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Spin–orbital separation in the quasi-one-dimensional Mott insulator Sr2CuO3

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When viewed as an elementary particle, the electron has spin and charge. When binding to the atomic nucleus, it also acquires an angular momentum quantum number corresponding to the quantized atomic orbital it occupies. Even if electrons in solids form bands and delocalize from the nuclei, in Mott insulators they retain their three fundamental quantum numbers: spin, charge and orbital1. The hallmark of one-dimensional physics is a breaking up of the elementary electron into its separate degrees of freedom2. The separation of the electron into independent quasi-particles that carry either spin (spinons) or charge (holons) was first observed fifteen years ago3. Here we report observation of the separation of the orbital degree of freedom (orbiton) using resonant inelastic X-ray scattering on the one-dimensional Mott insulator Sr2CuO3. We resolve an orbiton separating itself from spinons and propagating through the lattice as a distinct quasi-particle with a substantial dispersion in energy over momentum, of about 0.2 electronvolts, over nearly one Brillouin zone.

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Figure 1: Spin–orbital separation process in an antiferromagnetic spin chain, emerging after exciting an orbital.
Figure 2: Energy dependence of elementary excitations in Sr 2 CuO 3 observed with RIXS at the copper L 3 -edge resonance.
Figure 3: Dispersion of magnetic excitations: experimental data and simulation.
Figure 4: Dispersion of orbital excitations: comparison between experiment and ab initio calculations.

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  • 02 May 2011

    An axis label was replaced on Fig. 4c.


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This work was performed at the ADRESS beamline of the Swiss Light Source using the SAXES instrument jointly built by the Paul Scherrer Institut, Switzerland, and Politecnico di Milano, Italy. We acknowledge support from the Swiss National Science Foundation and its NCCR MaNEP. K.W. acknowledges support from the Alexander von Humboldt foundation and discussions with M. Daghofer and S.-L. Drechsler. J.-S.C. acknowledges support from the Foundation for Fundamental Research on Matter and from the Netherlands Organisation for Scientific Research. S.S. and A.R. acknowledge the support of the European contract NOVMAG. This research benefited from the RIXS collaboration supported by the Computational Materials Science Network programme of the Division of Materials Science and Engineering, US Department of Energy, grant no. DE-SC0007091.

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J.S., T.S. and H.M.R. planned the experiment. S.S. and A.R. fabricated the samples. J.S., K.J.Z., V.N.S. and T.S. carried out the experiment. J.S. and M.M. carried out the data analysis. C.M. helped with the data analysis. K.W. and J.v.d.B. developed the theory for the spin–orbital separation with assistance from M.W.H., L.H. and S.N. J.-S.C. provided the theory for the spin excitations. J.S., K.W., K.J.Z., H.M.R., J.v.d.B. and T.S. wrote the paper with contributions from all co-authors. L.P., H.M.R., J.v.d.B. and T.S. supervised the project.

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Correspondence to J. Schlappa or T. Schmitt.

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Schlappa, J., Wohlfeld, K., Zhou, K. et al. Spin–orbital separation in the quasi-one-dimensional Mott insulator Sr2CuO3. Nature 485, 82–85 (2012).

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