Where do the doped holes go in La[sub 2−x]Sr[sub x]CuO[sub 4]? A close look by XAFS

Abstract

Polarized x-ray absorption fine structure (XAFS) measurements at the La and Sr sites in La2−xSrxCuO4 (0.075≲x≲0.35) indicate that doped holes introduced with Sr are not uniformly distributed in the CuO2 planes but reside in impurity states with the majority of charge located on CuO6 octahedra coupled to the Sr dopants by an apical oxygen (denoted as Sr octahedra). A model based on doped hole-induced Jahn Teller (JT) distortions of these Sr octahedra indicates the impurity states are overlapping singlet and triplet ones resulting from pairings of intrinsic and extrinsic holes. The more mobile singlet pairs reside in the CuO2 planes, are bound by more than 0.1 eV and could Bose-condense into a superconducting state. The predominant appearance of (mostly) out-of plane triplet states at high x intimately related to the 2D-3D crossover in transport properties and could relate to the observed loss of high Tc in the overdoped regime, as confinement of carriers to the CuO2 planes is reduced.