  We investigate the multielectron effects on high-harmonic generation from
solid-state materials using the time-dependent Hartree-Fock theory. We find
qualitative change in harmonic spectra, in particular, multiple-plateau
formation at significantly lower laser intensities than within the
independent-electron approximation. We reveal its origin in terms of interband
polarization, i.e, electron-hole polarization, enabling interband excitation at
remote crystal momenta via Coulomb potential.
