Magnetothermal properties of Nb-W,Nb-Ti,and Nb-Mo alloys in the mixed state

Flux jumps and magnetothermal effects in single crystals of niobium and niobium doped with tungsten, titanium, and molybdenum have been studied in the mixed state in a vacuum calorimeter. It is observed that (a) the field at which the first flux jump occurs is independent of the sweep rate at low sweep rates (1–10 Oe sec) but increases sharply at higher sweep rates (～100 Oe/sec) of the magnetic field, (b) the heat evolved during the flux jump becomes larger as the sample's temperature is reduced, (c) the temperature fluctuations of the type observed by Zebouni et al. and Levy et al. are evidently due to the imperfect adiabatic conditions of experiments, and (d) the interval between the successive flux jumps is in good agreement with that expected from the critical state model of Swartz and Bean.     

Dephasing of Bose-Einstein condensates

Abstract

We discuss some recent work on the dephasing of Bose-Einstein condensates of interacting atoms due to fluctuations in the chemical potential μ. At vanishing temperature such fluctuations are due to the uncertainty of the number of particles in the condensate caused by the depletion effect. The dephasing is in this case a non-diffusive (and in principle reversible) effect called ‘collapse’ (and ‘revival’). Above a certain cross-over temperature the dephasing proceeds mainly via a diffusive process which follows in time a short and ineffective phase collapse. The phase-diffusion constant is dominated by the temperature dependent condensate-number fluctuations caused by the scattering of low-energy excitations off the condensate. We also discuss the (in general subdominant) contribution of the occupation-number fluctuations in quasi-particle states in the thermal cloud. For simplicity we discuss here results for a box-like trapping potential.     

MAGNETOSTRICTION EFFECTS IN CRACK LENGTH MEASUREMENTS

Abstract— Fatigue crack lengths were measured in a ferromagnetic material (a low alloy steel) using the d.c. potential drop technique. Significant cyclic fluctuations in potential were observed. Using discriminatory tests it has been established that the fluctuations may be attributed to inverse magnetostriction known as the Villari effect. The strain rate dependence of the fluctuations was investigated using sinusoidal, square and sawtooth waveforms, and the amplitude of the fluctuations was shown to be proportional to strain rate. In some cases the amplitude of the fluctuations was of similar magnitude to the crack length measurement potential.     

反应氯气气氛下卤化银光纤原料的超提纯

采用反应氯气气氛下水平区域熔融生长单晶的方法对卤化银原料进行超提纯,反应氯气气氛是由裂解低沸点碳氯化合物得到;化合物的通式为ＣＨ_ｎＣｌ_４－ｎ(ｎ＝０,１,２)．提纯的温度为４５０~５００℃,温区移动速度为０．２~０．８ｍｍ/ｍｉｎ;载流气体Ａｒ的流量为８０~１６０ｍＬ/ｍｉｎ．提纯后的卤化银原料红外吸收光谱显示,在４０００~８００ｃｍ^－１(２．５~１２．５μｍ)区域内的吸收随波长的增加而降低,中红外４０００~６００ｃｍ^－１(２．５~１６μｍ)区域是卤化银材料最佳的传输波段．用此原料制成的光纤可传输３．３~１６μｍ中红外光谱;传输１０．６μｍＣＯ_２激光的损耗为０．３~０．５ｄＢ/ｍ．     

Application of the embedded atom model to liquid metals: Liquid sodium

The procedure for the calculation of the embedded atom model (EAM) potential for liquid metal, which involves the use of diffraction data on the structure of material in the vicinity of the melting point, is applied to sodium. In fitting the parameters of EAM potential, use is made of the data on the structure of sodium at 378, 473, and 723 K, as well as on the thermodynamic properties of sodium at pressures up to 96 GPa. The use of the method of molecular dynamics (MD) and of the EAM potential produces good agreement with experiment as regards the structure, density, and potential energy of liquid metal along the p ? 0 isobar at temperatures up to 2300 K, as well as along the shock adiabat up to pressures of ～100 GPa and temperature of ～30 000 K. The melting temperature of bcc model of sodium with EAM potential is equal to 358 ± 1 K and close to real. The predicted value of bulk modulus at 378 K is close to the actual value. The self-diffusion coefficients under isobaric heating increase with temperature by the power law with exponent of 1.6546. The values of pressure, energy, heat capacity, and the temperature coefficient of pressure are calculated in a wide range of densities. The compression to 45–50% of normal volume causes a variation of the structure of liquid; this results in the emergence of atoms with a small radius of the first coordination sphere (～2.1 Å) and low coordination number, which form connected groups (clusters). Their concentration increases with decreasing volume and increasing temperature. The pre-peak of pair correlation functions, the height of which increases with heating, corresponds to these atoms. In the region of variation of the structure, the pressure decrease under isochoric heating follows the pattern of water anomaly.     

