Electron scattering and the lattice thermal conductivity of superconducting transition metal alloys

The mechanism of heat energy transport by phonon carriers which are primarily scattered by conduction electrons is re-examined for concentrated alloys in the normal and superconducting phases. The large increase in the normal state lattice thermal conductivity observed experimentally for concentrated niobium alloys, compared with the pure metal, is explained qualitatively as due to the diminished importance of scattering by d-electrons, compared with s-electrons, when the mean free path for the former becomes less than the typical phonon wavelength. Evidence is presented for the existence of a gap-less type of superconductivity in the concentrated alloys for both s and d-electrons.     

Kinetics of the current-induced phase transition in type I superconducting wires

We have measured the time required for the destruction of type I superconductivity in high purity indium wires by a current greater than the critical current. Our results are not in agreement with theoretical treatments presented by Pippard and by Rothen and Bestgen and we suggest possible inadequacies in the theoretical models.     

A simple model for the superconducting phase diagram near the metal-insulator transition

We solve a simple model of competing superconductivity and disorder in a mean-field approximation with two order parameters. A phase diagram showing superconducting-normal metal-insulator as well as superconducting-insulator transitions is obtained. Comparison with experiments shows a good qualitative agreement.     

On the creep of some metals at the superconducting transition

The influence of the superconducting transition on the creep of lead, indium, tin, and thallium was studied. It is shown that the transition to the superconducting state was accompanied by the jump-like creep-rate increase (sharp jump of the deformation creep curve). The jump of the deformation creep curve is indirectly dependent on the stress at which the metal transition from the normal (N) into the superconducting (S) state occurs and is observed when reaching some threshold value of the flow stress which differs for various metals. The regularities of the above phenomenon were investigated at transient and constant-strain creep rate. The time law of the creep in the N and S states and the character of the temperature dependence of the observed effect were determined. It is demonstrated that previously known experimental data and those described in the present paper can be explained without contradiction if they are interpreted in terms of the dynamic drag of dislocations, from the viewpoint of the disappearance of the force of their interaction with the conduction electrons during the N S transition, as well as the increase of the number of movable dislocations in the course of this transition.     

Use of proximity effect in iridium-gold superconducting phase transition thermometers

We have studied the proximity effect in bilayers of thin films of iridium covered by gold. These structures were evaporated onto sapphire single crystals for use as phase transition thermometers in cryogenic particle detectors. By varying the thicknesses of the iridium and gold layers, we achieved critical temperatures as low as 33 mK. The critical temperature of the bilayers is lower than predicted by the theory of de Gennes-Werthamer, but adding a free parameter to the theory allows good agreement. The transitions of the bilayers typically had widths of a few mK, with the narrowest reaching 0.2 mK, and were always sharper than those of pure iridium films evaporated simultaneously.     

The phenomenon of decrease in the friction force upon transition to the superconducting state

A simple explanation is given to the phenomenon of decrease in the friction force between an adsorbed gas film and the surface of a metal, which is observed when the metal is cooled below the superconducting transition temperature. It is pointed out that, below the critical temperature, the friction force must exhibit a characteristic extremum corresponding to the equality of the heat capacities of superconducting and normal phases.     

Fluctuations and the superconducting phase transition: Critical specific heat and paraconductivity

A general discussion is given of the role of fluctuations in introducing new effects in a superconductor at temperatures just above its transition temperature. In this region the BCS theory of superconductivity, a type of mean-field theory, does not predict any variation from normal state behavior until the critical point is reached. But the close analogy between the superconducting phase transition and other phase transitions requires the appearance of deviations from normal state behavior. The existence of a strong rise in the electrical conductivity, or paraconductivity, has been verified by Glover. A review of the phenomenological Landau-Ginzburg theory describing this effect is presented, as well as a calculation of the critical variation in the specific heat. A measurement of the latter effect would constitute an important check on the theory.Most of this material has been presented in a series of lectures given by the author at the Batsheva Seminar on Quantum Fluids, Technion, Haifa, Israël, August 1968; University of Maryland Department of Physics and Astronomy, Tech. Rept. no. 932 (January 1969). The work has been supported in part by the Air Force Office of Scientific Research and by the Office of Naval Research.National Science Foundation Senior Postdoctoral Fellow.     

Comparison of mechanisms of plasticity enhancement at the superconducting transition

The mechanisms of the plasticity enhancement at the N-S transition are considered, based on a comparison of the dependence of this effect on the concentration of defects limiting the dislocation mobility, which, as is shown, is different for competing dynamic models. A method is suggested for determining the role of quasistatic effects, arising due to the inhomogeneity of the dislocation structure of superconductors, in the plasticity enhancement at the N-S transition.     

The volume change at the superconducting transition of lead and aluminum

The volume change at the magnetic field-induced transition from the super-conducting to the normal state has been measured on single crystals of lead and aluminum between 0.3 K andT _c. From these data we have deduced the pressure dependence of the critical fieldH _c, of the critical temperatureT _c, and of the electronic specific heat coefficient . In lead, the results are in good agreement with theoretical calculations by Carbotte, where strong coupling effects are taken into account. We find ln / lnV=3.1±0.8, whereV is the volume. The measurements on aluminum giving ln / lnV=3±4 are consistent with results derived from thermal expansion experiments.This work was in part financially supported by the Schweizerischer Nationalfonds.     

