Fragile electronic superconductivity in Bi Single crystal

TL;DR

This study demonstrates the existence of fragile bulk superconductivity in ultra-pure Bismuth single crystals at extremely low temperatures, revealing unconventional pairing mechanisms and anisotropic critical field behavior.

Contribution

It provides the first detailed analysis of anisotropic critical fields in Bi, identifying light electrons as responsible for superconductivity and suggesting a non-BCS pairing mechanism.

Findings

Superconductivity observed at T_C = 0.53 mK in pure Bi

Anisotropic critical field data indicates light electrons drive superconductivity

Bi is an extremely weak, near type-I superconductor

Abstract

It was presumed that semimetal Bismuth (Bi) would not show superconductivity (SC) even at ultra-low temperatures ($<$10 mK) due to its very low carrier density ($\approx 3\times10^{17}$cm$^{-3}$). Recently, we have established bulk superconductivity in ultra-pure (99.9999\%) Bi single crystal at $\mathrm{T_C = 0.53}$ mK with an extrapolated upper critical field $\mathrm{H_C(0) = 5.2\mu}$T measured along the [$0001$] (trigonal) -crystallographic direction. At very low concentrations of the charge carriers, we are dealing with fragile Cooper pairs with an estimated large coherence length $\mathrm{\xi_{GL}(0)\approx 96 \mu}$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 semi-metal 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 [$01\bar 10$] (bisectrix)-crystallographic direction and compared it with the earlier data from measurements along the trigonal. Our theoretical analysis of the anisotropy of critical fields suggests that the light electrons in the three pockets of Bi bands are responsible for the SC and indicates that Bi is an extremely weak type-II (close to type-I) superconductor. Finally, we review the current theories proposed to explain the SC in Bi.