Diagrammatic quantum Monte Carlo for two-body problems: Applied to excitons - art.no. 186402

We present a novel method for the precise numeric solution of the irreducib le two-body problem and apply it to excitons in solids. The approach is bas ed on the Monte Carlo simulation of the two-body Green function specified b y Feynman's diagrammatic expansion. Our method does not rely on the specifi c form of the electron and hole dispersion laws and is valid for any attrac tive electron-hole potential, We establish limits of validity of the Wannie r (large radius) and Frenkel (small radius) approximations, present accurat e data for the intermediate radius excitons, and give evidence for the char ge transfer nature of the monopolar exciton in mixed valence materials.