PLASMAPOLARON SELFENERGY AND EFFECTIVE-MASS IN UNIAXIAL POLAR CRYSTALS

We study the polaron effects in a uniaxial polar semiconductor with el ectrons in the conduction band. We describe the system as an external charge interacting with three boson fields: two optical longitudinal p honons and a plasmon. The combined effects of these three fields lead to a quasiparticle which we call plasmapolaron. The selfenergy and the anisotropic effective mass of the plasmapolaron as a function of the anisotropy parameters and the electron density are calculated in the i ntermediate regime for the electron-phonon and electron-plasmon coupli ng. We find that: when the crystal has high polarizability along the c -axis and a large polaron radius in the a-b plane, the screening effec ts due to the plasmons on the electron-phonon interaction are small, s o that the latter is present even at relatively high electron density. In these conditions the induced charges are localized mainly on the a -b plane. We also calculate the selfenergy of the plasmapolaron in the strong coupling limit and compare it with that calculated in the inte rmediate regime. We find that, increasing the electron density, the se lfenergy in the intermediate regime becomes always lower than that in the strong limit. A possible connection of our results with the high-T -c superconductors is discussed.