Anharmonic effects in atomic hydrogen: Superconductivity and lattice dynamical stability

Abstract

We present first-principles calculations of metallic atomic hydrogen in the 400-600 GPa pressure range in a tetragonal structure with space group I41/amd, which is predicted to be its first atomic phase. Our calculations show a band structure close to the free-electron-like limit due to the high electronic kinetic energy induced by pressure. Bands are properly described even in the independent electron approximation fully neglecting the electron-electron interaction. Linear-response harmonic calculations show a dynamically stable phonon spectrum with marked Kohn anomalies. Even if the electron-electron interaction has a minor role in the electronic bands, the inclusion of electronic exchange and correlation in the density response is essential to obtain a dynamically stable structure. Anharmonic effects, which are calculated within the stochastic self-consistent harmonic approximation, harden high-energy optical modes and soften transverse acoustic modes up to a 20% in energy. Despite a large impact of anharmonicity has been predicted in several high-pressure hydrides, here the superconducting critical temperature is barely affected by anharmonicity, as it is lowered from its harmonic 318 K value only to 300 K at 500 GPa. We attribute the small impact of anharmonicity on superconductivity to the absence of softened optical modes and the fairly uniform distribution of the electron-phonon coupling among the vibrational modes.

Potential high- T c superconducting lanthanum and yttrium hydrides at high pressure

“…Recent first-principles calculations of metallic hydrogen in structures with space groups I4 1 /amd and Cmca-4 predict anharmonic behavior that differs from calculated results for hydrides with relatively low hydrogen content (51,52). Whereas in the I4 1 /amd the inclusion of anharmonicity slightly lowers T c (i.e., from 318 to 300 K at 500 GPa), the situation is drastically different in the Cmca-4 phase, where the calculated T c increases by a factor of 2 with anharmonicity introduced, such that the phase is predicted to be a superconductor above 200 K. In this context it is also worth noticing that anharmonic vibrations may enhance the electron-phonon matrix elements, e.g., in the case of disordered materials (53).…”

A perspective on conventional high-temperature superconductors at high pressure: Methods and materials

“…These compounds are in many ways all realizations of the Aschcroft-Ginzburg-Gilman's arguments, but with very different outcomes. In fact, the actual superconducting properties Figure 28:Éliashberg α 2 F function computed for selected hydrides: PdH (0 GPa) [208], PH 2 (120 GPa) [168], H 3 S (200 GPa) [173], LaH 10 (300 GPa) [461] and H 2 (500 GPa) [464]. A vertical shift is applied for plotting the functions.…”

Strong Electron-Phonon and Band Structure Effects in the Optical Properties of High Pressure Metallic Hydrogen

“…where σ intra and σ inter account, respectively, for the optical conductivity provided by electronic intraband and interband transitions, while σ phonons accounts for the direct phonon absorption contribution. As I4 1 /amd hydrogen lacks of IR active vibrational modes [1], we set σ phonons = 0. The interband and intraband contributions are computed in two stages.…”

“…The Eliashberg function α 2 F (ω) function needed to solve the ME equations is calculated as described in Ref. [1], but with the use of the same exchange and correlation potential as for the TDDFT calculation. α 2 F (ω) is calculated at the harmonic level as anharmonicity barely affects it [1].…”