Features in the dielectric properties and temperature-composition phase diagrams of NaNbO3-A0.5Bi0.5TiO3 (A = Li, Na, K, Ag) solid solutions

In the systems of solid solutions (1−x)NaNbO_3−x A_0.5Bi_0.5TiO₃, where A is Li (I), Na(II), K(III), and Ag(IV), the concentration dependence of the temperature T _m, corresponding to a maximum in the dielectric permittivity ε, is described by a curve with minimum. In systems II and IV, a “ break” is observed on the T _m(x) dependences in the region of x≈0.2, and the compositions with x<0.2 are characterized by anomalously large temperature hysteresis of ε(T) reaching 80–100 K. Some compositions of systems I and III have very diffuse peaks of ε(T) in the room temperature range, which is of interest from the standpoint of search for lead-free relaxer materials.     

Features of the relaxation polarization of a polar dielectric under conditions of large electrothermal loads

In a polar dielectric occurring under the conditions of large electrothermal loads, it is necessary to take into account the temperature dependence of the conductivity γ(T) and complex permittivity $\overset{\lower0.5em\hbox{$\overset{\lower0.5em\hbox{ . An allowance for this circumstance in the simplest model of the Debye relaxation polarization leads to deviation of the ε″=f(ε′) curve from the well-known Cole-Cole semicircle. Exact solutions of the model problems for a flat capacitor are presented for the cases when the temperature gradient is caused by an external source and by relaxation losses in the dielectric material.     

Low-frequency dielectric relaxation in 5TsB liquid crystals controlled by the ion adsorption-desorption processes

Stable liquid-crystalline states with various ε′ and ρ values were obtained in planar-oriented 5TsB liquid crystals by exposure to constant and alternating electric fields. The initial portion of the low-frequency dispersion of ε′ and ε″ obeys the Debye equation for both strictly planar and nonplanar (“oblique”) orientation of molecules.     

Refined parameters of the pair potential of interaction between nitrogen and water molecules

Taking into account new experimental data on the humidity of nitrogen, the second cross virial coefficient B _aw (T) is quite satisfactorily described in a broad temperature range in terms of the “6–12” Lennard-Jones potential.     

Thermal response of a fluid near its critical point:3He atT>T c

The local density response is studied in a simple fluid near the liquid-vapor critical point, subjected to temperature oscillations of its container. This investigation provides a new approach in the study of the adiabatic energy transfer (“piston effect”) in the fluid. The density response functionZ _F (ω, ε,z) is calculated for³He in the absence of stratification, where ω is the angular frequency, ε=(T−T _c )/T _c the reduced temperature,T _c =3.316 K the critical temperature, andz the vertical position in the container. Experiments are described where the density is measured by two superposed capacitive sensors in a cell of 3.5 mm height, and where the temperature oscillation frequencyf=ω/2π is varied between 10⁻⁴ and 2 Hz. Over the experimental range 5×10⁻⁴<ε<5×10⁻² there is in general reasonable agreement between predictions and experiments. The systematic departures might be accounted for by deviations from 1D geometry, which were not included in the calculations. Over the frequency and reduced temperature ranges, the damping effect from the critical bulk viscosity is predicted to be too small to be detectable. The observed effect of the stratification and its frequency dependence inZ _F are briefly discussed. In the appendix, the predicted critical acoustic attenuation from the bulk viscosity is compared with published data, the effect from finite thermal conductivity of the fluid container plates and also the corrections toZ _F for the effects of the cell sidewalls are calculated. F. Pobell     

Nonequilibrium Casimir force on a particle in cold vacuum background near a heated plate

General expressions for a nonequilibrium Casimir force in a stationary situation for the “small particle-plate” system with arbitrary local dielectric properties of materials are obtained for the given particle (T ₁) and plate (T ₂) temperatures and a “cold” (T ₃ = 0) vacuum background over the plate.     

