Condensed matter systems with coexisting Dirac cones and flat bands, and a switchable control between them within a single system, are desirable but remarkably uncommon. Here we report a layered quantum material system, KxNi4S2 (0 <= x <= 1), that simultaneously hosts both characteristics without involving typical Kagome/honeycomb lattices. Enabled by a topochemical K-deintercalation process, the Fermi surface can be fine-tuned continuously over a wide range of energies. Consequently, a non-magnetic Dirac-metal state with a topological nontrivial Z2 index of 1;(000), supported by first-principles calculations and high mobility up to 1471 cm2V-1s-1, is observed on the K-rich x = 1 side, whereas a flat-band induced antiferromagnetic state with TN up to 10.1 K emerges as K-content approaches 0. The KxNi4S2 system offers a versatile platform for exploring emerging phenomena and underscores a viable pathway for in-situ control of quantum materials dominated by Dirac cones, flat bands, and their interplay.
@article{arxiv.2509.09903,
title = {Evolution from Topological Dirac Metal to Flat-band-Induced Antiferromagnet in Layered KxNi4S2 (0<=x<=1)},
author = {Hengdi Zhao and Xiuquan Zhou and Hyowon Park and Tianqi Deng and Brandon Wilfong and Alann P. Au and Samuel E. Pate and Craig M. Brown and Hui Wu and Tushar Bhowmick and Tessa McNamee and Ravhi Kumar and Yu-Sheng Chen and Zhi-Li Xiao and Russell Hemley and Weizhao Cai and Shanti Deemyad and Duck-Young Chung and Stephan Rosenkranz and Mercouri G. Kanatzidis},
journal= {arXiv preprint arXiv:2509.09903},
year = {2025}
}