The effect of nonmagnetic 4f resonances on the superconducting transition temperature:ThCe alloys

It is shown that a recent theory due to Kaiser concerning the effect of nonmagnetic resonant states on superconductivity appropriately and accurately describes the dependence of the transition temperatureT _c ofThCe alloys on Ce concentration. The parameters describing thef-resonant states in the Anderson model are determined by fitting Kaiser's theory to the data, and reasonable values are obtained. With the assumption that the Hartree-Fock level width remains constant under pressure, it is shown that thef level moves further above the Fermi level as the pressure is increased. This is discussed in relation to recent pressure experiments on La and La compounds containing Ce impurities.Research sponsored by the Air Force Office of Scientific Research, Office of Aerospace Research, U.S. Air Force, under Grant No. AF-AFOSR-631-67-A.     

Partial substitution of Li for Cu in superconducting YBa2Cu3Oy

0.33–33% of the Cu in superconducting YBa₂Cu₃O _y has been replaced by Li (i.e.,x=0.01–1 in single-phase or nominal YBa₂Cu_3–x Li_xO_y). X-ray diffraction powder patterns remain the same as for YBn₂Cu₃O _y , with identical patterns up to about 17% substitution (i.e.,x=0.5). At higher percentages an additional phase appears. Electrical conductivity measurements indicate a small elevation ofT _c at low Li content. Starting at about 5% Li (x=0.15),T _c declines progressively and its width increases asx is raised.     

The influence of short-range magnetic correlations on the superconducting transition temperature

Taking into account correlations between well-defined localized magnetic moments in a dilute magnetic system, we derive an expression for the super-conducting transition temperature which is different from Bennemann's theory. In particular it is shown that the influence of short-range magnetic correlations in the vicinity of a magnetic phase transition on the superconducting transition temperature is twofold: (1) There are large corrections to the Abrikosov-Gor'kov theory. (2) For a certain choice of parameters the phase diagram in the temperature versus concentration plane shows a break (where the system is normally conducting) between two superconducting regions.     

The superconducting transition temperature of multilayer superconductors: The frustratedXY-Josephson model

The superconducting transition temperature of multilayered superconducting structures of the S-N(D)-S type in the RVB theory is defined taking into account the resonance pair tunneling between superconducting layers. The Josephson array defectivity is considered with the frustration parameters.     

The superconducting transition temperature of multilayered superconductors. The frustratedXY-Josephson model

The superconducting transition temperature of multilayered superconducting structures of the RVB theory for the S-N(D)-S type is defined taking into account the resonance pair tunneling between superconducting layers. The Josephson array defectivity is considered with the frustration parameters.     

The superconducting transition temperature of multilayer superconductors: The frustratedXY-Josephson model

The superconducting transition temperature of multilayered superconducting structures of the S-N(D)-S type in the RVB theory is defined taking into account the resonance pair “tunneling” between superconducting layers. The Josephson array defectivity is considered with the frustration parameters.     

The superconducting transition temperature of multilayered superconductors. The frustratedXY-Josephson model

The superconducting transition temperature of multilayered superconducting structures of the RVB theory for the S-N(D)-S type is defined taking into account the resonance pair “tunneling” between superconducting layers. The Josephson array defectivity is considered with the frustration parameters.     

The effect of the superconducting transition on the acoustic losses in audiofrequency niobium resonators

The mechanical Q of Nb disk samples has been measured as a function of temperature with different annealing treatments. A rapid increase in Q is seen on cooling through the superconducting transition temperature. This is shown to be consistent with the predictions of the theory of acoustic attenuation due to conduction electrons, and the measurements are believed to be the first audiofrequency observation of attenuation by this mechanism. Residual losses in the sample limit the possibility of a quantitative comparison with theory in the superconducting state. Maximum Q values obtained were 1.8 × 10⁷ at 690 Hz, 3.8 × 10⁷ at 1.6 kHz, and 4.8 × 10⁷ at 4.2 kHz. Optimization of the annealing treatment may increase the Q still further, thus enabling improved measurements of the electronic attenuation to be made.Work supported in part by the Australian Research Grants Committe.     

Quantum oscillations of the superconducting transition temperature in Al-SiO systems

We report measurements on the superconducting transition temperature T _cand the normal resistance R _nof quench-condensed pure and granular aluminum films. The changes of the superconducting transition temperature are characteristically dependent on the coating film thickness and are inversely dependent on the Al film thickness. The observed oscillations of the transition temperature are interpreted in the framework of the theory of quantum oscillations developed by Kagan and Dubovskii.This work was supported in part by the Deutsche Forschungsgemeinschaft.     

