Fragile electronic superconductivity in a Bi single crystal

It was presumed that semimetallic bismuth (Bi) would not show superconductivity (SC) even at ultralow temperatures (<10 mK ) due to its very low carrier density (approximate to 3 x10(17)cm(-3)) . Recently, we have established bulk superconductivity in an ultrapure (99.9999%) Bi single crystal at T-c=0.53 mK with an extrapolated upper critical field H-c(0 ) =5.2 T measured along the [0001] (trigonal) crystallographic direction [O. Prakash et al., Science 355, 52 (2017), 10.1126/science.aaf8227]. At very low concentrations of the charge carriers, we are dealing with fragile Cooper pairs with an estimated large coherence length xi(GL)(0 ) approximate to 96 mu m . We also stated that one needs to go beyond the conventional electron-phonon coupling (BCS-like) mechanism to understand the SC state in Bi. Bi is a compensated semimetal with electrons and holes as charge carriers. In order to find the charge carriers responsible for the SC, we report the temperature dependence of the anisotropic critical field along the H parallel to[01 1 0 ] (bisectrix) crystallographic direction and compare it with the earlier data from measurements along the H parallel to[0001 ] (trigonal) axis. Our theoretical analysis of the critical field anisotropy suggests that the light electrons in the three pockets of the Bi Fermi surface are responsible for the SC, which indicates that Bi is an extremely weak type-II (close to type-I) superconductor. Finally, we present a brief review of the current theories proposed to explain the SC in Bi.