Debye temperature in LaHx-LaDy superconductors

Abstract

The Debye temperature, T${_\theta}$=(h/2${\pi}$)/k${_B}$$\omega$$_\theta$, where the Debye frequency $\omega$$_\theta$ is integrated characteristic frequency of full phonon spectrum, $\alpha$$^2$($\omega$)F($\omega$). In the BCS theory, T${_\theta}$ in conjunction with electron-phonon coupling strength parameter, $\lambda$$_{e-ph}$, determines the superconducting transition temperature, Tc. Despite a fact that more accurate theory of electron-phonon mediated superconductivity requires the knowledge of full phonon spectrum, $\alpha$$^2$($\omega$)F($\omega$), which can be very accurately computed by first principles calculation technique, there is no experimental technique which can measure $\alpha$$^2$($\omega$)F($\omega$) in highly-compressed near-room-temperature (NRT) superconductors. Thus, $\omega$$_\theta$ remains to be the only measurable parameter of full phonon spectrum, $\alpha$$^2$($\omega$)F($\omega$), which can be deduced by the fit of experimental temperature-dependent resistance data, R(T), to Bloch-Gr\"uneisen equation. Taking in account that within electron-phonon mediated theory of superconductivity two isotopic counterparts (or, in case of NRT superconductors, the same superconducting phase at different pressures), designated by subscripts of 1 and 2, should be obey the relation of T${_\theta}_1$/T${_\theta}_2$=T${_c}_1$/T${_c}_2$, there is a way to reaffirm/disprove the electron-phonon mechanism of NRT superconductivity. In this paper, we perform the analysis for R3m-phase of H3S at different pressure, as well as for several superconductors in LaHx-LaDy system and show that there is a large disagreement between experimental data and T${_\theta}_1$/T${_\theta}_2$=T${_c}_1$/T${_c}_2$. Taking in account that similar disagreement has recently reported in H3S-D3S system, it can be concluded that primary origin for NRT superconductivity remains to be discovered.