SUPPRESSION OF TC IN YBNI2B2C AND Y0.9YB0.1NI2B2C

Our recent studies have shown that Yb ions are in anomalous magnetic s tate in YbNi2B2C. Though, on the basis of systematic of T-c's of super conducting members of RNi2B2C (R = rare earth) series, superconductivi ty is expected to occur in YbNi2B2C around 12 K, no superconductivity is found down to 2 K. As superconducting properties of some of the bor ocarbides are sample dependent, we have investigated off-stoichiometri c YbNi2B2C with compositions, Yb1.1Ni2B2C, YbNi1.8B2C, YbNi2B2.2C. We do not find signature of superconductivity down to 2 K in any of these samples. We show here that T-c of YNi2B2C (similar to 15.5 K) is dras tically reduced even at low concentration of Yb; T-c of Y0.9Yb0.1Ni2B2 C is similar to 2.5 K. This depression of T-c is anomalously large com pared to that expected on the basis of AG theory, perhaps the largest suppression observed on substitution of Yb in any superconductor, and implies strong 4f-conduction electron hybridization.