Role of oxygen in the orbital ordered state of La$_{0.5}$Sr$_{1.5}$MnO$_4$

In the single layer manganite, La$_{0.5}$Sr$_{1.5}$MnO$_4$ , experimental evidence points clearly to formation of orbital ordering but leaves the question open as to the exact nature of the state. Using oxygen K edge spectroscopy, we find that oxygen holes related to the Mn-O hybridization between O($2p$) and Mn($e_g$) states play an important role in the formation of the ordered groundstate. The large change in the number of $e_g$ related oxygen holes with the formation of the charge/orbital ordered states demonstrates it is not due to a locking in of orbitals which are fluctuating in orientation, but that the disordered state possesses a different orbital occupancy. The change in the number of $e_g$ holes occurs mainly within the ab plane and seems to be related to the crossover from ferromagnetic to anti-ferromagnetic correlations with the onset of the charge/orbital ordered state. This idea is supported by Mn$_4$O$_8$ cluster calculations. Work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.