Takasi Kawaguti

Superconducting and Normal State Properties of NiBi3.

The electrical resistivity, heat capacity, superconducting upper critical magnetic field and magnetic susceptibility of NiBi 3 have been measured for the polycrystal and needle crystal. The temperature dependence of the resistivity ρ( T ) above the superconducting transition temperature can be explained by the parallel resistor formula. From ρ( T ), using the sum of two Gruneisen-Bloch functions, we have obtained that the lower main value of the Debye temperature is 70 K and the higher one is 300 K. This is compatible with ω E ≡30.6 K and ω D ≡141 K obtained from the heat capacity using the hybrid Einstein-Debye model. We have compared the angular dependence of the critical magnetic field with that of the effective mass model and found an anisotropic mass ratio m ∥ b / m ⊥ b ≈0.17 for the needle (∥ b -axis) crystal. Almost the same value of 0.14 is also obtained from the analysis of ρ( T ).

Magnetoresistance and Inelastic Scattering Time in Thin Films of Silver and Gold in Weakly Localized Regime

The magnetoresistance in thin films of silver and gold has been measured in the temperature range 1.1 K–4.2 K. It is in good agreement with the theoretical results in weakly localized regime with suitable values of the spin-orbit scattering time. The inelastic scattering time is nearly in inverse proportion to temperature.

Positive Magnetoresistance of Thin Gold Films at Low Temperatures

The electrical resistance of ultra thin gold films increases as the temperature is lowered below 6 K. The magnetoresistance is positive and anisotropic. A logarithmic dependence of the resistance on magnetic field has been observed. The results are inqualitative agreement with the recent theories of the electron localization in two-dimensional systems.

Thickness-tuned superconductor-insulator transitions in quench-condensed Mo and MoRu films

The results of studies on superconductor–insulator transitions are reported for Mo and MoRu films of varying thickness deposited on SiO underlayers and on bare glass. The film samples were quench-condensed on glass substrates held at liquid He temperatures. MoRu films on SiO, which offer the most homogeneous film morphology, showed a critical sheet resistance of transition, Rc, of ~6.7 kΩ. Both series of Mo films on SiO and MoRu films on bare glass had . These values are larger than those previously reported for MoGe, MoSi, and MoC films. For the above films in the insulating region far from the transition, the exponent of the thermally activated resistance ranged from 0.5 to 1.0.

Electron-Phonon Scattering Effects on the Pair Breaking Parameter in Al, Sn and Pb Superconducting Films

The relation between the pair breaking parameter δ and the inelastic scattering rate 1/τ in has been studied for Al(sheet resistance \(R_{\box}<100 \Omega)\), Sn(<70 Ω) and Pb(<3 Ω) films. The δ III F agrees well with δ M independent of materials and dirtiness: The δ III F is determined by the analysis of the excess conductance with adding the correction term to the Maki-Thompson term and δ M is obtained from the relation \(\delta=\pi\hbar\)/8 k B T τ in , where the τ in is determined by the analysis of the magnetoconductance. The \(R_{\box}\) dependence of δ fluc,e-e M for Al and also Sn films, where the electron-phonon inelastic scattering contribution is subtracted experimentally, is in quantitative agreement with theories independent of materials. This implies that the material dependence of coefficients δ 0 and a in the experimental expression \(\delta=\delta_{0}+aR_{\box}\) is caused by e-p in-elastic scattering depending on materials and dirtiness of films.

Magnetoconductance near Superconducting Transition Temperature of Aluminum Films

In the wide range of the magnetic field, the magnetoconductance in thin Al films agrees well with the theory of Santos and Abrahams and also of Maekawa, extending Larkins theory to higher magnetic fields at temperatures slightly above superconducting transition temperature T c . The inelastic scattering rate 1/τ in obtained from magnetoconductance measurements increases rapidly with decreasing temperature near T c . The pair-breaking parameter δ, which is deduced from the measurements and related to the 1/τ in , implies the temperature dependence shown by Patton and also Keller and Korenman. It is about the half of that estimated from the Maki-Thompson contribution to the excess conductance due to fluctuations at zero magnetic field.

Localization and Interaction Effects in Weakly Localized Region in Aluminum Films

The temperature and magnetic field dependence of the electrical conductance σ in thin granular-aluminum films has been measured in the temperature range 4.2–35 K above the superconducting transition temperature. The temperature dependence of conductance σ in zero magnetic field has been compared with a theory including the higher order contribution of the interaction, by evaluating the localization term from the analyses of the magnetoconductance. The σ in zero field agrees with the theory in a temperature of minimum conductance and in the temperature dependence at high temperatures, with a reasonable value of the average matrix element of the Coulomb interaction.

Upward Curvature of the Temperature Dependence of Upper Critical Magnetic Field Perpendicular to Superconducting Aluminum Films

The temperature dependence of the electrical resistance of thin granular aluminum films shows the maximum far above the superconducting transition temperature due to both effects of thermodynamic fluctuations and electron localization, for the film with large normal-state sheet resistance R N . The temperature dependence of the upper critical field or the film with R N ≃10 3 (\(\varOmega\square\)) shows the upward curvature. The results on the upper critical field are explained by a current theory of dirty superconductors including electron localization and electron-electron interactions.

Excess electrical conductivity due to fluctuations of quench-condensed superconducting Nb films

The resistive transition of superconducting Nb films evaporated onto a liquid-He-cooled substrate has been investigated, and shows a rounding characteristic of the thermal fluctuations. Transition curves have been measured above and below the superconducting transition temperatureTc in the absence of a magnetic field. The excess electrical conductivity due to fluctuations aboveTc of quench-condensed Nb films is well described in terms of the sum of the contributions from the Aslamazov-Larkin term and from the Maki-Thompson term, in contradistinction to the case of quench-condensed Bi, Ga, and Pb films. This suggests that thermal phonons play an important role in the contribution of the Maki term.

Analysis of the superconducting-fluctuation conductance due to the Maki-Thompson contribution in aluminum films

Abstract The excess conductance σ′ and the magnetoconductance Δσ in a field H due to superconducting fluctuations have been measured at temperatures near Tc for Al films in the wide range of the sheet resistance R □ , 5.8 Ω/□ ⪅- R □ ⪅- 160 Ω/□ . The contributions due to the Maki-Thompson (MT) correction σ′MT and ΔσMT are well described by the recent Reizer theory, which was obtained from reconsidering the theory of Keller and Korenman. As predicted by the recent theory, the dirtiness dependence of σ′MT and ΔσMT of weak-coupling superconducting films is explained by only R□ without any adjustable parameter. The T dependence of σ′ and the T and H dependence of Δσ for all films are satisfactorily explained by theories including the Aslamazov-Larkin contributions, using R□ as the fitting parameter within a factor of 0.7–1.3 of experimentally determined values.