On the occurrence of superconductivity in compressed alkali metals. An indirect-exchange interpretation

Recently, superconductivity under high pressure in lithium, with a Tc as high as 20K at 48GPa(480kbar), as well as a Tc from about 10 to 16K over a wide range of pressures (23–65GPa), has been established in almost simultaneous experiments. Thus far, only cesium was known to become superconducting, with a Tc of 1.5K under a pressure of about 12.5 GPa. The present paper deals with an interpretation of these results. Refined electron–phonon interactions yield critical temperatures under pressure as high as 60–80K in lithium way above experimental results. In the same framework, even at ambient pressure a critical temperature of about 1K is predicted, whereas experiments fail to observe superconductivity down to at most a few times 10−3K. With emphasis on solid Li, we approach this problem adopting an indirect-exchange pairing formalism earlier introduced in (our) many papers on high- and low-Tc materials. This implies that we postulate that Cooper-pair formation can be mediated through electron-pair (antiparallel-spins) density arising from overlapping charge distributions for pairs of atoms (“molecular dimers”) formed at high pressures near the Fermi level, significantly enhanced by s-to-p (or s-to-d) transitions of valence electrons. General results are that the critical temperature is strictly zero at ambient pressure, and that a broad pressure region with low and almost constant Tc (because of steeply rising bulk modulus) should be observed, in agreement with experiment. In addition, no significant change of Tc is predicted across the family of alkali metals.