Anomalous quasi particles in a kagome Weyl ferromagnet

It is widely thought that flat-bands as well as Dirac and Weyl crossings can be supported in a kagomé layer. One model kagome system is Fe₃Sn₂, a ferromagnet with a high (~640 K) Curie temperature, which undergoes a first order spin reorientation around 120 K(1, 2). Our density functional theory (DFT) calculations predict Weyl nodes near the Fermi level E_F and electron pockets at the zone center(3). Magnetotransport of Fe₃Sn₂ displays anomalous behaviour at temperatures below 80 K, where the spin reorientation is complete, such as tunability of the carrier density via magnetization(4) and a 3-fold antisymmetric planar Hall effect(5). Until now, electron spectroscopy has not accounted for these interesting bulk properties: the angle-resolved photoemission spectroscopy (ARPES) data published so far appear 6-fold symmetric, with no sharp bands crossing E_F, and no studies showing obvious changes on cooling below 80 K. We report here on the use of micro-focused ARPES to resolve quasiparticles near E_F sufficiently well to make contact with the anomalous bulk properties of Fe₃Sn₂. With higher spatial-resolving capability, we discover that Fe₃Sn₂ has crystallographic twins for each of which ARPES is 3-fold symmetric(6). Furthermore, the larger electron escape depth and higher energy resolution of laser ARPES combined with the ability to image a single domain, finally allow the observation of sharp quasiparticle bands crossing E_F. The sharpest band, appearing only below 80 K, cannot be reproduced by our DFT calculation suggesting that its origin is from strong correlations, a view reinforced by our additional finding that it and its neighbouring bands display marginal Fermi liquid behaviour.

1. N. Kumar, Y. Soh, Y. Wang, Y. Xiong, Magnetotransport as a diagnostic of spin reorientation: Kagome ferromagnet as a case study. Physical Review B 100, 214420 (2019).

2. K. Heritage et al., Images of a first order spin-reorientation phase transition in a metallic kagome ferromagnet. Adv. Funct. Mater. 30, 1909163 (2020).

3. M. Yao et al., Switchable Weyl nodes in topological Kagome ferromagnet Fe3Sn2. ArXiv e-prints. 2018.

4. N. Kumar, Y. Soh, Y. Wang, J. Li, X. Y., Tuning the electronic band structure in a kagome ferromagnetic metal via magnetization. Phys Rev B 106, 045120 (2022).

5. N. Kumar, Y. Soh, Y. Wang, J. Li, Y. Xiong, Anomalour Planar Hall Effect in a kagome ferromagnet. arXiv:2005.14237, (2020).

6. S. A. Ekahana et al., Anomalous quasiparticles in the zone center electron pocket of the kagomé ferromagnet Fe3Sn2. arXiv2206.13750, (2022).