We present a comparative experimental and theoretical investigation of the two-dimensional charge transfer gap in the strongly correlated material ${\mathrm{Sr}}_2{\mathrm{CuO}}_2{\mathrm{Cl}}_2.$ We observe an Urbach behavior in the absorption profile over a surprisingly wide range of energies and temperatures. We present a model that accounts for phonon scattering to infinite order and that allows us to explain the data accurately by assuming coupling of the charge transfer gap exciton to lower-energy electronic excitations.

• Received 13 November 2000

DOI:https://doi.org/10.1103/PhysRevB.63.235104