Junichi Shiogai

Magnetic Field-Induced Insulator-Semimetal Transition in a Pyrochlore Nd2Ir2O7.

We investigate magnetotransport properties in a single crystal of pyrochore-type Nd2Ir2O7. The metallic conduction is observed on the antiferromagnetic domain walls of the all-in-all-out-type Ir 5d moment ordered insulating bulk state that can be finely controlled by an external magnetic field along [111]. On the other hand, an applied field along [001] induces the bulk phase transition from insulator to semimetal as a consequence of the field-induced modification of the Nd 4f and Ir 5d moment configurations. A theoretical calculation consistently describing the experimentally observed features suggests a variety of exotic topological states as functions of electron correlation and Ir 5d moment orders, which can be finely tuned by the choice of rare-earth ion and magnetic field, respectively.

Quantum Hall effect in a bulk antiferromagnet EuMnBi2 with magnetically confined two-dimensional Dirac fermions.

Quantum transport of quasi–two-dimensional Dirac fermions is largely controlled by magnetic order in a layered magnet. For the innovation of spintronic technologies, Dirac materials, in which low-energy excitation is described as relativistic Dirac fermions, are one of the most promising systems because of the fascinating magnetotransport associated with extremely high mobility. To incorporate Dirac fermions into spintronic applications, their quantum transport phenomena are desired to be manipulated to a large extent by magnetic order in a solid. We report a bulk half-integer quantum Hall effect in a layered antiferromagnet EuMnBi2, in which field-controllable Eu magnetic order significantly suppresses the interlayer coupling between the Bi layers with Dirac fermions. In addition to the high mobility of more than 10,000 cm2/V s, Landau level splittings presumably due to the lifting of spin and valley degeneracy are noticeable even in a bulk magnet. These results will pave a route to the engineering of magnetically functionalized Dirac materials.

Improvement of electron mobility in La:BaSnO3 thin films by insertion of an atomically flat insulating (Sr,Ba)SnO3 buffer layer

One perovskite oxide, ASnO3 (A = Sr, Ba), is a candidate for use as a transparent conductive oxide with high electron mobility in single crystalline form. However, the electron mobility of films grown on SrTiO3 substrates does not reach the bulk value, probably because of dislocation scattering that originates from the large lattice mismatch. This study investigates the effect of insertion of bilayer BaSnO3 / (Sr,Ba)SnO3 for buffering this large lattice mismatch between La:BaSnO3 and SrTiO3 substrate. The insertion of 200-nm-thick BaSnO3 on (Sr,Ba)SnO3 bilayer buffer structures reduces the number of dislocations and improves surface smoothness of the films after annealing as proved respectively by scanning transmission electron microscopy and atomic force microscopy. A systematic investigation of BaSnO3 buffer layer thickness dependence on Hall mobility of the electron transport in La:BaSnO3 shows that the highest obtained value of mobility is 78 cm2V−1s−1 because of its fewer dislocations. High electron mobility films based on perovskite BaSnO3 can provide a good platform for transparent-conducting-oxide electronic devices and for creation of fascinating perovskite heterostructures.

Shot Noise Induced by Nonequilibrium Spin Accumulation

When an electric current passes across a potential barrier, the partition process of electrons at the barrier gives rise to the shot noise, reflecting the discrete nature of the electric charge. Here we report the observation of excess shot noise connected with a spin current which is induced by a nonequilibrium spin accumulation in an all-semiconductor lateral spin-valve device. We find that this excess shot noise is proportional to the spin current. Additionally, we determine quantitatively the spin-injection-induced electron temperature by measuring the current noise. Our experiments show that spin accumulation driven shot noise provides a novel means of investigating nonequilibrium spin transport.

Dynamic nuclear spin polarization in an all-semiconductor spin injection device with (Ga,Mn)As/n-GaAs spin Esaki diode

We investigate the dynamic nuclear spinpolarization in an n-GaAs lateral channel induced by electrical spin injection from a (Ga,Mn)As/n-GaAs spin Esaki diode. Signatures of nuclear spinpolarization are studied in both three-terminal and non-local voltage signals, where a strong electron spin depolarization feature is observed close to zero magnetic field. This is due to the large nuclear field induced in the channel through hyperfine interaction between injected electron spins and localized nuclear spins. We study the time evolution of the dynamic nuclear spinpolarization and evaluate polarization and relaxation times of nuclear spins in the channel.

