Discovery of superconductivity in a low carrier density system: Bismuth*

Bismuth(Bi) has played a very important role in uncovering many interesting physical properties in condensed matter research, and still continues to draw enormous scientific interests due to its anomalous electronic properties. Many important phenomena such as Seebeck effect, Nernst effect, Shubnikov-de Haas effect, de Haas-van Alphen (dHvA) effect etc. were first discovered in Bi. Determination of the Fermi surface (FS) in Bi using dHvA measurements provided the basis to determine the Fermi surface of other elements\compounds. The layered structure of Bi plays a crucial role in observing many quantum phenomena rather easily. However, bulk rhombohedral Bi at ambient pressure is a compensated semimetal and it remains in the normal state down to 10 mK. The superconductivity (SC) in bulk Bi is thought to be very unlikely due to extremely low carrier density (n_e=n_p=3x10¹⁷/cm³ at 4.2 K) . The question of SC in Bi has remained unsolved both theoretically and experimentally even today. In this talk, I will describe the first-ever observation of bulk SC in highly pure Bi single crystals (99.9999%) below 0.53 mK under ambient pressure with an estimated critical magnetic field of 5.2 µT at 0 K ¹. The conventional Bardeen-Cooper-Schrieffer (BCS) theory or its extensions cannot explain the observed SC in Bi, since the adiabatic approximation used in the theories, ω_D/E_F<< 1 (where ω_D is the Debye frequency and E_F is the Fermi energy), does not hold true for Bi. One needs new theoretical ideas to understand SC in such a low carrier systems with unusual band structure in the non-adiabatic limit, ω_D/E_F≥1.
1. Om Prakash, Anil Kumar, A. Thamizhavel and S. Ramakrishnan, Science Vol. 355, Issue 6320, pp. 52-55 (2017).