Electronic Damping of Dislocations and Kinetic Phenomena in Superconductors Under Plastic Deformation

Interaction of a moving screw dislocation with conduction electrons is considered in the conventional pure superconductors. At temperaturesT≪T _c , both “slow” dislocations are considered, which interact only with thermally excited quasiparticles, and “fast” dislocations which also break the Cooper pairs. Near the critical temperature, two limiting cases are considered depending on the relation between Meissner penetration depth and anomalous skin-layer depth for the dislocation-induced electromagnetic field. Power dissipation dependence on temperature and dislocation velocity is obtained. Due to low field intensity in the short-wave part of spectrum, the superconducting state is shown not to be destructed by a moving dislocation in a broad range of temperatures. Non-equilibrium states of electronic system created by the dislocation field are analyzed in the paper. With this purpose, Eliashberg’s kinetic equation for a multi-mode excitation source is used. Dislocation field is shown to reduce the order parameter whenT≪T _c , or stimulate superconductivity when(T−T _c )/T _c≪ 1. Damping reduction due to stimulation effect is discussed. Power dissipation dependence on the dislocations concentration in non-equilibrium state is obtained.     

Mathematical Model for Transitional Processes in Josephson Cryotrons Based on Tunnel Junctions

In this work, we consider controlling of logical states of Josephson memory cells (cryotrons) based on superconductor-insulator-superconductor (SIS) Josephson tunnel junctions by external current impulses. A mathematical model for the transitional processes that take place during direct logical transitions “0” → “1” and inverse logical transitions “1” → “0” is proposed. By means of mathematical modeling, we investigate transitional processes in cryotrons during the change of their logical state and obtain their transitional characteristics for operational temperatures T ₁=11.6 K and T ₂=81.2 K, close to the boiling temperatures of helium and nitrogen, respectively. It is shown that such memory cells can efficiently operate under the temperature T ₂=81.2 K. The behavior of the Josephson cryotrons as well as their operational stability is explored.     

Temperature-dependent integral exciton absorption in semiconducting InP crystals

The temperature dependence of the fundamental absorption edge in free-standing “epitaxial” InP layers has been experimentally studied. The integral exciton absorption coefficient K(T) exhibits an increase at low temperatures, which is explained in terms of the exciton-polariton mechanism of light transfer in semi-conductor crystals with spatial dispersion. A critical temperature (T _c = 200 K), above which the integral absorption becomes constant, has been experimentally determined, and the corresponding critical decay parameter (Γ_c = 0.341 meV), longitudinal-transverse splitting (ħω_LT = 0.175 meV), and oscillator strength of the exciton transition (β = 0.237 × 10⁻⁴) have been calculated. The temperature dependence of the true dissipative decay has been determined.     

The effect of illumination on the current-voltage characteristics and conductivity of MnIn<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

The effect of illumination on the current-voltage (J-U) characteristics and the temperature dependence of conductivity σ(T) of MnIn₂S₄ single crystals has been studied. Under illumination, the J-U curves can be divided into three parts corresponding to the linear (J ∼ U), “3/2” (J ∼ U ^1.5) and “5/2” (J ∼ U ^2.5) laws. This behavior is explained by an increase in the density of free charge carriers under the joint action of electric field and illumination. The current J ₁ in the illuminated sample is 10⁴–10⁵ times the dark J ₂ value. The positions of energy levels in the semiconductor crystal under study are determined.     

Study of the dielectric parameters of aluminium ore bauxite of Mainpat area of Chhattisgarh at X-band frequency

A simple method for measuring the dielectric parameter of materials in the form of powders at microwave frequencies is suggested. Measurement of the permittivity ε′ and ge″ at 9·967 GHz on powder samples of the aluminum ore bauxite gives interesting results. It is found that ε′ and ε″ increases with packing densities (δ ). Further ε′ and ε″ also depend upon the percentage of Al₂O₃. These results show that the values of ε′ and ε″ can be used to set certain basic values for minability of the ore for a particular sample. Conductivities (σ) and relaxation (τ) are also calculated in the present studies     

Effect of manganese on the superconductivity of aluminum

The superconducting critical fields of pure aluminum and of two AlMn alloys, containing 440 and 900 ppm Mn, have been measured as a function of temperatureT from their transition temperaturesT _c down toT=0.07 K. The observedT _c depression can be understood in terms of Kaiser's theory for nonmagnetic localized states in superconducting alloys, with an effective Coulomb coupling constantN _d (0)U _eff equal to 0.43 for the AlMn alloys. Additional evidence that the Mn impurities do not possess a well defined magnetic moment in aluminum in this temperature range is obtained from the critical field results. In particular, the BCS law of corresponding states is obeyed by these alloys. We conclude that these AlMn alloys can be considered BCS superconductors, with a pairing interaction which is weaker than that in pure aluminum.Research supported by the National Science Foundation.     

