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We have calculated the thermoelectric conductivity tensor $\varepsilon_{ij}$ and the thermal conductivity tensor $\lambda_{ij}$ of a unidirectional lateral superlattice (ULSL) ($i,j = x,y$, with the $x$-axis aligned to the principal axis of the ULSL), %, given as the first- and the second-order moments, employing based on the asymptotic analytic formulas of the electrical conductivity tensor $\sigma_{ij}$ in the literature valid at low magnetic fields where large numbers of Landau levels are occupied. With the resulting analytic expressions, we clarify the conditions for the Mott formula (Wiedemann-Franz law) to be applicable with high precision to $\varepsilon_{ij}$ ($\lambda_{ij}$). We further present plots of the commensurability oscillations $\delta\varepsilon_{ij}$, $\delta\lambda_{ij}$, $\delta\kappa_{ij}$, and $\delta S_{ij}$ in $\varepsilon_{ij}$, $\lambda_{ij}$, (an alternative, more standard definition of) the thermal conductivity tensor $\kappa_{ij}$, and the thermopower tensor $S_{ij}$, calculated using typical parameters for a ULSL fabricated from a GaAs/AlGaAs two-dimensional electron gas (2DEG). Notable features of the $\delta S_{ij}$ are (i) anisotropic behavior ($\delta S_{xx} \ne \delta S_{yy}$) and (ii) the dominance of the $xy$ component over the other components ($|\delta S_{xy}| \gg |\delta S_{yx}|, |\delta S_{xx}|, |\delta S_{yy}|$). The latter clearly indicates that the two Nernst coefficients, $S_{xy}$ and $S_{yx}$, can be totally different from each other in an anisotropic system. Both (i) and (ii) are at variance with the previous theory and are attributable to the inclusion of a damping factor due to the small-angle scattering characteristic of GaAs/AlGaAs 2DEGs, which have not been taken into consideration in $\delta S_{ij}$ thus far., Comment: 14 pages, 9 figures, Title and Introduction altered to make the main point of the paper clearer. Minor revisions throughout the paper. Some additions to the IV Discussion. Explicit energy dependence of the zero-temperature conductivity newly presented in the Appendix

We report an experimental setup for simultaneously measuring specific heat and thermal conductivity in feedback-controlled pulsed magnetic fields of 50 msec duration at cryogenic temperatures. A stabilized magnetic field pulse obtained by the feedback control, which dramatically improves the thermal stability of the setup and sample, is used in combination with the flash method to obtain absolute values of thermal properties up to 37.2 T in the 2 K to 16 K temperature range. We describe the experimental setup and demonstrate the performance of the present method with measurements on single crystal samples of the geometrically frustrated quantum spin-dimer system SrCu$_2$(BO$_3$)$_2$. Our proof-of-principle results show excellent agreement with data taken using a standard steady-state method, confirming the validity and convenience of the present approach., Comment: 9 pages, 7 figures

During the standard ac lock-in measurement of the resistance of a two-dimensional electron gas (2DEG) applying an ac current $I = \sqrt{2} I_0 \sin(\omega t)$, the electron temperature $T_e$ oscillates with the angular frequency $2 \omega$ due to the Joule heating $\propto I^2$. We have shown that the highest ($T_\mathrm{H}$) and the lowest ($T_\mathrm{L}$) temperatures during a cycle of the oscillations can be deduced, at cryogenic temperatures, exploiting the third-harmonic (3$\omega$) component of the voltage drop generated by the ac current $I$ and employing the amplitude of the Shubnikov-de Haas oscillations as the measure of $T_e$. The temperatures $T_\mathrm{H}$ and $T_\mathrm{L}$ thus obtained allow us to roughly evaluate the thermal conductivity $\kappa_{xx}$ of the 2DEG via the modified 3$\omega$ method, in which the method originally devised for bulk materials is modified to be applicable to a 2DEG embedded in a semiconductor wafer. The $\kappa_{xx}$ thus deduced is found to be consistent with the Wiedemann-Franz law. The method provides a convenient way to access $\kappa_{xx}$ using only a standard Hall-bar device and the simple experimental setup for the resistance measurement., Comment: Main text: 10 pages, 5 figures. Supplementary material: 4 pages, 6 figures, minor revision

