Lifshitz transitions and zero point lattice fluctuations in sulfur hydride showing near room temperature superconductivity

Abstract: Emerets's experiments on pressurized sulfur hydride have shown that H3S metal has the highest known superconducting critical temperature Tc=203K. The Emerets data show pressure induced changes of the isotope coefficient between 0.25 and 0.5, in disagreement with Eliashberg theory which predicts a nearly constant isotope coefficient. We assign the pressure dependent isotope coefficient to Lifshitz transitions induced by pressure and zero point lattice fluctuations. It is known that pressure could induce changes of the topology of the Fermi surface, called Lifshitz transitions, but were neglected in previous papers on the H$_3$S superconductivity issue. Here we propose that H3S is a multi-gap superconductor with a first condensate in the BCS regime (in the large Fermi surface with high Fermi energy) which coexists with a second condensates in the BCS-BEC crossover regime (located on a small Fermi surface spots with small Fermi energy) near the $\Gamma$ and M point. We discuss the need of Bianconi-Perali-Valletta (BPV) superconductivity theory for superconductivity in H3S. It includes both the correction of the chemical potential due to pairing and the configuration interaction between different condensates, neglected by the Eliashberg theory. Here the shape resonance in superconducting gaps, similar to Feshbach resonance in ultracold gases, gives a relevant contribution to amplify the critical temperature. Therefore this work provides some key tools needed in the search for new room temperature superconductors.