EN
Emerets’s experiments on pressurized sulfur hydride
have shown that H3S metal has the highest known
superconducting critical temperature Tc = 203 K. 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 H3S superconductivity issue. Here we
propose thatH3S is a multi-gap superconductor with a first
condensate in the BCS regime (located in the large Fermi
surface with high Fermi energy) which coexists with second
condensates in the BCS-BEC crossover regime (located
on the Fermi surface spots with small Fermi energy) near
the and Mpoints.We discuss the Bianconi-Perali-Valletta
(BPV) superconductivity theory to understand superconductivity
in H3S since the BPV theory includes the corrections
of the chemical potential due to pairing and the configuration
interaction between different condensates, neglected
by the Eliashberg theory. These two terms in the
BPV theory give the shape resonance in superconducting
gaps, similar to Feshbach resonance in ultracold fermionic
gases, which is known to amplify the critical temperature.
Therefore this work provides some key tools useful in the
search for new room temperature superconductors.