A possible replacement of single crystal by melt-textured material in the study of YBa2Cu3O7−y

Improved superconducting properties were observed in melt-textured YBa₂Cu₃O_7-y ceramic material. The inductive current density has a small field dependence up to 5.5 T over a wide temperature range. The field and temperature dependences of the magnetic properties are similar to the behavior found in single crystals. Values of the intragrain critical current density in melt-texture samples have the same order of magnitude as in single crystals. The reduced porosity and improved grain alignment help to probe intrinsic physical properties that depend on crystallographic orientation. Thus experiments similar to those involving single crystals are possible. The modified Bean critical state model applied to single crystals can also be used in these melt-textured samples. Thus, it is possible to use melt-textured samples in some basic research which originally required the use of single crystals.     

直流电场中电流变体单链的电学行为及剪切强度

研究了由微小的潮湿玻璃球（直径为200—220μm）在硅油中加直流电场后形成的单“链”的电学行为及剪切强度．发现流过静态（无剪切）单链的电流随外加电场强度的增加而增加，受剪单链的剪切屈服应力与外加电场强度约1.4次幂成正比，这个电流和剪切屈服应力与组成链的玻璃球数（3—5个）几乎无关．实测单链的平均电流密度和剪切屈服应力随电场强度的变化与理论预言符合良好 关键词：     

High field effects in insulating CdF2: Gd crystals

CdF₂ crystals doped with Gd, before any conversion to semiconductor, exhibit at low temperatures electrical current and electroluminescence with d.c. voltage applied.The electrical conduction, attributed to electrons, is bulk limited and the i = i (V, T) characteristics are interpreted as a Poole-Frenkel conduction.The electroluminescence, due to intrinsic and impurity emission, is attributed to the impact process of field accelerated electrons, which give rise to electron-hole pair formation and impurity centre excitation.     

Ultraviolet lasing and field emission characteristics of ZnO nano-rods synthesized by nano-particle-assisted pulsed-laser ablation deposition

We describe the lasing and the field emission characteristics of ZnO nano-rod crystals synthesized by nano-particle-assisted pulsed-laser ablation deposition, where nano-particles formed in the gas phase by the condensation of the laser-ablated species are transported on to the substrate to form nanorods. The nano-rod crystals having a hexagonal shape with a pyramidal top were synthesized on a sapphire substrate by a single-step process. The nano-rod crystals act as an efficient ultraviolet emitter showing the stimulated emission under an optical excitation and also act as field emitters with a current density of 1 mA/cm² at an electric field strength of 16 V/μm. PACS 42.70.Hj; 85.45.Db; 81.05.Ea; 81.15.Fy     

Modeling of ionization–recombination processes in the atmospheric surface layer

The electrical structure of non-stationary horizontally-homogenous surface layer with multi-charged aerosol particles was mathematically modeled in the approximation of turbulent electrode effect. The profiles of positive and negative small ions and nuclei, electric field, polar air conductivity, current density and space charge density were computed in different time periods and various physical conditions. The mathematical model of non-stationary horizontally homogenous surface layer with aerosol particles was made regarding turbulent mixing and convective transport. The space-time distributions of positive and negative small ions and nuclei, electric field, electrical conductivity, current density and space charge density for various physical conditions (aerosol concentrations, turbulent mixing, convective transport, air ionization rate, electric field strength near surface, aerosol particles size) were received. Experimental data of electrical and meteorological parameters were measured and analyzed. It was received that theoretical results are in a good agreement with experimental data.     

Effect of a constant magnetic field and a pulse electric current on the mean linear density of twinning dislocations in bismuth crystals

Bismuth crystals are studied under the joint action of a pulse electric current and a constant magnetic field. It is shown that the combined effect of a constant magnetic field and pulse current leads to a substantial decrease in the mean linear density of twinning dislocations piled up at the boundaries of wedge twins. The decrease in the mean linear density of twinning dislocations is accompanied by a decrease in the microhardness of the samples.     

Electrical conductivity and thermally stimulated current in the paraelectric phase of (NH4)2SO4

Anomalous variations of d.c. electrical conductivity with temperature are observed in ammonium sulphate single crystals, suggesting a possible phase transition at 150°C. Measurements of thermally stimulated current also support these results. The mechanism of electrical conduction is explained on the basis of studies made on doped and quenched crystals.     

