Effect of Selenium doping on the superconductivity of Nb${}_2$Pd(S${}_{1-x}$Se${}_x$)${}_5$

Abstract

We study the isovalent substitution effect by partially introducing Se on S site in the newly discovered superconductor Nb${}_2$PdS${}_5$ ($T_c\sim$6 K) whose upper critical field is found to be far above its Pauli paramagnetic limit. In this Nb${}_2$Pd(S${}_{1-x}$Se${}_x$)${}_5$ (0$\le$$x$$\le$0.8) system, superconductivity is systematically suppressed by the Se concentration and ultimately disappears when $x\ge$ 0.5, after which a semiconducting-like 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.

5 pages, 4 figures