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In a hybrid system of topological insulator (TI)/superconductor (SC), the proximity-induced topological superconductivity is expected to appear at the interface. Here we propose and demonstrate that a TI/SC hybrid Bi2Te3/PdTe2 heterostructure serves as a platform for exploring topological superconductivity with various features: all made of tellurium compounds, epitaxial growth, and a small charge transfer interface. In the Bi2Te3/PdTe2 heterostructure films, we observe large nonreciprocal charge transport near the superconducting transition temperature under a transverse in-plane magnetic field. The observation indicates the interplay between the topological surface state and superconductivity, suggesting that the Bi2Te3/PdTe2 heterostructure is a candidate for a topological superconductor. Also observed is an unexpected sign reversal of the nonreciprocal coefficient when the in-plane magnetic field is slightly tilted toward the out-of-plane direction. The analysis reveals that the sign reversal occurs with the change of dominant vortex type, that is, the change from spontaneous vortices to external-field induced ones.

Spin-orbit coupling (SOC) is pivotal for various fundamental spin-dependent phenomena in solids and their technological applications. In semiconductors, these phenomena have been so far studied in relatively weak electron-electron interaction regimes, where the single electron picture holds. However, SOC can profoundly compete against Coulomb interaction, which could lead to the emergence of unconventional electronic phases. Since SOC depends on the electric field in the crystal including contributions of itinerant electrons, electron-electron interactions can modify this coupling. Here we demonstrate the emergence of SOC effect in a high-mobility two-dimensional electron system in a simple band structure MgZnO/ZnO semiconductor. This electron system features also strong electron-electron interaction effects. By changing the carrier density with Mg-content, we tune the SOC strength and achieve its interplay with electron-electron interaction. These systems pave a way to emergent spintronic phenomena in strong electron correlation regime and to the formation of novel quasiparticles with the electron spin strongly coupled to the density., Main Text (4 Figures), Supplementary notes (4 Figures)

The topological nature of the quantum anomalous Hall effect (QAHE) causes a dissipationless chiral edge current at the sample boundary1,2. Of fundamental interest is whether the chirality of the band structure manifests itself in charge transport properties. Here we report the observation of large non-reciprocal charge transport3 in a magnetic topological insulator, Cr-doped (Bi,Sb)2Te3. When the surface massive Dirac band is slightly carrier doped by a gate voltage, the edge state starts to dissipate and exhibits a current-direction-dependent resistance with a directional difference as large as 26%. The polarity of this diode effect depends on the magnetization direction as well as on the carrier type, electrons or holes. The correlation between the non-reciprocal resistance and the Hall resistance indicates that the non-reciprocity originates from the interplay between the chiral edge state and the Dirac surface state.

Electrons in conventional metals become less mobile under the influence of electron correlation. Contrary to this empirical knowledge, we report here that electrons with the highest mobility ever found in known bulk oxide semiconductors emerge in the strong-correlation regime of the Dirac semimetal of perovskite CaIrO3. The transport measurements reveal that the high mobility exceeding 60,000 cm2V−1s−1 originates from the proximity of the Fermi energy to the Dirac node (ΔE, Electron correlation normally makes electrons less mobile, but it is still not clear when correlation becomes very strong in Dirac semimetals. Here, Fujioka et al. report a very high electron mobility exceeding 60,000 cm2V−1s−1 in correlated Dirac semimetal of perovskite CaIrO3, due to the enhanced electron correlation nearby the Mott transition.

Superconductivity in the doped topological crystalline insulator ${\mathrm{Sn}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{Te}$ is studied by first-principles calculation based on superconducting density functional theory (SCDFT) and tunneling spectroscopy. By considering the spin-orbit coupling and frequency dependence of the screened Coulomb interaction in SCDFT, we succeed in reproducing the critical temperature of ${\mathrm{Sn}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{Te}$ quantitatively, in which the spin-orbit coupling is found to play an essential role. The leading gap function is a conventional $s$ wave with moderate anisotropy in $\mathbit{k}$ space, and we find that the subdominant odd-parity instability is significantly weaker than the $s$-wave instability. We perform tunneling spectroscopy measurement and confirm that the spectrum is consistent with the calculated gap function.

