Dr. Matteo Calandra, IMPMC-UPMC

Jeudi 15 Octobre 2015 14h
Amphi Holweck, Esc C, 1ème etage

Titre:High-Pressure Hydrogen Sulfide from First Principles:A Strongly Anharmonic Phonon-Mediated Superconductor

The recent discovery of superconductivity in high pressure HS2 with the
highest known Tc=205 K, opens new perspectives in the field by upturning
the conventional wisdom that an high Tc cannot be obtained via a phonon mediated pairing.

Here, we use first-principles calculations to study structural, vibrational,and superconducting properties of H2S at high pressures.

The inclusion of zero-point energy leads to two different possible dissociations of H2S,namely 3H2S in H3S+S and 5H2S in H3S+HS2, where both H3S and HS2 are metallic.

We show that HS2 is indeed superconductor, but with much lower Tc and cannot explain the measured Tc=205K at 200 GPa.
For H3S, we perform nonperturbative calculations of anharmonic effects within the self-consistent harmonic approximation and show that the harmonic approximation strongly overestimates the electron-phonon interaction (lambda=2.64 at 200 GPa) and Tc.
Anharmonicity hardens H-S bond-stretching modes and softens H-S bond-bending modes.
As a result, the electron-phonon coupling is suppressed by 30% (lambda=1.84 at 200 GPa).
Moreover, while at the harmonic level Tc decreases with increasing pressure, the inclusion of anharmonicity leads to a Tc that is almost independent of pressure.

High-pressure hydrogen sulfide is a strongly anharmonic superconductor.

[1] I. Errea et al. , Phys. Rev. Lett. 114, 157004 (2015)
[2] Li, et al. , arXiv:1508.03900
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