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VO2 is renowned for its electric transition from an insulating monoclinic (M1) phase, characterized by V–V dimerized structures, to a metallic rutile (R) phase above 340 K. This transition is accompanied by a magnetic change: the M1 phase exhibits a non-magnetic spin-singlet state, while the R phase exhibits a state with local magnetic moments. Simultaneous simulation of the structural, electric, and magnetic properties of this compound is of fundamental importance, but the M1 phase alone has posed a significant challenge to the density functional theory (DFT). In this study, we show none of the commonly used DFT functionals, including those combined with on-site Hubbard U to treat 3d electrons better, can accurately predict the V–V dimer length. The spin-restricted method tends to overestimate the strength of the V–V bonds, resulting in a small V–V bond length. Conversely, the spin-symmetry-breaking method exhibits the opposite trends. Each of these two bond-calculation methods underscores one of the two contentious mechanisms, i.e., Peierls lattice distortion or Mott localization due to electron–electron repulsion, involved in the metal–insulator transition in VO2. To elucidate the challenges encountered in DFT, we also employ an effective Hamiltonian that integrates one-dimensional magnetic sites, thereby revealing the inherent difficulties linked with the DFT computations.
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Phase contrast imaging of non-collinear spin textures with Lorentz microscopy
This article discusses the physical and mathematical background of phase contrast imaging with in‑line electron holography from a physics rather than a microscopy perspective and showcases the strength of non‑iterative and iterative approaches by application to magnetism research. A comprehensive derivation of magnetic and electric phase shift due to electromagnetic interaction with matter and electron wave propagation is presented as the foundation for phase retrieval algorithms based on the transport‑of‑intensity equation and Gerchberg–Saxton—an iterative exit wave reconstruction algorithm. The strength and potential of both algorithms are highlighted by experimental and numerical quantitative comparison using non‑collinear spin textures. Although the focus of this work is on magnetism research, the indifference of the exit wave reconstruction to the origin of the phase shift ensures applicability to study spatial variations in both electric and spin distributions in quantum, energy, and magnetic materials.
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- Award ID(s):
- 2203933
- PAR ID:
- 10493695
- Publisher / Repository:
- Springer Nature
- Date Published:
- Journal Name:
- Journal of Materials Research
- Volume:
- 38
- Issue:
- 23
- ISSN:
- 0884-2914
- Page Range / eLocation ID:
- 4977 to 4994
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1557/s43578-023-01216-1
