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

Abstract

Abstract The ab initio calculations suggest that YH 3 compound is a superconductor with the maximum value of the critical temperature ( T C ) 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 YH 3 have been determined in the framework of the Eliashberg formalism. It has been shown that T C 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: k B T C / ω ln ∈ 〈 0.138 , 0.082 〉 , where ω ln denotes the logarithmic phonon frequency. In particular, the dimensionless ratios: R Δ = 2 Δ ( 0 ) / k B T C , R C = Δ C ( T C ) / C N ( T C ) and R H = T C C N ( T C ) / H C 2 ( 0 ) take the following values: R Δ ∈ 〈 4.38 , 4.20 〉 , R C ∈ 〈 2.18 , 2.02 〉 and R H ∈ 〈 0.151 , 0.168 〉 . The symbol Δ represents the order parameter, Δ C is the specific heat jump, C N is the specific heat of the normal state, and H C is the thermodynamic critical field.