Nanshu Liu, Cong Wang, Changlin Yan, Changsong Xu, Jun Hu, Yanning Zhang, and Wei Ji
Abstract:
A recent experiment reported type-II multiferroicity in monolayer (ML) NiI2 based on a presumed spiral magnetic configuration (Spiral-B), which is, as we found here, under debate in the ML limit. Freestanding ML NiI2 breaks its C3 symmetry, as it prefers a striped antiferromagnetic order (AABB-AFM) along with an intralayer antiferroelectric (AFE) order. However, substrate confinement may preserve the C3 symmetry and/or apply tensile strain to the ML. This leads to another spiral magnetic order (SpiralIVX), while 2L shows a different order (SpiralVX) and Spiral-B dominates in thicker layers. Thus, three multiferroic phases, namely, SpiralB+FE, Spiral-IVX +FE, Spiral-VX+FE, and an anti-multiferroic AABB-AFM+AFE one, show layer-thickness-dependent and geometry-dependent dominance, ascribed to competitions among thickness-dependent Kitaev, biquadratic, and Heisenberg spin–exchange interactions and single-ion magnetic anisotropy. Our theoretical results clarify the debate on the multiferroicity of ML NiI2 and shed light on the role of layer-stacking-induced changes in noncollinear spin–exchange interactions and magnetic anisotropy in thickness-dependent magnetism.
Mao-Peng Miao, Nanshu Liu, Wen-Hao Zhang, Jian-Wang Zhou, Dao-Bo Wang, Cong Wang, Wei Ji, and Ying-Shuang Fu
Abstract:
Noncollinear magnetic orders in monolayer van der Waals magnets are crucial for probing delicate magnetic interactions under minimal spatial constraints and advancing miniaturized spintronic devices. Despite their significance, achieving atomic-scale identification remains challenging. In this study, we utilized spin-polarized scanning tunneling microscopy and density functional theory calculations to identify spin-spiral orders in mono- and bi-layer NiI2, grown on graphene-covered SiC(0001) substrates. We discovered two distinct spin-spiral states with Q vectors aligning and deviating by 7° from the lattice direction, exhibiting periodicities of 4.54 and 5.01 times the lattice constant, respectively. These findings contrast with bulk properties and align closely with our theoretical predictions. Surprisingly, the finite sizes of monolayers induce incommensurability with the spin-spiral period, facilitating collective spin switching behavior under magnetic fields. Our research reveals intrinsic noncollinear magnetism at the monolayer limit with unprecedented resolution, paving the way for exploring novel spin phenomena.
Zeyu Liu, Xianghua Kong, Zewen Wu, Linwei Zhou, Jingsi Qiao and Wei Ji
Abstract:
Many exotic electronic states were discovered in moiré superlattices hosted in twisted homo-bilayers in the past decade, including unconventional superconductivity and correlated insulating states. However, it is technically challenging to precisely and orderly stack two or more layers into certain twisting angles. Here, we presented a theoretical strategy that introduces moiré superlattices in untwisted homo-bilayers by applying different in-plane strains on the two layers of a graphene homo-bilayer, respectively. Our density functional theory calculations indicate that the graphene bilayer exhibits substantial out-of-plane corrugations that form a coloring-triangular structure in each moiré supercell under gradient in-plane strains. Such structure leads to a set of kagome bands, namely one flat-band and, at least, one Dirac band, developing along the M-K path after band-folding. For comparison, uniformly applied in-plane strain only yields a nearly flat band within path K-G, which is originated from local quantum confinement. These findings highlight the gradient strain as a route to feasibly fabricate exotic electronic states in untwisted flexible homo-bilayers.
Zemin Pan, Wenqi Xiong, Jiaqi Dai, Yunhua Wang, Tao Jian, Xingxia Cui, Jinghao Deng, Xiaoyu Lin, Zhengbo Cheng, Yusong Bai, Chao Zhu, Da Huo, Geng Li, Min Feng, Jun He, Wei Ji, Shengjun Yuan, Fengcheng Wu, Chendong Zhang, and Hong-Jun Gao
Abstract:
Flat electronic bands near the Fermi level provide a fertile playground for realizing interaction-driven correlated physics. To date, related experiments have mostly been limited to engineered multilayer systems (e.g., moiré systems). Herein, we report an experimental realization of nearly flat bands across the Fermi level in monolayer MoTe2-x by fabricating a uniformly ordered mirror twin boundary superlattice (corresponding to a stoichiometry of MoTe56/33). The kagome flat bands are discovered by combining scanning tunnelling microscopy and theoretical calculations. The partial filling nature of flat bands yields a correlated insulating state exhibiting a hard gap as large as 15 meV. Moreover, we observe pronounced responses of the correlated states to magnetic fields, providing evidence for a spin-polarized ground state. Our work introduces a monolayer platform that manifests strong correlation effects arising from flattened electronic bands.
Xiaocang Han, Mengmeng Niu, Yan Luo, Runlai Li, Jiadong Dan, Yanhui Hong, Xu Wu, Alex V. Trukhanov, Wei Ji, Yeliang Wang, Jiahuan Zhou, Jingsi Qiao*, Jin Zhang* & Xiaoxu Zhao*
Abstract:
Scanning probe microscopy and scanning transmission electron microscopy (STEM) are powerful tools to trigger atomic-scale motions, pattern atomic defects and lead to anomalous quantum phenomena in functional materials. However, these techniques have primarily manipulated surface atoms or atoms located at the beam exit plane, leaving buried atoms, which govern exotic quantum phenomena, largely unaffected. Here we propose an electron-beam-triggered chemical etching approach to engineer shielded metal atoms sandwiched between chalcogen layers in monolayer transition metal dichalcogenide (TMDC). Various metal vacancies (V_MX_n, n=0−6) have been fabricated via atomically focused electron beam in STEM. The parent TMDC surface was modified with surfactants, facilitating the ejection of sandwiched metal vacancies via charge transfer effect. In situ sequential STEM imaging corroborated that a combined chemical-induced knock-on effect and chalcogen vacancy-assisted metal diffusion process result in atom-by-atom vacancy formation. This approach is validated in 16 different TMDCs. The presence of metal vacancies strongly modified their magnetic and electronic properties, correlated with the unpaired chalcogen p and metal d electrons surrounding vacancies and adjacent distortions. These findings show a generic approach for engineering interior metal atoms with atomic precision, creating opportunities to exploit quantum phenomena at the atomic scale.
