Abstract
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 (Tc ~ 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 γn ~ 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 Hc2 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 Tc 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 ωln = 78.5 K, resulting in a theoretical Tc = 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.
Highlights
The discovery of superconductivity in the noncentrosymmetric compounds, i.e., crystals possessing no inversion center, has generated immense interest in condensed matter physics[1]
A moderately large electronic specific heat γn ~ 53.1 mJ/mol K2 was seen in the normal state and quasiparticle excitations in the superconducting state can be well characterized by a single s-wave gap with the magnitude of 2Δg/kBTc = 3.56, very close to the BCS value of 3.5
A sharp superconducting transition is observed around 2.3 K, which is consistent with the diamagnetization measurement shown in the inset of Fig. 2(a)
Summary
The discovery of superconductivity in the noncentrosymmetric compounds, i.e., crystals possessing no inversion center, has generated immense interest in condensed matter physics[1]. Recent muon spin relaxation (μ SR) measurements on the polycrystalline sample of a noncentrosymmetric superconductor La7Ir3 (Tc ~ 2.3 K) reveal spontaneous static or quasistatic magnetic fields, suggesting the breaking of TRS in its superconducting state[19,20]. This implies that La7r3 may be a candidate for unconventional superconductor as suggested. The triplet pairing with broken TRS often leads to an upper critical field far above the Pauli paramagnetic limit and has nodal or anisotropic gap functions. The Hall effect, high pressure effect and the band structure were investigated, which consistently support a phonon-mediated BCS pairing scenario for La7Ir3
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