Heavy-electron behavior in single-crystal YbNi2B2C

We have measured the magnetic susceptibility, specific heat, and electrical resistivity on single crystals of the intermetallic borocarbide YbNi2B2C. An enhanced linear contribution is observed in the specific heat with a Sommerfeld coefficient of 530 mJ/mol K-2, indicative of a heavy-electron system with a Kondo temperature similar to 10 K. The magnetic susceptibility, which is anisotropic and Curie-Weiss-like at high temperature, is also consistent with our interpretation of a strongly correlated ground state at low temperatures and crystal-electric-field excitations at higher temperatures. At T=1.8 K, the Wilson ratio is determined to be: 0.85 using the high-temperature effective moment. The resistivity shows a quadratic temperature dependence below 1.5 K with a T-2 coefficient of 1.2 mu Omega cm K-2. Unlike the other members of the series RNi(2)B(2)C (R=Y. Gd-Lu), YbNi2B2C does not order above our lowest measurement temperature of 0.34 K. The suppression of superconductivity in YbNi2B2C is consistent with a significantly enhanced hybridization between the conduction electrons and the 4f states.