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The cyclotron resonance of two-dimensional electron systems (2DES) of GaAs/AlGaAs heterostructures is investigated in a tilted magnetic field. In this study, the perpendicular (B⊥) and in-plane (B||) magnetic fields are changed systematically. More than two cyclotron resonances caused by B|| are observed. Further, the enhancement of cyclotron mass by B|| and its dependence on B⊥ and electron concentration of 2DES are also investigated in detail. These phenomena are well explained by the resonant subband Landau level coupling model.

We investigate the cyclotron resonance of corrugated lateral superlattices with ultrashort period (12 nm ) grown on (7 7 5)B GaAs substrates. In addition to the broad dips of cyclotron resonance compared with those of the unmodulated samples, the magnetic field dependence of observed cyclotron resonance spectra shows the anticross behavior or more complicated behavior. Several possible mechanisms on this behavior are discussed.

We investigate the influence of electron–electron interactions on cyclotron resonance (CR) of two-dimensional electron systems in tilted magnetic field. It is known that the electron–electron interactions affect the CR spectra in systems that have more than two cyclotron frequencies. In this study, this multi-component system is realized by applying the tilted magnetic field; the tilted magnetic field induces the resonant subband-Landau level coupling (RSLC) and a splitting of the CR spectrum is observed. Observed anomalous CR spectra cannot be explained by the RSLC with the single-electron picture. Present theoretical treatments cannot account for the result of our experiment.

We investigate different types of the breakdown of the quantum Hall effect (QHE), which are characterized by the sample width dependence of the critical current for the breakdown. The distinction of two different types of breakdown is thought to be the properties of the two-dimensional electron gas (2DEG) wafers, but it is not related directly to the 2DEG mobility and electron density determined by Hall measurement at low magnetic field. We observe the switching between linear and sublinear dependence of the QHE breakdown by varying temperature, gate voltage, and by illumination condition. The type of QHE breakdown is determined by the extent of potential fluctuations in the 2DEG, which are characterized by the random potential from ionized donors and by screening effects, comparing with those resulting from the thermal broadening of the electron distribution. The result of magnetocapacitance between 2DEG and Schottky gate also suggests that the 2DEG homogeneity play an important role in the breakdown of the QHE.

Variation in the temperature dependence of the resistance and magnetic-field dependence of negative magnetoresistance (MR) with increasing gate voltage has been studied on an n-channel poly-Si MOS inversion layer. We find a crossover from strongly-localized (hopping) to weakly-localized (diffusive) regime in two dimensions around the sheet resistance of ≈6 kΩ with increasing gate voltage. In both regimes, the negative MR due to orbital motion is observed and explained by the quantum interference effects.

Subalpine forest zone in the Japanese snowy mountains is characterized by a mosaic of coniferous forests and meadows.Abies mariesii thickets surround mesic meadows, forming a forest-meadow ecotone. Size and age structure of the thickets were studied to examine the stability of the forest-meadow boundaries on Mount Aizukomagatake in central Japan. The age structure indicates that the small height ofA. mariesii in the thickets was not due to the early stage of succession but due to adverse site conditions. In spite of their small stem height, they were as old as canopy trees in neighboringA. mariesii forests. Snow settling pressure madeA. mariesii thickets with broken main stems on the crest slopes, while snow glide pressure formed those with bending stems on leeward slopes. No evidence was found to show any major changes in the position of the forest-meadow boundaries in the size and age structure. The boundary position appears to have been almost steady for at least this century.

The quantum Hall effect for the GaAs/AlGaAs heterostrcture is investigated by an ac capacitance measurement between the two-dimensional electron system (2DES) and the gate on GaAs/AlGaAs. The capacitance minima at the quantum Hall plateaus are mainly determined not by the 2DES area under the gate but by the edge length of 2DES. There exists the high conductive region due to the edge states along the 2DES boundary, when the bulk conductivity σxx is small enough at low temperatures and high magnetic fields. From the temperature and frequency dependence of the capacitance minima, it is found that the measured capacitance consists of the contribution from the edge states and that of the bulk state, which is treated as a distributed circuit of a resistive plate with the conductivity σxx. The evaluated width of edge states from the capacitance is much larger than the magnetic length and the cyclotron radius expected from the one-electron picture. This wide width of edge states can be explained by the compressible-incompressible strip model, in which the screening effect is taken into account. Further the bulk conductivity of less than 10-12 S (S=1/Ω) is measured by the capacitance of the Corbino geometry sample, where the edge states are absent and the capacitance is determined by only σxx in this geometry. The localization of the bulk state is investigated by the obtained σxx.

This study was conducted to clarify the characteristics of landform effects on fire behavior and post-fire regeneration and to examine regional differences in the effects of fire within a mixed-forest area with various topographic conditions in Hokkaido, northern Japan. Fire spread was controlled by topographic barriers such as ridges and valleys, although this was not clear in the areas with serpentinite. Fire frequency was higher on windward slopes of the prevailing wind, and the size of the burned area varied with the underlying geology, which controlled the areal extent of hillside slopes. In areas with terrace deposits, Wakkanai Formation or serpentinized rock, burned areas were large and included few unburned stands. The type of vegetation that grew following the fires varied with slope aspect and topographic position.Sasa grasslands often formed at wind-exposed sites. The grasslands were well developed in the three areas with the previously mentioned geology, where large-scale burning had occurred. Structure of the re-established forest stands was also affected by landforms. Canopy height and the maximum diameter at breast height varied with slope aspect, topographic position and elevation, as well as with stand age. Tree size was found to decrease at wind-exposed sites within high-elevation zones.