We have observed commensurability oscillations (CO) in the Hall resistance $R_{yx}$ of a unidirectional lateral superlattice (ULSL). The CO, having small amplitudes ($\sim$ 1 $\Omega$) and being superposed on a roughly three-orders of magnitude larger background, are obtained by directly detecting the difference in $R_{yx}$ between the ULSL area and the adjacent unmodulated two-dimensional electron gas area, and then extracting the odd part with respect to the magnetic field. The CO thus obtained are compared with a theoretical calculation and turn out to have the amplitude much smaller than the theoretical prediction. The implication of the smaller-than-predicted CO in $R_{yx}$ on the thermoelectric power of ULSL is briefly discussed., Comment: 9 pages, 5 figures, minor revision

Spin and orbital freedoms of electrons traveling on spin-resolved quantum Hall edge states (quantum Hall ferromagnets) are maximally entangled. The unitary operations on these two freedoms are hence equivalent, which means one can manipulate the spins with non-magnetic methods through the orbitals. If one takes the quantization axis of spins along the magnetization axis, the zenith angle is determined by the partition rate of spin-separated edges while the azimuth angle is defined as the phase difference between the edges. Utilizing these properties, we have realized electrically controlled unitary operation on the electron spins on the quantum Hall ferromagnets. The zenith angle of the spin was controlled through the radius of gyration at a corner by means of applying voltage to a thin gate placed at one edge. The subsequent rotation in the azimuth angle was controlled via the distance between the edge channels also by a gate voltage. The combination of the two operations constitutes the first systematic electric operation on spins in the quantum Hall edge channels.

We report a cryogenic transimpedance amplifier (TA) suitable for cross-correlation current-noise measurements. The TA comprises homemade high-electron-mobility transistors with high transconductance and low noise characteristics, fabricated in an AlGaAs/GaAs heterostructure. The low input-referred noise and wide frequency band of the TA lead to a high resolution in current-noise measurements. The TA's low input impedance suppresses unwanted crosstalk between two distinct currents from a sample, justifying the advantage of the TA for cross-correlation measurements. We demonstrate the high resolution of a TA-based experimental setup by measuring the shot noise generated at a quantum point contact in a quantum Hall system.

We have performed low-temperature magnetotransport measurements on epitaxial graphene composed of domains of single-layer and bilayer areas simultaneously present on SiC(0 0 0 1). Positive magnetoresistance that tends to saturate at high magnetic fields is observed for longitudinal component , while the Hall resistance exhibits sublinear behaviour. The lineshapes for both and can be accounted for extremely well by the semiclassical magnetotransport model incorporating two types of carriers conducting in parallel. Two sets of mobilities and densities, corresponding to single-layer and bilayer regions, respectively, can be obtained by fitting the experimental traces to the two-carrier model formulae. From the carrier densities, in turn, the ratio of the single-layer to bilayer areas can be deduced, assuming the alignment of the Fermi levels in the two types of areas with differing dispersion relations and Dirac point energies. The ratio of areas thus obtained is consistent with the ratio directly ob...

Natural two-dimensional electron gas (2DEG) on the surface of n-type InAs has been used for studying superconductor-semiconductor junctions. For high mobility and easiness in sample design, however, 2DEGs in quantum wells are more favorable. Even at the edges of such thin InAs films for quantum wells, one-dimensional (1D) electrons may be formed and give rise to various phenomena in transport. Such conduction through the 1D edge states is of interest in itself, but on the other hand, may give some artifacts in the study of superconductor-semiconductor junctions. Hence it is important to separate the 1D conduction through the accumulation edges. Here we report such edge states carry supercurrent and affect quantum Hall effect. The experiment was carried out in a strip of InAs two-dimensional electron gas with a quantum point contact (QPC) on one of the edges. Supercurrent was observed in low magnetic field, and the critical current decreased when the conductivity of QPC was reduced. In high magnetic field, the resistance showed dip structures in the transition region between quantum Hall plateaus. We infer that these characteristic transport properties come from negative band bending of InAs, which leads to charge accumulation on edges.

In the last century the band-gap engineering including modulation doping technique, largely expanded the possibility of solid state devices. The concept of topology came into our view in the beginning of the present century. The first claim of topological insulator utilized quantum well to kill the topological anomaly in HgTe. Synthesis of artificial topological material is then one of the next target.

