Fingerprints of spin-orbital physics in cubic Mott insulators: Magnetic exchange interactions and optical spectral weights

Abstract

The temperature dependence and anisotropy of optical spectral weights associated with different multiplet transitions is determined by the spin and orbital correlations. To provide a systematic basis to exploit this close relationship between magnetism and optical spectra, we present and analyze the spin-orbital superexchange models for a series of representative orbital-degenerate transition metal oxides with different multiplet structure. For each case we derive the magnetic exchange constants, which determine the spin wave dispersions, as well as the partial optical sum rules. The magnetic and optical properties of early transition metal oxides with degenerate t 2g orbitals ͑titanates and vanadates with perovskite structure͒ are shown to depend only on two parameters, viz. the superexchange energy J and the ratio of Hund's exchange to the intraorbital Coulomb interaction, and on the actual orbital state. In e g systems important corrections follow from charge transfer excitations, and we show that KCuF 3 can be classified as a charge transfer insulator, while LaMnO 3 is a Mott insulator with moderate charge transfer contributions. In some cases orbital fluctuations are quenched and decoupling of spin and orbital degrees of freedom with static orbital order gives satisfactory results for the optical weights. On the example of cubic vanadates we describe a case where the full quantum spin-orbital physics must be considered. Thus information on optical excitations, their energies, temperature dependence, and anisotropy, combined with the results of magnetic neutron scattering experiments, provides an important consistency test of the spin-orbital models, and indicates whether orbital and/or spin fluctuations are important in a given compound. I. SUPEREXCHANGE AND OPTICAL EXCITATIONS AT ORBITAL DEGENERACYThe physical properties of Mott ͑or charge transfer͒ insulators are dominated by large on-site Coulomb interactions ϰU which suppress charge fluctuations. Quite generally, the Coulomb interactions lead then to strong electron correlations which frequently involve orbitally degenerate states, such as 3d ͑or 4d͒ states in transition metal compounds, and are responsible for quite complex behavior with often puzzling transport and magnetic properties. 1 The theoretical understanding of this class of compounds, with the colossal magnetoresistance ͑CMR͒ manganites as a prominent example, 2,3 has substantially advanced over the last decade, 4 after it became clear that orbital degrees of freedom play a crucial role in these materials and have to be treated on equal footing with the electron spins, which has led to a rapidly developing field, orbital physics. 5 Due to the strong onsite Coulomb repulsion, charge fluctuations in the undoped parent compounds are almost entirely suppressed, and an adequate description of these strongly correlated insulators appears possible in terms of superexchange. 6 At orbital degeneracy the superexchange interactions have a rather rich structure, represented by the so-called spin-orbita...

Unraveling Orbital Correlations with Magnetic Resonant Inelastic X-Ray Scattering

“…In a typical AO and FO case with large crystal-field Jahn-Teller interactions this should always be possible [2]. In the OL case this might be questionable but the experimental results suggest that also in that case it is a valid approach [3]. However, if this is not the case, then in principle it is not clear what is the nature of the elementary excitations in such systems [4].…”

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“…The method proposed here is of direct relevance to 2D orbital systems, e.g., K 2 CuF 4 or Cs 2 AgF 4 with FM layers and predicted (but not yet explicitly verified) AO ordering [43][44][45], as well as to three dimensional transition metal oxides with orbital degrees of freedom such as LaMnO 3 , KCuF 3 , LaTiO 3 or LaVO 3 [3]. In particular, in Part 3 of the Supplemental Material we predict magnetic RIXS spectra for two different polytypes of KCuF 3 with distinct orbitally ordered ground states.…”

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