Theoretical Study on Knight Shift and Nuclear Spin Relaxation Rate in the Charge Ordering State of the 1D Quarter-Filled Electron System

The magnetic susceptibility χ(T) in normal states of the quasi-one-dimensional (Q1D) superconductor is calculated by using the N-chain renormalization group technique developed newly. For temperatures above T _cross being of the order of the interchain hopping t┴, χ(T) of QID system agrees with that of one-dimensional systems. For temperatures below T _cross where the Fermi liquid state is expected, it is found that dχ(T)/dT>0 in contrast to the case of the normal Fermi liquid, i.e., dχ(T)/dT=0. The origin of such a “pseudo-gap like” behavior is discussed in terms of antiferromagnetic spin fluctuations.     

On “cold” and thermal Casimir-Polder forces for an atom interacting with a heated plate

Expressions for the “cold” and thermal parts of the Casimir-Polder force for an atom interacting with the surface of a plate possessing an arbitrary temperature T has been analyzed within the framework of a fluctuational-electromagnetic interaction theory that was previously developed for the “small spherical particle-metal plate” system. It is emphasized that this situation is nonequilibrium, since atom in the ground state has T = 0. A general expression for the nonequilibrium (temperature-dependent) part of the Casimir-Polder force is obtained.     

Theory of the Self-Organized Critical State in Nonequilibrium 4He

A near-critical sample of superfluid ⁴He subjected to a combination of a heat current Q and a gravitational field g has recently been confirmed experimentally to exhibit a self-organized critical (SOC) state, in which the local temperature profile T(z) parallels the local superfluid transition temperature, T (z), so that T(z)–T (z) is independent of the vertical coordinate z. We show that there is a particular heat current Q ₀(g)g such that the SOC state lies in the normal phase only when Q<Q ₀(g). The SOC state is shown to be dynamically stable and, surprisingly, to possess a propagating mode in which perturbations travel upstream at a particular speed c _SOC(Q). For Q>Q ₀(g) the SOC state is pushed below the superfluid transition. We present a model based on the Model F equations that shows that the resulting superfluid state is intrinsically dynamical and supports an average temperature gradient by way of an approximately uniform density of recurring phase slips. The role of a certain local dynamical instability temperature, T _c(Q, z), in the nucleation of the phase slips is emphasized.     

Simultaneous Measurements of Thermal,Electrical, and Acoustic Properties of BaTiO<Subscript>3</Subscript> – New Feature of 403 K Phase Transition

In order to obtain useful information on the transient process of phase transitions in ferroelectrics by various methods including calorimetry, the “mK-stabilized cell” of a small size has been developed. It is based on the heat flux differential scanning calorimeter (DSC) and has a temperature stability better than 1 mK. The “cell” can be used to change the temperature under nearly quasi-static conditions by an infinitesimally small rate not only on heating but also on cooling while passing through the transition points. It enables simultaneous measurements of endothermic heat and exothermic heat along with dielectric constants, displacement currents, etc. with a high degree of temperature resolution. X-ray diffraction measurements for sensing thermal anomalies are also possible by a minor modification of the “cell.” Precise and simultaneous measurements of thermal, electrical, and acoustic properties were carried out at the 403 K phase transition in BaTiO₃ single crystal grown by the top-seeded solution growth method. It has been clarified that the exothermic heat at the transition on cooling has more useful information than the endothermic heat on heating; in the cooling process two thermal anomalies occur separately at T₁ and T₂ although the transition is in a narrow temperature range. It is recognized from other methods that the nature of the transition on cooling is not of a single but of multiple steps. Resonant ultrasonic measurements using the “cell” were carried out, after developing a new excitation method. The sample does not have a simple softening approaching the transition point on cooling but has different elastic moduli for the two thermal anomalies at T₁ and T₂. The dielectric constant also has an intermediate constant value between T₁ and T₂. The crystal structure in the room temperature phase below the transition point has been determined by X-ray diffraction. In this region, tetragonal and monoclinic structures coordinating with each other exist. Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Low-Temperature (T=250–400 K) oxygen adsorption on YBa2Cu3O6.9 ceramics

