Effect of orbital currents on the restricted optical conductivity sum rule

We derive the restricted optical-conductivity sum rule for a model with circulating orbital currents. It is shown that an unusual coupling of the vector potential to the interaction term of the model Hamiltonian results in a non-standard form of the sum rule. As a consequence, the temperature dependence of the restricted spectral weight could be compatible with existing experimental data for high-T <Subscript> c </Subscript> cuprates above the critical temperature T <Subscript> c </Subscript>. We extend our results to the superconducting state, and comment on the differences and analogies between these two symmetry-breaking phenomena. Copyright Springer-Verlag Berlin/Heidelberg 2004