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

C. Q. Niu, J. H. Yang, Y. K. Li, Bin Chen, N. Zhou, J. Chen, L. L. Jiang, B. Chen, X. X. Yang, Chao Cao, Jianhui Dai, Xiaofeng Xu
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.
View original: http://arxiv.org/abs/1307.1933