The nonlinear influence of an electric field on phase transitions in ferromagnetic semiconductors: Lanthanum manganite

We investigate nonequilibrium processes of self-heating induced by electric current in ferromagnetic semiconductors exhibiting colossal magnetoresistance (CMR) in the vicinity of the Curie temperature. The heat balance equation is solved taking into consideration localized states that appear as a result of scattering from magnetic inhomogeneities and are characterized by a percolation threshold proportional to the amplitude of spin fluctuations. The appearance of N-shaped current-voltage characteristics and hysteresis in the dependence of magnetization on electric potential difference, which are caused by the emergence of a “hot” (with respect to internal temperature) semiconductor paramagnetic phase, is revealed in the steady-state regime. The possibility of suppression of the effect of colossal magnetoresistance with increasing potential difference is indicated. The onset of self-oscillation of current and magnetization with decreasing transverse dimensions of the sample is demonstrated.     

Potential fluctuations and metastabilities in hydrogenated amorphous silicon

The inhomogeneities present in hydrogenated amorphous silicon (a-Si : H) give rise to potential fluctuations. They influence the electronic properties of a-Si : H in a profound manner. Nevertheless, very few theoretical and experimental studies acknowledge their presence, because of the inherent difficulties in dealing with them. We find that the width of the potential fluctuations obtained from transport measurements is much smaller than that obtained by optical methods. This may be because the former (transport) depends on long-range potential fluctuations, whereas the latter is sensitive to short-range ones. External perturbations, e.g. light soaking and thermal quenching, are found to have different effects on the long-range potential fluctuations. Light soaking (the Staebler–Wronski effect) seems to increase them, whereas thermal quenching leaves them unchanged.     

Work function of vanadium dioxide thin films across the metal-insulator transition and the role of surface nonstoichiometry

Vanadium dioxide (VO(2)) undergoes a sharp metal-insulator transition (MIT) in the vicinity of room temperature and there is great interest in exploiting this effect in novel electronic and photonic devices. We have measured the work function of vanadium dioxide thin films across the phase transition using variable temperature Kelvin force microscopy (KFM). The work function is estimated to be ～5.15 eV in the insulating phase and increases by ～0.15 eV across the MIT. We further show that the work function change upon the phase transition is highly sensitive to near-surface stoichiometry studied by X-ray photoelectron spectroscopy. This change in work function is distinct from bulk resistance-versus temperature trends commonly used to evaluate synthesis protocols for such vanadium oxide films and optimize stoichiometry. The results are pertinent to understanding fundamental electronic properties of vanadium oxide as well as charge injection phenomena in solid-state devices incorporating complex oxides containing multivalence cations.     

Selective homoepitaxial growth and luminescent properties of ZnO nanopillars

High spatial density ZnO nanopillars (NPs) have been fabricated on catalyst- and pattern-free Si wafers using atmospheric pressure metal organic chemical vapor deposition (APMOCVD) at a moderate temperature (500?°C). The nanopillar diameter is ～ 35 nm and the length is ～ 150 nm, with a density of ～ 2 × 10(9) cm( - 2). The growth evolution of the nanopillars, providing the (0001)(NP) ? (0001)(ZNO grain) ? (100)(Si surface) epitaxial relationship, is extensively studied by scanning and high resolution transmission microscopy. The approach to obtaining the ZnO 1D structures is explained in terms of selective homoepitaxial growth via the crystallographic anisotropy of the seeding layer. The advanced PL properties of ZnO NPs, e.g. indications of free excitonic and absence of defect emission, are related to their single crystalline nature within one pillar and most probably better stoichiometry and less contamination. The observed efficient monochromatic UV emission from the ZnO NPs at room temperature points toward their potential application as building blocks for nanoscale optoelectronic devices.     

