Superconductivity of Bi-III phase of elemental Bismuth: insights from Muon-Spin Rotation and Density Functional Theory

Using muon-spin rotation the pressure-induced superconductivity in the Bi-III phase of elemental Bismuth (transition temperature $T_{rm c}simeq7.05$ K) was investigated. The Ginzburg-Landau parameter $kappa=lambda/xi=30(6)$ ($lambda$ is the magnetic penetration depth, $xi$ is the coherence length) was estimated which is the highest among single element superconductors. The temperature dependence of the superconducting energy gap [$Delta(T)$] reconstructed from $lambda^{-2}(T)$ deviates from the weak-coupled BCS prediction. The coupling strength $2Delta/k_{rm B}T_{rm c}simeq 4.34$ was estimated thus implying that Bi-III stays within the strong coupling regime. The Density Functional Theory calculations suggest that superconductivity in Bi-III could be described within the Eliashberg approach with the characteristic phonon frequency $omega_{rm ln}simeq 5.5$ meV. An alternative pairing mechanism to the electron-phonon coupling involves the possibility of Cooper pairing induced by the Fermi surface nesting.