The discovery of colossal magnetoresistance (CMR) in doped manganese oxides has attracted a lot of attention to this class of materials because of its scientific as well as technological relevance [1]. In order to develop a microscopic understanding of this new phenomenon and of the underlying interplay between charge, magnetic, and orbital order in the CMR materials [2], one could start from a model compound like the undoped ferromagnetic manganite LaMnO3, for which calculation of the ARPES spectra have been performed [2-5].
These systems would also provide a unique opportunity to study the signatures of orbital order in the electronic structure and to verify the existence of qualitatively new excitations, so called orbitons [6], which correspond to the perturbation of the orbital order and are predicted to affect the electron properties (see Fig.1). The dynamics of the orbital excitations could be studied in great detail by combining ARPES and electron-energy loss spectroscopy (EELS). However, because LaMnO3 is structurally 3D and only magnetically (electronically) layered, other more two dimensional compounds ‘isoelectronic’ to the undoped manganites (i.e., in the same 3d4 high-spin configuration and for this reason orbitally ordered) might be better suited for these experiments. In this regard, the growth of single crystals of potentially suitable systems has already been started in collaboration with Dr. H. Eisaki, from the AIST-Tsukuba Research Center in Japan.


[1] E. Dagotto, T. Hotta, and A. Moreo, Phys. Rep. 344, 1 (2001).
[2] Y. Tokura and N. Nagaosa, Nature 288, 462 (2000).
[3] J. van der Brink, P. Horsch, and A.M. Oles, Phys. Rev. Lett. 85, 5174 (2000).
[4] V. Perebeinos and P.B. Allen, Phys. Rev. Lett. 85, 5178 (2000).
[5] J. Bala et al., Phys. Rev. Lett. 87, 067204 (2001).
[6] E. Saitho et al., Nature 410, 180 (2001).