2026-03-24 アルゴンヌ国立研究所(ANL)
Top: Structure of KNi4S2. Left: Atoms of potassium (K), nickel (Ni) and sulfur (S) depicted in purple, red and yellow, respectively. Right: The removal of K atoms. Bottom: The transition between states and highlights flat bands (in squares) and Dirac cones (in circles). (Image by Hengdi Zhao.)
＜関連情報＞
- https://www.anl.gov/article/now-you-see-it-now-you-dont-material-can-transition-between-quantum-states
- https://www.cell.com/matter/abstract/S2590-2385(25)00461-8
- https://pubs.acs.org/doi/10.1021/jacs.1c05107
層状K xNi4S 2(0 ≤ x ≤ 1)におけるトポロジカルディラック金属からフラットバンド誘起反強磁性体への進化 Evolution from topological Dirac metal to flat-band-induced antiferromagnet in layered KxNi4S2 (0 ≤ x ≤ 1)
Hengdi Zhao ∙ Xiuquan Zhou ∙ Hyowon Park ∙ … ∙ Duck-Young Chung ∙ Stephan Rosenkranz ∙ Mercouri G. Kanatzidis
Matter Published:September 11, 2025
DOI:https://doi.org/10.1016/j.matt.2025.102418
Progress and potential
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.
Highlights
- Coexistence of flat bands and Dirac cones without Kagome/honeycomb lattices
- Continuously tunable Fermi surface through topochemical K-deintercalation
- Switchable ground state between Dirac-cone- and flat-band-dominated regimes
- Establishment of a new material design paradigm for correlated topological systems
Summary
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 1,471 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 the 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.
溶融水酸化物中の酸化状態制御による硫化ニッケルの新規化合物および相選択 New Compounds and Phase Selection of Nickel Sulfides via Oxidation State Control in Molten Hydroxides
Xiuquan Zhou,David J. Mandia,Hyowon Park,Mahalingam Balasubramanian,Lei Yu,Jianguo Wen,Andrey Yakovenko,Duck Young Chung,and Mercouri G. Kanatzidis
Journal of the American Chemical Society
DOI:https://doi.org/10.1021/jacs.1c05107
Abstract
Molten salts are promising reaction media candidates for the discovery of novel materials; however, they offer little control over oxidation state compared to aqueous solutions. Here, we demonstrated that when two hydroxides are mixed, their melts become fluxes with tunable solubility, which are surprisingly powerful solvents for ternary chalcogenides and offer effective paths for crystal growth to new compounds. We report that precise control of the oxidation state of Ni is achievable in mixed molten LiOH/KOH to grow single crystals of all known ternary K–Ni–S compounds. It is also possible to access several new phases, including a new polytope of β-K2Ni3S4, as well as low-valence KNi4S2 and K4Ni9S11. KNi4S2 is a two-dimensional low-valence nickel-rich sulfide, and β-K2Ni3S4 has a hexagonal lattice. Moreover, using KNi4S2 as a template, we obtained a new layered binary Ni2S by topotactic deintercalation of K. The new binary Ni2S has a van der Waals gap and can function as a new host layer for intercalation chemistry, as demonstrated by the intercalation of LiOH between its layers. The oxidation states of low-valence KNi4S2 and Ni2S were studied using X-ray absorption spectroscopy and X-ray photoelectron spectroscopy. Density functional theory calculations showed large antibonding interactions at the Fermi level for both KNi4S2 and Ni2S, corresponding to the flat-bands with large Ni-dx2–y2 character.
