Unusual electronic structure of the pseudoladder compound CaCu2O3

Experimental and theoretical studies of the unoccupied electronic structure of ${\mathrm{CaCu}}_{2}{\mathrm{O}}_{3}$ 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. ${\mathrm{CaCu}}_{2}{\mathrm{O}}_{3}$ deviates significantly from the standard $\mathrm{pd}\ensuremath{\sigma}$ cuprate picture. The corresponding strong interlayer exchange is responsible for the missing spin gap which is generic for other two-leg ladder cuprates.