Quantum anharmonic enhancement of superconductivity in P63/mmc ScH6 at high pressures: A first-principles study

Abstract

Making use of first-principles calculations, we analyze the effect of quantum ionic fluctuations and lattice anharmonicity on the crystal structure and superconductivity of P 63/mmc ScH6 in the 100-160 GPa pressure range within the stochastic self-consistent harmonic approximation. We predict a strong correction to the crystal structure, the phonon spectra, and the superconducting critical temperatures, which have been estimated in previous calculations without considering ionic fluctuations on the crystal structure and assuming the harmonic approximation for the lattice dynamics. Quantum ionic fluctuations have a large impact on the H2 molecular-like units present in the crystal by increasing the hydrogen-hydrogen distance about a 5%. According to our anharmonic phonon spectra, this structure will be dynamically stable at least above 85 GPa, which is 45 GPa lower than the pressure given by the harmonic approximation. Contrary to many superconducting hydrogen-rich compounds, where quantum ionic effects and the consequent anharmonicity tend to lower the superconducting critical temperature, our results show that it can be enhanced in P 63/mmc ScH6 by approximately 15%. We attribute the enhancement of the critical temperature to the stretching of the H2 molecular-like units and the associated increase of the electron-phonon interaction. Our results suggest that quantum ionic effects increase the superconducting critical temperature in hydrogen-rich materials with H2 units by increasing the hydrogen-hydrogen distance and, consequently, the electron-phonon interaction.

The physical significance of imaginary phonon modes in crystals

“…For example, it has been shown to effectively predict second-order phase transitions and to accurately reproduce phase diagrams [68]. It is also a reliable method for structure searching, as it can locate saddle points in complex systems with large numbers of atoms [69,70]. The scheme also has the capability to determine accurate phonon linewidths e.g.…”

Ab initio study of metastable occupation of tetrahedral sites in palladium hydrides and its impact on superconductivity

“…Due to the large computational effort to structurally relax in the quantum energy landscape these structures that lack symmetry, we limit ourselves to relax the five representative structures labeled from #1 to #5 in all figures. Despite ionic quantum effects and anharmonicity can impact the cell parameters [35,[45][46][47], considering the additional large computational effort required to relax the cell parameters for these systems with no symmetry, here we do not relax the unit cell further and just relax the internal centroid positions R in the quantum energy landscape. However, the stress tensor calculated including ionic quantum effects [35] in these structures shows that there is a weak anisotropy in the stress tensor for any of the five structures, irrespective of whether the cell remains fcc-like or not.…”

Strong Electron–Phonon Coupling in 3D WN and Coexistence of Intrinsic Superconductivity and Topological Nodal Line in Its 2D Limit

“…[ 38 ] Anharmonicity also reduces the total λ by more than 30% and T c by more than 59% in the range of 109–151 GPa for AlH 3 as compared with harmonic case. [ 39 ] Frozen‐phonon total energy of WN as a function of the displacement of W atom for α and β phonons are shown in Figure 2c,d. For α phonon, the distortion energy can be well fitted by the harmonic expression E δ = 7.0 δ 2 eV Å −2 up to displacement of 0.15 Å.…”