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Evidence for bulk superconductivity in pure bismuth single crystals at ambient pressure

Going cold with Bismuth Many elemental metals, such as lead and aluminum, become superconducting at low temperatures. Bismuth, a semimetal with very low carrier density, stays nonsuperconducting down to 10 mK. Prakash et al. performed tricky magnetization measurements to show that pure bulk bismuth does undergo the superconducting transition at a tiny temperature of about 0.5 mK (see the Perspective by Behnia). Because bismuth does not fit neatly into the standard picture of superconductivity, further theoretical work is necessary to explain the findings. Science, this issue p. 52; see also p. 26 Superconducting quantum interference device measurements show that bismuth is a superconductor at temperatures below 0.5 millikelvin. At ambient pressure, bulk rhombohedral bismuth is a semimetal that remains in the normal state down to at least 10 millikelvin. Superconductivity in bulk bismuth is thought to be unlikely because of the extremely low carrier density. We observed bulk superconductivity in pure bismuth single crystals below 0.53 millikelvin at ambient pressure, with an estimated critical magnetic field of 5.2 microteslas at 0 kelvin. Superconductivity in bismuth cannot be explained by the conventional Bardeen-Cooper-Schrieffer theory because its adiabatic approximation does not hold true for bismuth. Future theoretical work will be needed to understand superconductivity in the nonadiabatic limit in systems with low carrier densities and unusual band structures, such as bismuth.