Östen Rapp

Evidence for correlation effects: −√T behaviour in the low temperature electrical resistance of disordered metals

Abstract It is found that the electrical resistance for several metal-metalloid amorphous alloys decreases with increasing temperature as −√ T from about 20 mK to several K. The form and magnitude of this temperature dependence is in agreement with recent predictions from interaction theories.

Measurements of the electrical resistivity of amorphous ferromagnets

Abstract The electrical resistivity of two amorphous ferromagnets is measured below 60K. There is a minimum in both cases below which the resistivity increases as Δϱ/ϱ ∼ − 1.9 × 10 −3 10 log T . Above the minima the resistivity is proportional to T2 which may be due to electron-electron scattering. The resistivity does not saturate down to at least 70 mK.

Superconducting fluctuations in the resistivity of Bi-based 2:2:2:3

Abstract The electrical resistance was measured for two sintered samples of Bi(Pb)SrCaCuO in the temperature region above the super-conducting transitions at about 110 K. The excess conductivity Δσ was analysed in the form ln Δσ = x ln (T − T c ) T c . Different choices of Tc are discussed. A consistent behaviour for both samples was found only with a careful construction of a mean field critical temperature. Two temperatures T ∗ and TG could then be identified for each sample. T ∗ corresponds to a transition from two to three dimensional fluctuations in the mean field region, characterized by exponents x1 = −1 and x 2 = − 1 2 respectively. At TG, about 0.6 K below T ∗ , there is a cross-over to a critical region with an exponent x3 of roughly −0.3. It is argued that the minor differences between the results for samples of different quality indicate that impurity effects probably do not influence the analysis.

Excess conductivity in Y1−2xCaxTbxBa2Cu3O7−δ

Abstract Superconducting fluctuations have been studied in zero magnetic field in sintered samples of Y 1−2x Ca x Tb x Ba 2 Cu 3 O 7−δ by electrical resistivity measurements for x=0, 0.025, and 0.05. The excess conductivity Δσ was analysed as a function of reduced temperature ϵ =ln(T/T c ) in the range −5 ϵ Δσ ∝ ϵ α of α 1 =−1, α 2 =−0.5 and α 3 ≈ −0.4 respectively.

Neutron-diffraction studies and interatomic distances in CaPr doped NdBa2Cu3O7−δ

Abstract Sintered samples of Nd 1−2 x Ca x Pr x Ba 2 Cu 3 O 7−δ with x = 0, 0.025, 0.050, and 0.10 have been studied by electrical-resistivity measurements, X-ray and neutron diffraction and from calculated bond-valence sums (BVS). Doping of equal amounts of CaPr suppresses a parabolic term in the decrease of the superconducting T c and results in a strong linear suppression of −d T c /d x = 206 K. This observation indicates a single-impurity effect and suggests that CaPr is a charge neutral doping in Nd-123. The c -axis length and cell volume decrease with doping and the Cu2O4 distance increases. BVS calculations suggest that T c in Pr doped 123 may be governed by small changes in interatomic distances and corresponding charge transfers, rather than a contribution from magnetic pair breaking as previously conjectured. Similar calculations of published data for oxygen-deficient Y-123 and Pr doped Y-123, show that the hole density in the planes →0 at about the concentration where T c vanishes. The average CuO 2 plane distance supports that Pr has valence +4 at low dopings. Results are discussed which suggest that the Pr valence at higher doping levels is +3.

Structural, thermal and electrical studies of the Y1−2xCaxThxBa2Cu3O7−δ system with 0⩽x⩽0.20

STRUCTURAL, THERMAL AND ELECTRICAL STUDIES OF THE Y1-2XCAXTHXBA2CU3O7-DELTA SYSTEM WITH 0 LESS-THAN-OR-EQUAL-TO 0.20

New disordered superconductors

Abstract We have measured the low temperature resistance of zirconium-based metallic glasses. Superconductivity is found in: Zr65Cu35, Tc = 2.0 K; Zr70Ni30, Tc = 2.9 K; and Zr70Co30, Tc = 3.3 K.

Low temperature specific heat measurements of ZrV2 and HfV2

Abstract The C15 compounds ZrV 2 and HfV 2 show bulk superconductivity at 7.3 and 8.4 o K respectively, indicating a strong dependence of T c on the electron per atom number for C15 compounds. High electronic contributions to the specific heat are observed.

Superconductivity and electron-phonon interaction in TaV alloys

Abstract The electron-phonon coupling in TaV alloys is estimated from resistive measurements. It decreases rapidly with increasing V in Ta in qualitative agreement with the observed transition temperature, T c . A calculation of T c from the strong-coupling equation gives good agreement with observed values for a constant Coulomb pseudo-potential and one adjustable parameter.

Fluctuation conductivity of polycrystalline Hg, Tl-1223

Detailed resistivity measurements were made for polycrystalline Hg1-xTlxBa2Ca2Cu3O8+delta (x = 0 and 0.2) in zero field and above the superconducting T-c. The fluctuation conductivity Deltasigma is analyzed. With decreasing temperature two crossover temperatures could be identified for each sample; i.e. T* from two to three dimensional fluctuations in the mean field region (MFR), and T-G from the MFR to the critical region. All results are in agreement with the Lawrence-Doniach model. The c-axis coherence length and the interlayer coupling factor were obtained