@article{CTT100631622,
  author = {R. Schuster and S. Pyon and M. Knupfer and J. Fink and M. Azuma and M. Takano and H. Takagi and B. Buchner},
  title = {Charge-transfer excitons in underdoped Ca2-xNaxCuO2Cl2 studied by electron energy-loss spectroscopy},
  journal = {Phys. Rev. B},
  year = 2009,
}