Synopsis: Superconductors Under Pressure

The coupling of electrons to anharmonic crystal vibrations may explain the record high-temperature superconductivity in highly pressurized hydrogen sulfide.
Synopsis figure
Matteo Calandra/Sorbonne University

A recent report claims to have shown that pressurized H2S superconducts at 190K. If confirmed, this finding will knock cuprates from their position as the highest temperature superconductors (164K), and provide a step forward towards a room-temperature superconductor. High critical temperatures are thought to be associated with unconventional superconductivity, whose physical mechanisms are yet unknown. But Ion Errea at Donostia International Physics Center, Spain, and colleagues have now shown that this assumption is incorrect for H2S at high pressures. Their theoretical calculations suggest that H2S behaves much like a conventional superconductor, where superconductivity is driven by a phonon-mediated pairing mechanism.

The researchers studied the decomposition pathways of H2S when it is pressurized to 200 gigapascals. They found that H2S could form two stable metallic structures, H3S or HS2. The theory predicts that both exhibit superconductive behavior, but HS2 stops superconducting at 30K and thus cannot explain the recent experiments. Focusing on H3S, the authors study the interactions between the electrons and vibrational modes (phonons), which are known to be important for conventional superconductivity. They found that electron-phonon interactions explain high temperature superconductivity in H3S if the anharmonic vibrational motion of the hydrogen atoms is considered. Although high compression of H3S should limit the range of atomic displacement, the hydrogen atoms, because of their light mass, are significantly displaced from their equilibrium position. This shifts the harmonic vibrational frequencies through softening of certain bonds and hardening of others. The authors concluded that only if this anharmonicity is taken into account could they replicate the recently reported experimental results.

This research is published in Physical Review Letters.

–Katherine Wright.


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