energy of PWs(typically, ~600 eV) is required in standard PWbased methods.

-1086
-1088
-1090
-1092
0

200

400

600

800

1000

Cutoff planewaves, eV

Fig.1 The dependence of (a) the bond length
between C and H (Å) and (b) the total energy of
an isolated CH4 molecule on the cutoff energy ofPWs (eV). Red circles, blue squares, and green
triangles represent, respectively, the calculations
with 1s AO only, 1s and 2s AOs, and 1s, 2s, and 2p
AOs for the carbon atom.

Recently, we have applied TOMBO to thestudy of hydrogen storage materials [6]. To
simulate useful structures, we introduced simple
models of metal organic frameworks (MOFs)
that can expand hydrogen storage capacity
by lithium cation doping to clarify the physicalmechanisms
ofenhancinghydrogenadsorption energy. Fig. 2 shows the optimized
geometry of a useful calixarene structure(having a cup-like shape) including lithium
atoms. We found that the adsorption of lithium
atoms (red circles in the figures) increases thebinding energy of incorporating hydrogenmolecules much higher than the systems
without lithium doping. Besides this study, byusing high performance computers, we cansimulate and design various kinds of newhydrogen storage materials composed byrelatively light elements compared to thesystems considered so far in the literatures.

IMR KINKEN Research Highlights 2011

6.5

6.0

Y, Å

5.5

5.0

4.5

Fig.2 The optimized atomic geometry of a cup-likeshaped calixarene doped with lithium ions.

Another interesting application of TOMBO
is to simulate electron dynamics of an electronic
excited state in materials. The time evolution of
the electrons and holes in the electronic
excited states can be treated by using TOMBO
on the basis of the adiabatic local density
approximation (adiabatic LDA) in the timedependent density functional theory (TDDFT)
combined with the Ehrenfest theorem for the
adiabatic process. As a very simple example,
we considered a possibility of dissociation of ahydrogen molecule around a nickel dimer. We
put one hydrogen molecule and one nickel
dimer in a skew position. When one electron is
excited to the first excited state, where chargetransfer occurs from hydrogen to nickel, we
found that the hydrogen molecule starts todissociate, and the distance between nickel
and hydrogen atoms becomes shorter. In Fig.
3, dynamics of the dissociative process of ahydrogen molecule around a nickel dimer inthe first (optically) excited state is traced in a
red trajectory. The red point at the bottom(5.0Å, 4.0Å) denotes the nickel dimer pointing
perpendicular to the figure plane. Thisdissociative trajectory of hydrogen atoms
should be compared to the non-dissociative
trajectory in the ground state (although notdrown in the figure). This study supports to clarify
the dynamics of the spill-over process inhydrogen storage materials, which may
improve the hydrogen storage capacity tohigher densities (6wt.% or more) in order tofacilitate practical use [7].

4.0

3.5
3.5

4.0

4.5

5.0

X, Å

5.5

6.0

6.5

Fig.3 The trajectory (from up to down) of hydrogenatoms of a hydrogen molecule around a nickel
dimer (pointing perpendicular to the figure plane)
when one electron is excited from the highest
occupied molecular orbital level to the lowest
unoccupied molecular orbital level.

References[1] A. Jain, V. Kumar, M. H. F. Sluiter, and Y. Kawazoe, Comp.
Mat. Sci. 36, 171 (2006).
[2] T. Ohtsuki, K. Ohno, K. Shiga, Y. Kawazoe, Y. Maruyama,
and K. Masumoto, Phys. Rev. Lett. 81, 967 (1998).
[3] K. Ohno, Y. Maruyama, H. Kamiyama, E. Bei, K. Shiga,
Z.-Q. Li, K. Esfarjani, and Y. Kawazoe, in “Mesoscopic
Dynamics of Fracture: Computational Materials Design”,
edited by H. Kitagawa, T. Aihara Jr., and Y. Kawazoe,
Springer Series on Advances in Materials Research, Vol.1(Springer-Verlag, Berlin, Heidelberg, 1998) pp.210-219.
[4] K. Ohno, K. Esfarjani, and Y. Kawazoe, “Computational
Materials Science:
From Ab Initio to Monte Carlo
Methods”, Springer Series in Solid-State Sciences, Vol.129(Springer-Verlag, Berlin, Heidelberg, 1999) pp.1-325.
[5] M. S. Bahramy, M. H. F. Sluiter, and Y. Kawazoe, Phys. Rev.
B 73, 045111 (2006).
[6] R. Sahara, H. Mizuseki, and Y. Kawazoe, The Bulletin of
the Society of NANO Science and Technology (inJapanese), 9 (2), 45 (2011).
[7] R. Sahara, K. Ohno, and Y. Kawazoe, in preparation.

Keywords: TOMBO, first-principles method, all-electron mixed basis approach
R. Sahara1, M. H. F. Sluiter 2, K. Ohno3, H. Mizuseki1, and Y. Kawazoe1
1) IMR, Tohoku University, 2) Department of Materials Science and Engineering, Delft University of Technology,3) Department of Physics, Yokohama National University
E-mail: kawazoe@imr.edu
URL: http://www.kawazoe.imr.edu

IMR KINKEN Research Highlights 201152

Electronic Materials

Nitride Semiconductors: Saving Our Planet

N

itride semiconductors such as InN, GaN, AlN and their alloys are very promising for fabricating
devices with low power consumption, high-power, and high-frequency. InGaN-based blue lightemitting-diodes (LEDs) and laser-diodes (LDs) have been already commercially available. The new
trends of nitride semiconductors in the region of deep ultraviolet, green, and infrared wavelength,
will promote the device applications saving energy and creating new energies. Our researches are
related with these new technology trends, challenging the nitride semiconductor-based LDs for the
optical fiber communications system, phosphor-free white LEDs, brilliant green LEDs, and highefficiency solar cells. One of the key tasks for implementing the above devices is to improve thecrystalline quality of the InN film, using the pressurized-reactor metalorganic vapor phase epitaxy(PR-MOVPE) technology which has been developed by ourselves. This PR-MOVPE will become astrong tool for the thin film growth with high equilibrium pressure between gas and solid phases
because the partial pressure of sources makes it possible to control the facets of growth islands. Thiseffect is very important for getting high-quality epitaxial film grown on substrates with lattice mismatch
between substrates and epitaxially grown films. In the case of nitride semiconductors, this pressure
control is one of most important technology because blue, green, and ultraviolet LEDs are usuallyfabricated on sapphire substrates.