We have studied the fundamental properties of weak-link ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}/{\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ Josephson junctions fabricated by making a narrow constriction on superconducting ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ films through laser micropatterning. The junctions show a typical overdamped behavior with much higher critical current density compared with those previously reported for bulk ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}/s$-wave-superconductor junctions. Observed magnetic-field and temperature dependencies of the Josephson critical current suggest that the chiral $p$ wave is unlikely for the superconducting symmetry, encouraging further theoretical calculations of the ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}/{\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$-type junctions.

We report a current scaling study of a quantum phase transition between a quantum anomalous Hall insulator and a trivial insulator on the surface of a heterostructure film of magnetic topological insulators. The transition was observed by tilting the magnetization while measuring the Hall conductivity $\sigma_{xy}$. The transition curves of $\sigma_{xy}$ taken under various excitation currents cross each other at a single point, exemplifying a quantum critical behavior of the transition. The slopes of the transition curves follow a power law dependence of the excitation current, giving a scaling exponent. Combining with the result of the previous temperature scaling study, critical exponents $\nu$ for the localization length and $p$ for the coherence length are separately evaluated as $\nu$ = 2.8 $\pm$ 0.3 and $p$ = 3.3 $\pm$ 0.3., Comment: 6 pages, 4 figures

The impact of electron correlation on the Dirac semimetal state is investigated for perovskite ${\text{CaIrO}}_{3}$ in terms of the magnetotransport properties under varying pressures. The reduction of electron correlation with a pressure of 1 GPa enhances the Fermi velocity as much as 40%, but it reduces the mobility by an order of magnitude by detuning the Dirac node from the Fermi energy. Moreover, the giant magnetoresistance at the quantum limit due to the one-dimensional confinement of Dirac electrons is critically suppressed under pressure. These results indicate that the electron correlation is a crucial knob for controlling the transport of a correlated Dirac semimetal.

Quantum-relativistic materials often host electronic phenomena with exotic spatial distributions. In particular, quantum anomalous Hall (QAH) insulators feature topological boundary currents whose chirality is determined by the magnetization orientation. However, understanding the microscopic nature of edge vs. bulk currents has remained a challenge due to the emergence of multidomain states at the phase transitions. Here we use microwave impedance microscopy (MIM) to directly image chiral edge currents and phase transitions in a magnetic topological insulator. Our images reveal a dramatic change in the edge state structure and an unexpected microwave response at the topological phase transition between the Chern number [Formula: see text] and [Formula: see text] states, consistent with the emergence of an insulating [Formula: see text] state. The magnetic transition width is independent of film thickness, but the transition pattern is distinct in differently initiated field sweeps. This behavior suggests that the [Formula: see text] state has 2 surface states with Hall conductivities of [Formula: see text] but with opposite signs.

We report a proximity-driven large anomalous Hall effect in all-telluride heterostructures consisting of ferromagnetic insulator Cr2Ge2Te6 and topological insulator (Bi,Sb)2Te3. Despite small magnetization in the (Bi,Sb)2Te3 layer, the anomalous Hall conductivity reaches a large value of 0.2e2/h in accord with a ferromagnetic response of the Cr2Ge2Te6. The results show that the exchange coupling between the surface state of the topological insulator and the proximitized Cr2Ge2Te6 layer is effective and strong enough to open the sizable exchange gap in the surface state., 33 pages, 12 figures including supplemental material, accepted by PRL

We report large enhancement of upper critical field ${H}_{\mathrm{c}2}$ observed in superconducting ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ thin films. Through dimensional crossover approaching two dimensions, ${H}_{\mathrm{c}2}$ except the in-plane field direction is dramatically enhanced compared to bulks, following a definite relation distinct from bulk one between ${H}_{\mathrm{c}2}$ and the transition temperature. The anomalous enhancement of ${H}_{\mathrm{c}2}$ is highly suggestive of important changes of the superconducting properties, possibly accompanied with rotation of the triplet $d$ vector. Our findings will become a crucial step to further explore exotic properties by employing ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ thin films.

