Shingo Katsumoto

(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.

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.

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.

Observation of the Fano-Kondo antiresonance in a quantum wire with a side-coupled quantum dot.

We have observed the Fano-Kondo antiresonance in a quantum wire with a side-coupled quantum dot. In a weak coupling regime, dips due to the Fano effect appeared. As the coupling strength increased, conductance in the regions between the dips decreased alternately. From the temperature dependence and the response to the magnetic field, we conclude that the conductance reduction is due to the Fano-Kondo antiresonance. At a Kondo valley with the Fano parameter q approximately 0, the phase shift is locked to pi/2 against the gate voltage when the system is close to the unitary limit in agreement with theoretical predictions by Gerland et al. [Phys. Rev. Lett. 84, 3710 (2000)].

Mesoscopic Fano effect in a quantum dot embedded in an Aharonov-Bohm ring

The Fano effect, which occurs through the quantum-mechanical cooperation between resonance and interference, can be observed in electron transport through a hybrid system of a quantum dot and an Aharonov-Bohm ring. While a clear correlation appears between the height of the Coulomb peak and the real asymmetric parameter q for the corresponding Fano line shape, we need to introduce a complex q to describe the variation of the line shape by the magnetic and electrostatic fields. The present analysis demonstrates that the Fano effect with complex asymmetric parameters provides a good probe to detect a quantum-mechanical phase of traversing electrons.

Light-induced ferromagnetism in III-V-based diluted magnetic semiconductor heterostructures

We report for the first time the occurrence of light-induced ferromagnetic order in the III-V-based diluted magnetic semiconductor heterostructures (In,Mn)As/GaSb. We believe that the phenomenon is based on the generation of excess carriers (holes) in the (In,Mn)As layer by the irradiation of light, which enhances a carrier-mediated ferromagnetic interaction between Mn ions.

Magnetoresistance oscillation in two-dimensional electron gas under spatially modulated vector potential

We have observed an oscillatory magnetoresistance effect in a two-dimensional electron gas in GaAs/AlGaAs heterojunctions subjected to a periodically modulated magnetic field generated using a striped gate electrode of ferromagnetic metal. The hysteretic part of magnetoresistance clearly shows oscillations analogous to the so-called Weiss oscillation with the phase relation predicted for the case of vector potential modulation.

Metal-insulator transition and magnetotransport in III-V compound diluted magnetic semiconductors

Structural, magnetic and transport properties of diluted magnetic semiconductors, (Ga, Mn)As and (In, Mn)As, have been investigated. Manganese can be substitutionally doped into the group III site of the zincblend structure up to several percent. With Mn content of a few percent, these systems exhibit ferromagnetism at low temperatures. The highest Curie temperature so far achieved is 100 K for (Ga, Mn)As. The saturated magnetization values are consistent with S 5:2 local moment, suggesting divalent Mn which acts as an acceptor. The system becomes metallic with increasing Mn content, but a further increase of Mn content tends to decrease the hole density and increase disorder so that the system becomes nonmetallic again at higher Mn concentrations. Large negative magnetoresistance and highly anisotropic transport are observed in the semiconducting samples at low temperatures. The magnetic anisotropy in ultrathin films is found to be strongly affected by the lattice-mismatch-induced strain.