Hole distribution for (Sr,Ca,Y,La)_14 Cu_24 O_41 ladder compounds   studied by x-ray absorption spectroscopy
  The unoccupied electronic structure for the Sr_14Cu_24O_41 family of two-leg ladder compounds was investigated for different partial substitutions of Sr^2+ by Ca^2+, leaving the nominal hole count constant, and by Y^3+ or La^3+, reducing the nominal hole count from its full value of 6 per formula unit. Using polarization-dependent x-ray absorption spectroscopy on single crystals, hole states on both the chain and ladder sites could be studied. While for intermediate hole counts all holes reside on O sites of the chains, a partial hole occupation on the ladder sites in orbitals oriented along the legs is observed for the fully doped compound Sr_14Cu_24O_41. On substitution of Ca for Sr orbitals within the ladder planes but perpendicular to the legs receive some hole occupation as well. 
N. N\"ucker, M. Merz, C. A. Kuntscher, S. Gerhold, S. Schuppler, R.   Neudert, M. S. Golden, J. Fink, D. Schild, S. Stadler,...