Dopant Induced Enhancement of the Jahn-Teller Effect in Perovskite Cuprates

摘要

Polarized x-ray absorption fine structure (XAFS) measurements at La and Sr sites on powder La_(2-x)Sr_xCuO_4 finds two distinct local structures in the close vicinity of the impurity ion. The structures are explained in terms of two possible populations of lowest occupied states and, correspondingly, two values of elongation of the CuO_6 octahedrons. In the ground state, when the dopant-induced hole is in the x~2 - y~2 orbit, the elongation of the copper octahedron is about twice bigger compared to undoped La_2CuO_4. It is described as an enhanced Jahn-Teller (JT) effect. If the hole is in the hole-excited z~2 orbital, then the anti-H shortening of the octahedron takes place.