Investigation of the superconducting energy gap in the compound LuNi 2B2C by the method of point contact spectroscopy: Two-gap approximation
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It is shown that the two-gap approximation is applicable for describing the dV/dI(V) spectra of LuNi2B2C-Ag point contacts in a wide interval of temperatures. The values and the temperature dependences of the large and the small gaps in the ab plane and in the c direction were estimated using the generalized BTK model [A. Plecenik, M. Grajacar, S. Benacka P. Seidel, A. Pfuch, Phys. Rev. B49, 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 ab plane and 14.5 K in the c direction. The absolute values of the gaps are 0ab = 2.16 meV and 0c = 1.94 meV. For the large gaps the critical temperature Tc coincides with the bulk, Tcbulk = 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 ab plane the contribution from the small gap is much smaller and decreases rapidly as a temperature rises.
