The unusual electronic structure of the "pseudo-ladder" compound CaCu2O3
  Experimental and theoretical studies of the unoccupied electronic structure of CaCu2O3 single crystals have been performed using polarization-dependent x-ray absorption spectroscopy and band structure calculations. The measured hole distribution shows an unusual large number of holes in orbitals parallel to the interlayer direction which is in agreement with the theoretical analysis. CaCu2O3 deviates significantly from the standard pd-sigma cuprate picture. The corresponding strong interlayer exchange is responsible for the missing spin gap generic for other two-leg ladder cuprates. 
T.K. Kim, H. Rosner, S.-L. Drechsler, Z. Hu, C. Sekar, G. Krabbes, J.   Malek, M. Knupfer, J. Fink, H. Eschrig