High-Tc state of lanthanum hydrides

The high-T-c phase of lanthanum hydrides deserves special attention because this phase displays the highest observed critical temperature. We focus on the evaluation of the critical temperature, the isotope coefficient, and their pressure dependence in this phase (Fm (3) over barm phase of the LaH10 compound). By combining the method of two coupling constants, lambda(opt) and lambda(ac), and two characteristic frequencies with first-principle calculations we are able to analyze the role of light hydrogen in the amplitude of the critical temperature in this unique compound. Specifically, the critical temperature of 254 K and the isotope coefficient of 0.45 are evaluated at high pressure, and the results are in good agreement with the experimental data. The peculiar decrease in T-c upon further increase in pressure is explained by the presence of a flat region on the Fermi surface and the appearance of a two-gap structure, which should be experimentally observable by tunneling spectroscopy.