Evidence of s-wave superconductivity in the noncentrosymmetric LaIr.

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 Ir (T  ~ 2.3 K) that was recently proposed to break the time-reversal symmetry. It is found that LaIr displays a moderately large electronic heat capacity (Sommerfeld coefficient γ  ~ 53.1 mJ/mol K) and a significantly enhanced Kadowaki-Woods ratio (KWR 32 μΩ cm mol K J) that is greater than the typical value (10 μΩ cm mol K J) for strongly correlated electron systems. The upper critical field H 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 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  = 78.5 K, resulting in a theoretical T  = 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.