The accuracy of Hall resistance in the quantum anomalous Hall effect has been studied at zero magnetic field using Crx(Bi,Sb) 2 − x Te3-based magnetic heterostructure films of topological insulators. The measured deviation of the Hall resistance from its theoretical value h / e 2 was less than 2 ppm when the source drain current was 30 nA. This result has verified that the quantization of the Hall resistance is very accurate in the magnetic heterostructure films and in the previously reported uniformly doped films.

We theoretically investigate the dynamical nuclear spin polarization in a quantum point contact (QPC) at finite magnetic field. We find that when the QPC is tuned to be spin selective, at the conductance of e^2/h, a finite bias induces a dipole-like (spatially anti-symmetric) nuclear polarization: at the QPC center the polarization is zero, while, for GaAs parameters, the nuclear spins down (up) are induced on the source (drain) side. We predict that the dipole-like polarization pattern can be distinguished from a uniform polarization due to a qualitatively different response of the QPC conductance to the NMR field., Comment: 17 pages, 8 figures; referees' comments incorporated, some typos corrected, footnotes' format changed

Instability of quantum anomalous Hall (QAH) effect has been studied as function of electric current and temperature in ferromagnetic topological insulator thin films. We find that a characteristic current for the breakdown of the QAH effect is roughly proportional to the Hall-bar width, indicating that Hall electric field is relevant to the breakdown. We also find that electron transport is dominated by variable range hopping (VRH) at low temperatures. Combining the current and temperature dependences of the conductivity in the VRH regime, the localization length of the QAH state is evaluated to be about 5 $\mu$m. The long localization length suggests a marginally insulating nature of the QAH state due to a large number of in-gap states., Comment: 5 pages, 4 figures, accepted for publication in PRL

Gigantic magnetoresistance is shown in a Cr- and V-doped topological insulator multilayer, assuring robust axion insulator., Exploration of novel electromagnetic phenomena is a subject of great interest in topological quantum materials. One of the unprecedented effects to be experimentally verified is the topological magnetoelectric (TME) effect originating from an unusual coupling of electric and magnetic fields in materials. A magnetic heterostructure of topological insulator (TI) hosts such exotic magnetoelectric coupling and can be expected to realize the TME effect as an axion insulator. We designed a magnetic TI with a tricolor structure where a nonmagnetic layer of (Bi, Sb)2Te3 is sandwiched by a soft ferromagnetic Cr-doped (Bi, Sb)2Te3 and a hard ferromagnetic V-doped (Bi, Sb)2Te3. Accompanied by the quantum anomalous Hall (QAH) effect, we observe zero Hall conductivity plateaus, which are a hallmark of the axion insulator state, in a wide range of magnetic fields between the coercive fields of Cr- and V-doped layers. The resistance of the axion insulator state reaches as high as 109 ohms, leading to a gigantic magnetoresistance ratio exceeding 10,000,000% upon the transition from the QAH state. The tricolor structure of the TI may not only be an ideal arena for the topologically distinct phenomena but can also provide magnetoresistive applications for advancing dissipation-less topological electronics.

The quantum anomalous Hall effect (QAHE) is an exotic quantum phenomenon originating from dissipation-less chiral channels at the sample edge. While the QAHE has been observed in magnetically doped topological insulators (TIs), exploiting magnetic proximity effect on the TI surface from adjacent ferromagnet layers may provide an alternative approach to the QAHE by opening an exchange gap with less disorder than that in the doped system. Nevertheless, the engineering of a favorable heterointerface that realizes the QAHE based on the magnetic proximity effect remains to be achieved. Here, we report on the observation of the QAHE in a proximity coupled system of non-magnetic TI and ferromagnetic insulator (FMI). We have designed sandwich heterostructures of (Zn,Cr)Te/(Bi,Sb)2Te3/(Zn,Cr)Te that fulfills two prerequisites for the emergence of the QAHE; the formation of a sizable exchange gap at the TI surface state and the tuning of the Fermi energy into the exchange gap. The efficient proximity coupling in the all-telluride based heterostructure as demonstrated here will enable a realistic design of versatile tailor-made topological materials coupled with ferromagnetism, ferroelectricity, superconductivity, and so on., 18 pages, 4 figures

