Anomalous metallic state and anisotropic multiband superconductivity in Nb3Pd0.7Se7

Abstract

We report the discovery of superconductivity in Nb${}_3$Pd${}_x$Se${}_7$ with a $x$-dependent superconducting transition-temperature as high as $T_c\simeq 2.1$ K for $x\simeq 0.7$ (middle point of the resistive transition). Needle-like single crystals display anisotropic upper-critical fields with an anisotropy ${\hat{I}}^3=H_{c2}^b/H_{c2}^a$ as large as 6 between fields applied along their needle axis (or $b-$axis) or along the $a-$axis. As for the Fe based superconductors ${\hat{I}}^3$ is temperature-dependent suggesting that Nb${}_3$Pd${}_{0.7}$Se${}_7$ is a multi-band superconductor. This is supported by band structure calculations which reveal a Fermi surface composed of quasi-one-dimensional and quasi-two-dimensional sheets of hole character, as well as three-dimensional sheets of both hole- and electron-character. Remarkably, $H_{c2}^b$ is observed to saturate at $H_{c2}^b(T\to 0\text{K})\simeq 14.1$ T which is $4.26\times H_p$ where $H_p$ is the Pauli-limiting field in the weak-coupling regime. The synthesis procedure yields additional crystals belonging to the Nb${}_2$Pd${}_x$Se${}_5$ phase which also becomes superconducting when the fraction of Pd is varied. For both phases we find that superconductivity condenses out of an anomalous metallic state, i.e. displaying $\partial \ddot{I}/\partial T<0$ above $T_c$ similarly to what is observed in the pseudogap-phase of the underdoped cuprates. An anomalous metallic state, low-dimensionality, multi-band character, extremely high and anisotropic $H_{c2}$s, are all ingredients for unconventional superconductivity.

9 pages, 7 figures, to appear in Physical Review B