Meissner effect in nonstandard superconductors

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 [1,2]. Here we consider magnetic field expulsion in the first material in this class discovered in 2015, sulfur hydride [3]. A nuclear resonant scattering experiment has been interpreted as demonstration that the Meissner effect takes place in this material [4,5]. Here we point out that the observed effect, under the assumption that the system is in thermodynamic equilibrium, implies a Meissner pressure [6] in this material that is much larger than that of standard superconductors. This suggests that hydride superconductors are qualitatively different from the known standard superconductors if they are superconductors.