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

Abstract

Two hydrogen-rich materials: H 3 S and LaH 10 , synthesized at megabar pressures have revolutionized the field of superconductivity by providing a first glimpse into the solution for the hundred-year-old problem of room-temperature superconductivity. The mechanism governing these exceptional superconductors is the conventional electron-phonon coupling. Here, we describe recent advances in experimental techniques, superconductivity theory and first-principles computational methods, which made this discovery possible. The aim of this work is to provide an up-to-date compendium of the available results on superconducting hydrides and explain the synergy of different methodologies that led to the extraordinary discoveries in the field. Furthermore, in an attempt to evidence empirical rules governing superconductivity in binary hydrides under pressure, we discuss general trends in electronic structure and chemical bonding. The last part of the Review introduces possible strategies to optimize pressure and transition temperatures in conventional superconducting materials. Directions for future research in areas of theory, computational methods and high-pressure experiments are proposed, in order to advance our understanding of superconductivity.

Classifying hydrogen-rich superconductors

“…Despite a fact that H3S is widely classified as conventional [7] electron-phonon superconductor [1,[3][4][5][6][8][9][10][11][12][13], we performed [14] analysis of experimental upper critical field data, Bc2(T), reported by Mozaffari et al [15,16], and showed that H3S is unconventional superconductor which lies in common unconventional superconductors trend band of the Uemura plot [17,18] together with other unconventional superconductors, i.e. heavy fermions, fullerenes, pnictides, and cuprates.…”

Route to a Superconducting Phase above Room Temperature in Electron-Doped Hydride Compounds under High Pressure

“…However, this approach was not widely adopted until Ashcroft's suggestion [2] that high-pressure conditions can metallize hydrogen-rich materials that are insulators at ambient pressure. Subsequently, there has been much research on hydrogen-rich hydrides under high pressures [3][4][5][6]. Breakthroughs were achieved in SH 3 and LaH 10 systems, which had high T c of ∼200 and 250-260 K, respectively [7][8][9][10][11][12][13].…”

High-TC Superconductivity in Hydrogen Clathrates Mediated by Coulomb Interactions Between Hydrogen and Central-Atom Electrons

“…Mass isotope data for DH 10 reported in [12], deemed inconclusive in [4], are shown to more likely reflect the inverse correlation between T C and the fractional transition width T C /T C , as is found for D 3 S. Consequently, and notwithstanding the predictions of Éliashberg theory, the precise role played by phonons in the superconductivity of MH n awaits further investigation. Since quantitative scaling with the theoretical e 2 / is noted from optical evidence for compressed A15 Cs 3 C 60 [49] and H 3 S [41], and plausibly extending to LaH 10 , reflectivity measurements of MH n may prove invaluable in determining the nature of the mediating boson.…”

“…The LaH 10 clathrate, exhibiting superconductivity at temperatures of about 250 K [12] and above 260 K [13] for applied pressures P ~ 137 -218 GPa, currently holds the record for the highest transition temperature, T C . Electron-phonon interaction theory rooted in strong-coupling Migdal-Éliashberg methodology [17][18][19], density functional theory, or ab initio methods [4,20], predicts T C for LaH 10 spanning a broad range of 199 -288 K [4][5][6][21][22][23][24][25][26] and monotonically decreasing with increasing pressure [5,[24][25][26].…”

“…A review of conventional electron-phonon coupling theory in [4] includes a listing of numerous computational flaws that remain to be addressed. 1 As independent corroboration of phonon mediation has yet to be convincingly established [36], alternative considerations of electronic mechanisms are being developed, particularly in view of the high pressures [37], correspondences to high-T C cuprates [38] and limitations on the Fermi temperatures [39].…”