High temperature superconductivity in the candidate phases of solid hydrogen

Abstract

As the simplest element in nature, unraveling the phase diagram of hydrogen is a primary task for condensed matter physics. As conjectured many decades ago, in the low-temperature and high-pressure part of the phase diagram, solid hydrogen is expected to become metallic with a high superconducting transition temperature. The metallization may occur via band gap closure in the molecular solid or via a transition to the atomic solid. Recently, a few experimental studies pushed the achievable pressures into the 400 – 500 GPa range. There are strong indications that at some pressure in this range metallization via either of these mechanisms occurs, although there are disagreements between experimental reports. Furthermore, there are multiple good candidate crystal phases that have emerged from recent computational and experimental studies which may be realized in upcoming experiments. Therefore, it is crucial to determine the superconducting properties of these candidate phases. In a recent study, we reported the superconducting properties of the C2/c-24 phase, which we believe to be a strong candidate for metallization via band gap closure [M. Dogan et al., arXiv:2107.03889 (2021)]. Here, we report the superconducting properties of the Cmca-12, Cmca-4 and I41/amd-2 phases including the anharmonic effects using a Wannier function-based dense k-point and q-point sampling. We find that the Cmca-12 phase has a superconducting transition temperature that rises from 86 K at 400 GPa to 212 K at 500 GPa, whereas the Cmca-4 and I41/amd-2 phases show a less pressure-dependent behavior with their Tc in the 74 – 94 K and 307 – 343 K ranges, respectively. These properties can be used to distinguish between crystal phases in future experiments. Understanding superconductivity in pure hydrogen is also important in the study of high-Tc hydrides.

Anisotropy and isotope effect in superconducting solid hydrogen

“…As a result of this experimental and theoretical landscape, we include these four phases in our studies, which also aligns with the recent state-of-the-art attempts at computationally determining the phase diagram [28]. The superconducting properties of the phases with smaller unit cells (Cmca-4 and I4 1 /amd-2) had been investigated computationally by other researchers [6,[29][30][31][32][33], followed by our recent work on all the four phases [34,35], partly motivated by the rejuvenated interest in high-pressure superconducting hydrogen-rich systems [36][37][38][39].…”

“…In our previous treatment of the superconductivity of these phases of hydrogen, we employed the isotropic Eliashberg theory to estimate the superconducting temperatures of these systems [34,35]. Here, we expand our study to include anisotropic Eliashberg theory calculations.…”

“…The molecular phases C2/c-24, Cmca-12 and Cmca-4 consist of van der Waals-bonded layers of H 2 molecules, and the atomic phase consists of a highly symmetric 2-atom cell (the atomic structures can be found in the supplemental materials of [34,35]). Among these four phases, as the number of atoms in the unit cell decreases, the structure becomes more symmetric, which is reflected in the electronic band structure.…”

Universal diamond edge Raman scale to 0.5 terapascal and implications for the metallization of hydrogen

“…The alternative scenario implies the closure of the direct band in hydrogen and its transformation into a good metal at ∼450 GPa 44 , 45 or ∼430 GPa 46 within the same C 2/ c -24 molecular phase. This metal is predicted to be a superconductor with a T c ∼86 K at 400 GPa arising to ∼212 K at 500 GPa 49 . More experimental data are required to understand the nature of the reported observations.…”

Anharmonic theory of superconductivity and its applications to emerging quantum materials

“…While it is clear that phonon dispersion curves are strongly renormalized (i.e., lowered in energy) by anharmonicity in the hydrides [156,157,203], a clear picture about the effect of the ubiquitous large anharmonicity on the superconductivity of these systems is missing. In particular, systematic studies of the anharmonic phonon linewidths and the effect thereof on the Cooper pairing are currently lacking.…”