Superconducting characteristics of (Bi, Pb)-2223 single crystals with controlled oxygen-content

Critical current density was measured for oxygen-controlled (Bi, Pb)-2223 single crystals before and after the irradiation with gold ions in a magnetic field parallel to the irradiation-induced defects along the c-axis. Eleven specimens prepared in different annealing conditions were measured. The condensation energy density of each specimen was evaluated from the observed critical current density by using the summation theory of pinning forces of columnar defects and the flux creep theory. It was found that the specimen heat-treated at 1 atm in oxygen atmosphere has the highest condensation energy density among all specimens. Hence, it is speculated that the optimum oxygen pressure for the anneal is around 1 atm.     

Optically stimulated depolarization currents in Ca(NbO3)2 crystals

In Ca(NbO₃)₂ crystals a depolarization current is obtained in short-circuit measurements by light irradiation at 90 K after application of an external electrical field at cooling down. The spectral response of this current shows a broad band over the whole visible wavelength range. The position and height of the current maximum depend on the amount and direction of the spectral scanning speed. Since the short-circuit current is assumed to be associated with the rate of heterocharge neutralization by the drift of photo-excited carriers these phenomena are discussed in the model of persistent internal polarization rather than in that of moving zero-field plane. The results of calculation show good agreement with the experimental data.     

Effect of orographic features on global atmospheric electrical parameters over 160 different places of United States

Global atmospheric electrical parameters like atmospheric conductivity, air-earth current density, electric field, and atmospheric potential have been estimated over different places of United States (US) assuming fair weather conditions. Results show that atmospheric electrical conductivity and current density increase with height from sea level, while electric field remains constant at all altitudes. The atmospheric potential, on the other hand, decreases in US orography. New best fit regression lines between the atmospheric electrical parameters and height from sea level have been examined for US orography in both clean and clear atmosphere.     

Nature of the short wavelength excitations in vitreous silica: An X-Ray brillouin scattering study

Spontaneous emission in photonic crystals with anisotropic three-dimensional dispersion relation is studied. If the upper level is below a characteristic frequency omega(1), or above omega(2), or between omega(1) and omega(2), the radiation is a localized field with a frequency in the band gap, or a propagating field with a frquency in the band, or a diffusion field, respectively. An analytical expression for the Lamb shift is obtained. The Lamb shift for the current case is small compared to that in an ordinary vacuum or in one- or two-dimensional photonic crystals due to lower density of states.     

Anisotropic superconducting properties of FeSe0.5Te0.5 single crystals

We investigated the anisotropic electrical transport and magnetic properties of FeSe$_{0.5}$Te$_{0.5}$ single crystals grown by the self-flux method. The in-plane resistivity shows a metallic-like temperature dependence, while the out-of-plane resistivity shows a broad hump with a maximum at around 64 K. The magnetization loops for $H/\!/c$-axis and $H/\!/ab$-plane are also different, for example, there is a typical second peak for $H/\!/c$-axis. The in-plane critical current density is larger than the out-of-plane one. The coherence length and penetration depth were estimated by the Ginzburg-Landau theory. The anisotropic parameter $\gamma $ depends on the applied magnetic field and the temperature. The coupling of superconducting FeSe(Te) layers and the flux pinning mechanism relevant to anisotropy are also discussed.     

Contribution of Disclination Lines to Free Energy of Liquid Crystals in Single-Elastic Constant Approximation

In the light of C-mapping method and topological current theory, the contribution of disclination lines to free energy density of liquid crystals is studied in the single-elastic constant approximation. It is pointed out that the total free energy density can be divided into two parts. One is the usual distorted energy density of director field around the disclination lines. The other is the free energy density of disclination lines themselves, which is shown to be centralized at the disclination lines and to be topologically quantized in the unit of kn/2. The topological quantum numbers are determined by the Hopf indices and Brouwer degrees of the director l~eld at the disclination lines, i.e. the disclination strengths. From the Lagrange‘s method of multipliers, the equilibrium equation and the molecular field ofliquid crystals are also obtained. The physical meaning of the Lagrangian multiplier is just the distorted energy density.     