The axion insulator which may exhibit an exotic quantized magnetoelectric effect is one of the most interesting quantum phases predicted for the three-dimensional topological insulator (TI). The axion insulator state is expected to show up in magnetically doped TIs with magnetizations pointing inwards and outwards from the respective surfaces. Towards the realization of the axion insulator, we here engineered a TI heterostructure in which magnetic ions (Cr) are modulation-doped only in the vicinity of the top and bottom surfaces of the TI ((Bi,Sb)

We report on emergence of two-dimensional conduction and ferromagnetism at the interface of MnTe thin films and InP substrates. The MnTe/InP heterostructures grown by molecular-beam epitaxy show thickness-independent sheet conductivity above a critical thickness of about 20 nm, indicating the formation of a conducting layer at the interface. Furthermore, the ferromagnetic behavior is confirmed by both magnetization and anomalous Hall effect measurements below a critical temperature of 270 K. The critical temperature is also independent of the thickness. By the investigation of the atomic structure with transmission electron microscopy, we observe a structural anomaly near the interface which consists of an antiprism-type Mn network unlike a prism-type Mn network of conventional MnTe. The band structure calculation shows that the antiprism-type MnTe can host metallic conduction and ferromagnetism, which is consistent with the present experimental results. The interface engineering based on the chalcogenide compound will develop a new arena for designing the emergent low dimensional conduction and magnetism.

We have investigated the magnetic field and temperature dependence of the tunneling spectra of the eutectic system Sr2RuO4-Ru. Electric contacts to individual Ru lamellae embedded in Sr2RuO4 enable the tunneling spectra at the interface between ruthenate and a Ru microinclusion to be measured. A zero bias conductance peak (ZBCP) was observed in the bias voltage dependence of the differential conductance, suggesting that Andreev bound states are present at the interface. The ZBCP starts to appear at a temperature well below the superconducting transition temperature. The onset magnetic field of the ZBCP is also considerably smaller than the upper critical field when the magnetic field is parallel to the ab-plane. We propose that the difference between the onset of the ZBCP and the onset of superconductivity can be understood in terms of the existence of the single-component state predicted by Sigrist and Monien., Comment: 4 pages, 4 figures, to appear in J. Phys. Soc. Jpn. vol. 74 no. 2

We investigated a superconductor–semiconductor–superconductor junction formed by two superconducting NbN electrodes and a two-dimensional electron gas (2DEG) in an AlGaAs/GaAs heterostructure. We obtained a good ohmic contact between NbN/AuGeNi electrodes and 2DEG by annealing them at 450°C for 1 min in an N2 atmosphere. We observed a decrease in the resistance caused by Andreev reflection (AR) within the superconducting energy gap voltage in a zero magnetic field in this structure. We found that the peculiar features of the magnetoresistance in the transition region can be qualitatively explained by considering the existence of the AR in high magnetic fields.

