Characterization of the superconducting state in hafnium hydride under high pressure

The hydrogen-rich compounds at high pressure may exhibit notably high superconducting transition temperatures. In the paper, we have calculated the basic thermodynamic parameters of the superconducting state in two selected phases of HfH2 hydride under high-pressure respectively at 180 GPa for Cmma and 260 GPa for P21/m. Calculations has been conducted in the framework of the Eliashberg formalism. In particular, we have determined the values of the critical temperature (TC) to be equal to 8 K and 13 K for the Cmma and P21/m phases, respectively. Moreover, we have estimated other thermodynamic properties such as the order parameter (Δ(T)), the thermodynamic critical field (HC(T)), and the specific heat for the normal (CN) and superconducting (CS) state. Finally, we have shown that the characteristic ratios: RΔ=2Δ(0)/kBTC and RC=ΔC(TC)/CN(TC), which are related to the above thermodynamic functions, slightly differ from the predictions of the Bardeen-Cooper-Schrieffer theory due to the strong-coupling and retardation effects.