  Excitons in semiconductors and insulators consist of fermionic subsystems,
electrons and holes, whose attractive interaction facilitates bound
quasiparticles with quasi-bosonic character due to even-numbered pair spins. In
the presence of a degenerate electron gas, such excitons dissociate due to free
carrier screening, leaving a spectrally broad and faint optical signature
behind. Contrary to this expected behaviour, we have discovered pronounced
emission traces in bulk, germanium-doped GaN up to 100 K, mimicking excitonic
behaviour at high free electron concentrations from 3.4E19/cm3 to 8.9E19/cm3.
Consequently, we show that a degenerate, three-dimensional electron gas
stabilizes a novel class of quasiparticles, named collexons, by many-particle
effects dominated by exchange of electrons with the Fermi gas. The observation
of collexons and their stabilisation with rising doping concentration, is
facilitated by a superior crystal quality due to perfect substitution of the
host atom with the dopant.
