Pressure and high-Tc superconductivity in sulfur hydrides

Area of Science:

• Condensed Matter Physics
• Materials Science
• High-Pressure Physics

Background:

• Superconductivity in sulfur hydride (H3S) under high pressure has revealed record critical temperatures (TC).
• The rapid increase in TC near a critical pressure (Pcr ≈ 123 GPa) suggests a first-order structural transition.
• The cubic high-TC phase and a lower-TC phase with different periodicity indicate complex structural dynamics.

Purpose of the Study:

• To investigate the fundamentals of record-TC superconductivity in high-pressure sulfur hydride.
• To interpret the rapid TC increase as a signature of a first-order structural transition.
• To analyze the complex phonon spectrum and its influence on superconductivity.

Main Methods:

• Interpretation of TC increase as a fingerprint of a first-order structural transition.
• Analysis of the phonon spectrum, including acoustic and high-frequency hydrogen modes.
• Formulation of a modified approach for calculating TC due to spectral complexity.
• Investigation of the isotope effect using deuterium substitution.

Main Results:

• The structural transition at Pcr ≈ 123 GPa significantly impacts TC.
• A modified calculation method yields realistic TC values and quantifies contributions from optical and acoustic phonon branches.
• The isotope effect is linked to high-frequency phonons and varies between the two phases.
• Intermixing with pairing at hole-like pockets in the cubic phase leads to a second gap and a non-adiabatic regime.

Conclusions:

• The structural transition in H3S is crucial for its high-temperature superconductivity.
• The modified calculation method accurately captures TC and phonon contributions.
• The observed isotope effect and TC decrease are explained by phase intermixing and non-adiabatic pairing mechanisms.