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

## TL;DR

This study uses first-principles calculations to investigate anharmonic effects and isotope influence on superconductivity in lithium under high pressure, revealing significant anharmonic stabilization but no anomalous isotope effect.

## Contribution

It provides the first detailed anharmonic phonon and isotope effect analysis in high-pressure lithium, clarifying the role of anharmonicity and structural stability.

## Key findings

- Anharmonic phonon renormalization stabilizes the fcc phase above 25 GPa.
- No anomalous isotope effect observed in superconducting temperature.
- Different structural phases for isotopes do not explain the experimental anomaly.

## Abstract

Recent experiments [Schaeffer 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 $\mathrm{^7Li}$ and $\mathrm{^6Li}$ 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 $\Gamma$K 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.