The In$\text{}_{4}$Se$\text{}_{3}$ Crystal as a Three-Dimensional Imitative Model of Phenomena in One-Dimensional Crystals

For three-dimensional charge carriers described by the dispersion law with quartic terms of the wave vector, the density of states function similar as in the one-dimensional case was determined. This similarity allows the Pekar and Dejgen condenson states in the continuum approximation to exist. The calculated phonon spectrum reveals optical vibrations of a very low frequency, which favours the electron-phonon interaction and creation of the condenson states.