Hydrogen bond symmetrization by proton quantum motion and high Tc superconductivity in Sulfur hydrides at high pressure
Venue: Polo Scientifico e Tecnologico Fabio Ferrari – Room A205
Prof. Matteo Calandra CNRS and Université P. et M. Curie (France)
Atoms in a crystal are quantum particles that oscillate even at zero temperature. The quantum behavior of the atoms makes them very different from classical particles as they are not point charges and they do not necessarily sit at the minimum of the static energy potential. In other words, the vibrational energy associated to the quantum oscillations can strongly modify the static energy landscape, even changing the ground state derived from the Born-Oppenheimer energy surface (BOES) minimum. Here, making use of density-functional theory calculations and of the Stochastic Self-consistent Harmonic Approximation to treat the anharmonic and quantum effects of the ions beyond the perturbative limit, we show that the ground state of high Tc superconductor hydrogen sulfide at 155 GPa is completely determined by quantum fluctuations.
Indeed, despite the minimum of the BOES is obtained for a rhombohedral structure with covalently bonded H3S units and hydrogen bonds between them, quantum fluctuations favor a fully symmetric cubic structure in which the covalent and hydrogen bonds equalize. The quantum hydrogen-bond symmetrization and the large anharmonic effects are crucial to understand the pressure dependence of the observed extraordinary Tc=205 K at 155 GPa. We finally show how, the dependence of Tc as a function of pressure, as well as the isotope effect, can be completely explained in the framework of a phonon mediated pairing mechanism in the presence of large anharmonic effects [1,2,3].
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