In this notation, the diagonal contributions Δ Δ N λ λ , λ λ determine the population of λ λ excitons. The off-diagonal Δ Δ N ν ν , λ λ ≠ ≠ ν ν elements formally determine transition amplitudes between two exciton states ν ν and λ λ ≠ ≠ ν ν . For elevated densities, Δ Δ N ν ν , λ λ ≠ ≠ ν ν build up spontaneously and they describe correlated electron–hole plasma that is a state where electrons and holes move with respect to each other without forming bound pairs.