Transport and superconducting properties of RNi2B2C (R=Y,nLu) single crystals
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The in-plane resistivity, in-plane absolute thermopower, and upper critical field measurements are reported for single-crystal samples of (Formula presented)(Formula presented)C and (Formula presented)(Formula presented)C superconductors. The in-plane resistivity shows metallic behavior and varies approximately linearly with temperature near room temperature (RT) but shows nearly quadratic behavior in temperature at low temperatures. The (Formula presented)(Formula presented)C and (Formula presented)(Formula presented)C single-crystal samples exhibit large transverse magnetoresistance (6-8 % at 45 kOe) in the ab plane. The absolute thermopower S(T) is negative from RT to the superconducting transition temperature (Formula presented). Its magnitude at RT is a few times of the value for a typical good metal. S(T) is approximately linear in temperature between 150 K and RT. Extrapolation to T=0 gives large intercepts (few V/K) for both samples suggesting the presence of a much larger 'knee' than would be expected from electron-phonon interaction renormalization effects. The upper critical fields for H parallel and perpendicular to the c axis and the superconducting parameters derived from it do not show any anisotropy for the (Formula presented)(Formula presented)C single-crystal samples in agreement with magnetization and torque magnetometry measurements, but a small anisotropy is observed for the (Formula presented)(Formula presented)C single crystals. The analysis shows that these are moderately strong-coupling type-II superconductors (similar to the A-15 compounds) with a value of the electron-phonon coupling parameter (0) approximately equal to 1.2 for (Formula presented)(Formula presented)C and 1.0 for (Formula presented)(Formula presented)C, the Ginzburg-Landau coherence length (0) approximately equal to 70 , and (Formula presented)(0)60-70 kOe. The temperature dependence of the upper critical field shows a positive curvature near (Formula presented) in disagreement with the Werthamer, Helfand, Hohenberg, and Maki (WHHM) theory but in agreement with a recent solution of the Gorkov equation using a basis formed by Landau levels (Bahcall); however, the data show a severe disagreement between the observed low-temperature behavior of (Formula presented)(T) and that predicted either by WHHM or Bahcalls expressions. 1997 The American Physical Society.
