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

We measured the superconducting transition temperature of ⁶Li between 16 and 26 GPa, and report the lightest system to exhibit superconductivity to date. The superconducting phase diagram of ⁶Li is compared with that of ⁷Li 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.Light elements (low Z) and their compounds have been the subject of many recent studies for their potential as high-temperature superconductors (e.g., refs. 1–5). Due to their low mass, the physical properties of the low-Z compounds can be strongly influenced by zero-point effects (lattice quantum dynamics) (6), and mass-related isotope effects may be present in their thermodynamics of vibrational degrees of freedom. Such effects will influence the superconducting properties of these materials. Dependence of superconductivity on isotopic variations of low-Z compounds can be used to probe and determine the magnitude of mass-related effects. This in turn allows better development of models to determine their superconducting properties.Under ambient pressure, lithium is the lightest elemental metallic and superconducting system, and it exhibits one of the highest superconducting transition temperatures of any elemental superconductor under compression (7–11). Despite the large mass difference between the stable isotopes of lithium (∼15%), isotope effects in superconductivity of lithium have not been studied before.In systems with long-range attractive potential, the ratio of lattice zero-point displacements to interatomic distances may increase under compression (increase to the Lindemann ratio at high densities), provided they retain their long-range interactions (12, 13). (This is opposed to systems with short-range interactions, e.g., helium, in which the lattice becomes more classical under compression.) In these systems, more deviations from the static lattice behavior are expected at higher densities. At sufficiently low temperatures, where thermal energy is small, lattice quantum dynamics can play a more dominant role in the bulk properties. Sound velocity measurements on stable isotopes of lithium at 77 K and up to 1.6 GPa show that quantum solid effects in lithium, at least in the pressure range studied, do not decrease as a function of pressure (14). Raman spectroscopy studies between 40 and 123 GPa and at 177 K report a reduced isotope effect in high-frequency vibrational modes of Li, which may be related to quantum solid behavior (15). Up to this point, no experiments have reported a comparison of any physical properties of lithium isotopes at low temperatures and high pressures concurrently. Because the superconducting transition of lithium occurs in a relatively low temperature range (16–18), studying its superconducting isotope effect provides excellent conditions to search for zero-point lattice effects and their evolution as a function of pressure.In the present study, we have measured the superconducting isotope effects in the stable isotopes of lithium under pressure. Lithium is a simple metallic system that is expected to exhibit conventional phonon-mediated superconductivity and a well-defined superconducting isotope effect with nominal pressure dependence of the relative T_c values (19) according to the model by Treyeva and Trapezina (19), using theoretical values of Coulomb coupling constant, μ*(P) (20) by Christensen and Novikov and equation of state (EOS) of lithium (21) by Hanfland et al., assuming similar structures for the two isotopes, α should not vary by more than 10% for 15?GPa < P < 25?GPa]. Because phonon-mediated superconductivity depends on lattice and electronic properties of a material, any unusual isotopic mass dependence of the superconducting phase diagram can be indicative of the effects of large lattice dynamics on electronic and/or structural properties.