See also Absence of high temperature superconductivity in hydrides under pressure , Nonstandard superconductivity or no superconductivity in hydrides under high pressure

### Meissner effect in nonstandard superconductors

J.E. Hirsch and F. Marsiglio, arXiv:2101.01701 (2021), Physica C (2021).

It was recently pointed out that so-called superhydrides'', hydrogen-rich materials that appear to become superconducting at high temperatures and pressures, exhibit physical properties that are different from both conventional and unconventional standard type I and type II superconductors \cite{hm,dc}. Here we consider magnetic field expulsion in the first material in this class discovered in 2015, sulfur hydride \cite{eremetssh}. A nuclear resonant scattering experiment has been interpreted as demonstration that the Meissner effect takes place in this material \cite{nrs,nrs2}. Here we point out that the observed effect, under the assumption that the system is in thermodynamic equilibrium, implies a Meissner pressure \cite{londonbook} in this material that is {\it much larger} than that of standard superconductors. This suggests that hydride superconductors are qualitatively different from the known standard superconductors {\it if} they are superconductors.