Analysis of electron mobility versus temperature after photoexcitation in Si-doped AlxGa1−xAs

The low-temperature Hall mobility of photoexcited electrons has been measured in Si-doped MBE AlGaAs samples and compared with calculated data using the background acceptor density and the alloy scattering potential as free parameters. The possibility of discriminating between negative- or positive-U electron correlation energy for the DX centre has been investigated through a careful analysis of mobility versus temperature curves relating to different photoexcited electron densities. A crucial role of the acceptor density to explain the experimental data within the positive-U model has been evidenced.     

Characterization of heterogeneous interaction in binary mixtures of ethylene glycol oligomer with water, ethyl alcohol and dioxane by dielectric analysis

The associating behaviour of the binary mixtures of ethylene glycol oligomer (EGO), i.e. ethylene glycol (EG), diethylene glycol (DEG) and poly(ethylene glycol)s (PEG200, PEG300, PEG400 and PEG600) with water (W), ethyl alcohol (EA) and 1,4-dioxane (DX) over the entire concentration range at 25 °C have been investigated through their accurately measured values of dielectric constant. The static dielectric constant ε_o, high frequency limiting dielectric constant ε_∞, dielectric relaxation strength Δε, excess dielectric parameters ε^E₀ and ε^E_∞, effective Kirkwood correlation factor g^eff and corrective correlation factor g_f of EGO–W, EGO–EA and EGO–DX mixtures were determined to obtain qualitative and quantitative information about the complex formation through H-bond in these systems. Most of the evaluated dielectric parameters of EG and DEG in different ‘cosolvents’ have different characteristics as compared to the PEG–cosolvent mixture. The observed linear and non-linear behaviour of Δε against EGO monomer unit mole fraction X confirms the variation in the homogeneous structures in their binary mixtures with concentration variation. Appearance of the maximum in ε^E₀ against X plots indicates that a complex stable adduct is formed in the EGO–W mixtures at stoichiometric ratio 1:1.7 for lower oligomers but this ratio seems to be 1.7:1 for higher EGO molecules, which confirms that the EGO size and chain flexibility affects the complex formation between EGO and W. In case of EG–EA mixture 1:1 stoichiometric ratio form stable adduct whereas for higher EGO–EA, it is 3:1, at EGO monomer unit level. The complex formation behaviour of DEG–EA has entirely different characteristics when compared to the other studied EGO–EA mixtures. Although, 1,4-dioxane has weak polar behaviour dielectric properties of EG–DX and DEG–DX confirm the formation of stable adducts at the stoichiometric ratio 2:1 of EGO monomer unit mole fraction to the DX. For the higher EGO–DX mixtures, stable adduct forms at the stoichiometric ratio 9:1. Except DEG–EA mixtures, the EGO–W and EGO–EA form the complex with reduction in the effective number of dipoles. In EG–DX mixtures, the heterogeneous species form with a large reduction in the effective number of dipoles, which changes as the effective number of dipoles increases with the increase in monomer repeat units of EGO. Further the net electronic polarization in these binary mixtures increases due to heterogeneous interaction over the entire mixing concentration range.     

Density functional theory calculations establish the experimental evidence of the DX center atomic structure in CdTe

The In DX center and the DX-like configuration of the Cd host atom in CdTe are investigated using density functional theory. The simultaneous calculation of the atomic structure and the electric field gradient (EFG) allows one to correlate the theoretically predicted structure of the DX center with an experimental observable, namely, the EFG obtained from radioactive 111In/111Cd probe atoms in In doped CdTe. In this way, the experimental identification of the DX center structure is established.     

快速热处理对应变InGaAs/GaAs单量子阱激光二极管电子发射和DX中心的影响

研究分子束外延(MBE)生长的应变In0.2Ga0.8AsGaAs折射率梯度变化异质结单量子阱激光二极管的快速热处理(RTA)效应.结果表明,RTA移除了InGaAsGaAs界面非辐射中心,提高77K光致发光效率和有源层电子发射.同时Al和Ga原子互扩散,也增加了AlGaAs波导层DX中心浓度.RTA处理后样品电流冲击老化实验证明DX中心浓度呈现出相应的增加.这表明DX中心可能是激光二极管性能退化的原因之一. 关键词： 量子阱 快速热处理 电子发射 DX中心     

部分道路关闭引起的交通激波特性研究

本文根据实际交通中经常遇到的交通事故或部分道路施工等情况, 建立了部分道路关闭的交通流模型. 采用平均场理论分析和确定性NS元胞自动机规则分别对模型进行解析和数值模拟, 结果表明, 系统存在三种稳定的物理状态:低密度相、激波相和高密度相, 并找到了系统发生相变的临界密度. 理论分析和数值模拟能很好地符合.     