Measurements of Electrical Discharge Properties of a Transverse-Flow CO2 Laser

This paper deals with investigations of the spatial structure of the electrical potential, the current density, the gas temperature and the reduced electrical field strength in the discharge volume of a transverse gas flow CO₂-laser in dependence on the geometry of the electrodes. The data were obtained by means of Langmuir double probes. The results are compared with optical measurements applying a probe laser resonator.     

Topological Defects in Liquid Crystal Films

A topological theory of liquid crystal films in the presence of defects is developed based on the Ф-mapping topological current theory. By generalizing the free-energy density in ＂one-constant＂ approximation, a covariant free- energy density is obtained, from which the U（1） gauge field and the unified topological current for monopoles and strings in liquid crystals are derived. The inner topological structure of these topological defects is characterized by the winding numbers of Ф-mapping.     

Characterization of the hole transport and electrical properties in poly(9,9-dioctylfluorene)

A systematic study of the hole transport and electrical properties in blue-emitting polymers as poly(9,9-dioctylfluorene) (PFO) has been performed. We show that the temperature dependent and thickness dependent current density versus voltage characteristics of PFO hole-only devices can be accurately described using our recently introduced improved mobility model based on both the Arrhenius temperature dependence and non-Arrhenius temperature dependence. Within the improved model, the mobility depends on three important physical quantities: temperature, carrier density, and electric field. For the polymer studied, we find the width of the density of states σ=0.115 eV and the lattice constant a=1.2 nm. Furthermore, we show that the boundary carrier density has an important effect on the current density versus voltage characteristics. Too large or too small values of the boundary carrier density lead to incorrect current density versus voltage characteristics. The numerically calculated carrier density is a decreasing function of distance from the interface. The numerically calculated electric field is an increasing function of distance. Both the maximum of carrier density and minimum of electric field appear near the interface.     

Effective electrical and thermal conductivity of multifilament twisted superconductors

The effective electrical and thermal conductivity of composite wire with twisted superconducting filaments embedded into normal metal matrix is calculated using the extension of Bruggeman method. The resistive conductivity of superconducting filaments is described in terms of symmetric tensor, whereas the conductivity of a matrix is assumed to be isotropic and homogeneous. The dependence of the resistive electrical conductivity of superconducting filaments on temperature, magnetic field, and current density is implied to be parametric. The resulting effective conductivity tensor proved to be non-diagonal and symmetric. The non-diagonal transverse–longitudinal components of effective electrical conductivity tensor are responsible for the redistribution of current between filaments. In the limits of high and low electrical conductivity of filaments the transverse effective conductivity tends to that of obtained previously by Carr. The effective thermal conductivity of composite wires is non-diagonal and radius-dependent even for the isotropic and homogeneous thermal conductivities of matrix and filaments.     

Magnetic field penetration into half-space for type-II superconductors of the second kind

Equations that simulate the magnetic induction and current density distributions in half-space in view of the power I-V characteristic are derived. The magnetization front velocity is determined for a given mean rate of external magnetic field variation at the boundary of the sample. An integral condition for the electrical resistance (nonlinearly depending on the magnetic field) under which the magnetic flux penetrates into the sample with a finite rate is found. An analytical solution that simulates the power variation of the magnetic field at the boundary is given. The Bean generalized model describing the current density distribution near the critical current is considered. It is shown that solutions like shock waves may arise beyond the applicability domain of the Bean model.     

FeSe基超导单晶与薄膜研究新进展:自旋向列序、电子相分离及高临界参数

FeSe基超导体的超导临界温度可大范围调控,物理现象丰富,是非常规超导机理研究的热点.由于较高的超导临界参数及易于加工等特点,FeSe基超导体在超导应用开发方面也日益受到重视.大尺寸高质量的单晶和薄膜形态的FeSe基超导材料,对于相关基础科学研究和应用开发都极为重要.作者近年来先后开发和发明了水热离子交换（ion-exchange）、离子脱插（ion-deintercalation）、基底辅助水热外延生长方法,成功解决了二元FeSe和插层（Li,Fe）OHFeSe超导体高质量单晶和薄膜的生长和物性调控难题.进而在相关物理问题的研究中取得新进展,包括发现二元FeSe中自旋向列序与超导电性密切相关,观测到（Li,Fe）OHFeSe中的电子相分离现象.此外,（Li,Fe）OHFeSe超导薄膜呈现很高的超导临界电流密度和上临界磁场,其应用前景值得关注.