Spectroscopic evidence for polarons in partially occupied spin-orbit   states of YBa2Cu3O7
  The spin-orbit split white line, synchrotron X-ray spectra at the respective Cu and Ba L2,3 and Ba M4,5 edges in YBa2Cu3O7 crystal, powders, films and derivatives are compared to those for standard BaBr2 and CuO powders. The white line absorption integrated intensity, I ratios: r3,2 = I(L3)/I(L2) and r5,4 = I(M5)/I(M4) for BaBr2 are equal to the ratio of core states degeneracy, but r3,2(Ba in YBCO powder) < 2 < r3,2(Cu in CuO and YBCO film) and r5,4(Ba in YBCO film) < 1.5 (Table I) indicate an apparent overpopulation of the Cu:3d3/2 final states relative to the 3d5/2, and the Ba:5d5/2 over the 5d3/2, and the 4f7/2 over the 4f5/2 at room temperature. The difference spectrum DEL=A(YBCO)-A(CuO) with a leading sharp peak, (HWHH of 0.2 eV) at E0+0.5, and the temperature changes in the atomic X-ray absorption fine structure A-XAFS observed at 0.08 to 0.07 nm near the heaviest atom, Ba suggest that charge polarization contributes to the final state band occupancy in YBCO. 
J.V. Acrivos (1,2) L. Nguyen (2), H.S. Sahibudeen (2), P. Nachimuthu   (3) and M.A. Navacerrada (2,4) ((1,2) SJSU, (3)LBNL-A...