Imaging nano-scale electronic inhomogeneity in lightly doped Mott   insulator Ca_{2-x}Na_{x}CuO_{2}Cl_{2}
  The spatial variation of electronic states was imaged in the lightly doped Mott insulator Ca_{2-x}Na_{x}CuO_{2}Cl_{2} using scanning tunneling microscopy / spectroscopy (STM/STS). We observed nano-scale domains with a high local density of states within an insulating background. The observed domains have a characteristic length scale of 2 nm (~4-5a, a:lattice constant) with preferred orientations along the tetragonal [100] direction. We argue that such spatially inhomogeneous electronic states are inherent to slightly doped Mott insulators and play an important role for the insulator to metal transition. 
Y. Kohsaka, K. Iwaya, S. Satow, T. Hanaguri, M. Azuma, M. Takano, H.   Takagi