The Josephson supercurrent in ballistic superconductor/normal metal/superconductor (SNS) junctions is carried through the Andreev bound states (ABS) formed in the normal conductor. These virtual states of traversing electrons and holes are modulated but not destroyed by time-reversal symmetry conserving perturbation. However, we found that the critical current of the Nb/InAs/Nb junctions are strongly suppressed by the spin current, which keeps time-reversal symmetry.

Magnetic field and spin-orbit interaction (SOI) are representative freedoms that can be entangled to electron spins. In experiments, while magnetic field strong enough for the control of spin is difficult to be localized onto the area for single spins such as quantum dots, the orbit can be localized to narrow regions with mesoscopic structures. Hence for quantum information manipulation borne by electron spins, the control through SOI has an advantage in that sense.

Significance Weyl semimetals carry the promise of quantum electronic applications. Theoretical calculations have suggested that Luttinger semimetals recently found in iridium oxides may be a suitable group of materials where such topological phases, including Weyl semimetal state, may be found. For example, praseodymium-iridium oxide, P r 2 I r 2 O 7 , is one such Luttinger semimetal that can be tuned by perturbations such as strain into a Weyl semimetal state. Despite theoretical predictions of Weyl semimetal states in P r 2 I r 2 O 7 crystals, experimental proof remains elusive due to the difficulty of applying sufficient mechanical strain on single crystals. Our study provides strong experimental evidence that a Weyl semimetal state may indeed appear in strained pyrochlore iridium oxide films, opening a way to explore topological phases.

Atomically thin (or few-layer) two-dimensional transition metal dichalcogenide (TMDC) materials have various unique optoelectronic properties, which bring advantages for application to flexible solar cells and photodetectors, by bandgap engineering via van der Waals hybridization. TMDCs have crystal phase structures, such as the 2H semiconducting phase and the 1T (or 1T′) metallic phase. Recently, we demonstrated the creation of few-atom-layer 1T-metal/2H-semiconductor molybdenum disulphide (MoS2) lateral Schottky junctions by using electron beam (EB) irradiation and revealed their unique optoelectronic properties. However, the 1T phase is metastable, whereas the 1T′ phase is more stable and useful for various applications. Here, we create a few-layer 1T′-metal phase MoS2 by laser beam irradiation, which is a simpler, convenient, and low-cost method compared to EB irradiation. We observe unique optoelectronic features of the few-atom-layer 1T′-metal/2H-semiconductor lateral Schottky junctions in reverse bias voltage regions, such as an effective barrier height of ∼0.15 eV, highly efficient photogeneration ratios (>20%), and high sensitivity to photoirradiation angles without degradation for one month. These properties show great promise for application to highly efficient, flexible, and semitransparent photodetectors and solar cells with long-term reliability.

We provide evidence for spin-triplet electron pairing in proximity-induced superconductivity in a ferromagnetic semiconductor (In,Fe)As. As discovered in half-metallic materials, an extraordinarily long proximity range is observed. More surprising is a very strong concentration of supercurrent to the edges of the superconducting region, which is deduced from the extremely persistent oscillation of the critical current vs magnetic field. The maxima of the critical current appear not at the zero magnetic flux but at around the maximum magnetic disorder, reflecting the connectivity between the spin-triplet and singlet pairings. These spin-triplet natures in proximity superconductivity also reveal ferromagnetic properties of (In,Fe)As.

Identifying the two-dimensional (2D) topological insulating (TI) state in new materials and its control are crucial aspects towards the development of voltage-controlled spintronic devices with low-power dissipation. Members of the 2D transition metal dichalcogenides have been recently predicted and experimentally reported as a new class of 2D TI materials, but in most cases edge conduction seems fragile and limited to the monolayer phase fabricated on specified substrates. Here, we realize the controlled patterning of the 1T′ phase embedded into the 2H phase of thin semiconducting molybdenum-disulfide by laser beam irradiation. Integer fractions of the quantum of resistance, the dependence on laser-irradiation conditions, magnetic field, and temperature, as well as the bulk gap observation by scanning tunneling spectroscopy and theoretical calculations indicate the presence of the quantum spin Hall phase in our patterned 1T′ phases, The work carried out at Aoyama Gakuin University was partly supported by a grant for private universities and a Grant-in-Aid for Scientific Research (JP15K13277) awarded by MEXT. The work at the University of Tokyo was partly supported by Grantin-Aid for Scientific Research (JP17K05492, JP18H04218 and JP19H00652). J. J. P. and S. P. acknowledge Spanish MINECO through Grant No. FIS2016-80434-P, the Fundación Ramón Areces, the María de Maeztu Program for Units of Excellence in R&D (MDM-2014- 0377), the Comunidad Autónoma de Madrid through NANOMAGCOST Program, and the European Union Seventh Framework Programme under Grant Agreement No. 604391 Graphene Flagship. S. P. acknowledges the computer resources and assistance provided by the Centro de Computación Científica of the Universidad Autónoma de Madrid. S. P. was also supported by the VILLUM FONDEN via the Center of Excellence for Dirac Materials (Grant No. 11744). D. M. and E. G.-M. gratefully acknowledge support from the Graphene Flagship Graphene Core2 Contract No. 785219. E. G.-M also acknowledges IJCI-2017-32297 from Spanish MINECO/AEI