Determination of the range of alpha particles in SSNTD by optical density method

A technique based on the optical density (D) measurement of the etched track is useful for charged particles spectroscopy using SSNTD. It was shown that the stopping power of alpha particles in CR-39 is proportional to D. We measured the optical density and derived an expression to estimate the range of alpha particles in CR-39 detector as a function of the bulk etching rate and etching time. The relation between the etching time, track parameters (depth, radius) and D for different alpha particles energy and etching conditions were studied. A relation describing D as a function of track size is proposed.     

Transport in two dimensions. III self-diffusion coefficient at low densities

The low density form of the generalized frequency (s) and wavevector (k) dependent self-diffusion coefficient D(k, s) is calculated, from which the low density forms of related quantities, e.g. the velocity autocorrelation function, are derived. Agreement is obtained with the low density kinetic theory results. A closed form expression for D(k, s) valid over a wide range of densities and times is also given, showing consistency between the asymptotic long time results, obtained previously, and the low density kinetic theory results.     

金属Fe与间隙H原子相互作用的密度泛函研究

采用基于密度泛函理论的第一性原理方法, 研究了不同摩尔比下H在α-Fe和γ-Fe晶格中的间隙占位情况, 计算了稳态晶体的总能量、结合能、溶解热、电子态密度、电荷差分密度和电荷布居, 分析了间隙H原子和Fe晶格之间的相互作用, 讨论了H溶解对α-Fe和γ -Fe晶体电子结构的影响. 结果表明: H溶解引起α-Fe和γ-Fe晶体点阵晶格畸变, 体积膨胀率随着溶氢量的增加而增加. 从能量角度分析发现, H优先占据α-Fe的四面体间隙位, 而在γ -Fe中优先 占据八面体间隙位. 态密度、电荷差分密度以及电荷布居分析发现, 间隙H原子与Fe晶格的相互作用仅由H的1s轨道电子和Fe的4s轨道电子所贡献, 二者作用力相对较弱, 这是造成H在Fe晶格中固溶度较低的主要原因之一. 关键词： 金属Fe 间隙H原子 第一性原理 溶解热     

Effect of the current density on the volume expansion of the deposited thin films of aluminum during porous oxide formation

The effect of the anodic current density, temperature and concentration of the oxalic acid on the volume expansion of porous alumina films formed from 1.8 μm thick aluminum films has been investigated. The volume expansion of the aluminum during oxidation was determined by the step height between the aluminum surface and the porous alumina surface at the edge of the anodized region, which was measured with a mechanical profiler with computer signal processing. Experiments showed, that the volume expansion factor as well as the cell dimensions is proportional to the anodizing voltage. The dependence of the volume expansion factor (k) on the anodizing voltage (U) has the linear nature. The volume expansion factor in the 4% solution of oxalic acid can be defined by the following equation: k=1.092+0.007U. It was established, that the dependence of the logarithm of the ion current density on the inverse volume expansion factor of porous alumina has the linear nature.     

强关联效应下非磁性元素Ir掺杂的SmFeAsO电子结构理论研究

基于密度泛函理论, 采用广义梯度近似方法 (SGGA+U) 计算分析了SmOFeAs电子结构以及 Ir 掺杂对该体系晶体结构和电子结构的影响. 结果表明, 随着Ir的掺杂, SmOFeAs晶体结构中FeAs层与SmO层之间的耦合增强, 晶体内部所含的铁砷四面体随着Ir掺杂其畸变性程度逐步减小. Fe3d以及As4p杂化轨道对体系电子结构起主要影响作用. Ir掺杂所引入的电子使FeAs层的巡游电子增多、Fe3d轨道中的 d_z2轨道离域性增强. 当Ir掺杂量为20%时, 费米面处于电子态密度峰值附近, 费米面急剧变化使该体系的T_c值有所增高, 反映了体系费米能级移动与其超导电性的密切关联性. 计算的电子态密度与XPS所得价带谱实验结果一致, 进一步验证了采用SGGA+U方法其包含修正d轨道局域电子的库仑势, 使得计算结果与实验结果更加接近. 关键词： U')" href="#">GGA+U SmOFeAs 晶体结构 电子结构     