Etching studies on (1 0 0) faces of gel-grown ammonium dihydrogen orthophosphate crystals

Chemical etching employing specific etchants with varying etch times (20–60 s) has been successfully applied for the first time to reveal dislocation sites on the polished (1 0 0) faces of ammonium dihydrogen orthophosphate single crystals grown at ambient temperature, using a modified gel technique of double diffusion in a U-shaped beaker assembly. The selective behaviour of the etchant for straight and inclined dislocations has been demonstrated. Growth striations due to temperature fluctuations, low-angle tilt boundary and etch channels corresponding to stacking faults were clearly observed. Surface structures of the etched faces were photographed by optical and scanning electron microscopes and are discussed.     

Mathematical theory of molecular motors and a new approach for uncovering motor mechanism

Molecular motors operate in an environment dominated by thermal fluctuations. A molecular motor may produce an active force at the reaction site to directly move the motor forward. Alternatively a molecular motor may generate a unidirectional motion by rectifying thermal fluctuations. In this case, the chemical reaction establishes free energy barriers to block the backward fluctuations. The effect of the chemical reaction on the motor motion can be represented by the motor potential profile (rectifying barrier andor active driving force). Different motor mechanisms are characterised by different motor potential profiles. The mathematical theory and properties of molecular motors are discussed and a mathematical framework is developed for extracting the motor potential profile from measured time series of motor position. As an example, we discuss the binding zipper model for the F(1) ATPase, which was motivated mainly by the fact that the motor potential profile of the F(1) ATPase is nearly a constant slope.     

Quantum Fluctuations of Mesoscopic Damped Circuit Involving Capacitance-Inductance Coupling at a Finite Temperature

The quantization of mesoscopic damped circuit involving capacitance-inductance coupling is proposed by the method of thrice linear transformation and damped harmonic oscillator quantization. The quantum fluctuations of the charges and current of each loop are calculated by thermo-field dynamics (TFD) in thermal vacuum state, thermal coherent state and thermal squeezed state, respectively. It is shown that the quantum fluctuations of the charges and current not only depend on circuit inherent parameter and coupled magnitude, but also rely on squeezed coefficients, squeezed angle, environmental temperature and damped resistance. And, because of influence of environmental temperature and damped resistance, the quantum fluctuations increase with increasing temperature and decrease with prolonging time.      

Fluctuation Spectral Functions and Phase Transitions for Cold Fermi Gases in 2D

We study cold fermions in 2D that are interacting through the s-wave contact potential, using the path-integral formalism. The phase transitions are investigated using the joint solution of the gap and number equations. In the latter ones, the amplitude and phase fluctuations are taken into account within the t-matrix NSR-type formalism. It is shown that convergent solutions for the fermion density in 2D can be obtained even in the lowest-order (quadratic) approximation for fluctuations with a proper determination of the spectral functions. This approach leads, in particular, to a non-coherent quasicondensate which exists between the Berezinskii-Kosterlitz-Thouless (BKT) transition temperature below which the system is superfluid and a transition temperature T _Δ above which the quasicondensate is destroyed.     

Precursor Phenomena of Nucleations of Quantized Vortices in the Presence of a Uniformly Moving Obstacle in Bose-Einstein Condensates

We investigate excitations and fluctuations of Bose-Einstein condensates in a two-dimensional torus with a uniformly moving Gaussian potential by solving the Gross-Pitaevskii equation and the Bogoliubov equation. The energy gap Δ between the current-flowing metastable state (that reduces to the ground state for sufficiently slowly-moving potential) and the first excited state vanishes when the moving velocity v of the potential approaches a critical velocity $v_{\rm c}$ (>0). We find a scaling law $\Delta\propto(1-|v|/v_{\rm c})^{\frac{1}{4}}$ , which implies that a characteristic time scale diverges toward the critical velocity. Near the critical velocity, we show that low-energy local density fluctuations are enhanced. These behaviors can be regarded as precursor phenomena of the vortex nucleation.     

