High-Tc Hydrides: Interplay of Optical and Acoustic Modes and Comments Regarding the Upper Limit of Tc

Abstract

The phonon spectrum of many superconducting compounds and, especially, high-Tc hydrides, is broad and rather complicated, because of the presence of high-frequency optical modes. In order to analyze an interplay of optical and acoustic phonon branches, it is convenient to introduce two coupling constants, $\lambda _{\text {opt.}}$ and $\lambda _{\text {ac.}}$ , along with characteristic frequencies. The correlation between the value of Tc and this interplay is demonstrated for the family of tantal hydrides (TaH2/TaH4/TaH6). The problem of the upper limit of Tc is discussed. The phenomenon of room temperature superconductivity can be provided by the electron-phonon interaction and is described by the strong coupling theory.