AlK壳层等离子体辐射谱模型的比对

碰撞辐射模型的比对研究对校验发展等离子体辐射谱模型、提高等离子体参数的诊断精度具有重要意义. 基于Al等离子体, 对常用的辐射模型代码FAC和FLYCHK K壳层辐射谱模型开展了比对研究, 详细比较了它们的离子丰度、特征线强度、谱发射率曲线和吸收系数曲线等特征, 并根据各能态的速率方程, 从FAC和FLYCHK模型的结构特点出发, 分析了造成这些差异的原因. FAC 和FLYCHK计算得到的类H、类He离子n=2, n=3激发态数密度有显著差异, 进而引起特征线发射率及其比值(He-IC/He-αup, He-βup/H-βup)的差异, 从而对等离子体参数的诊断结果产生影响. 除了模型中采用的能级结构和碰撞辐射过程速率外, 计算结果显著地受到FAC 和FLYCHK 模型结构的影响. n=2激发态数密度的差异是由FAC和FLYCHK分别采用能级和超组态(组态)的方式构建n=2激发态的速率方程而引起的, 而FAC代码忽略了n=3与更高激发态之间的碰撞耦合过程, 是引起n=3激发态数密度差异的原因. 主要特征线的吸收系数与基态能级的数密度相关, 受到激发态数密度的影响较小, 因此与谱发射率曲线相比, FAC和FLYCHK计算结果的差异更小.     

Scaling behavior of the thermopower of the archetypal heavy-fermion metal YbRh2Si2

We reveal and explain the scaling behavior of the thermopower S/T exhibited by the archetypal heavy-fermion (HF) metal YbRh₂Si₂ under the application of magnetic field B at temperature T. We show that the same scaling is demonstrated by different HF compounds such as β-YbAlB₄ and the strongly correlated layered cobalt oxide [BiBa_0.66K_0.36O₂]CoO₂. Using YbRh₂Si₂ as an example, we demonstrate that the scaling behavior of S/T is violated at the antiferromagnetic phase transition, while both the residual resistivity ρ₀ and the density of states, N, experience jumps at the phase transition, causing the thermopower to make two jumps and change its sign. Our elucidation is based on flattening of the single-particle spectrum that profoundly affects ρ₀ and N. To depict the main features of the S/T behavior, we construct a T –B schematic phase diagram of YbRh₂Si₂. Our calculated S/T for the HF compounds are in good agreement with experimental facts and support our observations.     

Thermodynamic properties of MgO under high pressure from first-principles calculations

The equations of state (EOS) and other thermodynamic properties of the rocksalt (RS) structure MgO are investigated by ab initio plane-wave pseudopotential density functional theory method. The obtained results are consistent with the experimental data and those calculated by others. Through the quasi-harmonic Debye model, in which the phononic effects are considered, the dependences of relative volume V/V₀ on pressure P, cell volume V on temperature T, and Debye temperature Θ and specific heat C_V on pressure P are successfully obtained. The variation of the thermal expansion with temperature and pressure is investigated, which shows the temperature has hardly any effect on the thermal expansion at higher pressure.     

Induced absorption of light in the magnetoelectric-photo effect in InSb: Saturation at low temperatures

An induced absorption of photons observed in an electron-hole plasma of InSb in the transverse configuration (JH) of the current density J and the magnetic field H has been demonstrated to saturate at high values of J×H at low temperatures. Nevertheless, strong emission of photons having a central wavelength corresponding to the energy gap is still observed at 80 K, suggesting generation of excess electron-hole pairs by the action of the J×H force. The mechanism of the induced absorption is also discussed.     

稳定分层二层流非湍流/湍流层无平均剪切密度界面处的湍流

采用湍流统计理论、谱分析和快速畸变理论研究稳定分层二层流非湍流/湍流层无平均剪切密度界面处的湍流.分别在密度界面厚度（h）可忽略和很薄两种情况下,推导出任意理查森数（Ri）Ri→∞时,湍流层中水平、垂直方向速度的欧拉频谱和水平、垂直方向均方根速度的积分表达式.在h可忽略情况下：（1）任意Ri,Ri→∞时,密度界面对大尺度涡的影响显著,而对小尺度涡几乎无影响;距离密度界面越近,湍流层中水平方向均方根速度增大而垂直方向均方根速度减小;（2）任意Ri且在无量纲频率较大时,密度界面处、湍流层中水平、垂直方向速度的欧拉频谱满足-5/3幂次律,但是,它们不收敛于同一直线,表明密度界面处部分湍流转化为内波.在h很薄的情况下：（1）在水平方向上密度界面对湍流无显著的影响;湍流层中垂直方向速度的欧拉频谱出现过渡区,不满足-5/3幂次律,其幂次律增大,表明湍流过渡区的能量减少,但是,密度界面对线性小尺度涡仍几乎无影响;（2）距离密度界面越远,密度界面厚度对湍流的影响减弱并且偏向于线性中尺度涡;当远离密度界面时,过渡区消失,表明考虑密度界面厚度后密度界面对湍流的影响范围有限;（3）密度界面处垂直方向速度的欧拉频谱的幂次律减小,表明密度界面处线性内波的能量向线性低频区集中;（4）随着密度界面厚度增加,密度界面处垂直方向速度的欧拉频谱在整个线性内波频域里等幅度减小,湍流层中垂直方向速度的欧拉频谱只在线性低频区域减小且减小的幅度随着频率增大而减小;密度界面对湍流层中水平、垂直方向均方根速度影响的垂向范围随Ri增大而减小.     

