Enhanced upper critical fields in a new quasi-one-dimensional superconductor Nb₂Pd_xSe₅

We report the discovery of superconductivity with T_c = 5.5 K in Nb₂Pd_xSe₅, in which one-dimensional (1D) Nb-Se chains existing along the b-direction hybridize each other to form the conducting b-c* plane. The magnetic susceptibility and specific heat data in both single crystal and polycrystal show evidence of bulk superconductivity. The resistivity, Hall coefficient and magneto-resistance data all indicate the presence of an energy scale T* = ∼ 50 K, which becomes systematically lowered under hydrostatic pressure and competes with the stabilization of superconductivity. Combined with the band calculation results showing the Fermi surfaces with 1D character, we postulate that the energy scale T* is related to the formation of a density wave or a stabilization of low-dimensional electronic structure. The zero-temperature upper critical field, H_c2 (0), of the single crystal is found to be 10.5, 35 and 22 T in the a′, b and c* directions, respectively. While the linearly increasing H _c2 (T) for H//c* indicates the multi-band effect, H _c2 (0) for H//b and c* are found to be much bigger than the Bardeen-Cooper-Schrieffer (BCS) Pauli limiting field, 1.84 T_c ∼ 9 T. The suppressed Pauli paramagnetic effect points to the possibility of enhanced spin-orbit scattering related to the low-dimensional electronic structure or the presence of heavy elements such as Pd.