Peak Area (arb. units)

InN with a narrow band gap of about 0.65
eV can be expected as a novel material for LDs
in optical communications systems because
dEg/dT at room temperature can be expected
to be smaller than that of InGaAsP [1] from theoptical absorption measurement. Ideally,
electron-phonon
interactions
mainlycontribute to dEg/dT. T-dependent static
correlation functions of vibrational atomic
displacements <u2> of an InN film were
measured by X-ray diffraction (XRD).
The measurements were carried out using
a UHV chamber mounted on a six-circles
diffractometer in SPring-8. A 200-nm-thick InN
film, which was grown on (0001) sapphire
substrates by PR-MOVPE [2], was cooled down
to 60 K. Surface-normal and surface-parallel(radial and azimuthal) line scans in reciprocal
space around 222 and 034 reflections of the

500

_
2242
radial(o)

450

350
300

fit
0

50

azimuth( V )ref.
100 150 200 250 300

Temperature (K)Fig. 1: Peak area of InN 222 and 034 reflections as afunction of temperature. All of lines are calculated
from <u2> in Fig. 2

53

IMR KINKEN Research Highlights 2011



(1)

,

where m is an atomic mass and ω ischaracteristic frequency expressed as.(2)

The A, B and σ are parameters depending onelement, displacement direction, and material[3]. X-ray diffraction intensity is proportional tosquare of the absolute value of the structure
factor F(q)


_
0334

400

InN
film
were
repeatedly
measured.
T-dependent <u2> of wurtzite group-III nitrides
have been calculated and fitted to theanalytical form [3],(3)where q is scattering vector, fa is an atomic
scattering factor, r ja is the position of j-th atoma (In or N), and Ma is expressed as.

(4)

The temperature dependences of thepeak area of the X-ray line scans are shown in
Fig. 1. This dependence is reproduced fairly
well by the calculated <u2> for InN as shownwith dashed lines in Fig. 1. Parameter B’s informula (2) were refined to fit the measured
data as shown with solid lines in Fig. 1. The

IMR KINKEN Research Highlights 2011

0.007
ref.

AlN
GaN
InN

0.005

-0.05

0.004

2

o

// c
c

0.00

0.003
0.002
0.001
0

InP
GaAs

Eg(T)-Eg(0)
Eg(0)

2

<u > (A )

0.006

fit

N

-0.10

In

InAs

50

-0.15

100 150 200 250 300

Temperature (K)

0

100

200

300

400

TEMPERATURE (K)Fig. 2: <u2> of In and N atoms as a function oftemperature obtained from this work (solid lines)
and Ref. [3] (dashed line)

Fig. 4: Temperature dependence of band-gap
energy in III-V semiconductors

theoretical calculations on <u2>’s of InN(dashed lines in Fig. 2) and related materials(Fig. 3) show the <u2> of cation in InN are 0.40.8 times of that of InAs, InP, GaAs, and GaP[3,4]. For the anion, it is 0.6-1.0 times.
When the temperature is around or aboveis
inversely
Debye
temperature,<u2>
proportional to inter atomic force constant. The
force constant of III-N is large owing to small
anion N radius and bond length. Therefore,
small <u2> of III-N is quite reasonable.
This suggests that the small dEg/dT of InN
partly comes from small <u2> in comparison
with that of InGaAsP.

The temperature dependence of bandgap energy in III-V semiconductors is shown in
Fig. 4. This dependence is normalized with the
band-gap energy of each material at theabsolute zero temperature. The dependenceof InN, which is investigated from ourmeasurements of optical absorption, is also
shown. The above result can support Fig. 4.
Therefore, nitride semiconductors including InN
can be expected to be applied for temperaturestable devices. The optical characteristics of
InN show the perspective in the realization oflaser diodes for dense-wavelength-divisionmultiplexing (D-WDM) system, temperaturestable solar-cells, and so on.
References

0.014
anion
cation

0.01

InAs
InP
GaAs
GaP

2

<u > (A )

0.012

2

o

0.008
0.006

[1] J. Wu et al., J. Appl. Phys. 94, 4457 (2003).
[2] Y. Zhang et al., Phys. Stat. Sol. (c) 8, 482 (2011).
[3] M. Schowalter et al., Acta Cryst. A65, 227 (2009).
[4] M. Schowalter et al., Acta Cryst. A65, 5 (2009).

0.004
0.002
0

0

50

100 150 200 250 300

Temperature (K)Fig. 3: Theoretically calculated <u2>’s of InAs, InP,
GaAs, and GaP as a function of temperature.

Keywords: nitride semiconductors, energy gap, X-ray diffraction, white LEDs, green LEDs, solar Cells, infrared
LDs
Takashi Matsuoka (Physics of Electronic Materials Division)
E-mail: matsuoka@imr.tohoku.ac.jp
http://www.matsuoka-lab.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201154

Electronic Materials

Giant Spin-Hall Effect in Impurity Doped-Au

Highly efficient generation, control and detection of spin current, which is the flow of spin angularmomentum, are essential issues to develop spintronic devices with high performance and multifunctionality. Spin-Hall effect (SHE) enables us to convert between charge current and spin current
in nonmagnetic materials without using a ferromagnetic material. The perpendicularly spin-polarized
FePt/Au devices show giant SHE in Au. One of the possible origins for giant SHE is skew scattering byimpurities in Au. Pt-doped Au shows significant thickness dependence of SHE, suggesting theimportance of surface and/or interface scattering due to Pt atoms on the Au surface. In the case of
Fe-doped Au, on the other hand, the impurity concentration dependence and thickness dependenceof SHE imply that the resonant skew scattering in bulk mainly contributes to the large SHE.

“Spintronics”, which utilizes the freedom ofspins, provides electronic devices with novel
functionality. In the research field of spintronics,
the generation, control and detection of theflow of spin angular momentum (i.e. “spincurrent”) are key issues to develop highperformance devices. Recently, spin-Hall effect(SHE) in nonmagnetic materials has attracted
much attention since SHE enables us togenerate or detect pure spin current without
using a ferromagnetic material.
SHE is generally considered to the same
origin as that for the anomalous Hall effect(AHE). The most distinguished point of SHE is
that it does not require ferromagnetic order.
When charge current flows in a nonmagnetic
material having large spin-orbit coupling,
electrons are scattered in the transverse
direction. The scattered directions are opposite
between up- and down-spin electrons.
Consequently, spin current appears in thetransverse direction to the charge current,
which is called the direct SHE (DSHE). On theother hand, spin current induces the transverse
charge current, which is called the inverse SHE(ISHE). The electrical detection of SHE wasreported in several nonmagnetic metals [1-6].
Previously, we reported the giant SHE in Au
employing lateral devices with an FePt
perpendicular spin injector and a Au Hall cross[4]. Theories [7,8] have suggested that the
impurities in Au play a significant role for theenhancement of the spin-Hall angle (αH).
In order to understand the mechanism of
the giant SHE in Au, we have investigated SHEin Pt-doped and Fe-doped Au. The devices,consisting of an FePt perpendicular spin injector
and a doped Au Hall cross, were prepared onan MgO (100) substrate. Thin films wereprepared employing an ultrahigh vacuum
magnetron sputtering system. Nanometersized devices were fabricated using electron
beam lithography and Ar ion milling. The
thickness of the Hall cross (tN) was varied in therange from 10 nm to 30 nm. Fig. 1(a) shows ascanning microscope image for the device.
The measurement setup is also shown in Fig.1(a).
The electric current was applied from FePt to55

IMR KINKEN Research Highlights 2011

Fig. 1: (a) A scanning microscope image for thedevice with an FePt perpendicular spin injector
and a doped Au Hall cross. The measurement
circuit is also shown. (b) Non-local voltage (VH) asa function of perpendicular magnetic field for thedevices with 0.4 at.% Pt-doped, 0.4 at.% Fe-doped,
and undoped Au.

