Keimer's department > Research > Orbital physics

Orbital physics in transition metal oxides

The exceptionally rich phase behavior observed in transition metal oxides originates in a competition between many-body states with different spin, orbital, and charge ordering patterns. Based on ideas going back to the 1950's, it is possible to understand much of this behavior in a qualitative manner. However, the quantitative description of the interplay between spin, charge, and orbital correlations in transition metal oxides is still in its infancy. Thanks to the availability of high-quality single crystal samples and advanced spectroscopic methods, it is now possible to start developing quantitative models. Our approach has been to first conduct thorough experimental and theoretical studies of the magnetic and orbital states of insulating transition metal oxides with nearly degenerate transition metal d-orbitals. The outcome of these studies would then allow us to understand how spin and orbital correlations control charge transport in doped systems. Surprisingly, our early experiments revealed unexpectedly rich physics even in insulating materials. These results force us to qualitatively reconsider the ideas developed over the past 50 years.

Our current research is focused on systems with partially filled t2g levels. Due to the fact that t2g orbitals have their lobes directed away from the nearest-neighbor oxygen ligands, they are only weakly coupled to the crystal lattice. Their interaction with the spin degree of freedom is, on the contrary, relatively strong, with magnetic and orbital ordering often setting in at similar temperatures. This makes t2g materials more difficult to understand than systems with valence electrons in eg orbitals. Using appropriate spectroscopic techniques we systematically investigate the orbital ordering and orbital fluctuations in t2g electron systems, their interplay with magnetism and their role in the resulting electronic properties.

Examples from our recent research include the discovery of a new orbitally ordered paramagnetic phase above the magnetic ordering transition temperature in the 4d electron ruthenate system Ca2RuO4 [1]; the complete determination of the magnetic structure of its bilayered counterpart Ca3Ru2O7, which is characterized by a weaker orbital polarization [2]; as well as the determination of the non-trivial magnetic structure of RuSr2GdCu2O8, a particularly interesting system, in which long-range magnetic order coexists with superconductivity [3]. Resonant x-ray diffraction was successfully used in these investigations, proving it to be a valuable direct probe of orbital and magnetic order in transition metal oxides.

For the investigation of orbital excitations in t2g systems, momentum-resolved spectroscopy techniques are used. Both Raman spectroscopy [4] and resonant inelastic x-ray scattering (RIXS) [5,6] have been successfully employed in our department for the investigation of the 3d electron Mott-insulating titanates LaTiO3 and YTiO3, concluding on the theoretical model which best describes the driving force of orbital excitations in such systems [7] and revealing an unexpected universality of the electronic excitations in transition metal oxides in general. The studies were complemented by optical property investigations conducted with spectral ellipsometry [8,9].

Experimental work goes hand-in-hand with theoretical understanding in our department. Recent suggestions that the relativistic spin-orbit coupling, often neglected in transition metal oxides, may in fact lead to qualitatively new physics in some Mott insulating compounds, are of crucial importance for the interpretation of the electronic properties of systems with partially filled t2g orbitals. Indeed, the unusual magnetic properties of the 5d electron iridate Sr2IrO4 could be successfully described in terms of the locally entangled spin-orbital states formed by the spin-orbit interaction [10]. The latter may also induce a novel magnetically hidden, octupolar order predicted to be realized in Sr2VO4 [11]. Previous work pointed out the important role of the orbital degree of freedom in the formation of a dimerized spin-singlet state in 4d electron Li2RuO3 [12], and in stabilizing various magnetic phases in lattice-distorted 3d electron RVO3 vanadates [13].

References

  1. I. Zegkinoglou, J. Strempfer, C. S. Nelson, J. P. Hill, J. Chakhalian, C. Bernhard, J. C. Lang, G. Srajer, H. Fukazawa, S. Nakatsuji, Y. Maeno, B. Keimer. Phys. Rev. Lett. 95 (2005), 136401
  2. B. Bohnenbuck, I. Zegkinoglou, J. Strempfer, C. Schüßler-Langeheine, C. S. Nelson, Ph. Leininger, H.-H. Wu, E. Schierle, J. C. Lang, G. Srajer, S. I. Ikeda, Y. Yoshida, K. Iwata, S. Katano, N. Kikugawa, B. Keimer. Phys. Rev. B 77 (2008), 224412
  3. B. Bohnenbuck, I. Zegkinoglou, J. Strempfer, C. S. Nelson, H. H. Wu, C. Schüßler-Langeheine, M. Reehuis, E. Schierle, Ph. Leininger, T. Herrmannsdörfer, J. C. Lang, G. Srajer, C. T. Lin, B. Keimer. Phys. Rev. Lett. 102 (2009), 037205
  4. C. Ulrich, A. Gössling, M. Grüninger, M. Guennou, H. Roth, M. Cwik, T. Lorenz, G. Khaliullin, B. Keimer. Phys. Rev. Lett. 97 (2006), 157401
  5. C. Ulrich, J. P. Ament, G. Ghiringhelli, L. Braicovitch, M. Moretti Sala, N. Pezzotta, T. Schmitt, G. Khaliullin, J. van den Brink, H. Roth, T. Lorenz, B. Keimer. Phys. Rev. Lett. 103 (2009), 107205
  6. C. Ulrich, G. Ghiringhelli, A. Piazzalunga, L. Braicovich, N. B. Brookes, H. Roth, T. Lorenz, B. Keimer. Phys. Rev. B 77 (2008), 113102
  7. L. J. P. Ament, G. Khaliullin. Preprint: arxiv:0909.4656
  8. N. N. Kovaleva, A. V. Boris, L. Capogna, J. L. Gavartin, P. Popovich, P. Yordanov, A. Maljuk, A. M. Stoneham, B. Keimer. Phys. Rev. B 79 (2009), 045114
  9. N. N. Kovaleva, A. V. Boris, P. Yordanov, A. Maljuk, E. Brücher, J. Strempfer, M. Konuma, I. Zegkinoglou, C. Bernhard, A. M. Stoneham, B. Keimer. Phys. Rev. B 76 (2007), 155125
  10. G. Jackeli, G. Khaliullin. Phys. Rev. Lett. 102 (2009), 017205
  11. G. Jackeli, G. Khaliullin. Phys. Rev. Lett. 103 (2009), 067205
  12. G. Jackeli, D. I. Khomskii. Phys. Rev. Lett. 100 (2008), 147203
  13. P. Horsch, A. M. Oleś, L. F. Feiner, G. Khaliullin. Phys. Rev. Lett. 100 (2008), 167205
Figures
Fig. 1. The orbital ordering phase transition in Ca2RuO4 clearly manifested by a drop of the integrated intensity of the (1 0 0) reflection at 260 K, well above the magnetic ordering transition temperature (110 K) [1].
Fig. 2. Azimuthal-angle dependence of the scattered intensity from Ca3Ru2O7 at reflections (0 0 1) and (1 1 0), both above and below the metal to insulator transition temperature TMI, revealing a 90º-rotation of the magnetic moment direction at TMI [2].
Fig. 3. Comparison of the model calculations with the experimental RIXS data of YTiO3 for different scattering vectors along the [1 1 0] direction: (a) superexchange bond modulation, (b) local shakeup of collective orbital excitations, and (c) local crystal-field excitation model [6,7].