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

Phonon spectrum of many superconducting compounds and, especially, high Tc hydrides, is broad and rather complicated ,because of 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}opt. and {\lambda}ac., along with to characteristic frequencies. The correlation between the value of Tc with this interplay is demonstrated for the family of tantal hydrides (TaH2/ TaH4/ TaH6). The problem of upper limit of Tc is discussed. The phenomenon of room temperature superconductivity can be provided by the electron-phonon interaction and described by the strong coupling theory.