The Search for Superconductivity in High Pressure Hydrides

Abstract

The computational and experimental exploration of the phase diagrams of binary hydrides under high pressure has uncovered phases with novel stoichiometries and structures, some which are superconducting at quite high temperatures. Herein we review the plethora of studies that have been undertaken in the last decade on the main group and transition metal hydrides, as well as a few of the rare earth hydrides at pressures attainable in diamond anvil cells. The aggregate of data shows that the propensity for superconductivity is dependent upon the species used to "dope" hydrogen, with some of the highest values obtained for elements that belong to the alkaline and rare earth, or the pnictogen and chalcogen families.

Quantum crystal structure in the 250-kelvin superconducting lanthanum hydride

“…As we show in Extended Figure 3, setting (4) Φ = 0 has a negligible effect on the phonons obtained from the Hessian defined in Eq. (1).…”

“…Φ = 0, so that the phonon spectra corresponds to the one coming directly from the SSCHA variational force constants Φ. The results clearly show that while the effect of (3) Φ is important, setting (4) Φ = 0 is perfectly safe. All these phonon spectra in the figures are obtained by Fourier interpolating directly the real space anharmonic force constants in a 2 × 2 × 2 supercell.…”

“…(1). Therefore, (4) Φ is neglected throughout, and all superconductivity calculations in the F m-3m and R-3m phases are performed making use of the phonon frequencies and polarization vectors obtained from the Hessian of E(R) with (4) Φ = 0. We also estimated T c with the phonon frequencies and polarization vectors obtained instead from Φ, resulting in a critical temperature 12 K lower within Allen-Dynes modified McMillan equation.…”

“…The agreement between the calculated and experimental Tc values further supports this phase as responsible for the 250 K superconductivity. The relevance of quantum fluctuations in the energy landscape found here questions many of the crystal structure predictions made for hydrides within a classical approach that at the moment guide the experimental quest for room-temperature superconductivity [4][5][6]. Furthermore, quantum effects reveal crucial to sustain solids with extraordinary electron-phonon coupling that may otherwise be unstable [7].…”

“…These ideas have bloomed thanks to modern ab initio crystal structure prediction methods based on density-functional theory (DFT) [5,13,14]. Hundreds of hydrogen-rich compounds have been predicted to be thermodynamically stable at high pressures and, by calculating the electron-phonon interaction parameters, their T c 's have been estimated [4,5]. The success of this symbiosis between DFT crystal structure predictions and T c calculations is exemplified by the discovery of superconductivity in H 3 S at 200 K [1,15,16].…”