Dynamical screening of polar optical phonons in quantum wells

We show that in polar semiconductors near a quantum well a mixed phonon-plasmon vibration can be localized. The dispersion of the vibration is investigated for the simple case where one can neglect the difference of the lattice properties within and outside the well. The magnetic-field dependence of the dispersion law is discussed. Under the same conditions, we have analyzed penetration of a longitudinal electric field of a polar optical vibration into a quantum well. It appears that dynamical screening decreases considerably the field within the well. We give physical reasons for the decrease and come to the conclusion that in most cases of interest the localized vibrations are more effective scatterers of the conduction electrons of the quantum well than the bulk optical phonons. This can explain the observed shift of the magnetophonon resonance in the quantum wells.