Au. This induced spin accumulation around theinterface between FePt and Au, resulting in the
diffusion of spin current even in the Au Hall crosswhere charge current did not flow. This meansthat pure spin current is generated in the Au

IMR KINKEN Research Highlights 2011

Hall cross. The pure spin current was converted
into charge current due to ISHE in Au, and nonlocal voltage appeared in Au.
Fig. 1(b) displays the non-local voltage (VH)
as a function of perpendicular magnetic field
for the devices with 0.4 at.% Pt-doped, 0.4 at.%Fe-doped, and undoped Au. All the devices
show the clear hysteresis of VH, which reflects
the magnetization reversal of the FePt
perpendicular spin injector. These results
indicate that ISHE converts the spin current into
the charge current even for doped Au. From
the analysis of VH as a function of the distance
between FePt and Au Hall cross, the values ofαH were estimated for undoped and doped
Au. Undoped Au with tN = 10 nm showed αH =0.07 ± 0.02. The value of αH did not remarkably
depend on the concentration of the Fe impurity[9], which is consistent with the theory predicting
the resonant skew scattering [7]. On the other
hand, αH gradually increased with theconcentration of the Pt impurity, and αH = 0.14
± 0.04 was obtained for 3 at. % Pt-doped Au,
suggesting that the doping of Pt impurity leads
to the enhancement of αH.
Fig. 2 shows the thickness dependence ofαH for Pt-doped Au. In the case of Pt-doped
Au, αH decreased remarkably with increasingtN. This implies that the contribution of thesurface and/or interface scattering plays animportant role for the enhancement of αH. A
theory predicts that the Pt atoms on the Au(111) surface provides the enhancement of
spin-orbit coupling, which may explain theremarkable thickness dependence of SHE in
Pt-doped Au [10]. In contrast to the results of
Pt-doped Au, Fe-doped Au showed no
dependence of αH on tN, implying that theresonant skew scattering in bulk mainly
contributes to the large SHE in Fe-doped Au.
We
found
that
the
remarkableenhancement of αH was observed for Pt-doped
Au. Our finding is important to find a new routefor further enhancement of SHE, which opens
the way for the development of spintronic
devices with high performance and multifunctionality.

Fig. 2: The thickness (tN) dependence of spin-Hallangle (αH) for 1.4 at% Pt-doped Au. The
measurement was carried out at room
temperature.

References[1] S. O. Valenzuela and M. Tinkham, Nature 442, 176(2006).
[2] E. Saitoh, M. Ueda, H. Miyajima and G. Tatara, Appl.
Phys. Lett., 88, 182509 (2006).
[3] T. Kimura, Y. Otani, T. Sato, S. Takahashi and S. Maekawa,Phys. Rev. Lett., 98, 156601 (2007).
[4] T. Seki, Y. Hasegawa, S. Mitani, S. Takahashi, H. Imamura,
S. Maekawa, J. Nitta, and K. Takanashi, Nature Mater., 7,
125 (2008).
[5] M. Morota, K. Ohnishi, T. Kimura, and Y. Otani, J. Appl.
Phys. 105, 07C712 (2009).
[6] Y. Niimi, M. Morota, D. H. Wei, C. Deranlot, M. Basletic, A.
Hamzic, A. Fert, and Y. Otani, Phys. Rev. Lett., 106, 126601(2011)[7] G.Y. Guo, S. Maekawa, and N. Nagaosa, Phys. Rev. Lett.,
102, 036401 (2009).
[8] M. Gradhand, D. V. Fedorov, P. Zahn, and I. Mertig, Phys.
Rev. Lett., 104, 186403 (2010).
[9] I. Sugai, S. Mitani, and K. Takanashi, IEEE Trans. Magn.,
46, 2559 (2010).
[10] B. Gu, I. Sugai, T. Ziman, G. Y. Guo, N. Nagaosa, T. Seki,
K. Takanashi and S. Maekawa, Phys. Rev. Lett., 105, 216401(2010).

Keywords: spin current, nanostructure
Koki Takanashi (Magnetic Materials)
E-mail: koki@imr.tohoku.ac.jp
URL: http://www.magmatex.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201156

Electronic Materials

Fermi Surface Properties of Paramagnetic NpCd11 with aLarge Unit Cell.

We succeeded in growing a high-quality single crystal of NpCd with the cubic BaHg -type11

11

structure by the Cd-self flux method. From the magnetic susceptibility and specific heat experiments,
this compound is found to be a 5f-localized paramagnet with the singlet ground state in the crystalline
electric field (CEF) scheme. Fermi surface properties were measured using the de Haas-van Alphen(dHvA) technique. Long-period oscillations were observed in the dHvA frequency range of 9.1×105 to
1.9×107 Oe, indicating small cross-sectional areas of Fermi surfaces, which is consistent with a small
Brillouin zone based on a large unit cell. From the dHvA and magnetoresistance experiments, theFermi surface of NpCd11 is found to consist of many kinds of closed Fermi surfaces and a multiplyconnected-like Fermi surface, although the result of energy band calculations based on the
5f-localized Np3+(5f4) configuration reveals the existence of only closed Fermi surfaces. The
corresponding cyclotron effective mass is small, ranging from 0.1 to 0.7 m 0, which is consistent with asmall electronic specific heat coefficient γ ≈ 10 mJ/K 2∙mol, revealing no hybridization between the
5f electrons and conduction electrons.
UCd11 is one of the first discovered uraniumbased heavy fermion compounds with an
antiferromagnetic ordering at TN=5 K [1]. A
strongly enhanced value of the electronic
specific-heat coefficient γ (840 mJ/K 2∙mol) was
obtained from the specific heat above 7 K. The
hidden order transitions of UCd11 were observed
by applying hydrostatic pressure and magnetic
fields, using a high-quality single crystal [2].
The neptunium compound NpCd11 also
crystallizes in the BaHg11-type structure, as
shown in Fig. 1(a). The lattice parameter of
9.288 Å in NpCd11 is almost the same as that in
UCd11. The specific heat data with a small
electronic specific heat coefficient γ=10 mJ/
K 2∙mol indicates no evidence of magnetic
anomalies. The single crystals of NpCd11 weregrown by the Cd-self flux method, following the
Np-Cd alloy phase diagram. Many NpCd11
single crystals with about 5×4×2 mm3 wereobtained as shown in Fig. 1(b). We carried out
the measurements of electrical resistivity,transverse magnetoresistance, specific heat,magnetic susceptibility, magnetization and deHaas-van Alphen (dHvA) effect to clarify theelectronic properties, using a high-quality
single crystal of NpCd11 [3].
Fig. 2(a) shows the angular dependenceof the transverse magnetoresistance at 0.1 K
under 70 and 140 kOe, where the field is rotatedfrom <100> to <110>. The magnetoresistancebecomes minimum around <111> and <110>.
The corresponding field dependence of themagnetoresistance for three field directions isshown in Fig. 2(b).
Scince the 5f electrons in NpCd11 arelocalized, with the Np3+ (5f4) configuration, thevalence electrons are 6d17s2 of Np and 4d105s2
of Cd, revealing an uncompensated metal.
The magnetoresistance increases as H1.1,
revealing the existence of an open orbit along[110] in the configuration of J//[110] and H//[001]and also in the other wide field angle region.
57