Oxygen adsorption on YBa₂Cu₃O_6.9 ceramics at T=250–400 K was studied by thermal desorption (TD) mass spectrometry. It was established that, depending on the adsorption temperature (T _a), oxygen occurs on the substrate surface either in two forms (γ1 and γ2, for T _a < 350 K) or in a single form (γ2, for T _a ≥350 K). In the TD spectrum, γ1 adsorbed oxygen species are manifested by a peak with a maximum at T _max=330–340 K. When the adsorption temperature increases from 290 to 400 K, the TD peak of γ2 oxygen species shifts from T _max≈380 K to 440 K, which is accompanied by a decrease in the temperature “boundary” for the desorption of structural oxygen. In addition, γ2 oxygen species are capable of participating in isotope exchange with the oxygen of the ceramics. For both forms of adsorbed oxygen, TD most probably proceeds by the associative mechanism with an activation energy of 0.63±0.08 eV (γ1) and 1.13±0.02 eV (γ2). A model qualitatively explaining oxygen sorption in ceramics is proposed, according to which γ2 species are formed in the initial adsorption stage.     

On the role of vacancies in the process of self-diffusion at low temperatures

Expressions for the calculation of the parameters of self-diffusion in a crystal of simple substance at temperatures close to absolute zero are derived. It is established that, in “loose” crystal structures (with the first coordination number below 12), the amount of atoms involved in diffusion motion exceeds the number of vacant sites in the crystal lattice at T=0 K. The results of calculations of the vacancy and diffusion parameters at T=0 K for ten cryocrystals show that the phenomenon of “nonvacancy self-diffusion at T=0 K” can take place only in bcc helium.     

Flux trapping and its effect on AC susceptibility of granular superconductors

Experimental data on the real (x′) and imaginary (x″) parts of AC magnetic susceptibility as a function of DC bias field (H) shows the effect of trapped flux in granular highT _c superconductors. The aim was to substantiate our recent theoretical findings on the basis of a two-component critical state model suitable for granular highT _c superconductors. Stress has been given to understanding the origin of hysteresis inx′(H) andx″(H). It was seen in the experimental data that above a certain value of DC field range irreversibility appears inx′(H) andx″(H) creating hysteresis like loops. Comparison of data with calculated loops shows good agreement.x′(H) andx″(H) curves show considerable asymmetry in presence of trapped flux.     

Anharmonicity in the V<Subscript>2</Subscript>O<Subscript>3</Subscript> molecule and thermodynamic properties of V<Subscript>2</Subscript>O<Subscript>3</Subscript> in the gas phase

A quantum-mechanical calculation of the relative stability, structural parameters, and vibrational frequencies of V₂O₃ molecule isomers for different spin states was carried out using the BPW91/6-311+G(d, p) method. It was shown that the isomer with the C _s structure (nonplanar VOVO rectangle with an O atom attached to it) in the X ⁵ A″ electronic state possesses the maximum stability. The energy of the C _2v symmetry structure was higher than the lowest energy by just 23 cm⁻¹. It definitely indicated the impossibility of usage of the harmonic model in order to calculate the thermodynamic functions of V2O3 (g). A model is proposed based on which the energy levels and vibrational sums of states for this type of motion were calculated for the C _s → C _2v → C _s transition coordinate. These data, as well as results obtained from quantum-mechanical calculations, were used to calculate the thermodynamic functions of V₂O₃ (g) in the temperature range of T = 100–6000 K. The calculations were performed with the five excited electronic states with energies from 1000 to 9000 cm⁻¹ taken into account. A comparison with the data calculated in the “rigid rotator-harmonic oscillator” approximation was performed.