Weakly-correlated nodeless superconductivity in single crystals of Ca₃Ir₄Sn₁₃ and Sr₃Ir₄Sn₁₃ revealed by critical fields, Hall effect, and magnetoresistance measurements

We report a study of single crystal Ca₃Ir₄Sn₁₃ (CIS) and Sr₃Ir₄Sn₁₃ (SIS) by measuring the longitudinal and Hall resistivities, upper and lower critical fields and magnetoresistance, as well as the magnetization. The sign change in the Hall coefficient observed on both the CIS and SIS provides direct evidence for the Fermi surface reconstructing during the superlattice phase transition. Both materials are of current interest due to indications of superconductivity associated with charge-density-wave (CDW) ordering. Observations of the diamagnetic feature and the lower critical field H_c1(T) in both CIS and SIS can be realized by means of the nodeless single-gap BCS theory. In addition, a weak electronic correlation in both systems has been revealed by the small values of the spin exchange energy, upper critical field and Δ(0)/k_BT_c ratio, derived respectively from the normal-state Hall effect, resistive transition and temperature-dependent H_c1. It is noticeable that the magnetoresistance of SIS shows a rapid increase below T′ ∼ 40 K, following Kohler's scaling rule. The results of the magnetic susceptibility and Hall coefficient also exhibit anomalous features near T′. With respect to these observations, this suggests that the existence of an additional phonon mode with energy of about 4.0 meV in SIS is responsible for the presence of lattice instability toward a phase transition.