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

Abstract

Recent experiments1 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 7 Li and 6 Li 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.

Reactivity of lithium and platinum at elevated densities

“…Our observations may help explain the significant discrepancies that exist in the literature regarding the Li melting curve [7,44] and the reportedly large isotopic effect on T c between 6 Li and 7 Li [8]. It should be noted that a subsequent theoretical study has been unable to reproduce the reported effect [46].…”

Insight into Understanding the Inverse Isotope Effect in a One-Dimensional Spin-Peierls-Type Molecular Solid

“…The isotope effect generally occurs in a certain phase transition system, which is primarily linked to the factors including the zero-point energy, − amplitude of characteristic vibrations, electron–phonon coupling, − and tunneling effects . Extensive research attention has been devoted to both the normal and inverse isotope effects in superconductors, − , hydrogen-bonded ferroelectrics/antiferroelectrics, and other types of phase transition crystals ,,− owing to the phenomena facilitated by the phonon-assisted process.…”

“…The isotope effect generally occurs in a certain phase transition system, which is primarily linked to the factors including the zero-point energy, − amplitude of characteristic vibrations, electron–phonon coupling, − and tunneling effects . Extensive research attention has been devoted to both the normal and inverse isotope effects in superconductors, − , hydrogen-bonded ferroelectrics/antiferroelectrics, and other types of phase transition crystals ,,− owing to the phenomena facilitated by the phonon-assisted process. In contrast, within magnetic phase transition materials, the isotope effect has only been observed in a limited number of phonon-assisted phase transition systems, such as spin crossover , and spin-Peierls systems, , up to the present time.…”

Gaussian time-dependent variational principle for the finite-temperature anharmonic lattice dynamics

“…Recently, a stochastic implementation of the SCHA method has been developed to study the equilibrium crystal structure and phonon band structure of real anharmonic materials from first principles [16][17][18][19]. This method has been successfully applied to various lattice-related phenomena such as structural phase transitions [18][19][20][21], superconductivity [16,[22][23][24][25], and charge density waves [26][27][28][29][30][31] and to the dynamical properties such as the phonon spectral function [20,21,[32][33][34] and infrared and Raman spectra [35].…”