Yasuhiro Iye

(Ga,Mn)As: A new diluted magnetic semiconductor based on GaAs

A new GaAs‐based diluted magnetic semiconductor, (Ga,Mn)As, was prepared by molecular beam epitaxy. The lattice constant of (Ga,Mn)As films was determined by x‐ray diffraction and shown to increase with the increase of Mn composition, x. Well‐aligned in‐plane ferromagnetic order was observed by magnetization measurements. Magnetotransport measurements revealed the occurrence of anomalous Hall effect in the (Ga,Mn)As layer.

Tuning of the Fano Effect through a Quantum Dot in an Aharonov-Bohm Interferometer

The Fano effect, which arises from an interference between a localized state and the continuum, reveals a fundamental aspect of quantum mechanics. We have realized a tunable Fano system in a quantum dot (QD) in an Aharonov-Bohm interferometer, which is the first convincing demonstration of this effect in mesoscopic systems. With the aid of the continuum, the localized state inside the QD acquires itinerancy over the system even in the Coulomb blockade. Through tuning of the parameters, which is an advantage of the present system, unique properties of the Fano effect on the phase and coherence of electrons have been revealed.

Effect of low-temperature annealing on transport and magnetism of diluted magnetic semiconductor (Ga, Mn)As

We report improvements in the crystallinity of a III–V-based diluted magnetic semiconductor (Ga, Mn)As by heat treatment (annealing) after growth at comparatively low temperatures. This method can be used to raise the Curie temperature to 100 K without the need for severe optimization of growth conditions, as well as to adjust the material parameters to desired values.

The Anisotropic Upper Critical Field of Single Crystal YBa2Cu3Ox

We report a study of anisotropic upper critical field of single crystal samples of orthorhombic YBa2Cu3Ox. The critical field shows anisotropy characteristic of a quasi-two-dimensional superconductor, and is highest when the field is oriented perpendicular to the c-axis. For this field direction, the zero resistance state persists up to 86 K in a magnetic field of 90 kOe, which is lower by only 5 K than the zero resistance temperature in the absence of magnetic field. The anisotropy Hc2⊥/Hc2// is about 2 at 90 K, 5 at 86.5 K and tends to increase with decreasing temperature.

Epitaxy of (Ga, Mn)As, a new diluted magnetic semiconductor based on GaAs

Abstract GaAs-based diluted magnetic semiconductor, (Ga, Mn)As, with Mn composition x up to 0.07 was prepared by molecular-beam epitaxy on GaAs substrate at temperatures ranging from 160 to 320°C. Clear reflection high-energy electron diffraction oscillations were observed at the initial growth stage, indicating that the growth mode is two-dimensional. The lattice constant of (Ga, Mn)As films determined by X-ray diffraction showed a linear increase with the increase of Mn composition. Well-aligned in-plane ferromagnetic order was observed by magnetization measurements. Magnetotransport measurements also revealed the presence of ferromagnetic order in the (Ga, Mn)As layer. The easy axis of magnetization can be reversed by changing the strain direction in (Ga, Mn)As. GaAs (Ga, Mn)As superlattice structures with high crystal perfection and good interface quality were also prepared.

Anomalous peak effect in single crystal Bi2Sr2CaCu2O8+y studied by Hall probe magnetometry

Abstract We have studied an anomalous peak effect in a Bi 2 Sr 2 CaCu 2 O 8+ y single crystal using a specially designed Hall probe magnetometer. In order to investigate the origin of the anomalous peak effect which manifests itself in the magnetic hysteresis loop, the relaxation (10 -2 s t 4 s), and the local field imaging experiments were performed. Decoupling of 2D vortices in neighboring CuO layers, namely, a dimensional crossover in the pinning mechanism, is proposed as a possible mechanism of the peak effect.

Absence of current direction dependence of the resistive state of high temperature superconductors in magnetic fields

Resistive transition of Bi 2 Sr 2 CaCu 2 O 8+ y film in magnetic fields is studied in the light of the currently proposed flux creep model. Measurements with precise angular alignment have clarified that the resistive transition does not depend on the relative direction between magnetic field and transport current. This calls for a significant revision of the Lorentz-force-driven flux creep picture. A fluctuation induced phase-slip phenomenon as an alternative model is also discussed.

Nonmetal-metal-nonmetal transition and large negative magnetoresistance in (Ga, Mn)As/GaAs

Abstract We have studied magnetic and transport properties of a series of Ga 1−x Mn x As GaAs samples with different Mn concentrations (x = 0.015−0.071. For Mn content higher than about 0.02, carrier(hole)-induced ferromagnetism is observed. Samples with x = 0.035 and 0.043 behave as ferromagnetic dirty metals. With further increase of Mn content above x ∼ 0.05, the zero-field resistivity turns a semiconducting temperature dependence. Very large negative magnetoresistance is observed in non-metallic samples near the metal-nonmetal transitions both in the low and the high Mn content regimes.

A new type oscillatory phenomenon in the magnetotransport of θ-(BEDT―TTF)2I3

Abstract A new type oscillatory magnetotransport phenomenon has been observed in θ-type crystals of (BEDT-TTF) 2 I 3 at temperatures below 6 K and in the magnetic field above 3 T. The oscillation appears when the magnetic field of a fixed strength is rotated from the direction normal to the conductive two dimensional plane to a direction parallel to the plane. The period of the oscillation is described by an equation tan ( θ min )= sN ( s =0.39, N =0, 1, 2, 3,…), where θ min is the angle giving the position of the trough of the oscillation. The amplitude of the oscillation is primarily determined by the magnetic field component normal to the conductive plane. It is related to the temperature through the change in the resistivity ⪢ 0 .

Metal-Insulator Transition in the Bi2Sr2Ca1-xYxCu2O8+y System

Metal-insulator transition driven by the change of the carrier concentration was investigated in the 80 K superconductor Bi2Sr2CaCu2O8+y. The carrier concentration was controlled by the substitution of yttrium for calcium. Experimental results from structural, transport, and magnetic studies are discussed in comparison with other high-temperature superconductor systems, La2-xSrxCuO4 and YBa2Cu3O7-y.