IMR KINKEN Research Highlights 2011

Fig.1. (a) Crystal structure of NpCd11, where the Np
atom is surrounded by 20 nearest and next-nearest
neighbor Cd atoms, and (b) photograph of a
NpCd11 single crystal.

Fig.2. (a) Angular dependence of the transverse
magnetoresistance at 0.1 K under the field of 70
and 140 kOe , and (b) field dependence of thetransverse magnetoresistance at 0.1 K for H//<110>,<111> and <100>.

There exists, however, a question why themagnetoresistance does not indicate a clear
H2-dependence, even though the sample is ofhigh quality. The Fermi surface properties arealso discussed in the dHvA experiment.
In order to study furthermore the electronic
state of NpCd11, we performed the dHvA
experiment at low temperatures. Seven
fundamental dHvA branches are detected
together with their harmonics. Among them,three dHvA branches are named α, γ and δ. By

IMR KINKEN Research Highlights 2011

Fig. 3. Angular dependence of (a) the experimental and (b) theoretical dHvA frequencies in NpCd11. (c)Theoretical Fermi surface of NpCd11.

rotating the sample about the magnetic field,
we determined the angular dependence ofthe dHvA frequencies in NpCd11, as shown in
Fig. 3(a). Here, we could not observe clear
branch β for H//<110>, especially in the {100}plane. The dHvA frequencies are relatively
small, ranging from 9.1×105 to 1.9×107 Oe.
Since NpCd11 possesses small Brillouin zone
due to a large lattice constant a=9.288 Å, thebranches α, β and γ occupy small volumes,that is, approximately 0.97, 0.51 and 0.23%,
respectively. This indicates that branches α, βand γ correspond to the small pocket Fermi
surfaces. The dHvA frequency of branch δ hasa minimum for H//<110>, and increases by tilting
the field angle from <110>, indicating acylindrical or ellipsoidal Fermi surface
elongated along <110>. These branches aremost likely due to the multiply-connected Fermi
surfaces.
Here we note that the maximum dHvA
frequency allowed in the present small Brillouinzone is Fmax = 3.8×107 Oe for the spherical Fermi
surface. This means that the multiplyconnected Fermi surfaces contribute to alarge volume of the Fermi surface in thepresent small Brillouin zone of NpCd11.
We determined the cyclotron effective
mass from the temperature dependence ofthe dHvA amplitude for the principal field
directions. The cyclotron effective masses arein the range from 0.11 to 0.68 m0. These smallvalues are consistent with the small γ value. The
theoretical angular dependence of the dHvA
frequency and the corresponding Fermi

surface calculated from a fully-relativistic linear
augmented-plane-wave (LAPW) method areshown in Fig. 3(b) and (c), respectively.
All the theoretical Fermi surfaces are thus
closed Fermi surfaces, without multiplyconnected Fermi surfaces. If these Fermi
surfaces are correct, there exist no open orbits
in the magnetoresistance. Here we remark aband 44-hole Fermi surface centered at the Γpoint, possessing a slender arm along the <110>direction. One possibility is that this arm iselongated up to the Brillouin zone boundary
and is multiply-connected along <110>. In thiscase, the open orbits are formed only in a small
field-angle region around <110>.
References[1] Z. Fisk et al., Phys. Rev. B 30 (1984) 6360.
[2] D. Aoki et al., J. Phys. Soc. Jpn. 68 (1999) 3117.
[3] Y. Homma et al., IOP Conf. Series: Mater. Sci. & Eng. 9(2010) 012091.

Keywords: NpCd11, transuranium compound, magnetoresistance, CEF, dHvA, fermi surface
Y. Homma (Radiochemistry of Metals)
E-mail: yhomma@imr.tohoku.ac.jp
URL:http:// http://www.imr.tohoku.ac.jp/ja/org/research/24.html

IMR KINKEN Research Highlights 201158

Electronic Materials

Direct Evidence of Elastic Interaction between
Ferroelectric Domain and Misfit Dislocation

We have elucidated the elastic interaction between 90° domains and misfit dislocations in PbTiO /3

SrTiO3 (001) epitaxial thin films by local strain mapping based on high-resolution transmission electron
microscopy. A novel mechanism for the nucleation and growth of 90˚ domains has been proposed
based on the results, including the introduction of an additional dislocation perpendicular to a misfit
dislocation and the dissociation of these dislocations into two pairs of superlattice partial dislocations
with an anti-phase boundary.

