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

TL;DR

This study investigates how selenium doping affects the superconductivity of Nb$_2$PdS$_5$, revealing suppression of superconductivity with increased Se, a persistent high upper critical field, and insights into the superconducting gap structure.

Contribution

It provides the first systematic analysis of Se doping effects on Nb$_2$PdS$_5$, highlighting the robustness of the upper critical field and the nature of the superconducting gap.

Findings

Superconductivity is suppressed with increasing Se concentration.

The upper critical field to $T_c$ ratio remains constant despite doping.

The superconducting gap is nodeless and consistent with strong-coupling behavior.

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$\leq$$x$$\leq$0.8) system, superconductivity is systematically suppressed by the Se concentration and ultimately disappears when $x\geq$ 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.