Fig. 3

Suggested collexonic excitations in the Fermi sea. a The electronic ground state (G) is formed by the degenerate electron gas—the Fermi sea—in the Γ_7c conduction band of GaN:Ge. Upon optical excitation, interband excitations are formed and stabilized by the Fermi sea of electrons giving rise to the first excited state, the collexon (X^U) with a decay rate γ^U. In the high-density regime, such collexons scatter with electrons in the Fermi sea (γ_U→L), enabling the formation of the bicollexon (X^L). Hence, the bicollexon represents a mixed complex of interband and intraband electron–hole pairs that decays (γ^L), forming the second excited state. b Based on our modelling and the fitting function extracted from the imaginary part of Eq. 1 (solid, red lines), we can approximate the PL data (symbols) related to X^U and X^L. All spectra are displaced horizontally for clarity