Optical conductivity of one-dimensional doped Hubbard-Mott insulator
  We study the optical response of a strongly correlated electron system near the metal-insulator transition using a mapping to the sine-Gordon model. With semiclassical quantization, the spectral weight is distributed between a Drude peak and absorption lines due to breathers. We calculate the Drude weight, the optical gap, and the lineshape of breather absorption. 
D.N. Aristov, Vadim V. Cheianov, and A. Luther (NORDITA)