Takahiko Sasaki

Molecular beam epitaxy of wurtzite (Ga,Mn)N films on sapphire(0 0 0 1) showing the ferromagnetic behaviour at room temperature

Wurtzite (Ga,Mn)N films showing ferromagnetic behaviour at room temperature were successfully grown on sapphire(0001) substrates by molecular beam epitaxy using ammonia as nitrogen source. Magnetization measurements were carried out by a superconducting quantum interference device at the temperatures between 1.8K and 300K with magnetic field applied parallel to the film plane up to 7T. The magnetic-field dependence of magnetization of a (Ga,Mn)N film at 300K were ferromagnetic, while a GaN film showed Pauli paramagnetism like behaviour. The Curie temperatures of a (Ga,Mn)N film was estimated as 940K.Abstract Wurtzite (Ga,Mn)N films showing ferromagnetic behaviour at room temperature were successfully grown on sapphire(0xa00xa00xa01) substrates by molecular beam epitaxy using ammonia as a nitrogen source. Magnetization measurements were carried out by a superconducting quantum interference device at the temperatures between 1.8 and 300xa0K with a magnetic field applied parallel to the film plane up to 7xa0T. The magnetic-field dependence of magnetization of a (Ga,Mn)N film at 300xa0K was ferromagnetic, while a GaN film showed Pauli paramagnetism-like behaviour. The Curie temperature of a (Ga,Mn)N film was estimated as 940xa0K.

Magnetic and transport characteristics on high Curie temperature ferromagnet of Mn-doped GaN

Mn-doped GaN films on sapphire (0001) substrates were grown by molecular beam epitaxy system using ammonia as nitrogen source. The result of magnetization measurement gives Curie temperature as high as 940 K. The field and temperature dependencies of the magnetization show coexistence of ferromagnetic and paramagnetic phases. In addition, the temperature dependencies of electrical resistance and carrier concentration were measured to investigate the relation between the ferromagnetism and transport property. Below about 10 K, a similar anomalous increase of magnetization and resistance is observed.

High-Tc ferromagnetism in diluted magnetic semiconducting GaN: Mn films

Abstract Wurtzite GaN:Mn films with an extremely high Curie temperature of around 940xa0K and Mn concentration of only 3–5% are successfully grown on sapphire substrates by molecular beam epitaxy. Magnetization measurements are carried out using magnetic fields of up to 7xa0T parallel to the film surface. The magnetization process reveals the coexistence of ferromagnetic and paramagnetic contributions at low temperatures, with characteristic ferromagnetic magnetization at high temperatures. The observed transport characteristics demonstrate a close relation between magnetism and impurity conduction. The double exchange mechanism of the Mn-impurity band is presented as a possible model for the high-TC ferromagnetism in GaN:Mn.

Chaos-assisted directional light emission from microcavity lasers.

We study the effect of dynamical tunneling on emission from ray-chaotic microcavities by introducing a suitably designed deformed disk cavity. We focus on its high quality factor modes strongly localized along a stable periodic ray orbit confined by total internal reflection. It is shown that dominant emission originates from the tunneling from the periodic ray orbit to chaotic ones; the latter eventually escape from the cavity refractively, resulting in directional emission that is unexpected from the geometry of the periodic orbit, but fully explained by unstable manifolds of chaotic ray dynamics. Experimentally performing selective excitation of those modes, we succeeded in observing the directional emission in good agreement with theoretical prediction. This provides decisive experimental evidence of dynamical tunneling in a ray-chaotic microcavity.

Ray-wave correspondence in limacon-shaped semiconductor microcavities

The limacon-shaped semiconductor microcavity is a ray-chaotic cavity sustaining low-loss modes with mostly unidirectional emission patterns. Investigating these modes systematically, we show that the modes correspond to ray description collectively, rather than individually. In addition, we present experimental data on multimode lasing emission patterns that show high unidirectionality and closely agree with the ray description. The origin of this agreement is well explained by the collective correspondence mechanism.

Properties of ferromagnetic Ga/sub 1-x/Mn/sub x/N films grown by ammonia-MBE

Using ammonia as nitrogen source for molecular beam epitaxy, the GaN-based diluted magnetic semiconductor Ga/sub 1-x/Mn/sub x/N is successfully grown with Mn concentration up to x/spl sim/6.8% and with p-type conductivity. The films have wurtzite structure with substitutional Mn on Ga site in GaN. Magnetization measurements revealed that Ga/sub 1-x/Mn/sub x/N is ferromagnetic at temperatures higher than room temperature. The ferromagnetic-paramagnetic transition temperature, Tc, depends on the Mn concentration of the film. At low temperatures, the magnetization increases with increasing of magnetic field, implying that a paramagnetic-like phase coexists with ferromagnetic one. Possible explanations will be proposed for the coexistence of two magnetic phases in the grown films.

Local structure around Mn in ferromagnetic GaMnN film studied by X-ray absorption fine structure

The local structure of the Mn in a GaMnN film was investigated using extended X-ray absorption fine structure (EXAFS) analysis. This GaMnN film shows ferromagnetism with an extremely high Curie temperature above 740 K. We carried out EXAFS measurement of the K-edge of the Mn and Ga in this sample using a fluorescence-yield mode. The local structure of the atoms around the Mn is in good agreement with that around Ga, indicating that the Mn atoms substitute for Ga in GaN.

Pulsed high magnetic field ESR measurement on Ni/Pd nano-particles

An ESR experiment on Ni/Pd nano-particles was performed using high frequency and high magnetic field. As a result, special spin polarization in Pd/Ni nano-particles was observed in the Pd core around Ni, although the molecular orbits are made from eigenfunctions of a 3D-well potential. The spin polarization region forms localized or isolated states in the region with low Ni-concentration, but above a concentration of about 4 at%, coupling between the spin polarized regions around Ni appears. ESR-signals of Ni/Pd particles suggest that not only Ni atoms in the surface layer, but also Pd atoms in the core of the particles contribute to the magnetic polarization.

Magnetic and electrical properties of Mn-doped GaN

Summary form only given. The rapid development of growth techniques for III-nitride semiconductors has resulted in the successful fabrication of GaN-based optical and electronic devices. In turn, GaN has created much interest recently as a candidate for material of spintronic devices owing to theoretical predictions that GaN-based diluted magnetic semiconductors (DMS) have high Curie temperature Tc exceeding room temperature (RT) (Dietl et al. (2000)). Very recently, we have succeeded in the growth of Mn-doped GaN films showing ferromagnetic behaviour at RT by ammonia-MBE. Here we report magnetic (magnetization) and electrical properties (Hall measurements) of Mn-doped GaN (GaN:Mn) films with a summary of crystallographic characteristics.

Lasing emission patterns from Monza-shaped semiconductor microcavities

We fabricated Monza-shaped semiconductor microcavity lasers and evaluated their far-field characteristics. From the ray dynamical point of view, the Monza-shaped cavity belongs to a special class of fully chaotic cavities; its phase space is decomposed into two independent chaotic components, which yield two different emission patterns. In the present paper, we discuss which ensemble of ray trajectories (i.e., phase space measure) can reproduce far-field emission patterns observed in experiments.