Investigation of the superconducting energy gap in the compound LuNi 2 B 2 C by the method of point contact spectroscopy: two-gap approximation

It is shown that the two-gap approximation is applicable for describing the d V ∕ d I ( V ) spectra of LuNi 2 B 2 C – Ag point contacts in a wide interval of temperatures. The values and the temperature dependences of the large and the small gaps in the a b plane and in the c direction were estimated using the generalized BTK model [A. Plecenik, M. Grajacar, S. Benacka, P. Seidel, and A. Pfuch, Phys. Rev. B 49, 10016 (1994)] and the equations of S. I. Beloborodko, Fiz. Nizk. Temp. 29, 868 (2003 ) [ Low Temp. Phys. 29, 650 (2003)]. In the BCS extrapolation the critical temperature of the small gap is 10 K in the a b plane and 14.5 K in the c direction. The absolute values of the gaps are Δ 0 a b = 2.16 meV and Δ 0 c = 1.94 meV . For the large gaps the critical temperature T c coincides with the bulk, T c bulk = 16.8 K , and their absolute values are very close, being about 3 meV in both orientations. In the c direction the contributions to the conductivity from the small and the large gaps remain practically identical up to 10 – 11 K . In the a b plane the contribution from the small gap is much smaller and decreases rapidly as a temperature rises.