Evolution from topological Dirac metal to flat-band-induced antiferromagnet in layered KxNi4S2 (0 ≤ x ≤ 1)

Published

January 1, 2026

Author(s)

Hengdi Zhao, Xiuquan Zhou, Hyowon Park, Tianqi Deng, Brandon Wilfong, II Au, Samuel E. Pate, Craig Brown, Hui Wu, Tushar Bhowmick, Tessa McNamee, Ravhi Kumar, Yu-Sheng Chen, Zhi-Li Xiao, Russell Hemley, Weizhao Cai, Shanti Deemyad, Duck-Young Chung, Stephan Rosenkranz, Mercouri G. Kanatzidis

Abstract

Dirac materials and flat-band systems, each possessing distinct electronic structures, have captivated a wide range of scientific communities for their potential to host diverse emerging phenomena. In particular, a tunable ground state featuring a Fermi surface dominated by massive fermions from the flat band and massless fermions from the Dirac cone offers an ideal platform to study the interplay between these emerging phenomena. Despite great interest in such systems, materials with coexisting Dirac cones and flat bands are rare, relying on artificial lattice engineering, such as twisted bilayer graphene, or exotic structures, like Kagome or honeycomb lattices. In addition, the lack of an effective method for tuning the Fermi level poses another challenge. Here, we report a layered quantum material, KxNi4S2 (0 ≤ x ≤ 1), that simultaneously hosts both flat bands and Dirac cones at distinct energies without involving the typical Kagome or honeycomb lattice. Our molecular orbital bonding analysis suggests that the Ni–Ni bonding exclusively hosted by KxNi4S2 plays a vital role in the formation of Dirac cones. Notably, the K-content can be controlled through the K-deintercalation process, enabling the long-sought effective method of wide-range tuning of the Fermi level. With first-principles calculations and experimental confirmation, we demonstrate the versatile ground state that can be fine-tuned through the K-deintercalation process, from a non-magnetic topological Dirac metal (KNi4S2, x = 1) to a flat-band-induced antiferromagnet (Ni2S, x = 0). The KxNi4S2 (0 ≤ x ≤ 1) system offers an experimentally validated, versatile platform for exploring emerging phenomena from massless Dirac fermions, flat-band heavy electrons, and the interplay between them. This ex situ topochemical K-deintercalation study also establishes a highly tunable ground state, demonstrating a viable pathway for in situ control of quantum materials that can switch between Dirac-cone- and flat-band-dominated states via electrochemical intercalation and deintercalation.

Citation

Matter

Volume

9

Issue

1

Pub Type

Journals

Download Paper

https://doi.org/10.1016/j.matt.2025.102418

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Keywords

Dirac metal, non-trivial band topology, flat-band induced magnetism, topochemical deintercalation, non-Fermi liquids

Citation

Zhao, H. , Zhou, X. , Park, H. , Deng, T. , Wilfong, B. , Au, I. , Pate, S. , Brown, C. , Wu, H. , Bhowmick, T. , McNamee, T. , Kumar, R. , Chen, Y. , Xiao, Z. , Hemley, R. , Cai, W. , Deemyad, S. , Chung, D. , Rosenkranz, S. and Kanatzidis, M. (2026), Evolution from topological Dirac metal to flat-band-induced antiferromagnet in layered KxNi4S2 (0 ≤ x ≤ 1), Matter, [online], https://doi.org/10.1016/j.matt.2025.102418, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=959921 (Accessed February 21, 2026)