TI-polaron (translation-invariant polaron) – elementary quasiparticle in solid state. The ground state of TI-polaron is a delocalized state of electron-phonon system: the probabilities of electron's occurrence at any point of a space are similar. Both the electron density and the amplitudes of phonon modes (renormalized by an interaction with the electron) are delocalized. The concept of a polaron potential well (formed by local phonons) in which the electron is localized, i.e. self-trapped state is lacking. Accordingly, the induced polarization charge of the translation-invariant polaron is equal to zero.
The ground state energy of the translation-invariant polaron is lower than that of Pekar polaron and is E0 = -0.125720 α2 (for Pekar polaron E0 = -0.10851128 α2), where α is electron-phonon coupling.
TI-polarons can create bound TI-bipolaron states, which play an important role in the theory of superconductivity.