Multiphonon generation during photodissociation of slow Landau-Pekar polarons

Abstract

The spectra of the low-temperature photodissociation (photoionization) of Landau-Pekar polarons are calculated using the theory of quantum-coherent states and a new method of variation with respect to the parameters of phonon vacuum deformation. It is shown that the final polaron states upon photodissociation may have different numbers of phonons produced in a single dissociation event and different momenta of charge carriers. The spectrum of optical absorption related to the photodissociation of polarons exhibits a superposition of bands corresponding to various numbers of phonons formed as a result of dissociation of a single polaron. Due to a large width of the energy region corresponding to the final states of charge carriers, the halfwidth of each band is on the order of the energy of polaron coupling and is much greater than the phonon energy. For this reason, the individual phonon bands exhibit strong overlap. The very broad and, probably, structureless band formed as a result of the superposition of all these components begins at an energy equal to the sum of the polaron coupling energy (E p) and the phonon energy. This band has a maximum at a frequency of about 5.6E p/ħ and a halfwidth on the order of 5.6E p/ħ at a unit effective mass (m* = m e) of band electrons. For an effective charge carrier mass within m* = (1–3)m e, the energy of the polaron band maximum can be estimated as 5E p with an error of about 10%, and the halfwidth falls within 3.4E p < ħΩ1/2 < 5.6E p. The multiphonon character of this band is related to a decay of the phonon condensate after the escape of charge carrier from a polaron. Such polarons are likely to be observed in the spectra of complex metal oxides, including high-temperature superconductors. Examples of such polaron bands in the reported absorption and photoconductivity spectra of nonstoichiometric cuprates, manganites, nickelates, and titanates are presented. A theory of the formation of Landau-Pekar polarons with the participation of branches of the polarization oscillations of the medium is developed. It is shown that, under certain conditions, such a multiphonon-dressed polaron can possess a coupling energy on the order of 0.2–0.3 eV, so that the maximum of the corresponding absorption band may occur at 1–1.5 eV.