A simple theory of 40 K superconductivity in MgB2: first-principles calculations of Tc, its dependence on boron mass and pressure Original Research Article

The nickel borocarbides RNi2B2C, R=Y or Lu, have a superconducting Tc 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 YNi2B2C containing 11B or 10B. In A1g and B1g Raman symmetries, there is no shift in the position of the ‘2Δ’ peak with B-isotope. There is a distinct negative shift, however, in B2g 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 B2g Raman vertex is large.