Robust Mottness and tunable interlayer magnetism in Nb3X8 (X = F, Cl, Br, I) bilayers

Robust Mottness and tunable interlayer magnetism in Nb3X8 (X = F, Cl, Br, I) bilayers

Zhongqin Zhang, Jiaqi Dai, Cong Wang*, Zhihai Cheng, and Wei Ji*

Kagome materials have attracted extensive attention due to their correlated properties. The breathing kagome material system Nb3F8, Nb3Cl8, Nb3Br8, Nb3I8 is regarded as a Mott insulator. However, studies on the influence of interlayer coupling on its magnetic and Mott properties are lacking. In this work, we investigated the effect of interlayer coupling on bilayer properties of each Nb3X8 (X = F, Cl, Br, I) compound via density functional theory (DFT) calculations, considering 24 stacking configurations per material. We found that each bilayer material is a Mott insulator. Due to the competition between interlayer Pauli repulsion and hopping, most interlayer magnetism is AFM, a small number of cases show AFM-FM degeneracy, and the magnetic ground state of 3 configurations is interlayer FM, i.e., tunable interlayer magnetism occurs. This robustness of Mott states coexisting with tunable interlayer magnetism provide novel and comprehensive analysis and insights for the research of breathing kagome Mott insulators.

Mechanically and electrically switchable triferroic altermagnet in a pentagonal FeO2 monolayer

Mechanically and electrically switchable triferroic altermagnet in a pentagonal FeO2 monolayer

Deping Guo#,*, Jiaqi Dai#, Renhong Wang, Cong Wang*, and Wei Ji*

Two‑dimensional multiferroics promise low‑power, multifunctional devices, yet the intrinsic coexistence and mutual control of three coupled ferroic orders in a single layer remains elusive. Here, we identify pentagonal monolayer FeO2 as an intrinsic triferroic altermagnet where ferroelectric (FE), ferroelastic (FA), and altermagnetic (AM) orders coexist and tightly coupled, accompanied by a competing antiferroelectric (AFE) phase using first‑principles calculations. The solely presence of glide mirror Mx symmetry in a FeO2 sublayer, with the breaking of four‑fold rotation C4z symmetry, induces in‑plane vector ferroelectricity and twin‑related ferroelastic strains. Both FE and AFE phases break combined parity–time symmetry and display sizable altermagnetic spin splitting with Néel temperatures over 200 K. Electric‑field induced rotation of the FE polarization reverses the sign of the spin splitting, while in‑plane uniaxial strain triggers ferroelastic switching that simultaneously rotates the FE polarization vector by 90° and reverses the AM state. These electric‑field‑ and strain‑mediated pathways interlink six distinct polarization states that can be selected purely by electric fields and/or mechanical strain. This work extends intrinsic triferroicity to pentagonal monolayers and outlines a symmetry‑based route toward mechanically and electrically configurable altermagnetic spintronics.

Tunable altermagnetism via interchain engineering in parallel-assembled atomic chains

Tunable altermagnetism via interchain engineering in parallel-assembled atomic chains

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Deping Guo, Canbo Zong, Weihan Zhang, Cong Wang*, Junwei Liu*, and Wei Ji*

Altermagnetism has recently drawn considerable attention in three- and two-dimensional materials. Here we extend this concept to quasi-one-dimensional (Q1D) monolayers assembled from single-atomic magnetic chains. Through systematically examining nine types of structures, two stacking orders, intra- and interchain magnetic couplings, we identify four out of 30 promising structural prototypes for hosting altermagnetism, which yields 192 potential monolayer materials. We further confirm eight thermodynamically stable Q1D monolayers via high-throughput calculations. Using symmetry analysis and first-principles calculations, we find that the existence of altermagnetism is determined by the type of interchain magnetic coupling and predict three intrinsic altermagnets, CrBr3, VBr3, and MnBr3, due to their ferromagnetic interchain couplings and five extrinsic ones, CrF3, CrCl3, CrI3, FeCl3, and CoTe3, ascribed to their neglectable or antiferromagnetic interchain couplings. Moreover, the interchain magnetic coupling here is highly tunable by manipulating the interchain spacing, leading to experimentally feasible transitions between altermagnetic and nodal-line semiconducting states. In addition, applying external electric fields can further modulate the spin splitting. Our findings establish a highly tunable family of Q1D altermagnets, offering fundamental insights into the intricate relationship between geometry, electronic structure, and magnetism. These discoveries hold significant promises for experimental realization and future spintronic applications.