Massless Dirac electrons in condensed matter have attracted considerable attention. Unlike conventional electrons, Dirac electrons are described in the form of two-component wave functions. In the surface state of topological insulators, these two components are associated with the spin degrees of freedom, hence governing the magnetic properties. Therefore, the observation of the two-component wave function provides a useful clue for exploring the novel spin phenomena. Here we show that the two-component nature is manifested in the Landau levels (LLs) whose degeneracy is lifted by a Coulomb potential. Using spectroscopic-imaging scanning tunneling microscopy, we visualize energy and spatial structures of LLs in a topological insulator Bi2Se3. The observed potential-induced LL splitting and internal structures of Landau orbits are distinct from those in a conventional electron system and are well reproduced by a two-component model Dirac Hamiltonian. Our model further predicts non-trivial energy-dependent spin-magnetization textures in a potential variation. This provides a way to manipulate spins in the topological surface state., Comment: Revised version to appear in Nature Physics. Supplementary information is available at http://www.riken.jp/epmrt/Hanaguri/SI/Bi2Se3_2comp/SI.html

Two-dimensional electron gas (2DEG) subjected to a spatially modulated magnetic field exhibits commensurability oscillation of magnetoresistance, analysis of which furnishes information on the modulation amplitude. Under a periodically modulated magnetic field with zero mean, the resistivity of the 2DEG gains an extra term expressed as Δρ=AT2+C. The temperature quadratic term is attributed to electron–electron umklapp scattering, while the constant term arises from combined effect of the lateral magnetic superlattice and random impurities. In the composite Fermion (CF) regime near the half-filling of lowest Landau levels, spatial variation of the electron density is equivalent to magnetic field variation. Use of a short period lateral superlattice has enabled us to observe the commensurability oscillation of the magnetoresistance of CFs near the filling factor ν=3/2. The positions of the resistance minima are consistent with those calculated by assuming magnetic modulation and fully spin-polarized CFs.

The vertical transport in multilayered quantum Hall systems has been studied under a tilted magnetic field. The out-of-plane resistance peaks at integer filling, and the peak value increases with increasing in-plane field component. The magnitude of the effect is almost the same in the low temperature regime and high temperature regime, and the effect can be accounted for by the suppression of the interlayer charge transfer by the in-plane field.

The vertical transport in weakly coupled multilayered two-dimensional electron systems has been studied in the integer quantum Hall regime. The current–voltage characteristics show prominent non-ohmicity in the vertical transport when each layer is in the quantum Hall state. The voltage dependence curves of the differential conductivity scaled by the mesa perimeter collapse onto a single curve at lower voltages, while those scaled by mesa area are on a single curve at higher voltages. This shows that the current flows through the surface of the sample at low-voltage region and through the bulk at high-voltage region. A crossover from the surface transport to the bulk transport occurs in a fairly narrow range of the bias voltage.

Vertical transport in GaAs/AlGaAs superlattices has been studied in the quantum Hall regime. Both the in-plane and out-of-plane resistances exhibit an Arrhenius-type temperature dependence with small values of activation energy. At very low temperatures, the out-of-plane conductance becomes almost temperature independent and its values scale with the sample perimeter, indicating that the current is carried by the chiral surface states. The vertical transport in this regime exhibits distinct non-Ohmicity, suggesting the onset of bulk transport at high bias voltages. Rather strong non-Ohmicity is also observed in the low bias voltage region where the chiral surface state is responsible for the vertical transport.

We observe an adiabatic transport phenomena of electrons in valley-splitting quantum Hall edge channels for the first time using a high-mobility Si/SiGe two-dimensional electron system. We find that the scattering event between the valley-splitting edge channels is suppressed over a distance of ∼ 5 μ m , which is surprisingly longer than that expected from the valley-splitting energy gap in Si.

Vertical transport in GaAs/AlGaAs semiconductor superlattice has been studied as a function of magnetic field angle. A series of resistance peaks corresponding to the angular dependent magnetoresistance oscillation (ADMRO) effect were investigated. The present samples have higher carrier densities and wider well widths than previously studied. This allowed us to observe higher order ADMRO peaks. Another resistance peak is found at θ=90°(B‖layer). The origin of this peak structure is discussed in terms of occurrence of closed orbits in some part of a weakly corrugated cylindrical Fermi surface for this field orientation.

We have reproducibly fabricated suspended tungsten electrodes with a gap of less than 2nm on a suspended silicon nitride membrane with a slit. Our fabrication technique is based on the local deposition of tungsten by a focused ion beam. The length of the nanogap between the electrodes was controlled by detecting a tunneling current during the growth of the tungsten electrodes from both sides of the slit. The gap length can reach about 1nm, which corresponds to the size of a single molecule such as a fullerene.

