A. Oiwa

(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.

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.

Control of magnetization reversal process by light illumination in ferromagnetic semiconductor heterostructure p-(In, Mn)As/GaSb

The reduction in coercive force by light illumination has been found in ferromagnetic semiconductor heterostructure p-(In, Mn)As/GaSb prepared by molecular-beam epitaxy. Enhanced ferromagnetic coupling between Mn ions, arising from excess photogenerated holes, reduces the domain wall energy and changes the magnetization hysteresis characteristics. The value of coercive force returns to the original value when excess holes recombine with trapped 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.

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.

Ultrafast Quenching of Ferromagnetism in InMnAs Induced by Intense Laser Irradiation

Time-resolved magneto-optical Kerr spectroscopy of ferromagnetic InMnAs reveals two distinct demagnetization processes--fast (<1 ps) and slow (approximately 100 ps). Both components diminish with increasing temperature and are absent above the Curie temperature. The fast component rapidly grows with pump power and saturates at high fluences (>10 mJ/cm(2)); the saturation value indicates a complete quenching of ferromagnetism on a subpicosecond time scale. We attribute this fast dynamics to spin heating through p-d exchange interaction between photocarriers and Mn ions, while the approximately 100 ps component is interpreted as spin-lattice relaxation.

4π-periodic Josephson supercurrent in HgTe-based topological Josephson junctions.

The Josephson effect describes the generic appearance of a supercurrent in a weak link between two superconductors. Its exact physical nature deeply influences the properties of the supercurrent. In recent years, considerable efforts have focused on the coupling of superconductors to the surface states of a three-dimensional topological insulator. In such a material, an unconventional induced p-wave superconductivity should occur, with a doublet of topologically protected gapless Andreev bound states, whose energies vary 4π-periodically with the superconducting phase difference across the junction. In this article, we report the observation of an anomalous response to rf irradiation in a Josephson junction made of a HgTe weak link. The response is understood as due to a 4π-periodic contribution to the supercurrent, and its amplitude is compatible with the expected contribution of a gapless Andreev doublet. Our work opens the way to more elaborate experiments to investigate the induced superconductivity in a three-dimensional insulator.

Kondo universal scaling for a quantum dot coupled to superconducting leads.

We study competition between the Kondo effect and superconductivity in a single self-assembled InAs quantum dot contacted with Al lateral electrodes. Because of Kondo enhancement of Andreev reflections, the zero-bias anomaly develops side peaks, separated by the superconducting gap energy Delta. For ten valleys of different Kondo temperature T(K) we tune the gap Delta with an external magnetic field. We find that the zero-bias conductance in each case collapses onto a single curve with Delta/k(B)T(K) as the only relevant energy scale, providing experimental evidence for universal scaling in this system.

Ferromagnetic semiconductor (In, Ga, Mn)As with Curie temperature above 100 K

We have grown (InyGa1−y)1−xMnxAs ferromagnetic semiconductor layers with Mn composition of x up to 0.13 on InP substrates by molecular beam epitaxy. Near the lattice-matched composition, i.e., y∼0.53, the Curie temperature increases linearly with the ferromagnetically effective Mn composition xeff, following the empirical equation TC=1300×xeff. We obtained Curie temperatures above 100 K when x is relatively high (x>0.1; xeff⩾0.08) and the hole concentration is of the order of 1019 cm−3.