Normal and superconducting properties of PtSi films with thicknesses of d=2–20 nm have been investigated. The superconducting transition has been observed on the thin films down to d=4 nm. The systematic reduction of the transition temperature with decreasing d (increasing the sheet resistance) has been explained by the localization and Coulomb interaction effects on superconductivity. The temperature dependence of the upper critical field indicates that the PtSi films behave as homogeneous superconductors.

The in-plane magnetic field dependence of cyclotron resonance is measured both in single and double layer 2DES. In particular, we observe that the in-plane dependence of cyclotron mass in single-layer 2DES changes with carrier density and perpendicular magnetic field in the low electron density sample, which cannot be explained by the semiclassical calculation.

The effectiveness of estimating stand age from increment core samples taken at stump height (20 cm above ground) was tested in an even-aged stand ofBetula ermanii that had regenerated after a forest fire in 1945 in Hokkaido, Japan. Careful cross-dating revealed that annual rings were missing in 11 cores out of a total of 42 cores sampled, and that all these missing rings occurred in the outermost part of the core. These facts indicate that precaution has to be taken in selecting trees from which cores are to be sampled. The present work also revealed that those trees with missing rings have a characteristic appearance, with a thinner stem and less crown foliage than normal trees with complete rings. It was also found that even if this appearance test failed, the possibility of missing rings could be detected from a declining growth pattern, with extremely narrow rings on the increment core which normal trees did not show.

Characteristics of metal‐oxide‐semiconductor field‐effect transistors (MOSFETs) with sputter‐deposited gate‐oxide films at 4.2 K under high magnetic fields of up to 10 T are presented. The gate‐oxide films are deposited by oxygen‐argon sputtering of an SiO2 target at 200 °C, much lower than a conventional thermal oxidation temperature of 1000 °C. The MOSFET using a sputter‐deposited film has a Hall mobility of 7000 cm2/V s at 4.2 K. The quantum Hall effect and Shubnikov–de Haas oscillations are successfully observed. The magnetic angle dependence of the magnetoresistance and the Shubnikov–de Haas oscillation indicate that this MOSFET has a two‐dimensional electron gas. Analysis of the perpendicular magnetoresistance and the quantum Hall effect indicates that this MOSFET has a weakly localized state similar to that of thermal oxide film.

We have investigated the influence of antidot arrays on the breakdown of quantum Hall effect (QHE) in GaAs/AlGaAs heterostructures. In the samples without antidots, the value of the critical current remarkably decrease with increasing electron densities by illumination. In the samples with antidot arrays, the critical current still keeps its high value at relatively high electron density after illumination. These results indicate that the antidots can act as artificial potential fluctuations and modify the current distribution in QHE regime.

A narrow in‐plane gated constriction is defined by technique employing electron beam lithography and wet chemical etching. Two‐dimensional electron gas beside the narrow channel is used to control the number of occupied subbands in the constriction. A ballistic transport through the point contact is manifested by the observation of successive resistance steps when the subbands are depopulated.

The SnTe, Sn1−xEuxTe and Sn1−xSrxTe (x

SnTe-based films and superlattices (SLs) were prepared and their electrical properties were measured. A EuTe/SnTe SL exhibited a hole mobility of 2720 cm2/V s, which is the highest value reported for any semiconductor material at room temperature. The SnEuTe film also exhibited high hole mobility in contrast to the PbEuTe system. These properties are explained in terms of the band offsets of EuTe/SnTe heterojunction and a decrease in the number of Sn vacancies. In addition, SnTe/PbSe and SnTe/PbS SLs with thin SnTe layers displayed n-type conduction with Seebeck coefficients comparable to those for PbSe and PbS. These properties reflect the type II heterostructures.

Density and level of interface states are accompanied by growth interruption, which is inevitable in an in situ process using molecular beam epitaxy (MBE). This MBE process coupled with focused ion beam systems is investigated by means of the Hall measurements and capacitance–voltage measurements. The experimental results are compared to those of the self-consistent calculation. These results suggest that interface states are located at a shallow level from the conduction band edge.

Effect of the regrowth temperature of the cap layer grown successively on a Si focused ion beam (FIB) implanted GaAs surface on the dopant activation was investigated using a FIB/molecular beam epitaxy combined system. Indication of the reevaporation of the implanted Si was observed at high regrowth temperature and the fabrication process was improved by using low regrowth temperature. A high doping efficiency was obtained for the ion dose at about 1×1013 cm−2. Present results indicate the importance of controlling the regrowth condition to obtain high doping efficiency.

GaAs/GaAlAs quantum wires with a new type of side gates are fabricated. The gate electrodes are made of two‐dimensional electron gas (2DEG) and defined by Ar ion beam etching (IBE) or Ga focused ion beams (FIB) in the heterostructure wafers. The geometrical width of the conduction channel and the side gate is about 0.5–1.5 μm, and the side gate is separated by about 0. 5 μm from the conduction channel. It was observed that negative gate voltage could completely deplete the conduction channel, and, from Shubnikov–de Haas oscillation measurements, that both carrier density, and effective wire decreased with increasing the negative gate voltage. It is found that the FIB process can simplify the process steps, but induce more damage in the narrow channel than the Ar IBE process.