Influence of interstitial oxygen on the superconducting transition temperature of vanadium

The critical temperatureT _c and residual resistivity ₀ were measured in homogeneously oxidized thin vanadium foils. The per at % oxygen induced variations are respectively T _c =–1.2K and ₀=53 n-m. TheT _c data are used to calculate the electron-phonon coupling constant, which decreases 5% per at % of oxygen.     

Effect of stress on the superconducting transition temperature of SrTiO3

The superconducting transition temperatures of several specimens of reduced SrTiO₃ and of Nb-doped SrTiO₃ have been investigated as functions of hydrostatic and uniaxial compressive stresses up to 1.8 kbars. Decreases inT _c as large as 0.12 K have been observed in specimens under hydrostatic pressure. Because of the lowT _c and small compressibility of SrTiO₃, (lnT _c)/P and (lnT _c)/(lnV) are orders of magnitude greater than the corresponding effects in elemental superconductors. The effect of uniaxial stress onT _c varied with the direction of stress. Compression along a [111] direction caused large decreases inT _c, while both small increases and small decreases inT _c have been observed for [100] compression. It is believed that the present results reveal the presence of a sensitive volume dependence in one or more of the parameters important to superconductivity in SrTiO₃, and that no significant electron-transfer effects occurred in the range of stresses of this experiment.     

Pressure dependence of the superconducting transition temperature in alkali tungsten bronzes

The superconducting transition temperatures have been measured for the tetragonal I phase of Na_0.23WO₃, the hexagonal phase of K_0.33 WO₃, and the hexagonal phase of Rb_0.33WO₃ for pressures up to 20,000 bar. The slope of the curves is + 1.7 × 10^–5 K/bar for the sodium bronze, –5.8 × 10^–5 K /bar for the potassium bronze, and –1.5 × 10^–5 K/bar for the rubidium bronze. A sharp change in slope of the transition temperature vs. pressure curve for the potassium bronze at 4000 bar may indicate a phase change.Prepared for the U.S. Energy Research and Development Administration under Contract No. W-7405-eng-82.     

Effect of uniaxial stress on the superconducting transition temperature of NbSe2

The superconducting transition temperature T _c of 2H NbSe₂ was measured under uniaxial stress applied parallel to the crystal c axis. It was found that T _c decreases as the increasing applied compression, while it increases under hydrostatic pressure. It is concluded that T _c is strongly enhanced by the contractive strain within layers and is insensitive to the interlayer spacing.     

Specific-heat anomaly near superconducting transition in high temperature superconductors

A brief appraisal of the current state of understanding on the specific heat anomaly nearT _c in the oxide superconductors is presented. Spectacular specific heat double transitions are explained with plausible models.     

Fluctuation conductivity of tin films below the superconducting transition temperature

Measurements have been carried out of the temperature dependence of the fluctuation-induced excess electrical conductivity of thin (thickness much less than the Ginzburg-Landau coherence length) superconducting films of tin below the transition temperature. Two types of specimen were investigated: (a) films deposited on a glass substrate held at 300 K and (b) films deposited on a glass substrate at 300 K and then covered by a protective layer of germanium. Special care was taken to ensure sample homogeneity. Analysis of the results shows that the fluctuation parameter, ε_c, is not affected by the germanium layer. Its value is, however, much larger than that predicted by the theory of Kajimura, Micoshiba, and Yamaji.     

The superconducting transition temperature for La1−x Gd x

The deviation of the superconducting transition temperature from the Abrikosov-Gorkov behavior in the case of La_1−xGd _x is explained considering the presence of short range spin-glass at low temperature. The theoretical results show good agreement with the experimental data.     

The effect of composition on Tc in some high Tc A-15 structure super conductors

We discuss the effect of composition on T_c for Nb₃X where X = Ge, Ga, Al, Sn and Pt on the Nb rich side of stoichiometry. The concentration dependence (average slope) of T_c for all the materials is between ?1.3 and ?2.6 K per atomic % Nb. To within the accuracy of the data no systematic variation is found between the slopes of T_c and the highest values of T_c observed in these systems. The highest observed T_c in each system occurs at or near the stoichiometric composition. Comparison with radiation damage results suggests that the disorder introduced by deviations from stoichiometry is a major cause of the depression of T_c.