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.
Wenjuan Zhao#, Xieyu Zhou#, Dayu Yan#, Yuan Huang*, Cong Li, Qiang Gao, Paolo Moras, Polina M. Sheverdyaeva, Hongtao Rong, Yongqing Cai, Eike F. Schwier, Xixia Zhang, Cheng Shen, Yang Wang, Yu Xu, Wei Ji, Chen Liu, Youguo Shi, Lin Zhao, Lihong Bao, Qingyan Wang, Kenya Shimada, Xutang Tao, Guangyu Zhang, Hongjun Gao, Zuyan Xu, Xingjiang Zhou*, Guodong Liu*
Abstract:
An in-depth understanding of the electronic structure of 2H-MoTe2 at the atomic layer limit is a crucial step towards its exploitation in nanoscale devices. Here, we show that millimeter-sized monolayer (ML) MoTe2 samples, as well as smaller sized bilayer (BL) samples, can be obtained using the mechanical exfoliation technique. The electronic structure of these materials is investigated by Angle-Resolved Photoemission Spectroscopy (ARPES) for the first time and Density Functional Theory (DFT) calculations. The comparison between experiments and theory allows us to describe ML MoTe2 as a semiconductor with direct gap at K point. This scenario is reinforced by the experimental observation of the conduction band minimum at K in Rb-doped ML MoTe2, resulting in a gap of at least 0.924 eV. In the BL MoTe2 system the maxima of the bands at Γ and K display very similar energies, thus leaving the door open to a direct gap scenario, in analogy to WSe2. The monotonic increase in the separation between spin-split bands at K while moving from ML to BL and bulk-like MoTe2 is attributed to interlayer coupling. Our findings can be considered as a reference to understand Quantum Anomalous and Fractional Quantum Anomalous Hall Effects recently discovered in ML and BL MoTe2 based moir´ e heterostructures.
Renhong Wang (王人宏), Cong Wang (王聪)*, Ruixuan Li (李睿宣), Deping Guo (郭的坪), Jiaqi Dai (戴佳琦), Canbo Zong (宗灿波), Weihan Zhang (张伟 涵), and Wei Ji (季威)*
Abstract:
Kagome materials are known for hosting exotic quantum states, including quantum spin liquids, charge density waves, and unconventional superconductivity. The search for kagome monolayers is driven by their ability to exhibit neat and well-defined kagome bands near the Fermi level, which are more easily realized in the absence of interlayer interactions. However, this absence also destabilizes the monolayer forms of many bulk kagome materials, posing significant challenges to their discovery. In this work, we propose a strategy to address this challenge by utilizing oxygen vacancies in transition metal oxides within a “1+3” design framework. Through high-throughput computational screening of 349 candidate materials, we identified 12 thermodynamically stable kagome monolayers with diverse electronic and magnetic properties. These materials were classified into three categories based on their lattice geometry, symmetry, band gaps, and magnetic configurations. Detailed analysis of three representative monolayers revealed kagome band features near their Fermi levels, with orbital contributions varying between oxygen 2p and transition metal d states. This study demonstrates the feasibility of the “1+3” strategy, offering a promising approach to uncovering low-dimensional kagome materials and advancing the exploration of their quantum phenomena.
Xiaoqing Chen#, Huijuan Zhao#, Ruixiang Fei, Chun Huang, Jingsi Qiao, Cheng Sun, Haiming Zhu, Li Zhan, Zehua Hu, Songlin Li, Li Yang, Zemin Tang, Lianhui Wang, Yi Shi, Wei Ji, Jian-Bin Xu, Li Gao*, Xuetao Gan* & Xinran Wang*
Abstract:
Two-dimensional materials offer strong light-matter interaction and design flexibility beyond conventional bulk semiconductors, but an intrinsic limit is the low absorption imposed by the atomic thickness. A long-sought-after goal is to achieve complementary absorption enhancement through energy transfer (ET) to break this intrinsic limit. However, it is found challenging due to the competing charge transfer process and lack of resonance in exciton states. Here, we report highly efficient energy transfer (ET) in a 2D hybrid organic-inorganic heterostructure (HOIST) of Me-PTCDI/WS2. Resonant ET is observed leading to enhanced WS2 PL by as much as 124 times. We identify Dexter exchange between the Frenkel state in donor and an excited 2s state in acceptor as the main ET mechanism, as supported by density functional theory calculations. We further demonstrate ET-enhanced phototransistor devices with enhanced responsivity by nearly 1000 times without sacrificing the response time. Our results expand the understanding of inter-layer relaxation.
Xiangqi Wang†, Cong Wang†, Yupeng Wang†, Chunhui Ye, Azizur Rahman, Min Zhang, Suhan Son, Jun Tan*, Zengming Zhang*, Wei Ji*, Je-Geun Park6,7,8, and Kai-Xuan Zhang†*
Abstract:
Van der Waals (vdW) magnets, with their two-dimensional (2D) atomic structures, provide a unique platform for exploring magnetism at the nanoscale. Although there have been numerous reports on their diverse quantum properties, the emergent interfacial magnetism— artificially created at the interface between two layered magnets—remains largely unexplored. This work presents observations of such emergent interfacial magnetism at the ferromagnet/antiferromagnet interface in a vdW heterostructure. We report the discovery of an intermediate Hall resistance plateau in the anomalous Hall loop, indicative of emergent interfacial antiferromagnetism fostered by the heterointerface. This plateau can be stabilized and further manipulated under varying pressures but collapses under high pressures over 10 GPa. Our theoretical calculations reveal that charge transfer at the interface is pivotal in establishing the interlayer antiferromagnetic spin-exchange interaction. This work illuminates the previously unexplored emergent interfacial magnetism at a vdW interface comprised of a ferromagnetic metal and an antiferromagnetic insulator, and highlights its gradual evolution under increasing pressure. These findings enrich the portfolio of emergent interfacial magnetism and pave the way for future investigations on vdW magnetic interfaces and the development of next-generation spintronic devices.
