Wave - vector - dependent exchange interaction and its relevance for the effective exciton mass in Cu

The wave-vector dependence of electron-hole exchange interaction is investigated. For the yellow 1S exciton in Cu2O the exchange is derived up to the order k 2. The theoretical predictions are verified experimentally by high-resolution absorption experiments. In agreement with theory the fine structure shows a characteristic dependence on the direction of the wave vector. The exchange splitting of the orthoexciton triplet are distinguished from strain-induced perturbations. The exchange gives rise to an isotropic and an anisotropic correction of the effective exciton mass. This can explain the discrepancies in the measurements of the exciton mass in Cu2O.