Ryuichi Masutomi

Optical Hall effect in the integer quantum Hall regime.

Optical Hall conductivity σ{xy}(ω) is measured from the Faraday rotation for a GaAs/AlGaAs heterojunction quantum Hall system in the terahertz-frequency regime. The Faraday rotation angle (∼ fine-structure constant ∼u2009mrad) is found to significantly deviate from the Drude-like behavior to exhibit a plateaulike structure around the Landau-level filling ν=2. The result, which fits with the behavior expected from the carrier localization effect in the ac regime, indicates that the plateau structure, although not quantized, still exists in the terahertz regime.

Two-dimensional superconducting state of monolayer Pb films grown on GaAs(110) in a strong parallel magnetic field.

Two-dimensional (2D) superconductivity was studied by magnetotransport measurements on single-atomic-layer Pb films on a cleaved GaAs(110) surface. The superconducting transition temperature shows only a weak dependence on the parallel magnetic field up to 14T, which is higher than the Pauli paramagnetic limit. Furthermore, the perpendicular-magnetic-field dependence of the sheet resistance is almost independent of the presence of the parallel field component. These results are explained in terms of an inhomogeneous superconducting state predicted for 2D metals with a large Rashba spin splitting.

Strain dependence of hole effective mass and scattering mechanism in strained Ge channel structures

Strain dependence of hole effective mass (m∗) in the strained Ge channel was systematically studied, and monotonic m∗ reduction by more than 20% was clearly observed when the strain increased from 0.8% up to 2.8%. The scattering mechanism, which strongly depended on the modulation-doping structure as well as strains, was also investigated based on the Dingle ratio evaluation, and the interface roughness scattering was found to be effectively suppressed by adopting the inverted structure even for the largely strained channels.

Evidence for two-dimensional spin-glass ordering in submonolayer Fe films on cleaved InAs surfaces.

Magnetotransport measurements have been performed on two-dimensional electron gases formed at InAs(110) surfaces covered with a submonolayer of Fe. Hysteresis in the magnetoresistance, a difference in remanent magnetoresistance between zero-field-cooling procedures and field-cooling procedures, and logarithmic time-dependent relaxation after magnetic field sweep are clearly observed at 1.7 K for a coverage of 0.42 monolayer. These features are associated with spin-glass ordering in the Fe film.

Quantum Hall effect at cleaved InSb surfaces and low-temperature annealing effect

The authors have performed low-temperature in-plane magnetotransport measurements on two-dimensional electron systems induced by deposition of Ag at in situ cleaved surfaces of p-type InSb. The quantum Hall effect was observed even at low magnetic fields around 2T. The surface electron density and the electron mobility exhibit strong dependence on the Ag coverage and the annealing temperature in the range of 15–40K. The annealing effect suggests that the surface morphology strongly affects the properties of the two-dimensional electron systems.

Magnetic ordering of quasi-1 D Heisenberg antiferromagnet Cu benzoate at sub-mK temperatures

Abstract We have measured the AC susceptibility of quasi-1D S= 1 2 Heisenberg antiferromagnet Cu benzoate at temperatures down to 0.2 mK . A sharp susceptibility peak is observed at 0.8 mK under an earth field. This fact indicates a 3D ordering of linear chains coupled by a weak magnetic interaction between chains.

Magnetic-field-independent superconductivity of ultrathin Pb films on cleaved GaAs surface

We performed magnetotransport measurements on ultathin Pb films deposited onto cleaved GaAs surface and observed two-dimensional superconductivity for an amorphous 2.2-A-thick film, which is below one monolayer. The superconducting transition is almost independent of parallel magnetic field as high as 14 T. This means that the superconducting state has much larger critical magnetic field than Pauli paramagnetic limit. We consider two different mechanism relating Rashba spin splitting.

Well-width dependence of valley splitting in Si/SiGe quantum wells

The valley splitting in Si two-dimensional electron systems is studied using Si/SiGe single quantum wells (QWs) with different well widths. The energy gaps for 4 and 5.3 nm QWs, obtained from the temperature dependence of the longitudinal resistivity at the Landau level filling factor ν=1, are much larger than those for 10 and 20 nm QWs. This is consistent with the well-width dependence of the bare valley splitting estimated from the comparison with the Zeeman splitting in the Shubnikov–de Haas oscillations.The valley splitting in Si two-dimensional electron systems is studied using Si/SiGe single quantum wells (QWs) with different well widths. The energy gaps for 4 and 5.3 nm QWs, obtained from the temperature dependence of the longitudinal resistivity at the Landau level filling factor ν=1, are much larger than those for 10 and 20 nm QWs. This is consistent with the well-width dependence of the bare valley splitting estimated from the comparison with the Zeeman splitting in the Shubnikov–de Haas oscillations.

Alkali-metal-induced Fermi-level and two-dimensional electrons at cleaved InAs(110) surfaces

Low-temperature Hall measurements have been performed on two-dimensional electron gases (2DEGs) induced by deposition of Cs or Na on in situ cleaved surfaces of

Adsorbate-induced quantum Hall system probed by scanning tunneling spectroscopy combined with transport measurements

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