The nature of ferroelectric materials is
attributable to the switching of domain
configuration,inwhich
spontaneouspolarizations orientate in given directions. In
domain rotation, actually domain walls movein a crystal, or new domains nucleate. It has
been said that the motion or the nucleation of
domain walls are pinned by some defects,
such as misfit dislocations, owing to the elastic
interaction between them. Then, it is importantto quantify the elastic interaction between
domains and defects in order to understandthe dynamics and the nucleation and growthof the domains [1-4]. This paper illustrates the
structure of a 90° domain just on a misfit
dislocation at the atomic resolution, and elastic
interactions between them using electron
microscopy and related strain mapping
techniques [5].
Atomic configuration around 90˚ domain anddislocations [6]
PbTiO3 epitaxial thin films were depositedon SrTiO3(001) single crystal substrates using
pulsedmetalorganic
chemicalvapor
deposition (MOCVD) [7]. Fig. 1 shows aberrationcorrected HRTEM (high resolution transmission
electron microscope) and HAADF-STEM (highangle annular dark field-scanning transmission
electron microscope) images around an
a-domain, which means a 90˚ domain, near
the interface with edge-type dislocations. The
inset in (c) marks an anti-phase boundary (APB)
along a (101) plane, where atomic columns are
shifted by a half of the lattice period. This
a-domain is about 3 nm wide. The lattice planein the a-domain is strongly deflected inside the
3 nm band because of the large tetragonalityof PbTiO3. HAADF-STEM image of Fig. 1(c)clearly shows that lattice rotation takes placeacross the 90◦ domain wall, and that it stems
from the lattice rotation around the core of thedislocations. HRTEM images in Figs. 1(a) and (b)show weak contrasts at the PbTiO3/SrTiO3
interfaces. HAADF-STEM image of figure 1(c)clearly shows an atomic level relationshipbetween the interface and the dislocations. Inan early study, it was reported that, in thissystem, the misfit dislocation is often an edgetype perfect dislocation with Burgers vector
59

IMR KINKEN Research Highlights 2011

Fig. 1: (a) Aberration-corrected HRTEM imagearound the PbTiO3/SrTiO3 interface, (b) magnified
image of the squared region in (a). (c) HAADFSTEM image around edge dislocations with an
a-domain. The inset is a blow-up of the squared
region in (c). (d) shows an edge-type dislocation
without any a-domain. a- and c-domain havepolarizations in the in-plane and out-of-plane
directions, respectively.

b=a[100] [1]. Fig. 1(d) shows that a misfit
dislocation without an a-domain is the perfectdislocation with Burgers vector b=a[100]corresponding to the edge of the unit cell. The
extra double-plane is composed of a pair ofPb–O and Ti–O layers. However, the misfit
dislocation with an a-domain in figures 1(a)–(c)is dissociated into a couple of edge-typepartial dislocations at the bottom end of thea-domain (No. 1 and No. 2). Furthermore,
additional edge-type partial dislocations No. 3
and No. 4 are introduced after the growth of
a-domain. This result implies that edge-type
dislocation with Burgers vector b=a[001] canbe introduced and dissociated into a pair ofedge-type partial dislocations. Extra halfplanes of Pb–O or Ti–O layers are inserted at
the substrate and the left, parallel to (100) and(001) planes, respectively. Considering theIMR KINKEN Research Highlights 2011

contrast of the atomic columns in Fig. 1(c), extra
half-planes of Ti–O are inserted in the partial
dislocations No. 1 and No. 3, and those of Pb–O
in the partial dislocations No. 2 and No. 4. The
atomic arrangement on the (101) plane
between these partial dislocations is shifted bya half of the PbTiO3 unit cell. In our observations,
all the a-domains immediately above the
PbTiO3/SrTiO3 interface are accompanied withtwo pairs of superlattice partial dislocations
with an APB parallel to the (101) plane.
Elastic Interaction of 90˚ domain withdislocations [6]Fig. 2 shows a strain mapping by GPA of
Fig. 1(a); namely, (a) the normal strain in the inplane direction ε xx, (b) that in the out-of plane
direction εyy, (c) the share strain ε xy and (d) the
rigid rotation ωxy. These strain values show
deviation of the lattice parameter from theSrTiO3 reference. The ε xx map clearly shows thea-domain. The strain field ε xx around the partial
dislocation corresponds to that of thedislocation No. 1 and No. 2. The strain field ε xx
under tension around the partial dislocation iscoupled with that of the a-domain. The εyymap shows that the lattice parameter along
the a-axis in the a-domain is almost the sameas in the SrTiO3 substrate. The strain field εyyaround the partial dislocation corresponds tothat of the dislocations No. 3 and No. 4. The
compressed side of the strain field εyy is coupledwith the a-domain. The ε xy map shows no strainaround the a-domain, but around the misfit
dislocation. There is no elastic interaction
between the a-domain and the dislocations
through share strain. The ω xy map shows thatthe lattice of the a-domain rotatescounterclockwise against the c-domain andthe substrate. The lattice rotation around thedislocations is coupled with that of thea-domain. These strain maps elucidate thatthe 90° domain interacts with dislocations
attractively.
These results indicate that the lattice
rotation around the core of the misfit dislocation
with the Burgers vector b=a[100] triggers thenucleation of the 90° domain, and the strain
matching with the misfit dislocation assists
domain growth. Then, an additional edge
dislocation with Burgers vector b=a[001] isintroduced to compensate the lattice rotationof 90° domain. After that, an APB is introducedby the dissociation of full dislocations into twopair of superlattice partial dislocations for the

Fig. 2: Strain mapping of Fig. 1(a) around an a-domain
near the film/substrate interface; namely (a)normal strain in the in-plane direction ε xx, (b) thatin the out-of plane direction ε yy, (c) share strain ε xy
and (d) rigid rotation ωxy. Insets in (a)-(d) aremagnified strain fields around dislocations.

lattice matching at the bottom interface of
a-domains.
Thus, our atomic-scale analysis provides a
direct evidence on the elastic interaction
between an a-domain and a misfit dislocation,and on domain nucleation and growthmechanisms.
References[1] S. Stemmer, S. K. Streiffer, F. Ernst and M. Rühle, phys. stat.
sol. (a), 147, 135 (1995)[2] S. Stemmer, S. K. Streiffer, F. Ernst and M. Rühle, W. Y. Hsu
and R. Raj, Solid State Ionics (a), 75, 43 (1995)[3] I. B. Misirlioglu, A. L. Vasiliev, S. P. Alpay, M. Aindow and
R. Ramesh, J. Mater. Sci. 41, 697 (2006)[4] A. Yu. Emelyanov and N. A. Pertsev, Phys. Rev. B, 68,
214103 (2003)[5] M. J. Hÿtch, E. Snoeck and R. Kilaas, Ultramicroscopy, 74,
131 (1998)[6] T. Kiguchi, K. Aoyagi, Y. Ehara, H. Funakubo, T. Yamada,N. Usami, and T. J. Konno, Sci. Technol. Adv. Mater. 12,034413 (2011).
[7] H. Nakaki, YK. Kim, S. Yokoyama, R. Ikariyama, H.
Funakubo, K. Nishida, and KSaito, Appl. Phys. Lett. 91,
112904 (2007).