We study dynamic polarization and resistive detection of nuclear spins in a semiconductor quantum dot (QD) under the Kondo effect regime. We find that the differential conductance spectra of the QD exhibit hysteresis under the Kondo effect regime in magnetic fields. Relevance of nuclear spins to the hysteresis is confirmed by the detection of nuclear magnetic resonance signals by monitoring the differential conductance. We attribute the origin of the hysteresis to the dynamic nuclear spin polarization (DNP) induced in the QD. Using the DNP, we demonstrate nuclear spin relaxation rate measurements in the QD under the Kondo effect regime., Comment: 5 pages, 6 figures, accepted for publication in Phys. Rev. B

We investigate Dirac fermions on the surface of the topological insulator ${\text{Bi}}_{2}{\text{Se}}_{3}$ using scanning tunneling spectroscopy. Landau levels (LLs) are observed in the tunneling spectra in a magnetic field. In contrast to LLs of conventional electrons, a field-independent LL appears at the Dirac point, which is a hallmark of Dirac fermions. A scaling analysis of LLs based on the Bohr-Sommerfeld quantization condition allowed us to determine the dispersion of the surface band. Near the Fermi energy, fine peaks mixed with LLs appear in the spectra, which may be responsible for the anomalous magnetofingerprint effect [J. G. Checkelsky et al., Phys. Rev. Lett. 103, 246601 (2009)].

We studied spatial distribution of dynamic nuclear polarization (DNP) in a Hall-bar device in a breakdown regime of the quantum Hall effect (QHE). We detected nuclear magnetic resonance (NMR) signals from the polarized nuclear spins by measuring the Hall voltage $V_{xy}$ using three pairs of voltage probes attached to the conducting channel of the Hall bar. We find that the amplitude of the NMR signal depends on the position of the Hall voltage probes and that the largest NMR signal is obtained from the pair of probes farthest from the electron-injecting electrode. Combined with results on pump-and-probe measurements, we conclude that the DNP induced by QHE breakdown develops along the electron-drift direction., Comment: 4 pages, 4 figures, accepted for publication in PRB

We study dynamic nuclear polarization (DNP) induced by breakdown of the fractional quantum Hall (FQH) effect. We find that voltage-current characteristics depend on current sweep rates at the quantum Hall states of Landau level filling factors $\nu$ = 1, 2/3, and 1/3. The sweep rate dependence is attributed to DNP occurring in the breakdown regime of FQH states. Results of a pump and probe experiment show that the polarities of the DNP induced in the breakdown regimes of the FQH states is opposite to that of the DNP induced in the breakdown regimes of odd-integer quantum Hall states., Comment: 4 pages, 4 figures

We experimentally study the tunneling magnetoresistance (TMR) effect as a function of bias voltage $({V}_{\mathrm{SD}})$ in lateral $\mathrm{Ni}∕\mathrm{In}\mathrm{As}∕\mathrm{Ni}$ quantum-dot (QD) spin valves showing Coulomb blockade characteristics. With varying ${V}_{\mathrm{SD}}$, the TMR value oscillates and the oscillation period corresponds to conductance changes observed in the current-voltage $(I\text{\ensuremath{-}}{V}_{\mathrm{SD}})$ characteristics. We also find an inverse TMR effect near ${V}_{\mathrm{SD}}$ values where negative differential conductance is observed. A possible mechanism of the TMR oscillation is discussed in terms of spin accumulation on the QD and spin-dependent transport properties via excited states.

