Negative boron isotope effect of a ‘2Δ’ gap peak in Raman spectra from non-magnetic borocarbide superconductors

Abstract

The nickel borocarbides RNi 2B 2C, R=Y or Lu, have a superconducting T c of 15–16 K and exhibit properties consistent with an anisotropic s-wave gap. We briefly review their properties. Electronic Raman scattering results are presented on YNi 2B 2C containing 11B or 10B. In A 1 g and B 1 g Raman symmetries, there is no shift in the position of the ‘2Δ’ peak with B-isotope. There is a distinct negative shift, however, in B 2 g symmetry. This is interpreted as a negative isotope shift for the gap Δ on that portion of the Fermi surface where the magnitude of the B 2 g Raman vertex is large.