Positional Order and Diffusion Processes in Particle Systems

Nonequilibrium behaviors of positional order are discussed based on diffusion processes in particle systems. With the cumulant expansion method up to the second order, we obtain a relation between the positional order parameter $\Psi$ and the mean square displacement $M$ to be $\Psi \sim \exp(- {\bf K}^2 M /2d)$ with a reciprocal vector ${\bf K}$ and the dimension of the system $d$. On the basis of the relation, the behavior of positional order is predicted to be $\Psi \sim \exp(-{\bf K}^2Dt)$ when the system involves normal diffusion with a diffusion constant $D$. We also find that a diffusion process with swapping positions of particles contributes to higher orders of the cumulants. The swapping diffusion allows particle to diffuse without destroying the positional order while the normal diffusion destroys it.Comment: 4 pages, 4 figures. Submitted to Phys. Rev.

Positional order and diffusion processes in particle systems

The relaxation of a nonequilibrium solid to a fluid is determined by observing the positional order parameter in Monte Carlo simulations, and discussed based on diffusion processes in the hard-particle systems. From the cumulant expansion up to the second order, the relation between the positional order parameter Ψ and the mean square displacement is obtained to be Ψ～ exp(−K^2/2d) with a reciprocal vector K and the dimension of the system d. On the basis of this relation, the positional order should decay exponentially as Ψ ～ exp(−K^2Dt) when the system involves normal diffusion with a diffusion constant D. A diffusion process with swapping positions of particles is also discussed. The swapping of particles contributes to the higher orders of the cumulants, and swapping positions allows particles to diffuse without destroying the positional order while the normal diffusion destroys it.journal articl

Comparative study of drain-current collapse in AlGaN/GaN high-electron-mobility transistors on sapphire and semi-insulating SiC

The drain-current collapse at high drain voltage has been studied in AlGaN/GaN high-electron-mobility transistors (HEMTs) on both semi-insulating (SI)-SiC and sapphire substrates using small frequency (120 Hz) sinusoidal wave superimposed dc IDS-VDS characteristics. Low drain-current collapses were observed in AlGaN/GaN HEMTs on SI-SiC substrate when compared with the HEMTs on sapphire substrates. Two and three thermally activated deep traps were observed on SiC-based and sapphire-based HEMTs, respectively. The existence of an additional deep trap (ΔE = 0.61eV) could be associated with the material defects/ dislocations responsible for the severe drain current collapse in sapphire-based HEMTs. The white-light illuminated IDS-VDS characteristics support the existence of more number of deep traps in the sapphire-based HEMTs.application/pdfjournal articl

Temperature dependence of gate-leakage current in AlGaN/GaN high-electron-mobility transistors

We report on the studies of the temperature dependence of gate-leakage current in AlGaN/GaN high-electron-mobility transistors (HEMTs) for the temperature range 20-400°C. The results show that the temperature dependence of gate-leakage current for AlGaN/GaN HEMTs at subthreshold regime (VGS = -6.5V) have both negative and positive trends. It has been observed that the leakage current decreases with the temperature up to 80°C. Above 80°C, the leakage current increases with the temperature. The negative temperature dependence of leakage current with the activation energy +0.61 eV is due to the impact ionization. The positive temperature dependence of leakage current with the activation energy -0.20 eV is due to the surface related traps, and the activation energy -0.99 eV is due to the temperature assisted tunneling mechanism. The drain voltage at a fixed drain-leakage current reveals the occurrence of both positive (+0.28 V/K) and negative (-0.53 V/K) temperature coefficients.application/pdfjournal articl

High-temperature effects of AlGaN/GaN high-electron-mobility transistors on sapphire and semi-insulating SiC substrates

The high-electron-mobility transistors (HEMTs) have been demonstrated on both sapphire and semi-insulating (SI) SiC substrates, and the dc characteristics of the fabricated devices were examined at temperatures ranging from 25 to 500°C. The decrease in drain current and the transconductance with the increase of temperature have been observed. The decrease ratio of transconductance and drain current was similar for both the HEMTs on sapphire and SI-SiC substrates at and above 300°C. The HEMTs on SiC substrates showed better dc characteristics after being subjected to thermal stress up to 500°C. Although the SiC-based HEMTs showed better characteristics up to the temperature of 300°C, compared with the sapphire-based HEMTs, similar dc characteristics were observed on both at and above 300°C. For high-temperature applications (?300°C), additional cooling arrangements are essential for both devices.application/pdfjournal articl

