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In contrast to magnetic excitations, the dispersive orbital excitations (orbitons) are very hard to detect. While this is partially due
to the experimental problems, here we show [1] that there is also a deep theoretical reason for this situation: in compounds with antiferromagnetic ground states there is an inherent coupling between orbital excitations and spin fluctuations which leads to the failure of the orbital wave picture. While this coupling largely supresses orbiton motion in 2D and 3D and explains the lack of orbiton dispersion in many
transition metal oxides, in 1D the electron's spin and orbital degree of freedom separate allowing for large orbiton dipersion which can
be detected in e.g. quasi-1D cuprates [2]. This shows that not only spin and charge degrees of freedom can fractionalize in 1D but also the spin
and orbital degrees of freedom.
[1] K. Wohlfeld, M. Daghofer, S. Nishimoto, G. Khaliullin, and J. van den Brink, Phys. Rev. Lett. {\bf 107}, 147201 (2011). [2] J. Schlappa, K. Wohlfeld {\it et al.}, accepted in Nature (2012). |
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