Possible unconventional pairing in $(\text{Ca,Sr})_{3}(\text{Ir,Rh})_{4}\text{Sn}_{13}$ superconductors revealed by controlling disorder

TL;DR

This study investigates how controlled disorder affects charge-density wave and superconducting states in Ca,Sr,Ir,Rh,Sn13 compounds, revealing potential unconventional pairing mechanisms through irradiation experiments and theoretical analysis.

Contribution

It provides new insights into the interplay of CDW and superconductivity and suggests an unconventional multiband s^{+-} pairing state in these materials.

Findings

CDW transition temperature is suppressed by irradiation in all compounds.

Superconducting transition temperature shows complex behavior, initially increasing then saturating or decreasing.

Superfluid density indicates a weak-coupling, isotropic single-gap superconducting state.

Abstract

We study the evolution of temperature-dependent resistivity with controlled point-like disorder induced by 2.5 MeV electron irradiation in stoichiometric compositions of the "3-4-13" stannides, $(\text{Ca,Sr})_{3}(\text{Ir,Rh})_{4}\text{Sn}_{13}$.Three of these cubic compounds exhibit a microscopic coexistence of charge-density wave (CDW) order and superconductivity (SC), while $\text{Ca}_{3}\text{Rh}_{4}\text{Sn}_{13}$ does not develop CDW order. As expected, the CDW transition temperature, $T_{\text{CDW}}$, is universally suppressed by irradiation in all three compositions. The superconducting transition temperature, $T_{c}$, behaves in a more complex manner. In $\text{Sr}_{3}\text{Rh}_{4}\text{Sn}_{13}$, it increases initially in a way consistent with a direct competition of CDW and SC, but quickly saturates at higher irradiation doses. In the other three compounds, $T_{c}$ is monotonically suppressed by irradiation. The strongest suppression is found in $\text{Ca}_{3}\text{Rh}_{4}\text{Sn}_{13}$, which does not have CDW order. We further examine this composition by measuring the London penetration depth, $\lambda(T)$, from which we derive the superfluid density. The result unambiguously points to a weak-coupling, full single gap, isotropic superconducting state. Therefore, we must explain two seemingly incompatible experimental observations: a single isotropic superconducting gap and a significant suppression of $T_{c}$ by non-magnetic disorder. We conduct a quantitative theoretical analysis based on a generalized Anderson theorem which points to an unconventional multiband $s^{+-}$-pairing state where the sign of the order parameter is different on one (or a small subset) of the smaller Fermi surface sheets, but remains overall fully-gapped.