TRANSPORT AND SUPERCONDUCTING PROPERTIES OF RNI2B2C (R=Y, LU) SINGLE-CRYSTALS

The in-plane resistivity, in-plane absolute thermopower, and upper cri tical held measurements are reported for single-crystal samples of YNi 2B2C and LuNi2B2C superconductors. The in-plane resistivity shows meta llic behavior and varies approximately linearly with temperature near room temperature (RT) but shows nearly quadratic behavior in temperatu re at low temperatures. The YNi2B2C and LuNi2B2C single-crystal sample s exhibit large transverse magnetoresistance (approximate to 6-8% at 4 5 kOe) in the ab plane. The absolute thermopower S(T) is negative from RT to the superconducting transition temperature T-c. Its magnitude a t RT is a few times of the value for a typical good metal. S(T) is app roximately linear in temperature between approximate to 150 K and RT. Extrapolation to T = 0 gives large intercepts (few mu V/K) for both sa mples 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 ani sotropy for the YNi2B2C single-crystal samples in agreement with magne tization 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 pa rameter lambda(0) approximately equal to 1.2 for YNi2B2C and 1.0 for L uNi2B2C, the Ginzburg-Landau coherence length xi(0) approximately equa l to 70 Angstrom, and H-c2(0) similar to 60-70 kOe. The temperature de pendence of the upper critical field shows a positive curvature near T -c in disagreement with the Werthamer, Helfand, Hohenberg, and Maki (W HHM) theory but in agreement with a recent solution of the Gor'kov equ ation using a basis formed by Landau levels (Bahcall); however, the da ta show a severe disagreement between the observed low-temperature beh avior of H-c2(T) and that predicted either by WHHM or Bahcall's expres sions.