OPTICAL-ABSORPTION IN THE CANTED ANTIFERROMAGNETIC PHASE OF A T-J MODEL

We calculate the real part of the finite-frequency conductivity, i.e., the optical absorption, in the canted antiferromagnetic phase of a t-J model using a suitable 1/N expansion. We find that below a characteristic crossover frequency-DELTA the absorption is very small, while above DELTA it rises sharply to a broad maximum, in qualitative agreement with recent infrared absorption experiments in doped antiferromagnets. This crossover frequency is identified with an energy gap developing upon hole doping in one of the two branches of elementary excitations, at the corner of the square Brillouin zone.