Large N phase transition in the heat kernel on the U (N) group

The large N phase transition point is investigated in the heat kernel on the U(N) group with respect to arbitrary boundary conditions. A simple functional relation is found relating the density of eigenvalues of the boundary field to the saddle point shape of the typical Young tableaux in the large N limit of the character expansion of the heat kernel. Both strong coupling and weak coupling phases are investigated for some particular cases of the boundary holonomy.     

Density profile and flow of miscible fluid with dissimilar constituent masses

A computer simulation model is used to study the density profile and flow of a miscible gaseous fluid mixture consisting of differing constituent masses (m_A=m_B/3) through an open matrix. The density profile is found to decay with the height ∝exp (−m_A(B)h), consistent with the barometric height law. The flux density shows a power-law increase ∝(p_c−p)^μ with μ2.3 at the porosity 1−p above the pore percolation threshold 1−p_c.     

粒子数密度对飞秒Gauss型脉冲传播及光谱特性的影响

利用预估校正-时域有限差分(PC-FDTD)法求解全波Maxwell-Bloch方程,研究介质粒子数密度(N)对飞秒Gauss型激光脉冲在Λ型三能级原子介质中传播及光谱特性的影响.结果表明:小面积2π脉冲在不同N介质中都不发生分裂,脉冲频谱基本没有新的高频成分产生,随N增大中心频率附近光谱强度明显减小.面积4π脉冲,在N较大的稀疏介质及稠密介质中都产生分裂,在稀疏介质中随N增大频谱展宽幅度及高频成分强度增大,但在稠密介质中频谱展宽变小且远小于N较大时的稀疏介质情况.大面积8π脉冲,脉冲分裂情况与4π脉冲情况相似,但随N增大频谱展宽幅度及高频成分强度单调增大,且在稠密介质中的频谱展宽幅度及高频成分强度远大于N较小的稀疏介质情况.     

Numerical calculation of current density distributions in high temperature superconducting tapes with finite thickness in self field and external field

The current density distribution of high temperature superconducting (HTS) tapes is modeled for the combined case of an alternating self and applied magnetic field. This numerical analysis is based on the two-dimensional Poisson equation for the vector potential. A one-dimensional current (z-direction) and a one-dimensional applied field (y-direction) are assumed. The vector potential is rewritten into an equation of motion for the current density J(x,y,t). The model covers the finite thickness of the conductor and an n-power E–J relation. The magnetic field dependence of J_c is also included in this E–J relation. A time-dependent two-dimensional current distribution that is influenced by the aspect ratio of the conductor and the material properties in E=f(J,B) is calculated numerically. The numerical results are compared with the experimental results for the AC loss of a tape driven by a transport current. Finally, a total AC loss factor is given for two cases in magnetic field direction, perpendicular and parallel to the conductor broad side.     

Dynamical energy equipartition of the Toda model with additional on-site potentials

We study the dynamical energy equipartition properties in the integrable Toda model with additional uniform or disordered on-site energies by extensive numerical simulations. The total energy is initially equidistributed among some of the lowest frequency linear modes. For the Toda model with uniform on-site potentials, the energy spectrum keeps its profile nearly unchanged in a relatively short time scale. On a much longer time scale, the energies of tail modes increase slowly with time. Energy equipartition is far away from being attached in our studied time scale. For the Toda model with disordered on-site potentials, the energy transfers continuously to the high frequency modes and eventually towards energy equipartition. We further perform a systematic study of the equipartition time teq depending on the energy density εand the nonlinear parameter α in the thermodynamic limit for the Toda model with disordered on-site potentials. We find teq∝(1/ε)~a(1/α)~b, where b ≈ 2 a. The values of a and b are increased when increasing the strengths of disordered on-site potentials or decreasing the number of initially excited modes.