Phonon-mediated superconductivity in compressed NbH4 compound

There has been much interest in the superconducting properties of metal hydrides for the last few years. The hydrogen-rich compounds are regarded as a way of hydrogen metallization at lower pressures than that required for pure hydrogen. This paper deals with the thermodynamic properties of the superconducting state in niobium hydride (NbH4) at 300 GPa. All theoretical calculations have been conducted within the framework of the strong-coupling Eliashberg theory. It has been stated that the critical temperature (TC) is equal to 49.57 K when the Coulomb pseudopotential takes the commonly accepted value of 0.1. In the considered case, the ratio of the energy gap to the critical temperature (RΔ ≡ 2Δ(0)/kBTC), the ratio of the specific heat jump to the heat of the normal state (RC ≡ ΔC(TC)/CN(TC)) and the ratio connected with the thermodynamic critical field (RH ≡ TCCN(TC)/H2C(0)) are equal to 3.91, 2.12 and 0.154, respectively. Above results differ significantly from the values predicted by the weak-coupling Bardeen-Cooper-Schrieffer theory, which omits a very important role of the strong-coupling and phonon retardation effects.