SFB 1238 | August 26, 14:30
Crystal structure and electron density: insights into altermagnets, kagome metals, and nickel oxides
The crystal structure and electronic structure of quantum materials is the basis of condensed matter physics. We study clean bulk materials by synchrotron X-ray diffraction and report the atomic positions as well as the shape of the electronic charge cloud at each atomic site.
Our work benefits from recent improvements in the dynamic range of X-ray detectors (>108), increase in brightness of up to 100× at fourth-generation synchrotron light sources, and new methodology for separating the X-ray scattering from the (isotropic) core electrons and the (anisotropic) valence electrons in solids. The latter improvement, termed core-differential Fourier synthesis (CDFS), was developed in Japan by Prof. Sawa’s group at Nagoya University [1].
Using CDFS, we are able to access the unconventional anisotropy of atomic orbitals, or electronic multipole degrees of freedom in solids.
Solids with anisotropic p-, d-, f-, g- wave spin split bands have zero net magnetization but realize spintronics functionality akin to ferromagnets [2]. Nicola Spaldin and coworkers predict that the order parameter of d-wave altermagnets is a magnetic octupole (8 poles), and the order parameter of g-wave altermagnets is a magnetic triakontadipole (32 poles) [3,4]. These magnetic multipoles are created by electric multipoles in presence of collinear spins. We use CDFS to image the g-wave altermagnetic multipole order [5].
In kagome metals, we reveal superstructures with large unit cell, which satisfy a bonding analogue of the kagome ice rule characteristic for frustrated matter [6].
In La3Ni2O7, a layered nickelate, we overturn the prevalent structure model and reveal polar charge order as the parent state at ambient pressure, from which superconductivity emerges under finite pressure [7]. Finally, I will discuss our most recent work on a metal-insulator transition driven by multipole order [8].
[1] S. Kitou et al., Phys. Rev. Lett. 119, 065701 (2017). [2] R. Yamada, M. Birch, M.H. et al., Nature 646, 837–842 (2025). [3] S. Bhowal and N. A. Spaldin, Phys. Rev. X 14, 011019 (2024). [4] X. H. Verbeek et al., Phys. Rev. Research 6, 043157 (2024). [5] R. Misawa, M.H. et al., submitted (2026). [6] R. Misawa, M.H. et al., arXiv:2604.01691 (2026). [7] R. Misawa, M.H. et al., arXiv:2603.25119 (2026). [8] Y. Tanaka, R. Misawa, M.H. et al., in preparation.
University of Tokyo
PH2
Contact: Alexey Taskin