DESTRUCTION OF MAGNETOPHONON RESONANCE IN HIGH MAGNETIC-FIELDS FROM IMPURITY AND PHONON-SCATTERING IN HETEROJUNCTIONS

We present a detailed theoretical calculation to explain the collapse of the magnetophonon resonance (MPR) effect in high magnetic fields in certain high mobility Al-GaAs/GaAs heterojunction samples. The magnet oresistivity (rho(xx)) and the density of states (DOS) are calculated self-consistently. We include the interaction of electrons with backgr ound and remote impurities, with acoustic-phonons via deformation pote ntial and piezoelectric coupling, and with optic (LO) phonons. We find that: (i) the DOS is modified strongly by both elastic and inelastic scattering; (ii) the Landau-level (LL) width oscillates with magnetic fields commensurate with the MPR oscillations in rho(xx); and (iii) st rong impurity scattering leads to a shift of the DOS peaks to lower en ergy. An increase in LL width will reduce the additional resonant LO-p honon scattering, but also decrease the contribution to rho(xx) from a ll scattering mechanisms due to the reduction in DOS. Near the MPR con dition there are thus two competing processes: rho(xx) increases due t o LO-phonon scattering; but this increases the LL width and so reduces the contribution to rho(xx) from elastic scattering. In certain sampl es we found that the two effects balanced at high magnetic fields resu lting in the disappearance of MPR oscillations from rho(xx).