Transport and superconducting properties ofRNi2B2C (R=Y,nLu) single crystals

The in-plane resistivity, in-plane absolute thermopower, and upper critical field measurements are reported for single-crystal samples of ${\mathrm{YNi}}_2$ ${\mathrm{B}}_2$C and ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$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 ${\mathrm{YNi}}_2$ ${\mathrm{B}}_2$C and ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$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 ${\mathrm{T}}_{\mathrm{c}}$. Its magnitude at RT is a few times of the value for a typical good metal. S(T) is approximately linear in temperature between ≈150 K and RT. Extrapolation to T=0 gives large intercepts (few μV/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 ${\mathrm{YNi}}_2$ ${\mathrm{B}}_2$C single-crystal samples in agreement with magnetization and torque magnetometry measurements, but a small anisotropy is observed for the ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$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 λ(0) approximately equal to 1.2 for ${\mathrm{YNi}}_2$ ${\mathrm{B}}_2$C and 1.0 for ${\mathrm{LuNi}}_2$ ${\mathrm{B}}_2$C, the Ginzburg-Landau coherence length ξ(0) approximately equal to 70 Å, and ${\mathrm{H}}_{\mathrm{c}2\mathrm{}}$(0)∼60–70 kOe. The temperature dependence of the upper critical field shows a positive curvature near ${\mathrm{T}}_{\mathrm{c}}$ 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 ${\mathrm{H}}_{\mathrm{c}2\mathrm{}}$(T) and that predicted either by WHHM or Bahcall's expressions.