Keywords: ferroelectric, defects, transmission electron microscopy
T.Kiguchi (Advanced Analysis of Materials)
E-mail: tkiguchi@imr.tohoku.ac.jp
URL: http://konno-lab.imr.tohoku.ac.jp/

IMR KINKEN Research Highlights 201160

Electronic Materials

Electron-Hole Asymmetry of Spin Correlation in High
Transition Temperature Superconductors

Spin correlations in both electron- and hole-doped Mott insulator have been studied by single

crystal neutron scattering measurement. We succeeded in observing the spin fluctuation in the newtarget system of Bi2201. The qualitative analysis clarified the similarity of incommensurate correlationin Bi2201 and La214, suggesting a common doping-evolution of spin correlation in the single-layer
hole-dope system. On the other hand, commensurate spin fluctuation was observed in a wide
energy region of electron-dope PrLa214 system. Therefore, there exists intrinsic electron-holeasymmetry in the spin correlation against carrier doping.

High-transition-temperature(High-TC)superconductivity in cuprate oxide emerges
by doping sufficient charge carriers into an
antiferromagnetic
(AF)
Mott
insulator.
Therefore, the doping evolution of spincorrelation and its role in the mechanism of
superconductivity have been intensively
studied. Neutron-scattering measurement,
which is powerful technique for the investigationof spin correlations, clarified an intimate relation
between magnetism and superconductivity.
An existence of spin excitation showing thesimilar hourglass-shaped spectrum in the
different class of hole-doped superconductors
La2-xSr xCuO4 (La214) [1] and YBa2Cu3O6+σ [2]indeed suggests the importance of spin
fluctuations for the appearance of high-TC
superconductivity
To confirm the universal nature in the spin
excitation spectrum and to understand thesuperconducting
mechanism,
furthersystematic measurements on other referencecompounds are necessary. However, difficulties
in growing sizable single crystal becomebottleneck of the neutron-scattering study,
since inelastic measurement requires large
volume of sample over 1cc. We, therefore,
attermpted to grow single crystal of several
systems with the aim of investigation of entire
spin excitation at newly constructed high-flux
neutron facilities such as J-PARC.
Here, we introduce a recent progress inthe study of spin correlations in new target
systems of single-layer Bi2+x Sr xCuO6+σ (Bi2201)[3] and Pr1-x-yLayCexCuO4 (PrLa214) [4], whichis brought by the success of crystal-growth(Fig. 1(a)) and development of neutronscattering techniques.
Universal spin correlation in insulating phase ofhole-doped single-layer system
Dynamic spin correlation in the Bi2201 wasstudied to clarify the universal magnetism atlightly hole-doped region of cuprate oxide. We
have succeeded in observing incommensurate(IC) spin fluctuation in the spin-glass sample of
Bi2201 for the first time [5]. (Fig. 1(b)) The
modulation vector was found to be diagonal
to Cu-O bonding direction, similar to the61

IMR KINKEN Research Highlights 2011

observation in La214. Incommensurability
corresponding to the peak-splitting is also
consistent with the value for La124 with thecomparable hole concentration. Therefore, we
concluded that IC spin correlation commonly
develops in the spin-glass phase of single layer
cuprate by hole-doping. The geometry ofobserved IC peaks in the (h,k,0) reciprocal
plane is quite similar to that in insulating single
layer compound La2-xSr xNiO4 (Fig. 2), suggestingthat strong-coupling physics is more relevant to
the origin of IC magnetic peak in the spin-glass
phase of cuprate oxide.
Characteristic high-energy spin fluctuations in

Fig. 1: (a) Single crystal of Bi2.4 Sr1.6CuO6+δ and (b) theneutron scattering spectrum measured at ω=1
meV and 6 K.

IMR KINKEN Research Highlights 2011

Fig.2: Neutron scattering intensity from La1.67Sr 0.33 NiO4.
The spectrum sliced with the energy transfer
between 4meV and 6meV is shown in (h, k)
reciprocal plane.

electron-doped cuprate oxide
Up to now, many key measurements byneutron scattering have been lacking for theelectron-doped system, due to the experimental
difficulties. Therefore, the universal magnetic
properties of high-TC cuprate irrespective of thetype of carriers are still controversial. To make aprogress in the study of spin correlation, we havecarried out a comprehensive neutron scattering
measurement on the PrLa214 system [6].
Fig. 3 shows a neutron scattering intensity
measured for the electron-doped AF orderedphase of PrLa214. We clarified the existence ofhigh-energy spin fluctuations at least up to
150meV. Throughout the entire energy range,a single peak centered at the (0.5,0.5) positionin the tetragonal unit was observed (Figs. 3(b)and (c)), unlike to the hourglass-shaped
excitation in the hole-doped system. Therefore,
the doping dependence of spin fluctuations at
high-energy region in the two systems isdifferent and such a distinct doping evolution
suggests the existence of electron-holeasymmetry in the spin correlation in the wide
energy scale. The characteristics of themagnetism
anditsrelationtothesuperconductivity would be different in naturein the two systems.

Fig.3: (a) Inelastic neutron scattering spectra ofelectron-doped AF ordered Pr1.32La 0.6Ce0.08CuO4
in the energy and momentum space. Horizontal
axis is parallel to [110] direction. Constant energy
spectra sliced with (b) 130±10meV and (c)105±10meV.

References[1] J.M. Tranquada et al., Nature 375 (2004) 531.
[2] S.M. Hayden et al., Nature 375 (2004) 534.
[3] M. Enoki., et al., Physica C 470 (2010) S37.
[4] M. Fujita., et al., Phys. Rev. B 67 (2003) 014514.
[5] M. Enoki., et al., to appeare in JPSJ (Supplement).
[6] M. Fujita., et al., Phys. Rev. Lett. 101 (2008) 107003,
M. Fujita., et al., to appeare in JPSJ (Supplement).

Keywords: neutron scattering, high-tc iron-based pnictide superconductors, spin excitation, crystal growth,
M. Fujita (Materials Materials Processing and Characterization)
E-mail:fujita@imr.tohoku.ac.jp
URL:http://www.imr.tohoku.ac.jp/ja/org/research/27.html,http://www.yamada-lab.imr.tohoku.ac.jp/jp/
index.html

IMR KINKEN Research Highlights 201162

Electronic Materials

Effect of Current on Coulomb Oscillation in a Ni-Nb-Zr-H
Glassy Alloy with Multiple Junctions.

The effect of current on Coulomb oscillation in a (Ni0.36Nb 0.24Zr 0.40)90 H10 glassy alloy was carried out
under currents of 1 nA, 10 nA, 100 nA, 1μA, 10μA, 100μA, 1mA, 10 mA and 100 mA in the temperature
range of 300 to 6 K. The amplitude of the Coulomb oscillation showed maximum values at around
1μA. The discrete variation of the oscillation can be described as E=2x10 -4exp(2.34N) (r 2=0.993),
showing clear quantization of Coulomb oscillation, where E and N are the unit of h/2e2 and step
number, respectively. We suggested that quantization of conductance would be associated withnanometer-sized RC circuits in the alloy [1].