Investigations of SiO2/n-GaN and Si3N4/n-GaN insulator-semiconductor interfaces with low interface state density

The electrical properties of electron beam (EB) evaporated silicon dioxide (SiO2)/n-GaN, plasma enhanced chemical vapor deposited (PECVD) SiO2/n-GaN, and PECVD silicon nitride (Si3N4)/n-GaN interfaces were investigated using high frequency capacitance-voltage measurements. Compositions of the deposited insulating layers (SiO2 and Si3N4) were analyzed using x-ray photoelectron spectroscopy. Metal-insulator-semiconductor structures were fabricated on the metalorganic chemical vapor deposition grown n-type GaN layers using EB, PECVD grown SiO2 and PECVD grown Si3N4 layers. Minimum interface state density (2.5×1011eV-1cm-2) has been observed in the PECVD grown SiO2/n-GaN interface when it was compared with EB evaporated SiO2/n-GaN interface (5.3×1011eV-1cm-2) and PECVD Si3N4/n-GaN interface (6.5×1011eV-1cm-2). The interface state density (Nf) depends on the composition of deposited insulating layers.application/pdfjournal articl

Surface passivation effects on AlGaN/GaN high-electron-mobility transistors with SiO2, Si3N4, and silicon oxynitride

Surface passivation effects were studied on AlGaN/GaN high-electron-mobility transistors (HEMTs) using SiO2, Si3N4, and silicon oxynitride (SiON) formed by plasma enhanced chemical vapor deposition. An increase of IDmax and gmmax has been observed on the passivated (SiO2, Si3N4 and SiON) HEMTs when compared with the unpassivated HEMTs. About an order of magnitude low IgLeak and three orders of magnitude high IgLeak was observed on Si3N4 and SiO2 passivated HEMTs, respectively, when compared with the unpassivated HEMTs. The increase of IgLeak is due to the occurrence of surface related traps, which was confirmed by the observation of kink and hysteresis effect on dc and ac IDS-VDS characteristics, respectively. Though the Si3N4 passivated HEMTs show better dc characteristics, the breakdown voltage (BVgd) characteristics are not comparable with SiO2, SiON passivated and unpassivated HEMTs. The SiON is also a very promising candidate as a surface passivant for AlGaN/GaN HEMTs because it shows better BVgd with low hysteresis width and small ID collapse than Si3N4 passivated HEMTs.application/pdfjournal articl

「福島原発震災に関する研究フォーラム」

text紀要論文 / Departmental Bulletin Paperdepartmental bulletin pape

ゲンチョ コテイ プログラム オ モチイタ タイガイ ジュセイ ハイ イショク ノ 1993ネン ノ セイセキ

体外受精・肝移植(in vitro Fertilization-Embryo transfer: IVF-ET)の手技の簡便化を図るために,月経周期にとらわれず採卵日を固定した体外受精・肝移植プログラムを試みたので報告する。対象は1993年1月から12月までの間に,当科でIVF-ETを施行した92周期(57症例)である。採卵日を水曜日に固定し,その日から遡って10日前からHuman Menopausal Gonadotropin: hMG 225単位/日を7日間投与し,採卵36時間前にHuman Chorionic Gonadotropin: hCGを投与し,採卵を行った。6周期は卵胞発育不全のために採卵中止となったが86周期(57症例)を採卵し,68周期(48症例)に胚移植を施行した。臨床的妊娠数は20周期で,妊娠率は胚移植当たり29.4%,採卵当たり23.3%,患者数当たり35.0%,胚移植患者数当たり41.7%であった。このうち2例は流産,3例は子宮外妊娠であった。1例は内外同時妊娠で,子宮内の胎児は生存している。16例が妊娠継続した。このうち9例は単胎で,7例が多胎であった。採卵日を固定しても,他施設の成績と比べて,良好な成績を修めることができた。当科でのIVF-ETの成績の特徴は,子宮外妊娠と双胎の割合が高いことである。子宮外妊娠に関しては,患者の卵管機能障害に起因する側面もあったが,胚移植時の技術的な問題に起因する側面もあり,改善の余地があることが示唆された。双胎に関しては,当科のプログラムにおける受精卵の質の高さを示唆するものであった。採卵ができずに中止になった症例は,いずれも排卵誘発剤に対する反応性が低下している症例で,受精卵が得られずに中止になった症例の多くは男性不妊症であり,これらの症例に対する新たなる対応が必要と考えられた。以上1993年の当科における固定プログラムjournal articl

Improved measurements of the partial rate asymmetry in B→hh decays

journal articl