arXiv subjects

Tohru Okamoto

Publications and source records attributed to Tohru Okamoto.

At least 19 recordsLinked to original sources

arXiv2017

Magnetic-Field-Induced Superconductivity in Ultrathin Pb Films with Magnetic Impurities

It is well known that external magnetic fields and magnetic moments of impurities both suppress superconductivity. Here, we demonstrate that their combined effect enhances the superconductivity of a few atomic layer thick Pb films grown on a cleaved GaAs(110) surface. A Ce-doped film, where superconductivity is totally suppressed at zero-field, actually turns superconducting when an external magnetic field is applied parallel to the conducting plane. For films with Mn adatoms, the screening of the magnetic moment by conduction electrons, i.e., the Kondo singlet formation, becomes important. We found that the degree of screening can be reduced by capping the Pb film with a Au layer, and observed the positive magnetic field dependence of the superconducting transition temperature.

cond-mat.supr-con↗

arXiv2017

Effect of Parallel Magnetic Field on Superconductivity of Ultrathin Metal Films Grown on a Cleaved GaAs Surface

The parallel-magnetic-field $H_\parallel$ dependence of the superconducting transition temperature $T_c$ is studied for ultrathin films of In, Bi, and Al grown on GaAs(110). In the case of In films in the monolayer regime, $T_c$ exhibits a quadratic-like $H_\parallel$ dependence, which is one order of magnitude stronger than that previously observed in monolayer Pb films by the present authors [Phys. Rev. Lett. 111, 057005 (2013)]. The results are well reproduced by a model developed for an inhomogeneous two-dimensional superconducting state in the presence of a moderate Rashba spin-orbit interaction. The Rashba spin splitting is estimated to be 0.04 eV, which is much smaller than that expected for monolayer Pb films. In a few-monolayer Bi film, the suppression of $T_c$ with increasing $H_\parallel$ is comparable to that in monolayer Pb films. On the other hand, much stronger suppression, which is attributed to the Pauli paramagnetic effect, was observed for the Al film.

cond-mat.supr-con↗

arXiv2013

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 14 T, 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.

cond-mat.supr-con↗

arXiv2013

In-plane magnetic field dependence of cyclotron relaxation time in a Si two-dimensional electron system

Cyclotron resonance of two-dimensional electrons is studied for a high-mobility Si/SiGe quantum well in the presence of an in-plane magnetic field, which induces spin polarization. The relaxation time $τ_{CR}$ shows a negative in-plane magnetic field dependence, which is similar to that of the transport scattering time $τ_t$ obtained from dc resistivity. The resonance magnetic field shows an unexpected negative shift with increasing in-plane magnetic field.

cond-mat.mes-hall↗

arXiv2011

Metallic Behavior of Cyclotron Relaxation Time in Two-Dimensional Systems

Cyclotron resonance of two-dimensional electrons is studied at low temperatures down to 0.4 K for a high-mobility Si/SiGe quantum well which exhibits a metallic temperature dependence of dc resistivity $ρ$. The relaxation time $τ_{\rm CR}$ shows a negative temperature dependence, which is similar to that of the transport scattering time $τ_t$ obtained from $ρ$. The ratio $τ_{\rm CR}/τ_t$ at 0.4 K increases as the electron density $N_s$ decreases, and exceeds unity when $N_s$ approaches the critical density for the metal-insulator transition.

cond-mat.mes-hall↗

arXiv2010

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 ($\sim$ fine structure constant $\sim$ mrad) is found to significantly deviate from the Drude-like behavior to exhibit a plateau-like 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.

cond-mat.mes-hall↗

arXiv2009

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.

cond-mat.mes-hall↗

arXiv2009

Insulating Phases Induced by Crossing of Partially Filled Landau Levels in a Si Quantum Well

We study magnetotransport in a high mobility Si two-dimensional electron system by in situ tilting of the sample relative to the magnetic field. A pronounced dip in the longitudinal resistivity is observed during the Landau level crossing process for noninteger filling factors. Together with a Hall resistivity change which exhibits the particle-hole symmetry, this indicates that electrons or holes in the relevant Landau levels become localized at the coincidence where the pseudospin-unpolarized state is expected to be stable.

cond-mat.mes-hall↗

arXiv2009

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.

cond-mat.mes-hall↗

arXiv2008

Electronic Transport Properties of the Ising Quantum Hall Ferromagnet in a Si Quantum Well