We demonstrate an electric-field control of tunneling magnetoresistance (TMR) effect in a semiconductor quantum-dot (QD) spin-valve device. By using ferromagnetic Ni nano-gap electrodes, we observe the Coulomb blockade oscillations at a small bias voltage. In the vicinity of the Coulomb blockade peak, the TMR effect is significantly modulated and even its sign is switched by changing the gate voltage, where the sign of the TMR value changes at the resonant condition., 3 pages, 3 figures

We study the edge-channel transport at quantum Hall (QH) transition regions for a high-mobility $\mathrm{Si}∕\mathrm{Si}\mathrm{Ge}$ QH conductor by measuring nonlocal resistance $({R}_{\mathrm{NL}})$. The ${R}_{\mathrm{NL}}$ as a function of magnetic field changes drastically after Landau-level crossings. The features of the ${R}_{\mathrm{NL}}$ depend on the spin configuration between the innermost edge channel and the bulk state: the ${R}_{\mathrm{NL}}$ appears only when the relevant edge-bulk states have opposite spin orientations. Also, an origin of the spin-dependent resistivity [Phys. Rev. Lett. 94, 176402 (2005)] at QH transition regions is discussed in terms of the spin-dependent inter-edge-bulk scattering.

Electrical polarization of nuclear spins is studied in a Corbino disk under a breakdown regime of the quantum Hall effect (QHE). Since the edge channels are completely absent in the Corbino disk, we conclude that the electric current flowing in the bulk channel of a quantum Hall conductor is relevant to dynamic nuclear polarization (DNP). A pump and probe measurement demonstrates that DNP emerges near the critical voltage of the QHE breakdown. The agreement of the onset voltage of DNP with that of the QHE breakdown indicates that the underlying origin of DNP is closely related to that of the QHE breakdown., Comment: 3 pages, 4 figures

Nuclear spins are polarized electrically in a breakdown regime of an odd-integer quantum Hall effect (QHE). Electron excitation to the upper Landau subband with the opposite spin polarity flips nuclear spins through the hyperfine interaction. The polarized nuclear spins reduce the spin-splitting energy and accelerate the QHE breakdown. The Knight shift of the nuclear spins is also measured by tuning electron density during the irradiation of radio-frequency magnetic fields., Comment: 3 pages, 2 figures, EP2DS-17

We have fabricated a lateral double barrier magnetic tunnel junction (MTJ) which consists of a single self-assembled InAs quantum dot (QD) with ferromagnetic Co leads. The MTJ shows clear hysteretic tunnel magnetoresistance (TMR) effect, which is evidence for spin transport through a single semiconductor QD. The TMR ratio and the curve shapes are varied by changing the gate voltage., Comment: 4 pages, 3 figures

Using a laterally-fabricated quantum-dot (QD) spin-valve device, we experimentally study the Kondo effect in the electron transport through a semiconductor QD with an odd number of electrons (N). In a parallel magnetic configuration of the ferromagnetic electrodes, the Kondo resonance at N = 3 splits clearly without external magnetic fields. With applying magnetic fields (B), the splitting is gradually reduced, and then the Kondo effect is almost restored at B = 1.2 T. This means that, in the Kondo regime, an inverse effective magnetic field of B ~ 1.2 T can be applied to the QD in the parallel magnetic configuration of the ferromagnetic electrodes., Comment: 4 pages, 3 figures

We have investigated the tunnelling properties of the interface between superconducting Sr2RuO4 and a single Ru inclusion in eutectic. By using a micro‐fabrication technique, we have made Sr2RuO4/Ru junctions on the eutectic system that consists of Sr2RuO4 and Ru micro‐inclusions. Such a eutectic system exhibits surface superconductivity, called the 3‐K phase. A zero bias conductance peak (ZBCP) was observed in the 3‐K phase. We propose to use the onset of the ZBCP to delineate the phase boundary of a time‐reversal symmetry breaking state.

