Physics

Prof. S. Ramakrishnan
TIFR, Mumbai

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. Some of the key properties of Bi are: a small density of states (DOS; 2x10^-6 states/eV atom) at the Fermi level, very small Fermi surface (FS; 10^-5 of the Brillouin zone, consisting of small electron and hole pockets), low Fermi energy (E_F= 25 meV) ), low carrier density (n_e=n_p=3x10¹⁷/cm³ at 4.2 K) and small effective mass for charge carrier (m_eff~10^-3m_e). 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. 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 530 mK under ambient pressure with an estimated critical magnetic field of 5.2 mT at 0 K ¹. These measurements are carried out using the Copper nuclear refrigerator built at TIFR. The conventional Bardeen-Cooper-Schrieffer (BCS) theory cannot explain the observed SC in Bi, since the adiabatic approximation used in the BCS theory, w_D/E_F<< 1, does not hold true for Bi. Bi has a multi-valley type electronic band structure and SC in Bi could be brought about by the inter-valley electron-phonon coupling. Such a scenario calls for new theoretical ideas to understand SC in such low carrier systems with unusual band structure in the non-adiabatic limit, w_D/E_F≥1. Further, this observation of SC in Bi makes it the lowest carrier density superconductor surpassing the record held by doped SrTiO₃ for nearly 50 years.

1.  Om Prakash, Anil Kumar, A. Thamizhavel and S. Ramakrishnan, Science Vol. 355, Issue 6320, pp. 52-55 (2017).