  Discovery of superconductivity at megabar (MB) pressures in hydrogen sulfide
H3S, then in metal polyhydrides, starting with binary, LaH10, etc., and ending
with ternary ones, including (La, Y)H10, revolutionized the field of condensed
matter physics. These discoveries strengthen hopes for solution of the
century-old problem of creating materials that are superconducting at room
temperature. In experiments performed over the past 5 years at MB pressures, in
addition to the synthesis of hydrides itself, their physical properties were
studied using optical, X-ray and Mossbauer spectroscopy, as well as
galvanomagnetic measurement techniques. This paper presents the major results
of galvanomagnetic studies, including measurements in high static (up to 21T)
and pulsed (up to 70T) magnetic fields. Measurements of resistance drops to
vanishingly small level at temperatures below the critical Tc value, a decrease
in the critical temperature Tc with increasing magnetic field, as well as
diamagnetic screening, indicate the superconducting state of the polyhydrides.
The results of measurements of the isotope effect, together with the effect of
magnetic impurities on Tc, indicate the electron-phonon mechanism of electron
pairing. However, electron-electron correlations in polyhydrides are by no
means small, both in the superconducting and normal states. It is possible that
this is precisely what accounts for the unusual properties of polyhydrides that
have not yet received a satisfactory explanation, such as a linear temperature
dependence of the second critical field Hc2(T), a linear dependence of
resistance R(T), and a linear magnetoresistance, very similar to that
discovered by P. L. Kapitza in 1929.
