Dichotomy of flat bands in the van der Waals ferromagnet <math xmlns="http://www.w3.org/1998/Math/MathML"><mrow><msub><mi>Fe</mi><mn>5</mn></msub><msub><mi>GeTe</mi><mn>2</mn></msub></mrow></math>

Han Wu; Jianwei Huang; Chaowei Hu; Lei Chen; Yiqing Hao; Yue Shi; Paul Malinowski; Yucheng Guo; Bo Gyu Jang; Jian-Xin Zhu; Andrew F. May; Tyler Werner; Siqi Wang; Xiang Chen; Yaofeng Xie; Bin Gao; Yichen Zhang; Ziqin Yue; Zheng Ren; Makoto Hashimoto; Donghui Lu; Alexei Fedorov; Sung-Kwan Mo; Junichiro Kono; Yu He; Robert J. Birgeneau; Pengcheng Dai; Xiaodong Xu; Huibo Cao; Qimiao Si; Jiun-Haw Chu; Ming Yi

doi:10.1103/tbk8-zg11

Dichotomy of flat bands in the van der Waals ferromagnet ${Fe}_{5} {GeTe}_{2}$

Han Wu ¹, Jianwei Huang¹, Chaowei Hu^2,3, Lei Chen ¹, Yiqing Hao ⁴, Yue Shi², Paul Malinowski ², Yucheng Guo ¹, Bo Gyu Jang^5,6 et al.

Jian-Xin Zhu⁵, Andrew F. May ⁷, Tyler Werner⁸, Siqi Wang⁸, Xiang Chen⁹, Yaofeng Xie ¹, Bin Gao ¹, Yichen Zhang ¹, Ziqin Yue¹⁰, Zheng Ren ¹, Makoto Hashimoto ¹¹, Donghui Lu ¹¹, Alexei Fedorov ¹², Sung-Kwan Mo ¹², Junichiro Kono ^1,13,14,15, Yu He ⁸, Robert J. Birgeneau ^16,17,9, Pengcheng Dai^1,15,18, Xiaodong Xu^2,3, Huibo Cao⁴, Qimiao Si^1,15,18, Jiun-Haw Chu², and Ming Yi ^1,15,18,*

¹Department of Physics and Astronomy and Rice Center for Quantum Materials, Rice University, Houston, Texas 77005 USA
²Department of Physics, University of Washington, Seattle, Washington 98195, USA
³Department of Materials Science and Engineering, University of Washington, Seattle, Washington 98195, USA
⁴Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁵Theoretical Division and Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico, USA
⁶Department of Advanced Materials Engineering for Information and Electronics, Kyung Hee University, Yongin 17104, Republic of Korea
⁷Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁸Department of Applied Physics, Yale University, New Haven, Connecticut 06511, USA
⁹Department of Physics, University of California, Berkeley, Berkeley, California 94720, USA
¹⁰Applied Physics Graduate Program, Smalley-Curl Institute, Rice University, Houston, Texas 77005, USA
¹¹Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
¹²Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
¹³Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, USA
¹⁴Departments of Electrical and Computer Engineering, Rice University, Houston, Texas 77005, USA
¹⁵Smalley-Curl Institute, Rice University, Houston, Texas 77005, USA
¹⁶Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
¹⁷Department of Materials Science and Engineering, University of California, Berkeley, USA
¹⁸Rice Laboratory for Emergent Magnetic Materials, Rice University, Houston, Texas 77005, USA

^*Contact author: mingyi@rice.edu

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Phys. Rev. B 113, 125120 – Published 10 March, 2026

DOI: https://doi.org/10.1103/tbk8-zg11

Abstract

Quantum materials with bands of narrow bandwidth near the Fermi level represent a promising platform for exploring a diverse range of fascinating physical phenomena, as the high density of states within the small energy window often enables the emergence of many-body physics. On one hand, flat bands can arise from strong Coulomb interactions that localize atomic orbitals. On the other hand, quantum destructive interference can quench the electronic kinetic energy. Although both have a narrow bandwidth, the two types of flat bands should exhibit very distinct spectral properties arising from their distinctive origins. So far, the two types of flat bands have only been realized in very different material settings and chemical environments, preventing a direct comparison. Here we report the observation of the two types of flat bands within the same material system—an above-room-temperature van der Waals ferromagnet, ${Fe}_{5 - x} {GeTe}_{2}$ , distinguishable by a switchable iron site order. The contrasting nature of the flat bands is also identified by the remarkably distinctive temperature evolution of the spectral features, indicating that one arises from electron correlations in the Fe(1) site-disordered phase, while the other geometrical frustration in the Fe(1) site-ordered phase. Our results therefore provide a direct juxtaposition of the distinct formation mechanism of flat bands in quantum materials and an avenue for understanding the distinctive roles flat bands play in the presence of magnetism, topology, and lattice geometrical frustration, utilizing sublattice ordering as a key control parameter.

Physical Review B

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Dichotomy of flat bands in the van der Waals ferromagnet ${Fe}_{5} {GeTe}_{2}$

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Dichotomy of flat bands in the van der Waals ferromagnet Fe5GeTe2

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Dichotomy of flat bands in the van der Waals ferromagnet ${Fe}_{5} {GeTe}_{2}$