Anomalous suppression of T_c and moderate heavy fermion behaviour in YbNi₂B₂C

Abstract

We have synthesized single phase material YbNi₂B₂C. Results of our X-ray diffraction and L_III-edge studies are consistent with (nearly) trivalent state of Yb-ions in this material. Superconductivity is not observed down to 2 K in our resistivity and magnetic susceptibility studies. This is anomalous as, with Yb-ions in 3+ state, one expects T_c~12K in this material on the basis of the systematic of T_c in other magnetic superconductors belonging to the series RNi₂B₂C (R, a rare earth ion). Resistivity is relatively temperature independent in the temperature range 50K-300K. However, below 50K there is a large reduction of resistivity. The Sommerfeld coefficient, γ, obtained from the heat capacity data in the temperature range 12K-25K, is ~ 200 mJ/mol.K² which suggests the formation of a moderately heavy fermion state in YbNi₂B₂C at low temperatures. Suppression of superconductivity constitutes an independent evidence of anomalous state of Yb-ions in YbNi₂B₂C.