Amplification of microwave radiation in ice upon pressure-induced phase transition

Measurement results of propagation of microwave radiation with frequencies of 13.7 and 34 GHz through a microexplosion region in freshwater ice are reported. An increase in the power of the passing signal at 13.7 GHz by approximately 1 dB/m for about 1 ms was observed. This effect is presumably connected with formation of quasi-liquid films and their consequent freezing.     

Calculation of phase transition dynamics upon laser irradiation of semiconductors

Approximate expressions are obtained for calculations of the time characteristics of fusion and recrystallization, recrystallization rate, and depth to which the material fuses upon laser annealing of semiconductors. The expressions obtained may be used for selection of processing regimes.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 49, No. 1, pp. 132–136, July, 1985.     

On the study of phase formation and critical current density in superconducting MgB2

Superconducting bulk MgB₂ samples have been synthesized by employing sintering technique without using any additional process steps, generally undertaken in view of the substantial loss of magnesium, during heat treatment. Starting with Mg rich powders having different atomic ratios of Mg : B, as against the nominally required Mg : B = 1:2 ratio, we have obtained superconducting MgB₂ samples of different characteristics. The effect of excess Mg in the starting mixture and processing temperature on the phase-formation, transition temperature (T _C) and critical current density (J _C) have been investigated by electrical transport and a.c. susceptibility measurements. The X-ray diffraction and X-ray photoelectron spectroscopic analyses of MgB₂ bulk samples have been carried out to understand the role of excess Mg and the effect of processing temperature. It is established that MgB₂ samples with high critical current density can be synthesized from a Mg rich powder having Mg : B in 2:2 ratio, at temperatures around 790°C. Critical current density has been found to vary systematically with processing temperature.     

Doping-induced electron density modification at lattice sites of ZnO:Ga nanostructures: effects on vibrational and optical properties

Effects of Ga doping on the morphology, microstructure, electron density distribution, and optical properties of hydrothermally grown ZnO nanostructures have been studied by means of scanning electron microscopy, diffuse reflectance spectroscopy, X-ray diffraction, and the maximum entropy methods. It has been shown that while Ga incorporation in ZnO lattice does not result in a large distortion of its wurtzite structure, it affects substantially the electronic charge distribution along the Zn–O bonds. Anisotropic redistribution of the electron charge density around the cation sites consolidates the assumption that the Ga atoms in doped nanostructures incorporate by substituting Zn atoms. The formation of a high density of point defects modifies the lattice dynamics of ZnO; in addition, it introduces a pronounced band-tail in the forbidden band gap.     

Thermoelectric power above the superconducting transition

The thermoelectric power in the fluctuation region above the superconducting transition temperatureT _c is studied theoretically. Use is made of the time-dependent Ginzburg-Landau (TDGL) equation due to Ebisawa and Fukuyama, which includes the correction term of the order ofT/μ as well, whereT is the temperature and μ is the chemical potential. It is shown that the thermoelectric power tends to zero as (T?T _c ) ln [T/(T?T _c )] and as (T?T _c ) as the temperature approaches the transition temperatureT _c for the two-dimensional and the one-dimensional systems, respectively.     

High temperature phase transition in KH2PO4 crystal

DTA, weight loss, infrared and dielectric measurements have been performed on KH₂PO₄ (KDP) single crystal as well as on different particle size specimens. DTA result reveals two endothermic peaks. The lower peak at 180°C is particle size dependent and vanishes in specimen of particle size ≤ 0·1 mm. Dielectric measurements also show similar behaviour. No significant weight loss of the crystal was noticed when kept at 180°C. We are inclined to believe that fragmentation of crystal is likely to be responsible for the transition rather than PO₄-group rotation or decomposition of KDP.     

Fluctuation phenomena below the superconducting transition

The fluctuation propagator for the superconducting order parameter belowT _c is derived for a gapless superconductor. The fluctuation of the order parameter consists of two distinct modes: the fluctuations of the phase and the modulus of the order parameter. The former fluctuation obeys a phononlike dispersion, while the latter fluctuation is described by a damping diffusion type equation. The present theory gives an account of the fluctuation-induced Josephson effect belowT _c observed recently by Carlson and Goldman.     

Electron emission and phase transition of Zr-rich lead zirconate titanate ferroelectrics

This letter focuses on understanding the influences of antiferroelectric–ferroelectric (AFE–FE) phase transition on the ferroelectric electron emission process. The piezoelectric constant and electron emission current of un-prepoled PZT 96.5/3.5 ceramics as a function of emission times were studied. The hysteresis loops of PZT 96.5/3.5 ceramics before and after the electron emission process were presented. The results show that emission currents obtained from PZT 96.5/3.5 ferroelectrics in different phase states were similar. It was difficult to explain the experiment results using the fast AFE–FE phase transition model. Electron emission from PZT 96.5/3.5 ferroelectrics was independent of the AFE–FE phase transition.     

Linear relationship between crystal lattice parameters within one phase of ferroelastic lead orthophosphate

An analysis of experimental data on the temperature dependence of the crystal lattice parameters of lead orthophosphate Pb₃(PO₄)₂ shows that, within one (ferro-and paraelastic) phase, the unit cell parameter c is linearly related to the other lattice parameters. The bending of this linear dependence corresponds to a c value in the vicinity of a phase transition. An additional bending on the b versus c plots for the ferroelastic phase of lead orthophosphate is revealed, which is probably indicative of a second phase transition in the region of 90–120°C. A linear relationship between crystal lattice parameters has been also found in some other ferroelastic and ferroelectric crystals.