• 
  Schuster, R.; Pyon, S.; Knupfer, M.; Fink, J.; Azuma, M.; Takano, M.; Takagi, H.; Büchner, B.: Charge-transfer excitons in underdoped Ca_2-xNa_xCuO₂Cl₂ studied by electron energy-loss spectroscopy. Physical Review B 79 (2009), p. 214517/1-4

10.1103/PhysRevB.79.214517

Abstract:
Employing electron energy-loss spectroscopy we show that small values of doping in the system Ca2−xNaxCuO2Cl2 strongly influence the formation of charge-transfer excitons in the Cu-O plane. We find a remarkable redistribution of spectral weight between the two modes—a bound exciton and a continuum state—seen in the insulator yielding a single sharp feature for nonzero doping accompanied by a strong suppression of the dispersion. Our data may provide evidence for a prominent role of the magnetic background on the dynamics of charge-transfer excitations in underdoped cuprates.