Magnetotransport properties are investigated for a high mobility Si two dimensional electron systems in the vicinity of a Landau level crossing point. At low temperatures, the resistance peak having a strong anisotropy shows large hysteresis which is attributed to Ising quantum Hall ferromagnetism. The peak is split into two peaks in the paramagnetic regime. A mean field calculation for the peak positions indicates that electron scattering is strong when the pseudospin is partially polarized. We also study the current-voltage characteristics which exhibit a wide voltage plateau.

cond-mat.mes-hall↗

arXiv2008

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 p-type InAs. The surface donor level, at which the Fermi energy of the 2DEG is pinned, is calculated from the observed saturation surface electron density using a surface potential determined self-consistently. The results are compared to those of previous photoelectron spectroscopy measurements.

cond-mat.mes-hall↗

arXiv2007

Ettingshausen Effect around Landau Level Filling Factor nu=3 Studied by Dynamic Nuclear Polarization

Spin current perpendicular to the electric current is investigated around Landau level filling factor $ν=3$ in a GaAs/AlGaAs two-dimensional electron system. Measurements of dynamic nuclear polarization in the vicinity of the edge of a specially designed Hall bar sample indicate that the direction of the spin current with respect to the Hall electric field reverses its polarity at $ν=3$, where the dissipative current carried by holes in the spin up Landau level is replaced with that by electrons in the spin down Landau level.

cond-mat.mes-hall↗

arXiv2007

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

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

cond-mat.mes-hall↗

arXiv2006

Dynamic nuclear polarization induced by hot electrons

A new method for local dynamic nuclear polarization is demonstrated in a GaAs/AlGaAs heterostructure at the Landau level filling factor $ν=3$. Using a narrow channel sample, where the width varies stepwise along the electron flow, we find that electron cooling (heating) causes the polarization of nuclear spins against (toward) the external magnetic field at liquid helium temperatures. The longitudinal nuclear spin relaxation rate varies exponentially with inverse temperature.

cond-mat.mes-hall↗

arXiv2006

Electrically detected electron spin resonance in a high mobility silicon quantum well

The resistivity change due to electron spin resonance (ESR) absorption is investigated in a high-mobility two-dimensional electron system formed in a Si/SiGe heterostructure. Results for a specific Landau level configuration demonstrate that the primary cause of the ESR signal is a reduction of the spin polarization, not the effect of electron heating. The longitudinal spin relaxation time T_1 is obtained to be of the order of 1 ms in an in-plane magnetic field of 3.55 T. The suppression of the effect of the Rashba fields due to high-frequency spin precession explains the very long T_1.

cond-mat.mes-hall↗

arXiv2005

Observation of the Ettingshausen effect in quantum Hall systems

Evidence of the Ettingshausen effect in the breakdown regime of the integer quantum Hall effect has been observed in a GaAs/AlGaAs two-dimensional electron system. Resistance of micro Hall bars attached to both edges of a current channel shows remarkable asymmetric behaviors which indicate an electron temperature difference between the edges. The sign of the difference depends on the direction of the electric current and the polarity of the magnetic field. The results are consistent with the recent theory of Akera.

cond-mat.mes-hall↗

arXiv2005

Spin polarization and metallic behavior of a silicon two-dimensional electron system

We have studied the magnetic and transport properties of an ultra-low-resistivity two-dimensional electron system in a Si/SiGe quantum well. The spin polarization increases linearly with the in-plane magnetic field and the enhancement of the spin susceptibility is consistent with that in Si-MOS structures. Temperature dependence of resistivity remains metallic even in strong magnetic fields where the spin degree of freedom is frozen out. We also found a magnetoresistance anisotropy with respect to an angle between the current and the in-plane magnetic field.

cond-mat.mes-hall↗

arXiv2005

Two-dimensional electrons at a cleaved semiconductor surface: Observation of the quantum Hall effect

Low-temperature in-plane magnetotransport measurements have been performed on adsorbate-induced electron systems formed at in-situ cleaved surfaces of p-type InAs. The Ag-coverage dependence of the surface electron density strongly supports a simple model based on a surface donor level lying above the conduction band minimum. The observations of the quantized Hall resistance and zero longitudinal resistivity demonstrate the perfect two-dimensionality of the surface electron system. We also observed the Rashba effect due to the strong asymmetry of the confining potential well.

cond-mat.mes-hall↗