Superconductivity with extremely large upper critical fields in Nb$_{2}$Pd$_{0.81}$S$_{5}$

Here, we report the discovery of superconductivity in a new transition metal-chalcogenide compound, i.e. Nb$_{2}$Pd$_{0.81}$S$_{5}$, with a transition temperature $T_{c} \cong $ 6.6 K. Despite its relatively low $T_{c}$, it displays remarkably high and anisotropic superconducting upper critical fields, e.g. $\mu_{0}H_{c2}$ ($T \to $ 0 K) \textgreater\ 37 T for fields applied along the crystallographic $b$-axis. This value is considerably larger than the value reported for the technologically relevant Nb$_{3}$Sn compound ($\mu_{0}H_{c2}$ $\sim$ 30 T, with $T_{c} =$ 18 K)$^{1,2}$. Its ratio of $\mu_{0}H_{c2}$ ($T$ $\to $ 0 K) to $T_{c}$, is also larger than those of the new Fe based superconductors, e.g. $\beta $-FeSe (20 T/8.7 K)$^{3}$, Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ ($\sim$ 70 T/28 K)$^{4}$, and even higher than the reported ratio for the Chevrel-phase PbMo$_{6}$S$_{8}$(60T/13.3 K)$^{5}$ compound. For a field applied perpendicularly to the $b$-axis, $\mu_{0}H_{c2}$ shows a linear dependence in temperature which coupled to a temperature-dependent anisotropy of the upper critical fields, suggests that Nb$_{2}$Pd$_{0.81}$S$_{5}$ is a multi-band superconductor. This is confirmed by band structure calculations which reveal nearly cylindrical and quasi-one-dimensional Fermi surface sheets having hole and electron character, respectively.