The introduction of spin-orbit interactions (SOIs) and the subsequent appearance of a two-dimensional (2D) topological phase are crucial for voltage-controlled and zero-emission energy spintronic devices. In contrast, graphene basically lacks SOIs due to the small mass of the carbon atom, and appropriate experimental reports for SOIs are rare. Here, we control small-amount (cover ratios < 8%) random decoration of heavy nanoparticles [platinum (Pt) or bismuth telluride (Bi2Te3)] onto mono-layer graphene by developing an original nanoneedle method. X-ray photoelectron spectra support low-damage and low-contamination decoration of the nanoparticles, suggesting the presence of Bi-C and Te-C coupling orbitals. In the samples, we find particle-density-dependent non-local resistance (RNL) peaks, which are attributed to the (inverse) spin Hall effect (SHE) arising from SOI with energies as large as about 30 meV. This is a larger value than in previous reports and supported by scanning tunneling spectroscopy. The present observation should lead to topological phases of graphene, which can be introduced by random decoration with controlled small amounts of heavy nanoparticles, and their applications.

Realization of the quantum-spin-Hall effect in graphene devices has remained an outstanding challenge dating back to the inception of the field of topological insulators. Graphene's exceptionally weak spin-orbit coupling -stemming from carbon's low mass- poses the primary obstacle. We experimentally and theoretically study artificially enhanced spin-orbit coupling in graphene via random decoration with dilute Bi2Te3 nanoparticles. Remarkably, multi-terminal resistance measurements suggest the presence of helical edge states characteristic of a quantum-spin-Hall phase; the magnetic-field and temperature dependence of the resistance peaks, X-ray photoelectron spectra, scanning tunneling spectroscopy, and first-principles calculations further support this scenario. These observations highlight a pathway to spintronics and quantum-information applications in graphene-based quantum-spin-Hall platforms.

Electron spin transport in graphene is extremely sensitive to foreign atoms and ripples of the SiO2 substrate. Indeed, the observed spin diffusion- and relaxation-length (time) were smaller than theoretically expected owing to this, although a large spin diffusion length has been recently realized in graphene synthesized on a SiC substrate. It is, thus, crucial to enhance the spin phase coherence and spin diffusion (relaxation) length of a graphene/SiO2 substrate particularly for future graphene spintronics. One of the approaches to realize this is the investigation of the spin phase in the phase interference phenomena of electron spin waves (such as weak localization (WL)) and its correlation with the spin–orbit-interaction (SOI). However, their coexistence in graphene is difficult to be realized experimentally. Here, we have realized the extremely light hydrogenation of a graphene surface (≪0.1%) on SiO2 by precisely controlling the amount of electron beam (EB) irradiation to a specific EB resist including hydrogen atoms, treated on graphene. It allows the coexistence of WL and the SOI. We find spin phase protection (suppression of dephasing) of the electron-spin-waves in the WL on temperature and external magnetic-field dependence in the graphenes with hydrogenation volumes (NH) as small as 0.06%. As an origin, correlation of the WL with a Rashba-type SOI, which can be introduced by out-of-plane symmetry breaking due to the formation of sp3 bonds derived from the small NH, is discussed. The present finding in lightly hydrogenated graphene must be beneficial for graphene spintronics, which requests a long spin diffusion- and coherence-length. It will realize a possible 2D-topological insulating state in graphene.

