Charge-transfer excitons in underdoped Ca2-xNaxCuO2Cl2 studied by electron energy-loss spectroscopy

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.