High-T{sub c} Hydrides: Interplay of Optical and Acoustic Modes and Comments Regarding the Upper Limit of T{sub c}

The phonon spectrum of many superconducting compounds and, especially, high-T{sub c} 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, λ{sub opt.} and λ{sub ac.}, along with characteristic frequencies. The correlation between the value of T{sub c} and this interplay is demonstrated for the family of tantal hydrides (TaH{sub 2}/TaH{sub 4}/TaH{sub 6}). The problem of the upper limit of T{sub c} is discussed. The phenomenon of room temperature superconductivity can be provided by the electron-phonon interaction and is described by the strong coupling theory.