  The millicharged particle has become an attractive topic to probe physics
beyond the Standard Model. In direct detection experiments, the parameter space
of millicharged particles can be constrained from the atomic ionization
process. In this work, we develop the relativistic impulse approximation (RIA)
approach, which can duel with atomic many-body effects effectively, in the
atomic ionization process induced by millicharged particles. The formulation of
RIA in the atomic ionization induced by millicharged particles is derived, and
the numerical calculations are obtained and compared with those from free
electron approximation and equivalent photon approximation. Concretely, the
atomic ionizations induced by mllicharged dark matter particles and
millicharged neutrinos in high-purity germanium (HPGe) and liquid xenon (LXe)
detectors are carefully studied in this work. The differential cross sections,
reaction event rates in HPGe and LXe detectors, and detecting sensitivities on
dark matter particle and neutrino millicharge in next-generation HPGe and LXe
based experiments are estimated and calculated to give a comprehensive study.
Our results suggested that the next-generation experiments would improve 2-3
orders of magnitude on dark matter particle millicharge $\delta_{\chi}$ than
the current best experimental bounds in direct detection experiments.
Furthermore, the next-generation experiments would also improve 2-3 times on
neutrino millicharge $\delta_{\nu}$ than the current experimental bounds.
