Unconventional superconductivity in quasi-one-dimensional Rb$_2$Cr$_3$As$_3$

TL;DR

This paper reports the synthesis and characterization of Rb$_2$Cr$_3$As$_3$, a quasi-one-dimensional superconductor with unconventional properties, expanding the family of Cr$_3$As$_3$-chain based superconductors.

Contribution

The study introduces Rb$_2$Cr$_3$As$_3$ as a new superconductor with unique properties, demonstrating the effects of Rb substitution on structure and superconductivity.

Findings

Superconductivity at 4.8 K in Rb$_2$Cr$_3$As$_3$

Upper critical field exceeds Pauli limit by two times

Evidence of nodes in the superconducting gap

Abstract

Following the discovery of superconductivity in quasi-one-dimensional K$_2$Cr$_3$As$_3$ containing [(Cr$_3$As$_3$)$^{2-}$]$_{\infty}$ chains [J. K. Bao et al., arXiv: 1412.0067 (2014)], we succeeded in synthesizing an analogous compound, Rb$_2$Cr$_3$As$_3$, which also crystallizes in a hexagonal lattice. The replacement of K by Rb results in an expansion of $a$ axis by 3\%, indicating a weaker interchain coupling in Rb$_2$Cr$_3$As$_3$. Bulk superconductivity emerges at 4.8 K, above which the normal-state resistivity shows a linear temperature dependence up to 35 K. The estimated upper critical field at zero temperature exceeds the Pauli paramagnetic limit by a factor of two. Furthermore, the electronic specific-heat coefficient extrapolated to zero temperature in the mixed state increases with $\sqrt{H}$, suggesting existence of nodes in the superconducting energy gap. Hence Rb$_2$Cr$_3$As$_3$ manifests itself as another example of unconventional superconductor in the Cr$_3$As$_3$-chain based system.