We have investigated microwave transmission through the edge of quantum Hall systems by employing a coplanar waveguide (CPW) fabricated on the surface of a GaAs/AlGaAs two-dimensional electron gas (2DEG) wafer. An edge is introduced to the slot region of the CPW by applying a negative bias $V_\mathrm{g}$ to the central electrode (CE) and depleting the 2DEG below the CE. We observe peaks attributable to the excitation of edge magnetoplasmons (EMP) at a fundamental frequency $f_0$ and at its harmonics $i f_0$ ($i$ = 2, 3,...). The frequency $f_0$ increases with decreasing $V_\mathrm{g}$, indicating that EMP propagates with higher velocity for more negative $V_\mathrm{g}$. The dependence of $f_0$ on $V_\mathrm{g}$ is interpreted in terms of the variation in the distance between the edge state and the CE, which alters the velocity by varying the capacitive coupling between them. The peaks are observed to continue, albeit with less clarity, up to the regions of $V_\mathrm{g}$ where 2DEG still remains below the CE., Comment: 8 pages, 8 figures

Pristine topological insulators (TIs) with no carrier doping principally suffer from a lack of magnetic ordering. We create a nanomesh structure, a honeycomblike array of hexagonal nanopores with extremely high density, on a nondoped two-dimensional (2D) thin TI (Bi2Te3). We observe antiferromagnetism (AFM) in completely hydrogen-terminated nanomeshes, while it is eliminated by ferromagnetism (FM) in completely O-terminated nanomeshes. Interplay between the AFM and FM is observed in partially O-terminated nanomeshes. As a possible origin for the observed magnetisms, Kramers doublets, which may exist around nanopore edges, for AFM and those eliminated by FM spin alignment due to O-Te coupling along pore edges are discussed based on the structure and annealing dependence of the magnetisms. The nondoped 2D nanomesh formed on a TI will lead to new avenues of research on topological magnetisms and spintronics.

We have investigated the spatial distribution of the electron temperature generated in a two-dimensional electron system (2DES) subjected to a perpendicular magnetic field. We measure thermoelectric voltages between Ohmic contacts located at the end of the voltage-probe arms of a Hall bar fabricated from a GaAs/AlGaAs 2DES wafer, immersed in the mixing chamber of a dilution refrigerator held at 20 mK. Magneto-oscillations due to the Landau quantization are examined for the thermoelectric voltages between the contact pairs straddling the main bar (arrangement to measure the transverse component $V_{yx}$), and between the pairs located along the same side of the main bar (arrangement for the longitudinal component $V_{xx}$). For the former arrangement, the oscillation amplitude diminishes with the distance from the heater. For the latter arrangement, the pair on one side exhibits much larger amplitude than the pair on the opposite side, and the relation becomes reversed by inverting the magnetic field. The behaviours of the oscillation amplitude are qualitatively explained by the spatial distribution of the electron temperature numerically calculated taking into consideration the thermal diffusion into the voltage contacts and the electron-phonon interaction. For both arrangements, the oscillations are shown to derive predominantly from the transverse (Nernst) component, $S_{yx}$, of the thermopower tensor. The calculation also reveals that the voltage probes, introducing only minor disturbance at zero magnetic field, substantially reduce the temperature once a magnetic field is applied, and the thermoelectric voltages generated at the voltage arms account for a significant part of the measured voltages., Comment: 17 pages, 18 figures (including Supplementary data), minor corrections

One of the mechanisms to cause the Josephson effect between two superconductors through a normal conductor is virtual traversal of electrons and holes through Andreev bound states. The spin Hall effect, on the other hand, disturbs the formation of Cooper pairs at the interfaces through the discrimination of up and down spins. We have found that the Josephson effect is strongly suppressed with the spin Hall effect caused by an electric current crossing transversally through the normal strip. This result proposes a spintronic control of the Josephson effect in superconducting devices. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have found large effect of mechanical modifications on the magnetic anisotropy in thin films of (Ga,Mn)As on GaAs substrates and on (In,Ga)As buffer layers. Such modifications are simply brought about with etching two-dimensional films into thin-wires, which process partly releases compressive or tensile strains in the thin films. The effect of such mechanical processing depends on the etching depth, through which we can control the magnetic anisotropy. Uniaxial in-plane magnetic anisotropy has disappeared with releasing the strain, which result suggests that the uniaxial anisotropy is due to some extrinsic effect in one hand, that the dimer formation model is not the case on the other hand.

