Transport and superconducting properties of RNi2B2C (R=Y, Lu) single crystals

The in-plane resistivity, in-plane absolute thermopower, and upper critical held measurements are reported for single-crystal samples of YNi2B2C and LuNi2B2C 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 YNi2B2C and LuNi2B2C single-crystal samples exhibit large transverse magnetoresistance (approximate to 6-8% at 45 kOe) in the ab plane. The absolute thermopower S(T) is negative from RT to the superconducting transition temperature T-c. Its magnitude at RT is a few times of the value for a typical good metal. S(T) is approximately linear in temperature between approximate to 150 K and RT. Extrapolation to T = 0 gives large intercepts (few mu 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 YNi2B2C single-crystal samples in agreement with magnetization and torque magnetometry measurements, but a small anisotropy is observed for the LuNi2B2C 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 lambda(0) approximately equal to 1.2 for YNi2B2C and 1.0 for LuNi2B2C, the Ginzburg-Landau coherence length xi(0) approximately equal to 70 Angstrom, and H-c2(0) similar to 60-70 kOe. The temperature dependence of the upper critical field shows a positive curvature near T-c in disagreement with the Werthamer, Helfand, Hohenberg, and Maki (WHHM) theory but in agreement with a recent solution of the Gor'kov equation using a basis formed by Landau levels (Bahcall); however, the data show a severe disagreement between the observed low-temperature behavior of H-c2(T) and that predicted either by WHHM or Bahcall's expressions.