On the superconducting nature of the Bi-II phase of elemental Bismuth

The superconductivity in the Bi-II phase of elemental Bismuth (transition temperature $T_{rm c}simeq3.92$ K at pressure $psimeq 2.80$ GPa) was studied experimentally by means of the muon-spin rotation as well as theoretically by using the Eliashberg theory in combination with Density Functional Theory calculations. Experiments reveal that Bi-II is a type-I superconductor with a zero temperature value of the thermodynamic critical field $B_{rm c}(0)simeq31.97$~mT. The Eliashberg theory approach provides a good agreement with the experimental $T_{rm c}$ and the temperature evolution of $B_{rm c}$. The estimated value for the retardation (coupling) parameter $k_{rm B}T_{rm c}/omega_{rm ln} approx 0.07$ ($omega_{rm ln}$ is the logarithmically averaged phonon frequency) suggests that Bi-II is an intermediately-coupled superconductor.