Anharmonicity and the isotope effect in superconducting lithium at high pressures: A first-principles approach

Recent experiments [A. M. Schaeffer et al., Proc. Natl. Acad. Sci. USA 112, 60 (2015)] have shown that lithium presents an extremely anomalous isotope effect in the 15–25 GPa pressure range. In this article we have calculated the anharmonic phonon dispersion of 7Li and 6Li under pressure, their superconducting transition temperatures, and the associated isotope effect. We have found a huge anharmonic renormalization of a transverse acoustic soft mode along ΓK in the fcc phase, the expected structure at the pressure range of interest. In fact, the anharmonic correction dynamically stabilizes the fcc phase above 25 GPa. However, we have not found any anomalous scaling of the superconducting temperature with the isotopic mass. Additionally, we have also analyzed whether the two lithium isotopes adopting different structures could explain the observed anomalous behavior. According to our enthalpy calculations including zero-point motion and anharmonicity it would not be possible in a stable regime.The authors acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (Grant No. FIS2016-76617-P) and the Department of Education, Universities and Research of the Basque Government and the University of the Basque Country (Grant No. IT756-13). M.B. is also thankful to the Department of Education, Language Policy and Culture of the Basque Government for a predoctoral fellowship (Grant No. PRE-2015-2-0269).Peer Reviewe