The quantum blockade effect by quantumdot tunneling is one of the topics currently
attracting a great deal of interest in the field ofphysics. Recently, Fukuhara et al. have observed
the electric current-induced voltage oscillationin ((Ni 0.6Nb 0.4)0.7Zr 0.3)1-yHy glassy alloys (with
0.052 ≤ y ≤ 0.152) at temperatures below 240
K[2] and at room temperature[3]. From atomic
bonding data in XAFS(X-ray Absorption Fine
Structure)[4] using strong radiation photons of
Spring 8, and a Nyquist diagram (Cole-Coleplot) by ac impedance analysis using a LCR
meter [5], we proposed that quantum dot
tunneling is charging and discharging thevacancy capacitance of Zr(Nb)-H-□-H- Zr(Nb)atomic bond arrays among the electricconducting distorted icosahedral Zr5Ni5Nb3
clusters (dots of ~0.55 nm in size [4]), where □ isthe vacancy tunnel with regular prism of 0.23
nm, in the glassy alloys. Thus, we regarded it asa DC/AC converting device with a large
number of nsnometer size capacitors.
To investigate the effect of Current on
Coulomb Oscillation in eccentric glassy alloys,
we determined the specific electrical resistanceof (Ni 0.36Nb 0.24Zr 0.40)90 H10 glassy alloy consistingof nanometer-sized clusters by the four-probe
method with a dc current of ±1 nA, ±10 nA, ±100
nA, ±1 μA, ±10 μA, ±100 μA, ±1 mA, ±10 mA and±100 mA at cooling and heating rates of 1 K/sfrom 373 K to 6 K [1].

Fig.1 Electrical resistivities of (Ni 0.36Nb 0.24Zr 0.40)90 H10glassy alloys under (a) ±1 nA, (b) ±1 μA, (c) ±1 mA
and (d) ±100 mA dc current.

63

IMR KINKEN Research Highlights 2011

The representative current-dependent
resistivities are shown as a function oftemperature (Fig.1). These curves are composedof two kinds of curves, linear and abnormal
oscillation ones. The linear curves follow Ohmiclaw whereas the abnormal curves do not. The
abnormal curves represent dc current-induced
Coulomb oscillation, which is associated with
the quantum mechanism of solute hydrogen,although the restricted temperature region isdifferent.
The temperature coefficients of resistivity(TCR) for linear curves with a semiconducting
character in Fig. 1 are presented in Fig. 2(a).
The decrement in TCR suggests anenhancement of the Debye-Waller factor,
which is a characteristic of amorphous solids.
The reason for the enhancement may be dueto the pseudo-localization effect of an electron
in an extremely small electric current such as 1
nA. It is known that an electron-electroninteraction by localization effect causes anincrease in electrical resistivity accompaniedby elastic scattering.
The maximum amplitude (voltage) of the
Coulomb oscillation (Fig. 2(b)) in Fig. 1 increases
as the current decreases down to 1mA andthen shows a plateau between 1 mA and 1 μA,
and finally decreases in the lower current
region from 100 to 1 nA, indicating an
occurrence of the optimum current for quantum
dot tunneling between the icosahedral
Zr5Ni5Nb3 cluster islands.
As can be seen from Fig. 1, the oscillation
showed discrete variation. The pronounced
resistances quantized in the unit of h/2e2 (R) for
currents of 1 and 10 nA in Fig.1 are plotted inFig. 3, as a function of step number (N). The

Fig.2 Effects of current on TCR (a) and maximum
resistivity (b).

IMR KINKEN Research Highlights 2011

Fig.3 Relation between quantized resistance (R) andstep number (N) for the (Ni 0.36Nb 0.24Zr 0.40)90 H10glassy alloys under ±1 and 10 nA. Insert: Temperaturedependent resistivity for 10 nA.

Coulomb oscillation for 10 nA is shown in theinsert in Fig. 3. The discrete voltage jumps by
one order for each step. Both relations for 1
and 10 nA can be described as R=0.0002exp(2.34N) (r 2= 0.993), showing clear fractional
quantization of resistance in Coulomb
oscillation. We infer that the width of a narrow
channelconnecting
three-dimensional
reservoirs of electrons among the distorted
icosahedral Zr5Ni5Nb3 clusters increases when
the current decreases, provided that the glassy
alloy is composed of a high number of ananometer- sized RC circuits constructed by apair of pointed Zr5Ni5Nb3 channels andperpendicularly connecting cluster array
reservoirs which contain subnanometer-sized
capacitors (Fig. 4).
To inspect quantum transport in glassy
alloys of interest, we lastly analyzed the
dynamic evolution of an ensemble of electrons
performing macroscopic resonant tunnelingfor room-temperature millimeter-sized Coulomb
oscillation in Ni-Nb-Zr-H glassy alloys as afunction of the cluster size and boundary
length, using a one-dimensional (1D) KronigPenny model with a stepwise potential [6]. Fig.
5 (a) shows the barrier number dependence ofTn. Tn shows nonstationary counter-rotating
sinusoidal chaotic behavior in periodic units
below and over a boundary number of around3000. The reflection component of S matrix,
S11(n), shows a quasi-periodic torus portrait in
the complex plane of Fig.5 (b). Since the torus
is an attractor of three-dimensional phasespace, it manifests as macroscopic resonant
tunneling of an ensemble of electrons throughFig.4 Configuration pattern of a nanometer-sized RC
circuit constructed by a pair of pointed, negatively
charged, distorted-icosahedral Zr5Ni5Nb3 clusterchannels connecting both perpendicular cluster
array reservoirs which are surrounded by capacitors
shifting electron potential. Note: the Voronoi-type
polyhedral portion is not shown.

Fig.5 Transport properties of δ-function model for
a=0.79, d=0.23 and F=3.52x10 -6. Tn; Insert: cluster
array with a size of 0.55 nm and boundary distanceof 0.23 nm; (b) S11(n) =ρnexp(iθn);multi-barriers. Strictly speaking, the component
wanders in a chaotic way in the torus space.
Thus we believe that these results successfully
explain the macroscopic resonant tunneling in
Ni-Nb-Zr-H glassy alloys
References[1] M.Fukuhara, and A.Inoue, Appl.Phys.Lett., 97, 243108(2010).
[2] M.Fukuhara, A.Kawashima,S.Yamaura and A.Inoue,
Appl.Phys.Lett., 90, 20311 (2007)[3] M.Fukuhara and A.Inoue, J.Appl.Pjys., 105, 063715(2009).
[4] M.Fukuhara, N.Fujima, H.Oji, A.Inoue and S.Emura, J.
Alloy Comp., 497, 182 (2010).
[5] M.Fukuhara, M.Seto and A.Inoue, Appl.Phys.Lett., 96,043103 (2010).
[6] M.Fukuhara and M.Ban, Chaos, 20, 033107 (2010).

