Evidence of s-wave superconductivity in the noncentrosymmetric La₇Ir₃

Superconductivity in noncentrosymmetric compounds has attracted sustained interest in the last decades. Here we present a detailed study on the transport, thermodynamic properties and the band structure of the noncentrosymmetric superconductor La ₇ Ir ₃ (T _c  ~ 2.3 K) that was recently proposed to break the time-reversal symmetry. It is found that La₇Ir₃ displays a moderately large electronic heat capacity (Sommerfeld coefficient γ _n  ~ 53.1 mJ/mol K²) and a significantly enhanced Kadowaki-Woods ratio (KWR ~32 μΩ cm mol² K² J^-2) that is greater than the typical value (~10 μΩ cm mol² K² J^-2) for strongly correlated electron systems. The upper critical field H_c2 was seen to be nicely described by the single-band Werthamer-Helfand-Hohenberg model down to very low temperatures. The hydrostatic pressure effects on the superconductivity were also investigated. The heat capacity below T _c reveals a dominant s-wave gap with the magnitude close to the BCS value. The first-principles calculations yield the electron-phonon coupling constant λ = 0.81 and the logarithmically averaged frequency ω _ln  = 78.5 K, resulting in a theoretical T _c  = 2.5 K, close to the experimental value. Our calculations suggest that the enhanced electronic heat capacity is more likely due to electron-phonon coupling, rather than the electron-electron correlation effects. Collectively, these results place severe constraints on any theory of exotic superconductivity in this system.