Superconductivity in palladium-noble metal-hydrogen systems

The apparently puzzling observations in the superconducting behavior of palladium—noble metal—hydrogen (deuterium) systems, viz., the occurrence of higherT _c and a monotonic increase inT _c for Au, Ag and Cu systems; the absence of the H(D) induced opposite isotope effect; the existence of an optimum noble metal content, different for different alloy systems, beyond which superconductivity ceases; and the existence of an optimum hydrogen content, different for different alloys, giving the highestT _c , have been accounted for by taking due cognition of the electronic and the crystallographic behavior of these alloys and their role in superconductivity, the participation of H(D)-optic phonons, in conjunction with anharmonicity, in BCS-type pairing interaction and the manifestations of the metallurgical and thermodynamical properties of these alloys in superconductivity.     

The superconducting transition temperatures of the thorium-hydrides

The contributions to the quantity ² F() coming from the different vibration modes are calculated for the compounds Th₄H₁₅ and Th H₂. The electronic quantities have been evaluated in a cluster approximation while simple models are used for the phonons. The high transition temperature of Th₄H₁₅ is explained by the large contribution of the hydrogen modes to the McMillan, which overcompensates the deterioration of the Th-lattice in comparison to the Th-metal. In Th H₂ the hydrogen contribution turns out to be insufficient for a highT _c . An important property of these substances is the significant admixture off-states at the Fermi level.     

Energy gaps and transition temperatures of ultrathin superconducting films

Energy gaps and transition temperatures of extremely thin aluminum, indium and lead films, deposited on SiO or naphthalene, depend on the film thickness up to about 300 Å. This dependence is due to quantum size effects and to surface interactions.     

Elastic interaction and the phase transition in coherent metal-hydrogen systems

We study the statistical mechanics of hydrogen dissolved in metals. The underlying model is based on the assumption that the dominant attractive interaction between the protons in the metal is of an elastic nature.

In the first part of the paper we review some general properties of the elastic interaction. We then discuss the importance of boundary conditions for the form of the elastic interaction, which turns out to be of the Curie-Weiss type with macroscopic range.

In the second part we investigate the a-a' (‘gas-liquid’) phase transition in the hydrogen lattice fluid. The long-range part of the elastic interaction is treated in mean field approximation. In the canonical ensemble as opposed to the grand canonical ensemble one finds no co-existing phases near the critical point. Instead there is a continuous transition which changes into a first-order transition at tricritical points. In the temperature-density region which normally corresponds to the two-phase co-existence region the hydrogen density is inhomogeneous and varies on a macroscopic scale.

The peculiar nature of the a-a' phase transition is due to the long-range character of the elastic interaction, which ultimately results from the requirement of coherency of the host crystal. We argue that coherent metal-hydrogen systems offer examples of real systems where the classical theory of phase transitions applies.     

Electrotransport in a coherent metal-hydrogen system

The growth and decay kinetics of macroscopic hydrogen density modes in single crystals of niobium excited by an applied electric field was measured. The samples were prepared in-situ as to prevent incoherent precipitations. At a low hydrogen concentration (c=0.04 H/Nb) the diffusion coefficient and the effective charge agree well with results from experiments using polycrystalline samples. At a high hydrogen concentration (c=0.28 H/Nb) the electrotransport experiments could be extended into the unstable region of the incoherent phase diagram. The spinodal temperature obtained from the concentration amplitude (350±12 K) is significantly lower than the incoherent spinodal temperature. Important difficulties in evaluating electrotransport experiments at high hydrogen concentrations are discussed.     

Effect of pressure on the superconducting transition in Bi-Sr-Ca-Cu-O system

Abstract

The effect of pressure on T_c was studied on 110 K class Bi-Sr-Ca-Cu-O (1:1:1:2) compounds. T_c was varied with current in I^3/2 , and defined at the value extrapolated to zero current. dT_c /dP = + 0.18 K/kbar was obtained up to 10.9 kbars.     

Effect of nuclear irradiation on the superconducting transition temperatures of A-15 materials

A mechanism for the degradation of superconducting transition temperatures T_c by high energy nuclear irradiation in A-15 materials is proposed. The radiation is supposed to produce small “disordered” regions having a very low T_c, in a matrix having the original T_c of the material. The composite system has a well defined but lower T_c due to the proximity effect of the “disordered” regions.     

