Mechanism for the Structural Transformation to the Modulated Superconducting Phase of Compressed Hydrogen Sulfide

Abstract

A comprehensive description of crystal and electronic structures, structural transformations, and pressure-dependent superconducting temperature (T_c) of hydrogen sulfide (H₂S) compressed from low pressure is presented through the analysis of the results from metadynamics simulations. It is shown that local minimum metastable crystal structures obtained are dependent on the choice of pressure-temperature thermodynamic paths. The origin of the recently proposed 'high-T_c' superconducting phase with a modulated structure and a diffraction pattern reproducing two independent experiments was the low pressure Pmc2₁ structure. This Pmc2₁ structure is found to transform to a Pc structure at 80 K and 80 GPa which becomes metallic and superconductive above 100 GPa. This structure becomes dynamically unstable above 140 GPa beyond which phonon instability sets in at about a quarter in the Γ to Y segment. This explains the transformation to a 1:3 modulation structure at high pressures proposed previously. The pressure trend of the calculated T_c for the Pc structure is consistent with the experimentally measured 'low-T_c phase'. Fermi surface analysis hints that pressurized hydrogen sulfide may be a multi-band superconductor. The theoretical results reproduced many experimental characteristics, suggesting that the dissociation of H₂S is unrequired to explain the superconductivity of compressed H₂S at any pressure.

Figure 1

Crystal structures of H₂S obtained from metadynamics simulations carried out at different pressures and temperatures, starting from the Pmc2₁ structure. Big and small spheres represent S and H atoms, respectively. Unit cells are marked with black boxes.

Figure 2

(a) Evolution of the electronic band structure of the Pc structure near the Fermi level at 80, 100 and 120 GPa. (b) Electronic band structure and projected DOS of the Pc structure at 80 GPa. The Fermi level and band gap are indicated by dash lines.

Figure 3

Phonon dispersion curves of the Pc structure curve at 180 GPa.

Figure 4

Temporal positions of the S and H atoms indicated by different colors at different slices from molecular dynamics simulation (see text) showing the rapid hopping and diffusing motion of the hydrogen atoms at 160 GPa.

Figure 5

(a) The α²F(ω) and λ(ω) of the Pc structure calculated at 120 GPa. (b) Comparison between the calculated T_c for various H_xS species (all but refs^,) and the measured T_c in compressed H₂S^,. The shaded red region indicates the range of estimate for the T_c at 200 GPa for the modulated 1:3 structure.

Figure 6

(a) Total and atom projected phonon density of states, (b) electronic band structure, (c) Fermi surfaces, and (d) nesting function ξ (Q) for the Pc structure at 160 GPa.