Using muon-spin rotation the pressure-induced superconductivity in the Bi-III phase of elemental bismuth (transition temperature $T_{\mathrm{c}}\simeq 7.05$ K) was investigated. A Ginzburg-Landau parameter $\kappa =\lambda /\xi =30\left(6\right)$ ($\lambda$ is the magnetic penetration depth, $\xi$ is the coherence length) was estimated, which turns out to be the highest among known single element superconductors. The temperature dependence of the superconducting energy gap $\left[\mathrm{\Delta }\right(T\left)\right]$ reconstructed from ${\lambda }^{-2}\left(T\right)$ deviates from the weakly coupled BCS prediction. The coupling strength $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{c}}\simeq 4.34$ 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 a characteristic phonon frequency ${\omega }_{\ln }\simeq 5.5$ meV. An alternative pairing mechanism to the electron-phonon coupling involves the possibility of Cooper pairing induced by Fermi-surface nesting.

• Received 20 February 2018
• Revised 27 September 2018

DOI:https://doi.org/10.1103/PhysRevB.98.140504