OPTICAL CONDUCTIVITY OF STRONGLY CORRELATED ELECTRON-SYSTEMS

We present an exact-diagonalization study of the frequency- and wave-v ector-dependent conductivity sigma(q,omega) in small clusters in the t wo-dimensional t-J model. Unlike the related dynamical density correla tion function, sigma(q=0,omega) in the underdoped regime has the excha nge constant J as its characteristic energy scale and is dominated by a resonancelike excitation with frequency similar to 1.7 J. We interpr et this as transition to a p-like excited state of a spin-bag-type qua siparticle (or, alternatively, a tightly bound spinon-holon pair) and show that a simple calculation based on the string picture explains th e numerical results semiquantitatively. For doping levels greater than or equal to 25% t remains the only energy scale of sigma(q=0,omega).