Evidence for an orbital dependent Mott transition in the ladders of (La,Ca)${}_x$Sr${}_{14-x}$Cu${}_{24}$O${}_{41}$ derived by electron energy-loss spectroscopy

Abstract

The knowledge of the charge carrier distribution among the different orbitals of Cu and O is a precondition for the understanding of the physical properties of various Cu-O frameworks. We employ electron energy-loss spectroscopy to elucidate the charge carrier plasmon dispersion in (La, Ca)${}_x$Sr${}_{14-x}$Cu${}_{24}$O${}_{41}$ in dependency of $x$ as well as temperature. We observe that the energy of the plasmon increases upon increasing Ca content, which signals an internal charge redistribution between the two Cu-O subsystems. Moreover, contrary to an uncorrelated model we come to the conclusion that the holes transferred to the Cu${}_2$O${}_3$ ladders are mainly located in the bonding and not in the anti-bonding band. This is caused by an orbital dependent Mott transition.