In a superconductor–normal–superconductor (SNS) structure, the effect of transverse current across the normal part on the transport through Andreev bound states (ABSs) has been examined. Here, a ballistic InAs two-dimensional electron system (2DES) is used as the N-layer to form ABSs. At the same time the 2DES has strong spin–orbit interaction, hence there should emerge the spin-Hall effect associated with the transverse current. We have observed strong reduction of characteristic oscillation in the conductance versus bias voltage, which may be attributed to spin polarization due to the spin-Hall effect.

The highly successful Dirac equation predicts peculiar phenomena such as Klein tunnelling and Zitterbewegung (ZB) of electrons. From its conception by Erwin Schrödinger, ZB has been considered key in understanding relativistic quantum mechanics. However, observing the ZB of electrons has proved difficult, and instead various emulations of the phenomenon have been proposed producing several successes. Concerning charge transport in semiconductors and graphene, expectations were high but little has been reported. Here, we report a surprisingly large ZB effect on charge transport in a semiconductor nanostructure playing “flat pinball”. The setup is a narrow strip of InAs two-dimensional electron gas with strong Rashba spin–orbit coupling. Six quantum point contacts act as pinball pockets. In transiting between two contacts, ZB appears as a large reproducible conductance fluctuation that depends on the in-plane magnetic field. Numerical simulations successfully reproduced our experimental observations confirming that ZB causes this conductance fluctuation.

et al., The formation and control of a room-temperature magnetic order in two-dimensional (2D) materials is a challenging quest for the advent of innovative magnetic- and spintronic-based technologies. To date, edge magnetism in 2D materials has been experimentally observed in hydrogen (H)-terminated graphene nanoribbons (GNRs) and graphene nanomeshes (GNMs), but the measured magnetization remains far too small to allow envisioning practical applications. Herein, we report experimental evidences of large room-temperature edge ferromagnetism (FM) obtained from oxygen (O)-terminated zigzag pore edges of few-layer black phosphorus (P) nanomeshes (BPNMs). The magnetization values per unit area are ~100 times larger than those reported for H-terminated GNMs, while the magnetism is absent for H-terminated BPNMs. The magnetization measurements and the first-principles simulations suggest that the origin of such a magnetic order could stem from ferromagnetic spin coupling between edge P with O atoms, resulting in a strong spin localization at the edge valence band, and from uniform oxidation of full pore edges over a large area and interlayer spin interaction. Our findings pave the way for realizing high-efficiency 2D flexible magnetic and spintronic devices without the use of rare magnetic elements., This work at Aoyama Gakuin was partly supported by a Grant-in-aid for Scientific Research (Basic research A: 24241046 and Challenging Exploratory Research: 15K13277) and grant for private University in MEXT and AOARD grant (No. 135049) in U.S. Air Force Office of Scientific Research. The work in the University of Tokyo was also supported by Grant-in-Aid for Scientific Research on Innovative Area, “Nano Spin Conversion Science” (No. 26103003), and by Grants (Nos. 25247051 and 15K17676). S. R. acknowledges Funding from the Spanish Ministry of Economy and Competitiveness and the European Regional Development Fund (No. FIS2015-67767-P (MINECO/FEDER)), the Secretaria de Universidades e Investigación del Departamento de Economía y Conocimiento de la Generalidad de Cataluña, and the Severo Ochoa Program (MINECO, No. SEV-2013-0295).

We demonstrate detection of spin polarization in a quantum wire utilizing a side coupled quantum dot. By applying in-plane magnetic fields, spin split conducting channels are formed and produce spin polarization in the wire. The magnetic fields also create Zeeman split spin levels in the dot and these states are used as a spin polarization detector. Such kind of spin polarization detection with a side coupled quantum dot will operate even in the zero magnetic field by using two-electron singlet and triplet levels.

Core/shell heterostructure nanowires are one of the most interesting mesoscopic systems potentially suitable for the study of quantum interference phenomena. Here, we report on experimental observations of both the Aharonov-Bohm (h/e) and the Altshuler-Aronov-Spivak (h/2e) oscillations in radial core/shell (In2O3/InOx) heterostructure nanowires. For a long channel device with a length-to-width ratio of about 33, the magnetoresistance curves at low temperatures exhibited a crossover from low-field h/2e oscillation to high-field h/ e oscillation. The relationship between the oscillation period and the core width was investigated for freestanding or substrate-supported devices and indicated that the current flows dominantly through the core/shell interface.