Atomic to mesoscale hierarchical structures and magnetic states in an anisotropic layered ferromagnet FePd2Te2

Atomic to mesoscale hierarchical structures and magnetic states in an anisotropic layered ferromagnet FePd2Te2

Shuo Mi#, Manyu Wang#, Bingxian Shi#, Songyang Li, Xiaoxiao Pei, Yanyan Geng, Shumin Meng, Rui Xu, Li Huang, Wei Ji, Fei Pang, Peng Cheng*, Jianfeng Guo*, and Zhihai Cheng*

Two-dimensional (2D) magnetic materials have predominantly exhibited easy-axis or easy-plane anisotropy and display a high sensitivity to the underlying crystal structure and lattice symmetry. Recently, an in-plane anisotropic 2D ferromagnet of FePd2Te2 has been discovered with intriguing structure and quasi-one-dimensional spin system. Here, we report a real-space investigation of its twinning structure and magnetic states using atomic/magnetic force microscopy (AFM/MFM) combined with scanning tunneling microscopy (STM). The atomic to mesoscale hierarchical structures with the orthogonal and corrugated compressive /tensile(C/T) regions are directly observed due to the intrinsic twinning-domain characteristic. The structure-related intact ferromagnetic (FM), field-induced polarized-FM states and their transitions are comparatively discussed at the mesoscale with the corresponding macroscopic magnetic measurements. Temperature- and field-dependent evolution of magnetic phase are further investigated at the FM and PM states, and summarized to obtain a unique H-T phase diagram of FePd2Te2. Our work provides key results for understanding the complicated magnetic properties of FePd2Te2, and suggests new directions for manipulating magnetic states through the atomic and mesoscale structure engineering.

Evidence of Ferroelectricity in an Antiferromagnetic Vanadium Trichloride Monolayer

Evidence of Ferroelectricity in an Antiferromagnetic Vanadium Trichloride Monolayer

Science Advances 11, eado6538 (2025); arXiv:2404.13513 (2024)

Jinghao Deng#, Deping Guo#, Yao Wen, Shuangzan Lu, Zhengbo Cheng, Zemin Pan, Tao Jian, Yusong Bai, Hui Zhang, Wei Ji*, Jun He*, Chendong Zhang*

Multiferroicity allows magnetism to be controlled using electric fields or vice versa, which has gained tremendous interest in both fundamental research and device applications. A reduced dimensionality of multiferroic materials is highly desired for device miniaturization, but the coexistence of ferroelectricity and magnetism at the two-dimensional limit is still debated. Here, we used a NbSe2 substrate to break both the C3 rotational and inversion symmetries in monolayer VCl3 and thus introduced exceptional in-plane ferroelectricity into a two dimensional magnet. Scanning tunnelling spectroscopy directly visualized ferroelectric domains and manipulated their domain boundaries in monolayer VCl3, where coexisting antiferromagnetic order with canted magnetic moments was verified by vibrating sample magnetometer measurements. Our density functional theory calculations highlight the crucial role that highly directional interfacial Cl–Se interactions play in breaking the symmetries and thus in introducing in-plane ferroelectricity, which was further verified by examining an ML-VCl3/graphene sample. Our work demonstrates an approach to manipulate the ferroelectric states in monolayered magnets through van der Waals interfacial interactions.