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Matteo Calandra - Université Pierre et Marie Curie

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=190 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 pressures P> 200 GPa.
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
measure Tc=190K at 200 GPa.

For H3S, we perform nonperturbative calculations of anharmonic effects within the selfconsistent
harmonic approximation and show that the harmonic approximation strongly
overestimates the electron-phonon interaction (l=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% (l =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.

Reference :
Ion Errea, Matteo Calandra, Chris J. Pickard, Joseph Nelson, Richard J. Needs, Yinwei Li, Hanyu Liu,
Yunwei Zhang, Yanming Ma, and Francesco Mauri, Phys. Rev. Lett. 114, 157004 (2015)


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