We have developed a method for electrical polarization of nuclear spins in quantum Hall systems. In a breakdown regime of odd-integer quantum Hall effect (QHE), excitation of electrons to the upper Landau subband with opposite spin polarity dynamically polarizes nuclear spins through the hyperfine interaction. The polarized nuclear spins in turn accelerate the QHE breakdown, leading to hysteretic voltage-current characteristics of the quantum Hall conductor., Comment: 3 pages, 4 figures, submitted to Appl. Phys. Lett

We have investigated transport properties of single‐electron transistors made from colloidal nanoparticles. The single‐electron transistors were fabricated depositing chemically synthesized gold nanoparticles between pairs of electrodes. Current‐voltage characteristics were measured at liquid He temperature. The observed Coulomb diamond indicates that electron transport occurs through a single gold nanoparticle. Anomalous increase of current was observed in a device. The origin of the current increase is discussed in connection with a suppression of molecular tunneling barriers.

The understanding of the zero bias conductance peak (ZBCP) in the tunnelling spectra of S/N junctions involving d-wave cuprate superconductors has been important in the determination of the phase structure of the superconducting order parameter. In this context, the involvement of a p-wave superconductor such as Sr2RuO4 in tunnelling studies is indeed of great importance. We have recently succeeded in fabricating devices that enable S/N junctions forming at interfaces between Sr2RuO4 and Ru micro-inclusions in eutectic crystals to be investigated.3 We have observed a ZBCP and have interpreted it as due to the Andreev bound state, commonly seen in unconventional superconductors. Also we have proposed that the onset of the ZBCP may be used to delineate the phase boundary for the onset of a time reversal symmetry broken (TRSB) state within the superconducting state, which does not always coincide with the onset of the superconducting state. However, these measurements always involved two interfaces between Sr2RuO4 and Ru. In the present study, we have extended the previous measurements to obtain a deeper insight into the properties of a single interface between Sr2RuO4 and Ru., Comment: To appear in J. Phys. Soc. Jpn. Vol. 75 No.12 issue

We report low-temperature transport measurements through molecules of Gd metallofullerenes between superconducting suspended electrodes. The presence and number of molecules in the 2 nm-wide gap between electrodes was determined by high resolution transmission electron microscopy. We find that a junction containing a single metallofullerene dimer between superconducting electrodes displays signs of proximity-induced superconductivity. In contrast, no proximity effects develops in junctions containing larger cluster of metallofulerenes. These results can be understood by taking into account multiple Andreev reflections, and the spin states of the Gd atoms., Comment: 5 pages, 4 figures. Submitted to Phys.Rev.Lett

We present the results of experimental study of superconductivity in individual molecules of carbon nanotubes, DNAs and metallofullerenes. Critical currents of supeconductor‐molecule‐superconductor junctions were extensively studied as a function of temperature and magnetic field. The mechanism of current induced superconductor‐normal state transition for a long molecule (carbon nanotubes and DNAs) is the creation of phase slip centres and for a short molecule (metallofullerens) — multiple Andreev reflections. We observe an influence of spin state of encapsulated atom on the induced superconductvity in a metallofullerene molecule.

We report magntetotranport measurements around a filling factor $\ensuremath{\nu}=\frac{3}{2}$ of two-dimensional electron gas under weak unidirectional periodic potential modulation. Composite fermions (CF's) at $\ensuremath{\nu}=\frac{3}{2}$ which are the electron-hole conjugate of $\ensuremath{\nu}=\frac{1}{2}$ CF's, show positive magnetoresistance (PMR) and oscillatory magnetoresistance---commensurability oscillation (CO)---for a modulation period of $a=92\mathrm{nm}.$ Minima of the CO occur at the positions expected for commensurability with effective magnetic-field modulation, and are consistent with the field-dependent Fermi wave number ${k}_{F}$ resulting from field-dependent density of the $\ensuremath{\nu}=\frac{3}{2}$ CF's. The positions are consistent with ${k}_{F}$ for fully spin-polarized CF's, except for one dip which is better explained by a fully spin-unpolarized ${k}_{F},$ smaller by factor $\sqrt{2}$ than its spin-polarized counterpart. With the introduction of in-plane magnetic field by tilting, the PMR shows unexpected asymmetry: it grows both in its magnitude and extent on the lower-field side, while it shrinks on the higher-field side.