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 a 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.

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.

Dynamic properties of microemulsions in the single-phase channels.

We have studied the dynamic and rheological properties in the single-phase channels of a microemulsion system with a mixed anionic/nonionic surfactant system and decane from the aqueous to the oil phase. One isotropic channel, called the "upper" channel, begins at the L(3) phase (sponge-like phase) of the binary surfactant mixture on the water side and passes with a shallow minimum for the surfactant composition to the oil side. The other "lower" single-phase channel begins at the micellar L(1) phase and ends in the middle of the phase diagram. Both isotropic channels are separated by a huge anisotropic single phase L(α) channel that reaches from the water side to 90% of oil in the solvent mixture. The structural relaxation time of the viscous fluids could be measured with electric birefringence (EB) measurements, where a signal is caused by the deformation of the internal nanostructure of the fluids by an electric field. For the L(3) phase, the EB signal can be fitted with a single time constant. With increasing oil in the upper channel, the main structural relaxation time passes over a maximum and correlates with the viscosity. Obviously, this time constant controls the viscosity of the fluid (η(o) = G'·τ). It is remarkable that the longest structural relaxation time increases three decades, and the viscosity increases two decades when 10% of oil is solubilized into the L(3) phase. Conductivity data imply that the fluid in the upper channel has a bicontinuous structure from the L(3) phase to the microemulsion with only 10% oil. In this oil range, the conductivity decreases three decades, and the electric birefringence signals are complicated because of a superposition of up to three processes. For higher oil ratios, the structure obviously changes to a HIPE (high internal phase emulsion) structure with water droplets in the oil matrix.

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(CR) shows a negative temperature dependence, which is similar to that of the transport scattering time τ(t) obtained from ρ. The ratio τ(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.

Microemulsions from silicone oil with an anionic/nonionic surfactant mixture.

Microemulsion phases have been prepared for the first time from the silicone oil "M(2)" (hexamethyldisiloxane) and a surfactant mixture of a nonionic surfactant "IT 3" (isotridecyltriethyleneglycolether) and an ionic surfactant Ca(DS)(2) (calciumdodecylsulfate). For such a surfactant mixture the hydrophilicity of the system can be tuned by the mixing ratio of the two components. With increasing IT 3 content, the surfactant mixtures show a L(1)-phase, a wide L(α)-region and a narrow L(3) sponge phase. For constant temperature, two single phase channels exist in the microemulsion system. The lower channel (low IT 3 content) ends in the middle of the phase diagram with equal amounts of water and oil, the upper channel begins with the L(3)-phase and passes all the way to the oil phase. Conductivity data show that the upper channel has a bi-continuous morphology up to 40% oil while the lower channel consists of oil droplets in water. In contrast to previous studies on nonionic systems, the two single phase channels are not connected and microemulsions with equal amount of oil and water do not have a bicontinuous structure.

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 system 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.

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.

Ettingshausen effect around a Landau level filling factor nu = 3 studied by dynamic nuclear polarization.

A spin current perpendicular to the electric current is investigated around a Landau level filling factor nu=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 nu=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.

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 T1 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 T1.