Superconductivity in lithium below 0.4 millikelvin at ambient pressure

Elements in the alkali metal series are regarded as unfavourable for superconductivity due to their monovalent character. The superconducting transition at temperatures as high as 20 K recently found in compressed lithium, the lightest alkali element, is considered to occur due to pressure induced changes in the conduction-electron band structure. The condition at the ambient pressure in lithium had remained unresolved, both theoretically and experimentally. Here we report that lithium is a superconductor also at zero pressure at extremely low temperatures below 0.4 mK. This is the lowest superconducting transition temperature for any pure metal ever observed. Lithium, as a particularly simple host for the conduction electron system, represents an important case for any attempts to classify the superconductors and transition temperatures, especially in judging if any nonmagnetic configuration can be assumed to exclude superconductivity down to zero temperature. Such a fundamental system provides a stringent test case for already highly developed computational methods in predicting the transition temperatures from first principles. Furthermore, the combination of extremely weak superconductivity and relatively strong nuclear magnetism in lithium would evidently lead to mutual competition between these two ordering phenomena under suitably prepared conditions.