Superconducting wire network subjected to a uniform and a checkerboard-patterned magnetic field is investigated. The checkerboard field is created by an array of ferromagnetic dots placed on top of the superconducting network. The superconducting transition at α = 1 / 2 (α being the frustration parameter) is studied in detail by measuring the current–voltage (I–V) characteristics. The vortex configuration in this case is doubly degenerate. We investigated the change of the transition according to lifting this degeneracy by applying the checkerboard field. The results are consistent with the scenario proposed by Korshnov [S.E. Korshnov, Phys. Rev. Lett. 88 (2002) 167007].

We report a study of the Aharonov–Bohm (AB) type oscillation of weak field magnetoresistance in an anti-dot lattice (ADL) of a two-dimensional hole system in the presence of strong spin–orbit interaction. The Fourier transform of the oscillations reveals non-monotonic evolutions with the different range of harmonic component, which are indicative of the existence of additional phase factors. A simulation considering Berry’s phase and the phase arising from the spin–orbit shift in the momentum space shows qualitative agreement with the experiment.

We have developed a novel technique for detection of spin polarization with a quantum dot weakly coupled to the objective device. The disturbance to the object in this technique is very small since the detection is performed through sampling of single electrons in the object with very slow rate. We have applied the method to a quantum point contact (QPC) under a spin-orbit interaction. A high degree of spin polarization in the vicinity of the QPC was detected when the conductance stayed on a plateau at a half of the unit conductance quantum ($G_{\rm q}/2\equiv e^2/h$), and also on another plateau at $2e^2/h$. On the half-quantum plateau, the degree of polarization $P$ decreased with the bias source-drain voltage of the QPC while $P$ increased on the single-quantum plateau, manifesting that different mechanisms of polarization were working on these plateaus. Very long spin relaxation times in the detector quantum dot probably due to dynamical nuclear spin polarization were observed. Anomalous decrease of $P$ around zero-bias was observed at a Kondo-like resonance peak., Comment: 8 pages

We have observed hysteretic behavior of magnetoresistance with slow dynamics in a GaAs/AlGaAs two-dimensional hole system (2DHS) in quantum Hall regime. The relaxation towards the stable state exhibits a distinct change in behaviour between the filling ranges ν > 1 and ν < 1, suggesting a spin-related mechanism. The relaxation time scale on the order of an hour was observed at the lowest temperature. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report resistively-detected nuclear magnetic resonance studies on GaAs/AlGaAs two-dimensional electron systems in the quantum Hall regime. A peculiar ‘dispersive’ lineshape that occurs in the vicinity of Landau level filling ν = 1 is found to evolve into a simple dip structure with increasing |ν – 1|. The nuclear spin-lattice relaxation rate exhibits distinct temperature dependences in the respective regions. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report experimental results and a theoretical model of the Fano-Kondo effect in a quantum dot embedded in an Aharonov-Bohm (AB) ring. A clear Fano-Kondo lineshape is observed in the coherent conductance as a function of the dot gate voltage. And the phase shift locking to π /2, which is a touchstone of the SU(2) Kondo theory is clearly observed in the middle of Kondo valleys. The variation of the lineshape versus the magnetic flux piercing the ring is similar to that expected for a simple double-slit system rather than that under “phase rigidity” for two-terminal quantum interference devices. We have extended the theoretical model of single-channel AB ring to that of a multi-channel case and succeeded to explain the above puzzling results. This demonstrates that useful information can be extracted even from transport through two terminal devices. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated Aharonov–Bohm-type oscillation in the quantum Hall plateau transition region in three types of square arrays of antidots; a large ( 50 × 160 antidots) array, a small ( 5 × 10 antidots) array, and the sample with antidots placed only near the side edges. The temperature dependence of the amplitude confirmed that the oscillation originates from the fine structure in the density of single particle states circumnavigating around each antidot. In addition, we have also observed Altshuler–Aronov–Spivak oscillation near zero magnetic field in square arrays of antidots.

