Abstract
Interband optical absorption spectra of a two-dimensional spin-1/2 Fermi-degenerate system with a bound state are investigated theoretically. We examine how the optical spectra depend on the carrier density to understand the crossover from a normal exciton-peak structure in the low-density (insulator) limit to the power-law singularity in the high-density case. An interplay between the doubly-degenerate bound state and the electronic spin degrees of freedom is of essential importance in determining the threshold behavior of the spectra. Whole spectral shape is calculated with the time-dependent coupled-cluster expansion method, and an exact expression of the critical exponents at these four thresholds is analytically obtained.