Orbital Imaging across the Metal-Insulator Transitions in Ti<sub>2</sub>O<sub>3</sub>, VO<sub>2</sub>, and V<sub>2</sub>O<sub>3</sub>.
ORAL · Invited
Abstract
The metal-insulator transitions (MIT) in oxides such as Ti2O3, V2O3 and VO2, are considered to be classical examples of Mott transitions. Despite the large amount of research in the last 50 years, several fundamental characteristics of these MIT’s seems to have been neglected. For example, the band gaps are orders of magnitude larger in energy scale than that of the transition temperatures. Based on early ideas of Daniel Khomskii and coworkers [1,2] we infer that we need to consider inter-site spin-spin and orbital-orbital correlations in these systems.
So far, the electronic occupation of the orbitals has mostly been deduced from polarization dependent spectroscopies in which spectra must be analyzed using theory or modelling. This, however, is also a challenge in itself, since calculations hit their limits due to the extended many-body nature of the problem. Here we have determined the Ti/V 3d orbital occupation across the MIT’s using the recently developed orbital-imaging method [3-5] based on x-ray Raman spectroscopy or non-resonant inelastic x-ray scattering with high momentum transfers. This new experimental method circumvents the need for calculations and provides instead the information as measured. We thus can make a direct image of the active orbitals in the various phases.
In all three compounds we observed switching of the orbital occupations across the transitions. The multiplet structure of the Coulomb interactions involved in the associated virtual inter-site excitations produces a massive change of the effective Hubbard U and band width W parameters. While in Ti2O3 and VO2 the formation and break-up of the c-axis dimers and chains can be established, the much-believed c-axis dimer in V2O3 is electronically non-existent. The role of entropy as the driving force behind the MIT’s is exemplified.
[1] L.N. Bulaevvskii and D.I. Khomskii, Soviet physics / Solid state 9, 2422 (1968)
[2] K. Kugel and D. I. Khomskii, Sov. Phys. Usp. 25, 231 (1982)
[3] H. Yavaş et al., Nature Physics 15, 559 (2019)
[4] B. Leedahl et al., Nature Commun. 10, 5447 (2019)
[5] A. Amorese et al., Phys. Rev. X 11, 011002 (2021)
So far, the electronic occupation of the orbitals has mostly been deduced from polarization dependent spectroscopies in which spectra must be analyzed using theory or modelling. This, however, is also a challenge in itself, since calculations hit their limits due to the extended many-body nature of the problem. Here we have determined the Ti/V 3d orbital occupation across the MIT’s using the recently developed orbital-imaging method [3-5] based on x-ray Raman spectroscopy or non-resonant inelastic x-ray scattering with high momentum transfers. This new experimental method circumvents the need for calculations and provides instead the information as measured. We thus can make a direct image of the active orbitals in the various phases.
In all three compounds we observed switching of the orbital occupations across the transitions. The multiplet structure of the Coulomb interactions involved in the associated virtual inter-site excitations produces a massive change of the effective Hubbard U and band width W parameters. While in Ti2O3 and VO2 the formation and break-up of the c-axis dimers and chains can be established, the much-believed c-axis dimer in V2O3 is electronically non-existent. The role of entropy as the driving force behind the MIT’s is exemplified.
[1] L.N. Bulaevvskii and D.I. Khomskii, Soviet physics / Solid state 9, 2422 (1968)
[2] K. Kugel and D. I. Khomskii, Sov. Phys. Usp. 25, 231 (1982)
[3] H. Yavaş et al., Nature Physics 15, 559 (2019)
[4] B. Leedahl et al., Nature Commun. 10, 5447 (2019)
[5] A. Amorese et al., Phys. Rev. X 11, 011002 (2021)
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Presenters
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Liu H Tjeng
Max Planck Institute for Chemical Physics of Solids
Authors
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Liu H Tjeng
Max Planck Institute for Chemical Physics of Solids