The Superconducting Phase Diagram of Li Metal to 67 Gpa

The dependence of the superconducting transition temperature T c on nearly hydrostatic pressure has been determined to 67 GPa in an ac susceptibility measurement for a Li sample embedded in helium pressure medium. With increasing pressure, superconductivity appears at 5.47 K for 20.3 GPa, T c rising rapidly to ∼ 14 K at 30 GPa. The T c (P)-dependence to 67 GPa differs significantly from that observed in previous studies where no pressure medium was used. Evidence is given that superconductivity in Li competes with symmetry breaking structural phase transitions which occur near 20, 30, and 62 GPa. T c is found to decrease rapidly in a dc magnetic field, the first evidence that Li is a type I superconductor. Of the 52 elemental solids known to be superconducting, 23 enter this state only if compressed under sufficiently high pressures [1]. The latest confirmed member of this ever growing family is the alkali metal Li, an element devoid of superconductivity at ambient pressure to temperatures as low as 4 mK [2]. Following an early report of possible pressure-induced superconductivity in Li near 7 K [3], recent electrical resistivity studies by Shimizu et al [4] to 48 GPa (480 kbar) followed by ac susceptibility and resistivity studies by Struzhkin et al [5] to 80 GPa confirmed the onset of a superconducting state for pressures above 20 GPa. The observed dependences of T c on pressure in these three experiments, however, are in poor agreement. This may result from the fact that no pressure medium was used, the ultrahard diamond anvils [4, 5] or BN spacers [3] pressing directly onto the Li sample, subjecting it to shear stress and plastic flow. Shear-stress effects on T c (P) are well known from studies on such diverse superconducting materials as organic metals [6], high-T c oxides [7], MgB 2 [8], and Re metal [9]. In a substance like Li, where a multitude of potential phases lie very close in energy [10, 11], the shear stress may be sufficient to induce structural phase transitions. Neaton and Ashcroft [10] have obtained the counterintuitive result that under sufficient compression the electronic structure of fcc Li departs radically from free-electron-like behavior. The anomalous increase in the magnitude of the pseudopo-tential (and the electron-ion interaction) in the fcc phase under pressure leads not only to the possibility of a superconducting state in Li, one where T c might increase …