Renormalization of the Excitation Spectrum and the Migdal Effect in a 2D Electron System with Strong Interaction

The dependence of the Fermi energy of quasiparticles on the electron concentration is studied from analysis of the spectra of radiative recombination of 2D electrons with photoexcited holes bound at remote acceptors; in this way, the dependence of the renormalized mass of quasiparticles on the 2D electron concentration is determined. It is found that upon a decrease in the electron concentration (with decreasing parameter r s to 4.5), the effective mass of the density of quasiparticle states increases by 35% as compared to the cyclotron mass of electrons. It is shown that in a transverse magnetic field, the concept of quasiparticles in a 2D Fermi liquid is preserved not only near the Fermi surface, but at a large depth under it (down to the bottom of the size-quantization band) because the broadening of excitations turns out to be much smaller than their energy. It is found that the mass of quasiparticles as well as their broadening noticeably depend on their energy measured from the Fermi surface to the bulk down to the very bottom of the size-quantization band. The Migdal effect is detected: in the regime of the strong electron–electron interaction at a very low temperature (25 mK), the electron distribution function acquires tails on both sides of the Fermi energy ( E F ); in addition, for E = E F , a sharp jump is observed in the electron distribution function. It is shown that upon a decrease in the 2D electron concentration, the contribution of the tails detected in the distribution function increases, and amplitude Z of the Migdal jump decreases significantly. The dependence of the Migdal jump amplitude on the electron concentration is investigated experimentally.