Effect of selenium doping on the superconductivity of Nb2Pd(S1-xSex)5

We study the isovalent substitution effect by partially introducing Se on the S site in the newly discovered superconductor Nb2PdS5 (T-c similar to 6 K) whose upper critical field is found to be far above its Pauli paramagnetic limit. In this Nb2Pd(S1-xSex)(5) (0 <= x <= 0.8) system, superconductivity is systematically suppressed by the Se concentration and ultimately disappears when x >= 0.5, after which a semiconductinglike ground state emerges. In spite of the considerably reduced T-c with Se doping, the ratio of the upper critical field H-c2 to T-c remains unaffected. Moreover, the size of the heat capacity jump at T-c is smaller than that expected for a BCS superconductor, implying that a strong-coupling theory cannot be the origin of this large upper critical field. In addition, the low-lying quasiparticle excitations are consistent with a nodeless gap opening over the Fermi surface. These results combined impose severe constraints on any theory of exotic superconductivity in this system.