Possible unconventional pairing in (Ca,Sr)$_3$ (Ir,Rh)$_4$Sn$_{13}$ superconductors revealed by controlling disorder

International audienceWe study the evolution of temperature-dependent resistivity with added pointlike disorder induced by 2.5 MeVelectron irradiation in stoichiometric compositions of the “3-4-13” stannides, (Ca,Sr)3(Ir,Rh)4Sn13. Three ofthese cubic compounds exhibit a proposed microscopic coexistence of charge density wave (CDW) order andsuperconductivity (SC), while Ca3Rh4Sn13 does not develop CDW order. As expected, the CDW transitiontemperature TCDW is universally suppressed by irradiation in all three compositions. The superconductingtransition temperature, Tc, behaves in a more complex manner. In Sr3Rh4Sn13, it increases initially in a wayconsistent with a direct competition of CDW and SC, but quickly saturates at higher irradiation doses. In theother three compounds, Tc is monotonically suppressed by irradiation. The strongest suppression is found inCa3Rh4Sn13, which does not have CDW order. We further examine this composition by measuring the Londonpenetration depth λ(T ), from which we derive the superfluid density. The result unambiguously points to aweak-coupling, full single gap, isotropic superconducting state. Therefore we must explain two seeminglyincompatible experimental observations: a single isotropic superconducting gap and a significant suppressionof Tc by nonmagnetic disorder. We conduct a quantitative theoretical analysis based on a generalized Andersontheorem which points to an unconventional multiband s+−-pairing state where the sign of the order parameter isdifferent on one (or a small subset) of the smaller Fermi surface sheets but remains isotropic and overall fullygapped