Non-BCS superconducting state in yttrium hydride at a record low value of the external pressure

The ab initio calculations suggest that YH3 compound is a superconductor with the maximum value of the critical temperature (TC) for the record low external pressure equal to 17.7 GPa. Due to the high value of the electron-phonon coupling constant, the thermodynamic properties of the superconducting state in YH3 have been determined in the framework of the Eliashberg formalism. It has been shown that TC changes in the range from 45.91K to 27.29 K, for the value of the Coulomb pseudopotential μ⋆∈〈0.1,0.3〉. The parameters of the superconducting phase differ significantly from the values predicted by the BCS theory due to the significant strong-coupling and retardation effects: kBTC/ωln∈〈0.138,0.082〉, where ωln denotes the logarithmic phonon frequency. In particular, the dimensionless ratios: RΔ=2Δ(0)/kBTC, RC=ΔC(TC)/CN(TC) and RH=TCCN(TC)/HC2(0) take the following values: RΔ∈〈4.38,4.20〉, RC∈〈2.18,2.02〉 and RH∈〈0.151,0.168〉. The symbol Δ represents the order parameter, ΔC is the specific heat jump, CN is the specific heat of the normal state, and HC is the thermodynamic critical field.