Keywords: amorphous, electronic material, nanocluster
M.Fukuhara(Adv. Mater. Devel.& Integ. of Novel Struct. Met. and Inorg. Mat)
E-mail: Fukuhara@imr.tohoku.ac.jp
URL: http://www.xraylab.imr.tohoku.ac.jp

IMR KINKEN Research Highlights 201164

Electronic Materials

Conversion of Conduction Carriers from Holes to Electrons
in YBa2Cu3Oy Films Using an Electrochemical Technique.

A

strong reduction of holes and an accumulation of electrons in YBa2Cu3Oy films were madepossible at room temperature by using an electrochemical doping technique. Using this technique,we succeeded in observing the superconductor-to-insulator transition in the hole-doped region,and in converting the sign of the conduction carriers from holes to electrons. Eventually we obtaineda ‘metallic’ electron-doped YBCO film with the carrier density of 2.5 × 1020 cm -3 and the resistivity
below 40 mΩcm.

Counter
Electrode(Pt)
V EC
Substrate(SrTiO3)IMR KINKEN Research Highlights 201120

as grown(ttotal = 0 h)

as grown: YBa2Cu3O6.45
V EC = 1.0 V

2.0 h
4.0 h
6.0 h
8.0 h
9.5 h
10.5 h11.5 h12.5 h13.5 h
14.5 h15.5 h10Electrolyte

Sample(YBCO)

Fig. 1: Schematic drawing for the concept of theelectrochemical reaction.

65

electric-field-induced superconductivity with
the EDL structure has been reported [8,9]. On
the other hand, when VEC is higher, as is the
case
in
this
study,aconventional
electrochemical
reaction
occurs.
Theadvantage of this reaction is that it can be
performed at a low temperature, and offerdifferent reaction routes from the usual heat
treatment for controlling materials.
The samples used in this work were c-axis
oriented YBCO films with a thickness of 50 nm
grown by rf magnetron sputtering on the (100)surfaces of SrTiO3. As the starting materials, weprepared films with various y (oxygen contents;
doping levels) [4]. In the electrolyte of KClO4/
PEO or Cs/PEO (where PEO = polyethyleneoxide), we repeated the sequence of (i)applying the voltage VEC between the YBCO
film and the Pt electrode for a time tEC at 300 K,(ii) measuring the temperature dependence ofthe resistivity ρ(T), and (iii) measuring the Hall
coefficient RH at 100 K.
Fig. 2 shows the typical evolution of ρ(T)with the total reaction time ttotal = ΣtEC when yof the initial sample was ~6.45 (Tc = 54K) andVEC =1.0 V was applied in KClO4/PEO electrolyte.

ρ (10-3 Ω cm)

High transition temperature (high-Tc)
superconductivity in cooper oxides appears
when the parent Mott insulators are dopedwith holes (p-type carriers) or electrons (n-typecarriers). However, p-type and n-type cooperoxides in most cases differ not only in their
doping dependence of magnetic andsuperconducting transitions but also incrystallographic structure [1-3]. For precise
comparison with the theories for the origin of
high-Tc superconductivity, a wide-range controlof the carrier density, which contains the signchange of carriers, in an identical parent
material is highly desired. Here, we demonstrate
the first realization of an n-type metallic state inan YBa2Cu3Oy (YBCO) film, which is known tobe typical p-type superconductor with amaximum Tc ~ 90 K [4].
The standard methods for carrier control in
the high-Tc copper oxides are chemical
substitution and variation of oxygen content
with heat treatment [5,6]. For further expansionof the doping region, we employed anelectrochemical technique [4], as is shown in
Fig. 1. In this technique, the sample and counterelectrode (typically Pt) are immersed in anelectrolyte and the strong electric field
between the sample and the ions moved at
the surface region by the voltage VEC plays animportant role. When VEC is smaller than athreshold value, the ions at the sample surface
form an electric double layer (EDL), whichfunctions as an effective capacitor [7]. Recently,

0
0

100

200

300

T (K)
Fig. 2: Variation of the temperature dependence ofresistivity ρ(T) with total reaction time ttotal in KClO4/
PEO electrolyte for the sample with the as-growncomposition of YBa2Cu3O6.45 (Tc = 54 K).

IMR KINKEN Research Highlights 2011

106
105

ρ (10-3 Ω cm)

The remarkable features are the increase in ρ
in the normal state by more than one order ofmagnitude and the decrease in Tc followed by
the superconductor-to-insulator transition.
Generally, such behaviour is associated withthe reduction in the hole concentration.
Indeed, a decrease in 1/eRH(100K), giving a
relative measure for the carrier concentration,with ttotal was observed.
Using this strong reduction process, we
tried to realize n-type YBCO. Fig. 3 shows thevariation of ρ(T) as a function of ttotal when y of
the initial sample was ~6.2 (heavily
underdoped) and VEC =1.0 V was applied in
CsClO4/PEO. The as-grown sample exhibits asemiconductive behaviour of ρ(T) with positive
RH. With increasing ttotal up to 4 h, ρ increased
significantly and became beyond the extent ofour measurement. With a further increase of
ttotal, we observed a remarkable recovery of
the electric conduction. It was also checkedthat 1/eRH(100K) decreases with ttotal andcrosses zero-doped point to a negative value
at ttotal > 4 h. These results strongly indicate thatthe carrier type changed from p-type to n-typearound ttotal = 4 h. ρ continued to decrease by
more than four orders of magnitude, andeventually at ttotal = 250 h, we obtained ann-type metallic YBCO film with 1/eRH(100K) =2.5 × 1020 cm -3.
Based on the data of X-ray diffraction and
SIMS measurements [4], we conclude that the
reduction of the oxygen content is the mostplausible mechanism of the electrochemical
doping, and that especially in the n-type
region, YBCO films with y < 6, which cannot be
attained in the usual heat treatment, isprepared. The present results provide firm
evidence that the electrochemical method
can pave a way toward ambipolar High-Tc
copper oxide superconductors, which havenever been synthesized yet.

as grown(ttotal = 0 h)

as grown: YBa2Cu3O6.2
V EC = 1.0 V

4h
6h
8h
16 h
24 h
40 h
66 h
116 h
174 h
253 h

104
103
102
101
0

100

200

300

T (K)
Fig. 3: Variation of the temperature dependence ofresistivity ρ(T) with total reaction time ttotal in
CsClO4/PEO electrolyte for the sample with theas-grown composition of YBa2Cu3O6.2.

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Keywords: electrical resistivity, doping, superconducting
T. Nojima (Laboratory of Low Temperature Materials Science)
E-mail: nojima@imr.tohoku.ac.jp

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