Elements in the alkali metal series are regarded as unfavourable for superconductivity due to their monovalent character. 1,2 The superconducting transition at temperatures as high as 20 K recently found in compressed lithium, 3-6 the lightest alkali element, is considered to occur due to pressure induced changes in the conduction-electron band structure. 6-12 The condition at the ambient pressure in lithium had remained unresolved, both theoretically and experimentally. 11-16 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

The effective interaction between 3 He quasiparticles in dilute liquid 3 He-4 He mixtures affects many of its physical properties. The interaction potential is determined here from the saturation solubility and osmotic pressure data reported recently. The interactions are examined consistently over the entire pressure range of liquid-helium mixtures at very low temperatures, that is, from 0 to 25.6 bar, where the solid phase appears. To reproduce all experimental data, it was necessary to include a concentration dependence in the potential. This dependence, however, turned out to be almost the opposite to what has been proposed earlier. The deduced potential can be used to calculate, among other things, an estimate for the superfluid transition temperature of 3 He-4 He mixtures. In addition, we also find values for some less well-established parameters for helium mixtures, such as the binding energy of a 3 He atom in superfluid 4 He.

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