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.
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.
The properties of two-dimensional (2D) van der Waals materials can be tuned through nanostructuring or controlled layer stacking, where interlayer hybridization induces exotic electronic states and transport phenomena. Here we describe a viable approach and underlying mechanism for the assisted self-assembly of twisted layer graphene. The process, which can be implemented in standard chemical vapour deposition growth, is best described by analogy to origami and kirigami with paper. It involves the controlled induction of wrinkle formation in single-layer graphene with subsequent wrinkle folding, tearing and re-growth. Inherent to the process is the formation of intertwined graphene spirals and conversion of the chiral angle of 1D wrinkles into a 2D twist angle of a 3D superlattice. The approach can be extended to other foldable 2D materials and facilitates the production of miniaturized electronic components, including capacitors, resistors, inductors and superconductors.