Face shear piezoelectric properties of relaxor-PbTiO(3) single crystals

Poling relaxor-PbTiO(3) single crystals along pseudocubic [011] results in a macroscopic symmetry of mm2, enabling a large face shear d(36) in Zt±45° cut crystals. In order to allow the determination of electrical properties by the resonance method, square samples are required. Using Pb(In(0.5)Nb(0.5))O(3)-Pb(Mg(1∕3)Nb(2∕3))O(3)-PbTiO(3) crystals, piezoelectric d(36) coefficients were determined to be in the range of 2000-2500 pC∕N, with electromechanical coupling factor k(36)～0.80-0.83. Mechanical quality factor Q～180 and ultralow frequency constant of ～500 Hz m were obtained. Together with the wide temperature usage range (up to ～110 °C) and high ac driving field stability (～5 kV∕cm), such face shear crystals have a promising potential for ultralow-frequency-transducer applications.     

Thermal Amplitude Fluctuations in Arrays of Small Josephson Junctions

In studies on Josephson Junctions arrays (JJA), it is usually assumed that finite size effects enter only through the grain capacitance and that the amplitude of the superconducting gap is well described by the bulk Bardeeen–Cooper–Schrieber (BCS) prediction. Here, we employ path integral techniques to show analytically that for sufficiently small grains thermal fluctuations modify substantially the amplitude of the order parameter and the critical temperature of the JJA. We identify a region of sizes and temperatures for which quantum fluctuations are negligible and thermal fluctuations enhance the critical temperature of the JJA.     

Temperature and excitation density dependent photoluminescence of sputtering-induced GaAs/AlGaAs quantum dots

GaAs/AlGaAs quantum dots (QDs) are fabricated by low-energy ion beam sputtering and molecular beam epitaxy (MBE) re-growth. Temperature (6.5-78?K) and excitation power density (0.49-3.06?W?cm(-2)) dependent photoluminescence (PL) are presented and discussed in detail. The low-temperature PL emission at 720?nm is attributed to GaAs QDs with height of ～6.1?nm and base width of ～23?nm, calculated based on the quantum box model with infinite potential barrier. The calculated QD dimensions are in good agreement with those obtained from atomic force microscopy (AFM) analysis. Nonradiative recombination and Auger-assisted recombination are found to be the main PL quenching mechanisms at high temperature.     

Possible High-Temperature Superconductivity in Multilayer Graphane: Can the Cuprates be Beaten?

We analyze a possible superconductivity in the hole-doped system of layered hydrogenized graphene by taking into account thermal fluctuations of the order parameter. In particular, we demonstrate that in the one-layer case the values of the high mean-field (MF) critical temperature $T_{\mathrm{\mathrm{c}}}^{\mathrm{MF}}\sim 80$ ?C90 K, predicted recently by Savini et al (Phys. Rev. Lett. 105:037002, 2010), do not alter significantly due to the fluctuations, and the Berezinskii-Kosterlitz-Thouless (BKT) critical temperature of the vortex superconductivity is almost the same as the MF temperature at doping 0.01?C0.1. We show that in the case of multilayer system, when the coupling between the layers stabilizes the superconducting phase in the form of fluxon superconductivity, the critical temperature T _c can increase dramatically to the values ??150 K, higher than the corresponding values in cuprates under ambient pressure.     

聚乳酸玻璃化转变行为的分子模拟

采用等温等压(NPT)正则系综,用分子动力学方法研究了聚乳酸的玻璃化转变行为,获得了玻璃化转变温度.在230～440K范围内,通过模拟体系的密度、比体积及相关径向分布函数等参数,最终确定了聚乳酸的玻璃化温度,模拟计算的结果与实验值吻合.     

IN690合金管热挤压温升与挤压力的研究

温升和挤压力是影响钢管挤压过程的重要指标,利用热模拟实验获得了IN690合金的热加工本构关系,建立了IN690合金钢管热挤压过程的有限元模型.采用正交实验设计的仿真实验系统分析了坯料温度（T b=1000～1200℃）、挤压速度（v=20～200 mm/s）和模具预热温度（T d=300～500℃）对管材成形过程中温升...