Doping of a One-Dimensional Mott Insulator: Photoemision and Optical   Studies of Sr$_2$CuO$_{3+\delta}$
  The spectral properties of a one-dimensional (1D) single-chain Mott insulator Sr$_2$CuO$_{3}$ have been studied in angle-resolved photoemission and optical spectroscopy, at half filling and with small concentrations of extra charge doped into the chains via high oxygen pressure growth. The single- particle gap is reduced with oxygen doping, but the metallic state is not reached. The bandwidth of the charge-transfer band increases with doping, while the state becomes narrower, allowing unambiguous observation of separated spinon and holon branches in the doped system. The optical gap is not changed upon doping, indicating that a shift of chemical potential rather than decrease of corelation gap is responsible for the apparent reduction of the photoemission gap. 
T. Valla, T. E. Kidd, P. D. Johnson, K. W. Kim, C. C. Homes and G. D.   Gu