Shigeki Kobayashi

Self-Assembled Growth of GaSb Type II Quantum Ring Structures

We demonstrate the self-assembled growth of GaSb quantum ring structures by molecular beam epitaxy. GaSb rings with the internal and external diameters of about 20 nm and 60 nm are successfully formed on GaAs by a growth procedure different from that for InAs rings reported earlier. The shape of GaSb structures can be controlled from a ring-like to an elongated disk-like geometry by changing the amount of deposited GaSb. A possible growth mechanism of GaSb rings is discussed. Photoluminescence spectra of the rings are presented and their features are discussed in terms of the type II band alignment, in which only holes are confined in the ring.

Electrical and Optical Control of Ferromagnetism in III-V Semiconductor Heterostructures at High Temperature (?100 K)

We demonstrate the electrical and optical control of ferromagnetism in semiconductor heterostructures at high temperatures of 100–117 K. The heterostructures consist of Mn delta (δ)-doped GaAs and p-type AlGaAs. We are able to isothermally change the paramagnetic state to the ferromagnetic state and vice versa, by applying a gate electric-field or by light irradiation. The large modulation of TC (ΔTC~15 K) at high temperatures (>~100 K) demonstrated here may pave the way for functional device applications compatible with the present semiconductor technology.

The Effect of Sintering Conditions on the Formation of the High-Tc Phase in the Bi-Sr-Ca-Cu-O System

The effect of heat schedule on the formation of high-Tc phase has been studied. The two-step pattern held at 880°C for 20 min and at 870°C for a long time was chosen as the optimum heat schedule. After 200 hours of treatment at 870°C, zero resistivity was attained at 104 K and the value of AC susceptibility at 4.2 K was as high as 86% of the complete diamagnetism. Preheating at the temperature range of partial melting enhanced the formation of the high-Tc phase due to the densification, texture formation and the nucleus formation of the high-Tc phase.

Absolute gravity change associated with the March 1997 earthquake swarm in the Izu Peninsula, Japan

We carried out both absolute and relative gravity measurements in the Izu Peninsula just before and after the March 1997 earthquake swarm occurred. The measurements revealed significant absolute gravity changes, which we find to be made of three spatial components. The first one is located near Cape Kawana, and would be associated with the volcanic activity that caused the earthquake swarm. The second one would be associated with shallow and localized magma intrusion just beneath Ito. The third one may be due to a change in the deep region beneath the Kita-Izu fault system, which is considered to be a major tectonic line of this region. The gravity changes can be used to detect underground mass movement. For this purpose, we first use crustal movement observations to construct an elastic dislocation model with two tensile faults and a left lateral fault. Then we use the gravity changes to constrain the density of the material which filled the tensile faults. We find that the density is likely to be small, and that the gravity changes of the first component are reproduced well by the fault model. The smallness of the density implies that highly vesiculated magma or water would have injected into the faults.

Detection of volcano deformations and coseismic movements using JERS-1 L-band SAR differential interferometry; combination with other geodetic measurements

We have carried out JERS-1 differential interferometry (DinSAR) research in volcano and earthquake monitoring as The Earthquake Remote Sensing Frontier Project since 1997. The potential of L-band SAR interferometry to detect subtle crustal movements even in mountainous regions in Japan and its geodetic accuracy were evaluated by comparison and combination with other geodetic and seismological observations (e.g. GPS network, leveling survey, precise gravity measurement, strong motion seismology). So far we have succeeded in several cases; (1) lateral fault motion by the Kagoshima earthquake (M=6.3) in 1997, (2) volcano inflation and the following earthquake (M=6.1) around Mt. Iwate in 1998, (3) tensile deformation associated with earthquake swarms around the monogenetic volcano area in the Izu Peninsula in 1997 and 1998.

Crustal movement imaging by D-InSAR, reflector array, dense GPS, leveling and gravity changes in the plate convergent zone, at the Tokai region, central Japan

In order to obtain more precise and quantitative crustal deformations, we combine the results of spaceborne differential interferometric synthetic aperture radar (D-InSAR), with the dense distribution of continuous Global Positioning System (GPS) observations, precise gravity measurements, and leveling surveys in the Tokai region, where dense and continuous geophysical and geochemical monitoring has been carried out due to the expected huge interplate earthquake. Seasonal uplift-subsidence motions of 2 to 6 cm are obtained from winter to the beginning of summer for both 1995 and 1997, that might be concordant with the leveling survey, although the atmospheric effects should play an important role for these conclusions. At the Sagara corner reflector, we have experimentally sifted the surface of the reflector and obtained fairly reasonable results, although the discrepancy of about 1.5 cm (along the line of sight) between theoretical estimate could be attributed to the unexpected contamination of noise from the surroundings and/or analytical processes.

Detection of singular corner reflectors in residential and mountainous areas from SAR images

In a mountainous and highly populated country like Japan, corner reflectors frequently must be placed near confounding objects. A procedure to detect singular corner reflectors in such situations is described. It is found that the initial reduction of speckle by the Enhanced SFP filter greatly facilitated the detection of these corner reflectors from both JERS-1 SAR images and ERS-2 images.

Molecular beam epitaxy of self-assembled GaSb-based quantum dot structures for the control of photoluminescence wavelengths towards 1.3 /spl mu/m range

Self-assembled GaSb quantum dots were fabricated on [001] GaAs by molecular beam epitaxy using Sb/sub 4/ as V group element. GaSb dots showed anisotropic shape elongating along <110> orientation. The Sb/sub 4/ beam fluxes during growth of GaSb affect the dots elongation ratio and the densities. The quantum dot PL emission energy showed a blueshift when the GaSb dots were covered a GaAs cap layer grown at 580/spl deg/C compared with that grown at 500/spl deg/C. By varying the GaSb coverage, the PL peaks from quantum dots can be controlled to reach emission wavelength on optical communication range.

Hidden Fault Scarp Inferred from Gravity Analysis and Disaster Belt of the 1995 Hyogo-Ken Nanbu Earthquake

Dense gravity measurements were carried out together with the GPS positioning along five survey lines across the Rokko fault system. The subsurface structure relevant to the 1995 Hyogo-ken Nanbu earthquake was inferred from the Bouguer anomaly. A hidden fault was discovered on the southwestward extension of the Koyo fault underneath the sedimentary layer. The extension runs just on the edge of the earthquake disaster belt. The thickness of the sedimentary layer was estimated to decrease gradually toward the mountain side. The wedge-like structure of the soft layer and the hidden fault scarp under the Kobe plain may serve as a focusing lens of seismic rays during the earthquake.