High-Tc superconductivity above 130 K in cubic MH4 compounds at ambient pressure

TL;DR

This work addresses the challenge of achieving high-$T_c$ superconductivity under ambient pressure by proposing a new class of hydrogen-rich MH4 hydrides with PtHg4-type structure. Using high-throughput first-principles calculations, PtH4, PdH4, and AuH4 are identified as dynamically stable at ambient pressure, with $T_c$ values reaching up to $T_c\approx$133 K for PdH4 according to Migdal-Eliashberg theory. The mechanism is dominated by hydrogen-dominated electron-phonon coupling and phonon softening, aided by pronounced Fermi-surface nesting. The results suggest a design pathway that combines chemical templating and lattice symmetry to realize high-$T_c$ hydrides at ambient pressure, with potential synthesis via stabilization of a bcc M lattice and quenching, and possible alloying to facilitate formation.

Abstract

Hydrides have long been considered promising candidates for achieving room-temperature superconductivity; however, the extremely high pressures typically required for high critical temperatures remain a major challenge in experiment. Here, we propose a class of high-Tc ambient-pressure superconductors with MH4 stoichiometry. These hydrogen-based compounds adopt the bcc PtHg4 structure type, in which hydrogen atoms occupy the one-quarter body-diagonal sites of metal lattices, with the metal atoms acting as chemical templates for hydrogen assembly. Through comprehensive first-principles calculations, we identify three promising superconductors, PtH4, AuH4 and PdH4, with superconducting critical temperatures of 84 K, 89 K, and 133 K, respectively, all surpassing the liquid-nitrogen temperature threshold of 77 K. The remarkable superconducting properties originate from strong electron-phonon coupling associated with hydrogen vibrations, which in turn arise from phonon softening in the mid-frequency range. Our results provide crucial insights into the design of high-Tc superconductors suitable for future experiments and applications at ambient pressure.

Figures (5)

• Figure 1: (a) Three dimensional electron localization function of the bcc lattice formed by the M atoms (isosurface = 0.326). The circle highlights the interstitial quasi-atoms (ISQ). (b) The structure of MH$_4$ at ambient pressure. The M and H atoms are colored in gray and pink, respectively. (c) Schematic of our screening workflow. Panels (a) and (b) were produced with VESTA Momma:ko5060.
• Figure 2: Calculated band structures and projected density of states (PDOS) of (a-c) PtH$_4$, PdH4 and AuH$_4$ at 0 GPa. The circles highlight the triply degenerate (spinless) point where three energy bands meet together. (d-f) Fermi surfaces of PtH$_4$, PdH4 and AuH$_4$, colored with respect to the Fermi velocity $\left\langle{v}\right\rangle$ (10$^5$ m/s). The color bars use the same scale of $\left\langle{v}\right\rangle$.
• Figure 3: Calculated phonon band structures, phonon density of states (PHDOS), electron-phonon coupling coefficient $\lambda(\omega)$ and Elisberg spectral function $\alpha^2F(\omega)$ for (a) PtH$_4$, (b) PdH$_4$, and (c) AuH4 at 0 GPa. The red solid circles show the phonon linewidth with a radius proportional to the EPC strength. The arrows denote the significant vibration modes in phonon dispersion curves.
• Figure 4: (a) The calculated nesting function $\xi$(Q) of MH4 at 0 GPa along some particular $q$ trajectories. (b) Isotropic superconducting gap of MH4 obtained by numerically solving the isotropic Migdal-Eliashberg equations.
• Figure 5: (a) Ab initio molecular dynamics simulation of PtH$_4$ at 0 GPa and 300 K. The inserts are snapshots of structures at 0 ps and 8 ps simulations, respectively. (b) Calculated electrostatic potential on the Pt--H plane.