Evidence of s-wave superconductivity in the noncentrosymmetric La(7)Ir(3)

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 La(7)Ir(3) displays a moderately large electronic heat capacity (Sommerfeld coefficient γ(n) ~ 53.1 mJ/mol K(2)) and a significantly enhanced Kadowaki-Woods ratio (KWR ~32 μΩ cm mol(2) K(2) J(−2)) that is greater than the typical value (~10 μΩ cm mol(2) K(2) 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.