On the phase transitions for the large polaron in a magnetic field

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Abstract
The large polaron in a magnetic field is treated within the Fock approximation. In order to investigate the nature of the phase transitions that can occur in the internal structure of the polaron, model spectra of various symmetries are used to calculate the ground-state energy. These models involve an electron in a magnetic field (external or internal in origin) and harmonically bound to a fixed point (isotropically or anisotropically). A variational procedure is used to determine, within a specific model, the symmetry that gives the lowest energy for given parameters. The authors find that phase transitions occur that are functions of the magnetic field or of electron-phonon coupling constants. For some models, these transitions are similar to those found by Peeters and Devreese (1981, 1982), using a Feynman integral formalism for a large electron-phonon coupling: strong lattice polarisation for a small magnetic field and small polarisation for large external fields. Furthermore, they also find transitions for smaller electron-phonon coupling, using models involving the presence of an internal magnetic field. This makes the observation of these transitions more susceptible to experimental verification.