High-pressure superconducting phase diagram of 6 Li: Isotope effects in dense lithium

The emergence of exotic quantum states, such as fluid ground state and two-component superconductivity and superfluidity, in a compressed light metallic system has been entertained theoretically for metallic phases of hydrogen. The difficulty of compressing hydrogen to metallization densities has prevented experimental proof of these effects. Studying lithium, which is isovalent to hydrogen and the lightest metal, is considered as a route to studying the lattice quantum effects in a dense light metallic system. Here, by comparing the superconductivity of lithium isotopes under pressure, we present evidence that properties of lithium at low temperature may significantly be dominated by its lattice quantum dynamics. This study is the first experimental report on superconducting properties of 6 Li, the lightest superconducting material. We measured the superconducting transition temperature of 6 Li between 16 and 26 GPa, and report the lightest system to exhibit superconductivity to date. The superconducting phase diagram of 6 Li is compared with that of 7 Li through simultaneous measurement in a diamond anvil cell (DAC). Below 21 GPa, Li exhibits a direct (the superconducting coefficient, α , T c ∝ M − α , is positive), but unusually large isotope effect, whereas between 21 and 26 GPa, lithium shows an inverse superconducting isotope effect. The unusual dependence of the superconducting phase diagram of lithium on its atomic mass opens up the question of whether the lattice quantum dynamic effects dominate the low-temperature properties of dense lithium.