Constraints on Sub-GeV Dark Matter--Electron Scattering from the CDEX-10 Experiment

We present improved germanium-based constraints on sub-GeV dark matter via dark matter--electron ($\chi$-$e$) scattering using the 205.4 kg$\cdot$day dataset from the CDEX-10 experiment. Using a novel calculation technique, we attain predicted $\chi$-$e$ scattering spectra observable in high-purity germanium detectors. In the heavy mediator scenario, our results achieve 3 orders of magnitude of improvement for $m_{\chi}$ larger than 80 MeV/c$^2$ compared to previous germanium-based $\chi$-$e$ results. We also present the most stringent $\chi$-$e$ cross-section limit to date among experiments using solid-state detectors for $m_{\chi}$ larger than 90 MeV/c$^2$ with heavy mediators and $m_{\chi}$ larger than 100 MeV/c$^2$ with electric dipole coupling. The result proves the feasibility and demonstrates the vast potential of a new $\chi$-$e$ detection method with high-purity germanium detectors in ultralow radioactive background.