We have investigated the scaling behavior of the quantum Hall plateau transition in double quantum well systems with different interlayer tunneling strengths. The scaling behavior of the localization property is found to be similar between the case when the relevant Landau level (LL) is non-degenerate and the case when two LLs associated with the two layers are accidentally degenerate. In both cases, the scaling exponent κ∼0.4 close to the canonical value is obtained, and it is unaffected by the in-plane magnetic field which changes the interlayer tunneling strength.

Magnetoresistance (MR) of GaAs two-dimensional hole systems (2DHSs) is investigated in the strongly insulating regime. By rotating the samples, the total magnetic field B tot and the perpendicular component B ⊥ to the 2DHS are controlled independently. In the low- B ⊥ region, a large positive B ⊥ -dependence of the longitudinal resistivity ρ xx is observed when B tot is small, while it is replaced by a negative one when B tot is large. For higher B ⊥ , dips are observed in the ρ xx vs B ⊥ curve and the values of B ⊥ at the dip depend on B tot . The results are discussed both in the framework of single-particle localization and in the framework of Wigner crystallization.

The effect of localized spins on the quantum coherence in solids is discussed. A quantum dot with an odd number of electrons can be a model system for a localized spin. It is experimentally shown that a spin flip scattering by a quantum dot pulls the trigger of quantum decoherence. On the other hand, spin flip scattering is the basic process to construct the Kondo singlet state around a magnetic impurity. Through an interference effect of the Kondo state (the Fano–Kondo effect) in a side-coupled dot system, we show experimentally that the Kondo singlet state is quantum mechanically coherent. The analysis of the Fano–Kondo lineshape indicates the locking of the phase shift to π / 2 , which is in agreement with theoretical predictions. The Fano–Kondo effect is also observed in an Aharonov–Bohm ring, in which a quantum dot is embedded, and also indicates the phase shift locking to π / 2 .

We present experimental results and a model to solve the problem of "in-phase Coulomb peaks" observed in transport through a quantum dot. In a marginal region between Coulomb-blockade and open-dot, we have observed Fano-type interference through two energy levels inside the dot, which manifest themselves in two overlapped Coulomb-diamond-like structures in the excitation spectrum. One of the two levels is strongly coupled to the leads and the phase of traversing electrons is locked to it. We have detected the phase change at the vertices and the centers of the larger diamonds through the sign of the Fano's asymmetric parameters supporting the above deduction., 4 pages, 4 figures, to appear in J. Phys. Soc. Jpn. (December 15, 2004)

Three kinds of Aharonov–Bohm (AB)-type oscillation have been investigated in triangular antidot lattice fabricated from a GaAs/AlGaAs two-dimensional electron gas sample. The oscillation periods of Altshuler–Aronov–Spivak (AAS) effect and AB-type effect near zero magnetic field are determined by the unit cell area, whereas those of AB-type oscillations in the quantum Hall plateau transition regime are governed by the effective area of antidot. The evolution of the high-field AB-type oscillation as a function of gate voltage gives infomation on the profile of the self-consistent potential associated with compressible edge channels formed around antidot. The temperature dependences and decoherence mechanisms of the AAS and AB-type oscillations near zero magnetic field as well as the high-field AB-type oscillation are discussed.

We have studied transport in a narrow Hall bar subjected to a transversally modulated magnetic field in quantum Hall regime. Landau level spatially varies across the channel and suppresses back scattering between oppositely directed edge channels. The resistance suppressed by the "magnetic barrier" recovers with increasing temperature, showing thermally activated temperature dependence.

We have investigated universal conductance fluctuations (UCF) in a narrow channel of two-dimensional electron gas subjected to a gradient magnetic field with a vanishing average, generated from a cobalt strip deposited along the center line of the channel. We find that the fluctuation amplitude Δ G tends to be a larger value in the low-temperature limit and the characteristic field B c is less temperature sensitive, compared with those for UCF in a uniform magnetic field.

We have investigated magnetotransport behavior of a double quantum well system with weak interlayer tunneling. The scaling behavior of the localization property for the Landau levels associated with either layer is in agreement with earlier reports. For the case of Landau level coincidence between the two layers, an anomalous value of the exponent κ≈0.50 is observed. We have observed a peculiar resistance peak at B≃9T for which the corresponding Hall resistance remains unchanged on both sides of the resistance peak. Hysteretic transition has been found in the region of crossing of the LLs originating from different layers.