High pressure induced shifts in the superconducting transition temperature of sputtered films

Both negative and positive shifts in the superconducting transition temperature with negligibly small changes in the resistance ratio were observed in the NbGe and V₃Si films, respectively, under pressure up to ～ 22 kbar. The results are discussed along with the roles of defects recently proposed in these materials.     

First order phase transition in the system of two coupled fields with different transition temperatures

It is shown that for the case of a strong field coupling in the system of two fields the low temperature phase may be stable in the fluctuation region of the high temperature phase.     

High temperature superconducting behaviour in PbTePb system

We have observed a strongly field dependant diamagnetic susceptibility in PbTe up to temperatures reaching 20° K. This could be explained through anomalous superconductivity of lead precipitates in subgrain boundaries of PbTe crystals.     

Internal stresses in the α -α′ phase transition of coherent metal-hydrogen systems

In anα-α′ phase transition of the type discussed by Wagner and Horner, the smoothly varying hydrogen distribution generates coherency stresses in the metal crystal. The system is in a metastable state, which is expected to have a long lifetime if the internal stresses are smaller than some critical stress for destroying coherency. A comparison of the calculated internal shear stress with the experimental value of the critical shear stress for the onset of plastic deformation in a Nb crystal suggests that there may well be an appreciable region in which a coherent transition is experimentally observable.     

Weak electron localization and electron-electron interaction effects in metallic glasses at temperatures just above superconducting transition

Electrical resistance and magnetoresistance (MR) of metallic glasses Zr_xBe_1−x (x = 0.6; 0.7) and Zr₇₅Rh₂₅ in the temperature range of 4.2–12 K and in the magnetic field of 7.4T have been studied. It has experimentally been shown that in accordance with the theory of weak localization of electrons (WEL) and electron-electron interaction (EEI) temperature dependences of conductivity and magnetic conductivity (MC) are determined by localization effects and EEI in the Cooper and Diffusion channels. The comparison of theoretical and experimental values of coefficients at temperature laws of the conductivity variation points out to the need to take into account the Maki-Thompson (MT) temperature correction in the absence of a magnetic field.     

Superconducting transition temperatures of the system V1−xTixHy

T_c measurements for V_1?xTi_xH_y show that a decrease in T_c always occurs upon hydrogenation, but with slopes ($\text{?T}{}_{\mathrm{c}}\text{?y}\text{)}{}_{\mathrm{x}}$ that change with x. A phenomenological model is suggested, associating the T_c decrease in most of the range of x to changes in nominal number of conducting electrons and in the volume induced by the hydrogenation. The model roughly accounts for the variations in T_c in the V-rich alloys. It fails to explain the T_c variation at the Ti end.     

X-ray scattering and theα-α′ phase transition in coherent metal-hydrogen systems

Metallic singlet ground state systems have often anomalous elastic properties which result from the coupling of the phonons to the crystal field energy levels of the rare earth ions. It is shown that structural and magnetic phase transitions should occur in the same way in those systems. They are caused by a sufficiently strong electron-ion interaction. While the orbital part of this interaction is responsible for possible structural phase transitions, the spin dependent part is responsible for magnetic phase transitions. This is demonstrated in detail by considering as an example the aspherical Coulomb charge scattering and the isotropic exchange interaction. We calculate the sound velocity, the sound attenuation and the excitations in the presence of the aspherical Coulomb charge scattering. Furthermore we discuss the mutual coupling of structural and magnetic phase transitions. This includes a consideration of other types of coupling than aspherical Coulomb scattering and isotropic exchange.     

Kosterlitz-Thouless-like transition in granular superconducting films

A Kosterlitz-Thouless-like transition in granular superconducting films is considered. A magnetic field fluctuations influence on thermodynamic properties is analyzed. A renormalization of an interaction constant because of “spin-wave” fluctuations is calculated. A role of a weak disorder is considered. A relationship between the transition temperature and the film sheet resistance is found to be in good agreement with experimental data. If the transition temperature is not too low the results are applicable to uniform superconducting films.