Exciton absorption in disordered crystals with a singular spectrum (the Urbach problem)
Kusmartsev F. V., Shchukin V. A.
The state-density tails in disordered crystals with a singular exciton spectrum exhibit an Urbach behavior at к = 0, Inp(E) осE. This behavior is caused by an increase in the dimension of space resulting from the singular term in the spectrum.