Electronic states of doped holes in La(2-x)SrxCuO4: a unique application of XAFS.
Polarized XAFS measurements on powder of La2_=Sr=CuO4 with aligned c-axes find that apical oxygens neighboring only the Sr dopants have a double-site distribution. The occupation of both sites totals one oxygen, is independent of temperature, but changes with x. A theory of the double-site is presented in terms of Jahn-Teller distortions (JT) induced by the extrinsic doped holes paired with the intrinsic holes present in the undoped x = 0 sample. These pairs reside in singlet and triplet impurity states and satisfy Bose statistics. It is speculated that high Tc occurs because of Bose-condensation of the singlet pairs. Because of the strong electron-lattice interaction of the JT effect, XAFS can determine the corresponding electronic states in this special case.