Ab initio evidence for the formation of impurity d3z2Àr2 holes in doped La2ÀxSrxCuO4

Using the spin unrestricted Becke-3-Lee-Yang-Parr density functional, we computed the electronic structure of explicitly doped La22xSrxCuO4 (x50.125, 0.25, and 0.5!. At each doping level, an impurity hole band is formed within the undoped insulating gap. This band is well localized to CuO6 octahedra adjacent to the Sr impurities. The nature of the impurity hole is A1g in symmetry, formed primarily from the z 2 orbital on the Cu and pz orbitals on the apical O’s. There is a strong triplet coupling of this hole with the intrinsic B1g Cu x-y/O1 ps hole on the same site. Optimization of the c coordinate of the apical O’s in the doped CuO6 octahedron leads to an asymmetric anti-Jahn-Teller distortion of the O2 atoms toward the central Cu. In particular, the O2 atom between the Cu and Sr is displaced 0.26 Å while the O2 atom between the Cu and La is displaced 0.10 Å. Contrary to expectations, investigation of a 0.1 Å enhanced Jahn-Teller distortion of this octahedron does not force formation of an x-y hole, but instead leads to migration of the z hole to the four other CuO6 octahedra surrounding the Sr impurity. This latter observation offers a simple explanation for the bifurcation of the Sr-O2 distance revealed in x-ray absorption fine structure data.