Superconductivity in Dense Lithium

Abstract

Superconductivity in compressed lithium is observed by magnetic susceptibility and electrical resistivity measurements. A superconducting critical temperature (Tc) is found ranging from 9 to 16 kelvin at 23 to 80 gigapascals. The pressure dependence of Tc suggests multiple phase transitions, consistent with theoretical predictions and reported x-ray diffraction results. The observed values for Tc are much lower than those theoretically predicted, indicating that more sophisticated theoretical treatments similar to those proposed for metallic hydrogen may be required to understand superconductivity in dense phases of lithium.

Dense Low-Coordination Phases of Lithium

“…These features are consistent with the picture that reduced coordination numbers at high pressures arise from a Jahn-Teller-like distortion [2]. Calculations of the high-pressure phases also show an accumulation of electronic charge density within the interstitial region as the 2s electron density is pushed away from the atomic cores by Coulomb repulsion, Pauli exclusion, and the orthogonality of core and valence orbitals [2,3,5].Experiments suggest another phase transition at 60-70 GPa, although the new structure has not been determined [6,7]. A number of theoretical studies have sought to identify candidate phases at higher pressures [2,8,9].…”

“…This structure contains sixfold coordinated atoms. On the other hand, recent hightemperature DFT molecular dynamics simulations found fourfold coordinated Li atoms in the pressure range 150-810 GPa [10].Superconductivity has been observed in Li up to $85 GPa, indicating metallic behavior [6,11,12]. However, calculations have suggested that semimetallic or even semiconducting behavior might occur at high pressures [2].…”

High Pressure Effects on Superconductivity

“…Early high-pressure experiments [9] revealed a drop of resistivity in Li at 7 K between 22 and 32 GPa but the data were not conclusive enough to be interpreted in terms of superconductivity. Only recent measurements of electrical transport and magnetic properties have unambiguously shown that superconductivity exists in Li at pressures above 20 GPa and at temperatures up to 20 K [10,11,12]. The large interest in the superconductivity in dense lithium is also motivated by the fact that it is the "simplest" metal with only one valence electron, no d-electrons in sight, and it is the superconducting element closest to hydrogen.…”

Origin of strong coupling in lithium under pressure

“…Elemental lithium, with superconducting T c ∼ 15 K around [1,2,3] 30 GPa and up to 20 K in strained samples [1] at 50 GPa, joins MgB 2 (T c =40 K), metallic Y (T c =20 K) [4], and PuCoGa 5 (T c =19 K) [5] as the big surprises in critical temperature in this century. Transformation from a simple nearly-free-electron metal not superconducting [6] above 100 µK to a strongly coupled electron-phonon superconductor at 20-50 GPa has been explained, at least semiquantitatively, in recent work by Profeta et al [7] and by Kasinathan et al [8].…”