Diagrammatic Monte Carlo scheme for dielectric losses in metals

Bethe-Salpeter equation (BSE) in the self-consistent Hartree-Fock (HF) basis is often used for describing complex many-body effects in material science applications. Its numerically exact solution on the real-frequency axis at finite temperature for polarization using the diagrammatic Monte Carlo method [I. S. Tupitsyn and N. V. Prokof'ev, Phys. Rev. B 109, 045152 (2024)] captures effects of multiple Coulomb scattering of a single particle-hole excitation, but does not account for multiple pair excitations important for studying dielectric loses in metals at frequencies comparable to the plasmon mode. In this paper we report technical developments which allow one to efficiently compute the dielectric response in a wide frequency range from zero to a few Fermi energies without systematic bias at finite $T$. By applying it to the homogeneous electron gas we demonstrate how at small momenta the gap in the spectral density between the electron-hole and plasmon excitations, existing within the HF-BSE approach, is filled with two particle-hole excitations and is completely washed out already at temperature $T\ensuremath{\sim}{\ensuremath{\varepsilon}}_{F}/10$.