MnK-edge XANES study of the La1-xCaxMnO3 colossal magnetoresistive manganites - art. no. 014413

We report Mn K-edge x-ray absorption near edge structure (XANES) calculatio ns of the La1-xCaxMnO3 manganites considering three pail. correlations, acc ording to the three fundamental degrees of freedom governing their unusual electronic properties, namely, the electronic structure of the unoccupied s tates probed by the Mn K-edge profile versus (i) local atomic distortions; (ii) local magnetic ordering; and (iii) the charge-transfer nature of the M n-O bonds. The calculations are accompanied by Mn K-edge XANES measurements in the temperature range 30-300 K. The main features of the absorption edg e can be qualitatively reproduced in terms of single-electron multiple-scat tering calculations for an 87-atom cluster. Lattice polaronic distortions i n La0.7Ca0.3MnO3 are simulated assuming a strongly distorted orthorhombic s tructure above and an almost undistorted rhombohedral structure below T-c. The results roughly reproduce the energy "shift" across T-c observed experi mentally. Mn K-edge spin-polarized XANES spectra of the x = 0, 0.3, and 1 s amples are presented. An energy splitting between the majority- and minorit y-spin spectra of 0.5-1.1 eV contributes to the total XANES broadening belo w the Neel (Curie) temperature. A small feature B-3 Standing approximately 6 eV above the main absorption peak is beyond the scope of single-electron calculations; it is assigned to a shake-up transition. To illustrate, the c alculated Mn K edge is obtained as the convolution product of the single-el ectron XANES and the spectrum of many-body excitations in the Mn-O electron ic states upon the sudden switching on of the Mn Is core hole. We investiga te the charge-transfer (CT) versus. Mott-Hubbard-type ground state using th e U-c, Delta, and the T-dp parameters determined by previous Mn 2p x-ray ph otoemission spectroscopy measurements and find that LaMnO3 should be viewed as a CT-type insulator with a substantial O 2p component in the ground sta te. In light of these results the controversial issue of Mn valence states in the manganites is critically reexamined. We argue that the disproportion ation may be understood as a mixture of the CT many-body electronic configu rations coupled with spin and lattice degrees of freedom.