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.
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*
Abstract:
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.
Two-dimensional (2D) non-van der Waals (vdW) Cr5Te8 has attracted widespread research interest for its air stability and thickness-dependent magnetic properties. However, the growth of large-scale ultrathin 2D Cr5Te8 remains challenging. Here, we selected GaTe powder as the precursor to supply Te monomers and fabricated submillimeter 2D Cr5Te8 nanosheets. By optimizing the growth temperature and source–substrate distance (DSS), we successfully achieved Cr5Te8 nanosheets with a lateral size of up to ∼0.19 mm and corresponding thickness down to ∼4.8 nm. The role of GaTe is to enhance the efficient Te atom concentration, which promotes the lateral growth of Cr5Te8 nanosheets. Furthermore, our findings reveal the appearance of Cr5Te8 nanosheets exhibiting serrated edges and a stacked structure like those of wedding cakes. Magnetic property measurement revealed the intense out-of-plane ferromagnetism in Cr5Te8, with a Curie temperature (TC) of 172 K. This work paves the way for the controllable growth of submillimeter ultrathin 2D ferromagnetic crystals and lays the foundation for the future synthesis of millimeter ultrathin ferromagnets.
Yating Li, Mengmeng Niu, Junpeng Zeng, Quan Zhou, Xu Wu, Wei Ji, Yeliang Wang, Ren Zhu, Jingsi Qiao, Jianbin Xu, Yi Shi, Xinran Wang, and Daowei He
Abstract:
Organic semiconductors are highly promising as channel materials for energy-efficient, cost-effective, and flexible electronics. However, grain boundaries (GBs) can cause significant device performance variation, posing a major challenge for the development of high-performance organic circuits. In this work, we effectively passivated GB-induced traps in monolayer organic thin-film transistors (OTFTs) via p-type doping with the organic salt TrTPFB. The doping strategy broadens the mobility edge, effectively shielding GB-induced energy barriers and Coulomb scattering, and promotes deeper nonlocalized hybridization states for conduction. Consequently, the charge transport mechanism transitions from multiple trapping and release (MTR) to a more band-like behavior, even when GBs are present within the device channel. The doped OTFTs demonstrate ultralow mobility variation (1.4%) and threshold voltage variation (4.9%), as well as record-low contact resistant of RC = 0.6 Ω·cm, outperforming most single-crystal technologies. These performance metrics render doped monolayer polycrystalline films highly promising candidates for industrial-scale organic electronics.
Zemin Pan#, Wenqi Xiong#, Jiaqi Dai#, Hui Zhang#, 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:
Although the kagome model is fundamentally two-dimensional, the essential kagome physics, i.e., the kagome-bands-driven emergent electronic states, has yet to be explored in the monolayer limit. Here, we present the experimental realization of kagome physics in monolayer Mo33Te56, showcasing both ferromagnetic ordering and a correlated insulating state with an energy gap of up to 15 meV. Using a combination of scanning tunnelling microscopy and theoretical calculations, we find a structural phase of the monolayer Mo-Te compound, which forms a mirror-twin boundary loop superlattice exhibiting kagome geometry and multiple sets of kagome bands. The partial occupancy of these nearly flat bands results in Fermi surface instability, counteracted by the emergence of ferromagnetic order (with a coercive field ~0.1 T, as observed by spin-polarized STM) and the opening of a correlated hard gap. Our work establishes a robust framework featuring well-defined atomic and band structures, alongside the intrinsic two-dimensional nature, essential for the rigorous examination of kagome physics.
