Many-Body Effects in a Nonequilibrium Electron-Lattice System: Coupling of Quasiparticle Excitations and LO Phonons

The coupling of quasiparticle excitations in an electron gas to the LO phonons gives rise to novel phonon modes with energies below $\lesssim q{v}_{\mathrm{F}}$. We propose that these low-energy "phonon" modes, in addition to the usual plasmon-phonon coupled modes, are responsible for a large (by orders of magnitude) enhancement of the hot-electron energy-loss rate to the lattice at low electron temperatures, as observed in several recent experiments. Experimental conditions are outlined for an unambiguous observation of this predicted many-body effect.