The role of Ni vacancies on the physical properties of CeNi_xBi_2-y single crystals

The search for new superconductors (SC) represents an exciting field in condensed matter physics. Recently, CeNi_0.8Bi₂ has been reported as a new bulk SC (T_c ~ 4.2 K) in which Ni vacancies play an essential role for the emergence of superconductivity. Here we report experiments of magnetic susceptibility and heat capacity combined with energy dispersive X- ray spectroscopy analysis on CeNi_xBi_2-y (x = 0.62, 0.70, 0.74, 0.78) single crystals. Our data show a systematic enhancement of the antiferromagnetic transition temperature (T_N) with x, as well as the low temperature Ce³⁺ magnetic anisotropy. In particular, we find that the crystal- field ground state doublet becomes more isolated from the excited states as one approaches the Ni-rich end. As a consequence of such subtle crystal-field evolution, our analyses suggest that both the magnetic frustration and the hybridization between Ce³⁺ 4f and conduction electrons are decreasing along the series. In addition, SC transitions are observed in both La and Ce members with no bulk signature, strongly indicating the presence of impurity SC phases.