High field-effect mobility at the (Sr,Ba)SnO3/BaSnO3 interface

A perovskite oxide, BaSnO3, has been classified as one of transparent conducting materials with high electron mobility, and its application for field-effect transistors has been the focus of recent research. Here we report transistor operation in BaSnO3-based heterostructures with atomically smooth surfaces, fabricated on SrTiO3 substrates by the (Sr,Ba)SnO3 buffer technique. Indeed, modulation of band profiles at the channel interfaces with the insertion of wide bandgap (Sr,Ba)SnO3 as a barrier layer results in a significant improvement of field-effect mobility, implying effective carrier doping at the regulated heterointerface. These results provide an important step towards realization of high-performance BaSnO3-based field-effect transistors.

Giant enhancement of spin detection sensitivity in (Ga,Mn)As/GaAs Esaki diodes

We investigate the correlation between spin signals measured in three-terminal (3T) geometry by the Hanle effect and the spin accumulation generated in a semiconductor channel in a lateral (Ga,Mn)As/GaAs Esaki diode device. We systematically compare measurements using a 3T configuration, probing spin accumulation directly beneath the injecting contact, with results from nonlocal measurements, where solely spin accumulation in the GaAs channel is probed. We find that the spin signal detected in the 3T configuration is dominated by a bias-dependent spin detection sensitivity, which in turn is strongly correlated with charge-transport properties of the junction. This results in a particularly strong enhancement of the detected spin signal in a region of increased differential resistance. We find additionally that two-step tunneling via localized states in the gap of (Ga,Mn)As does not compromise spin injection into the semiconductor conduction band.

Enhanced electron mobility at the two-dimensional metallic surface of BaSnO3 electric-double-layer transistor at low temperatures

Wide-bandgap oxides exhibiting high electron mobility hold promise for the development of useful electronic and optoelectronic devices as well as for basic research on two-dimensional electron transport phenomena. A perovskite-type tin oxide, BaSnO3, is currently one of such targets owing to distinctly high mobility at room temperature. The challenge to overcome towards the use of BaSnO3 thin films in applications is suppression of dislocation scattering, which is one of the dominant scattering origins for electron transport. Here, we show that the mobility of the BaSnO3 electric-double-layer transistor reaches 300 cm2 V−1 s−1 at 50 K. The improved mobility indicates that charged dislocation scattering is effectively screened by electrostatically doped high-density charge carriers. We also observed metallic conduction persisting down to 2 K, which is attributed to the transition to the degenerate semiconductor. The experimental verification of bulk-level mobility at the densely accumulated surface sheds m...

Magnitude and sign control of lithography-induced uniaxial anisotropy in ultra-thin (Ga,Mn)As wires

We were able to control the magnitude and sign of the uniaxial anisotropy in 5-nm-thin (Ga,Mn)As wires by changing the crystallographic direction of the lithography-induced strain relaxation. The 1-μm-wide (Ga,Mn)As wires, oriented in [110] and [11¯0] directions, were fabricated using electron beam lithography. Their magnetic anisotropies were studied by a coherent rotation method at temperatures between 4.5 and 75 K. Depending on the orientation of the wire, the additional uniaxial anisotropy observed along the axis of the 1-μm-wide samples either increased or decreased the total uniaxial anisotropy.

In-plane tunneling anisotropic magnetoresistance in (Ga,Mn)As/GaAs Esaki diodes in the regime of the excess current

We investigate the angular dependence of the tunneling anisotropic magnetoresistance in (Ga,Mn)As/n-GaAs spin Esaki diodes in the regime where the tunneling process is dominated by the excess current through midgap states in (Ga,Mn)As. We compare it to similar measurements performed in the regime of band-to-band tunneling. Whereas the latter show biaxial symmetry typical for magnetic anisotropy observed in (Ga,Mn)As samples, the former is dominated by uniaxial anisotropy along the 〈110〉 axes.