Kazuto Akiba

Quantum Hall states observed in thin films of Dirac semimetal Cd 3 As 2

A well known semiconductor Cd3As2 has reentered the spotlight due to its unique electronic structure and quantum transport phenomena as a topological Dirac semimetal. For elucidating and controlling its topological quantum state, high-quality Cd3As2 thin films have been highly desired. Here we report the development of an elaborate growth technique of high-crystallinity and high-mobility Cd3As2 films with controlled thicknesses and the observation of quantum Hall effect dependent on the film thickness. With decreasing the film thickness to 10 nm, the quantum Hall states exhibit variations such as a change in the spin degeneracy reflecting the Dirac dispersion with a large Fermi velocity. Details of the electronic structure including subband splitting and gap opening are identified from the quantum transport depending on the confinement thickness, suggesting the presence of a two-dimensional topological insulating phase. The demonstration of quantum Hall states in our high-quality Cd3As2 films paves a road to study quantum transport and device application in topological Dirac semimetal and its derivative phases.Despite many achievements in the topological semimetal Cd3As2, the high-quality Cd3As2 films are still rare. Here, Uchida et al. grow high-crystallinity and high-mobility Cd3As2 thin films and observe quantum Hall states dependent on the confinement thickness.

Anisotropic upper critical field of theBiS2-based superconductorLaO0.5F0.5BiS2

Superconducting properties under high magnetic fields for the novel BiS2-based layered superconductor LaO0.5F0.5BiS2 (polycrystalline sample prepared using the high-pressure synthesis method) have been investigated. The anisotropy of the upper critical field was discussed by analyzing the temperature dependence of the temperature derivative of resistivity. The temperature dependence of upper critical field showed an anomalous behavior with a characteristic magnetic field of 8 T. We revealed that the anomalous behavior was caused by the existence of three kinds of upper critical field, HC2max, HC2mid and HC2min in LaO0.5F0.5BiS2. The lowest upper critical field HC2min is regarded as the Hc2 where the superconducting states of the grains with an orientation of H // c are suppressed. The HC2max and HC2mid are regarded as the Hc2 where the superconducting states of the grains with an orientation of H // ab are suppressed. The difference between the HC2max and HC2mid could be explained by the anisotropy of superconducting states within the Bi-S plane. The estimated anisotropy parameter of the upper critical fields was about 7.4 for the high-pressure-synthesized LaO0.5F0.5BiS2 polycrystalline sample. Since the value was clearly lower than the anisotropy parameter of over 30 observed in the previous report on the LaO0.5F0.5BiS2 single crystals grown under ambient pressure, we concluded that a larger anisotropy of superconductivity was not essential for the enhancement of Tc, and lower anisotropy might be important for a higher Tc in LaO1-xFxBiS2.

Possible Excitonic Phase of Graphite in the Quantum Limit State

UTokyo Research掲載「量子極限状態における励起子のBCS的状態の可能性」 URI: http://www.u-tokyo.ac.jp/ja/utokyo-research/research-news/possible-excitonic-bcs-like-state-in-the-quantum-limit-state.html

Two-carrier analyses of the transport properties of black phosphorus under pressure

We report on the electronic transport properties of black phosphorus and analyze them using a two-carrier model in a wide range of pressure up to 2.5 GPa. In semiconducting state at 0.29 GPa, the remarkable non-linear behavior in the Hall resistance is reasonably reproduced by assuming the coexistence of two kinds of hole with different densities and mobilities. On the other hand, two-carrier analyses of the magnetotransport properties above 1.01 GPa suggest the coexistence of high mobility electron and hole carriers that have almost the same densities, i.e., nearly compensated semimetallic nature of black phosphorus. In the semimetallic state, analyses of both the two-carrier model and quantum oscillations indicate a systematic increase in the carrier densities as pressure increases. An observed sign inversion of Hall resistivity at low magnetic fields suggests the existence of high mobility electrons (\sim105 cm2 V-1 s-1) that is roughly ten times larger than that of holes, in the semimetallic black phosphorus. We conclude that the extremely large positive magnetoresistance that has been observed in semimetallic state cannot be reproduced by a conventional two-carrier model.

Anomalous Quantum Transport Properties in Semimetallic Black Phosphorus

Magnetoresistance in single crystals of black phosphorus is studied at ambient and hydrostatic pressures. In the semiconducting states at pressures below 0.71 GPa, the magnetoresistance shows periodic oscillations, which can be ascribed to the magneto-phonon resonance that is characteristic of high mobility semiconductors. In the metallic state above 1.64 GPa, the both transverse and longitudinal magnetoresistance show titanic increase with exhibiting superposed Shubnikov-de Haas oscillations. The observed small Fermi surfaces, high mobilities and light effective masses of carriers in semimetallic black phosphorus are comparable to those in the representative elemental semimetals of bismuth and graphite.

Observation of Poiseuille flow of phonons in black phosphorus

Hydrodynamic flow of phonons, previously detected only in a handful of solids, is observed in black phosphorus. The travel of heat in insulators is commonly pictured as a flow of phonons scattered along their individual trajectory. In rare circumstances, momentum-conserving collision events dominate, and thermal transport becomes hydrodynamic. One of these cases, dubbed the Poiseuille flow of phonons, can occur in a temperature window just below the peak temperature of thermal conductivity. We report on a study of heat flow in bulk black phosphorus between 0.1 and 80 K. We find a thermal conductivity showing a faster than cubic temperature dependence between 5 and 12 K. Consequently, the effective phonon mean free path shows a nonmonotonic temperature dependence at the onset of the ballistic regime, with a size-dependent Knudsen minimum. These are hallmarks of Poiseuille flow previously observed in a handful of solids. Comparing the phonon dispersion in black phosphorus and silicon, we show that the phase space for normal scattering events in black phosphorus is much larger. Our results imply that the most important requirement for the emergence of Poiseuille flow is the facility of momentum exchange between acoustic phonon branches. Proximity to a structural transition can be beneficial for the emergence of this behavior in clean systems, even when they do not exceed silicon in purity.

Large magneto-thermopower in MnGe with topological spin texture

Quantum states characterized by nontrivial topology produce interesting electrodynamics and versatile electronic functionalities. One source for such remarkable phenomena is emergent electromagnetic field, which is the outcome of interplay between topological spin structures with scalar spin chirality and conduction electrons. However, it has scarcely been exploited for emergent function related to heat-electricity conversion. Here we report an unusually enhanced thermopower by application of magnetic field in MnGe hosting topological spin textures. By considering all conceivable origins through quantitative investigations of electronic structures and properties, a possible origin of large magneto-thermopower is assigned to the strong energy dependence of charge-transport lifetime caused by unconventional carrier scattering via the dynamics of emergent magnetic field. Furthermore, high-magnetic-field measurements corroborate the presence of residual magnetic fluctuations even in the nominally ferromagnetic region, leading to a subsisting behavior of field-enhanced thermopower. The present finding may pave a way for thermoelectric function of topological magnets.Topological spin textures produce versatile electronic functionalities, but are scarcely exploited for achieving heat to electricity conversion. Here, Fujishiro et al. attribute an enhanced magneto-thermopower in MnGe with topological spin hedgehogs, to electron scattering via the dynamics of an emergent magnetic field.