Determination of hole distribution in Sr14-xCaxCu24O41 using soft x-ray absorption spectroscopy at the Cu L3 edge

The physical properties of Sr${}_{14\ensuremath{-}x}$Ca${}_{x}$Cu${}_{24}$O${}_{41}$ are determined by the hole distribution between the edge-shared CuO${}_{2}$ chain and the corner-shared Cu${}_{2}$O${}_{3}$ two-leg ladder, but inconsistent results on the hole distribution were obtained in various experimental works in recent decades. In this work we reinvestigate the hole distribution by soft x-ray absorption spectroscopy at the Cu ${L}_{3}$ edge. By comparing with the pure ladder and the pure chain systems, we can unambiguously distinguish between the hole distributions in the chain and the ladder. We have found that there are 5.3 holes in the edge-shared chain and 0.7 holes in the corner-shared ladder on average for Sr${}_{14}$Cu${}_{24}$O${}_{41}$. Upon Ca substitution, the holes gradually transfer from the edge-shared chain to the corner-shared ladder, and there are up to 1.21 holes in the corner-shared ladder for Sr${}_{1.8}$Ca${}_{12.2}$Cu${}_{24}$O${}_{41}$.