Hole distribution in (Sr,Ca,Y,La)14Cu24O41 ladder compounds studied by X-ray absorption spectroscopy

The unoccupied electronic structure for the ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ family of two-leg ladder compounds was investigated for different partial substitutions of ${\mathrm{Sr}}^{2+}$ by ${\mathrm{Ca}}^{2+},$ leaving the nominal hole count constant, and by ${\mathrm{Y}}^{3+}$ or ${\mathrm{La}}^{3+},$ 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 ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}.$ On substitution of Ca for Sr orbitals within the ladder planes but perpendicular to the legs receive some hole occupation as well.