After the preparation of 2D electronic flat band (EFB) in van der Waals (vdW) superlattices, recent measurements suggest the existence of 1D electronic flat bands (1D-EFBs) in twisted vdW bilayers. However, the realization of 1D-EFBs is experimentally elusive in untwisted 2D layers, which is desired considering their fabrication and scalability. Herein, the discovery of 1D-EFBs is reported in an untwisted in situ-grown two atomic-layer Bi(110)superlattice self-aligned on an SnSe(001) substrate using scanning probe microscopy measurements and density functional theory calculations. While the Bi-Bi dimers of Bi zigzag (ZZ) chains are buckled, the epitaxial lattice mismatch between the Bi and SnSe layers induces two 1D buckling reversal regions (BRRs) extending along the ZZ direction in each Bi(110)-11 x 17 supercell. A series of 1D-EFBs arises spatially following BRRs that isolate electronic states along the armchair (AC) direction and localize electrons in 1D extended states along ZZ due to quantum interference at a topological node. This work provides a generalized strategy for engineering 1D-EFBs in utilizing lattice mismatch between untwisted rectangular vdW layers.
Deping Guo#, Pengjie Guo#, Shijing Tan, Min Feng, Limin Cao, Zheng-Xin Liu*, Kai Liu*,
Zhong-Yi Lu, Wei Ji*
Abstract
Dirac nodal-line semimetals (DNLSMs) host novel quasiparticle excitations and intriguing transport properties, which are, however, easily perturbed under strong spin-orbit coupling (SOC), especially in low-dimensions. Two-dimensional (2D) layers have numerous advantages and are under continuous development; however, 2D-DNLSMs resistant to SOC are yet to be discovered. Here, we report the C_2v×Z_2^T little co-group, a non-symmorphic symmetry we found in 2D, guarantees a robust 2D-DNLSM against SOC, which could be imposed in three-atomic-layer (3-AL) Bismuth (the brick phase, a novel Bi allotrope) and other layered materials. Intriguingly, (4n+2) valence electrons fill the electronic bands in 3-AL Bi, such that the nodal line passes the Fermi level where other low-energy states are gapped, allowing feasible observation of DNLSM-induced phenomena without interference from other bands in future transport measurements. Thus, our study demonstrates an unprecedented category of layered materials, allowing for the exploration of nearly isolated DNL states in 2D.
Zheng, Fangyuan; Guo, Deping; Huang, Lingli; Wong, Lok Wing; Chen, Xin; Wang, Cong; Cai, Yuan; Wang, Ning; Lee, Chun-Sing; Lau, Shu Ping; Ly, Thuc Hue; Ji, Wei and Zhao, Jiong
Abstract
Phase patterning in polymorphic two-dimensional (2D) materials offers diverse properties that extend beyond what their pristine structures can achieve. If precisely controllable, phase transitions can bring exciting new applications for nanometer-scale devices and ultra-large-scale integrations. Here, the focused electron beam is capable of triggering the phase transition from the semiconducting T” phase to metallic T’ and T phases in 2D rhenium disulfide (ReS2) and rhenium diselenide (ReSe2) monolayers, rendering ultra-precise phase patterning technique even in sub-nanometer scale is found. Based on knock-on effects and strain analysis, the phase transition mechanism on the created atomic vacancies and the introduced substantial in-plane compressive strain in 2D layers are clarified. This in situ high-resolution scanning transmission electron microscopy (STEM) and in situ electrical characterizations agree well with the density functional theory (DFT) calculation results for the atomic structures, electronic properties, and phase transition mechanisms. Grain boundary engineering and electrical contact engineering in 2D are thus developed based on this patterning technique. The patterning method exhibits great potential in ultra-precise electron beam lithography as a scalable top-down manufacturing method for future atomic-scale devices. DOI:10.1002/advs.202200702
Kui Huang (黄逵), Zhenxian Li (李政贤), Deping Guo (郭的坪), Haifeng Yang (杨海峰), Yiwei Li (李一苇), Aiji Liang (梁爱基), Fan Wu (吴凡), Lixuan Xu (徐丽璇), Lexian Yang (杨乐仙), Wei Ji (季威), Yanfeng Guo (郭艳峰), Yulin Chen (陈宇林)* and Zhongkai Liu (柳仲楷)*
Abstract
As a van der Waals ferromagnet with high Curie temperature, Fe5–xGeTe2 has attracted tremendous interests recently. Here, using high-resolution angle-resolved photoemission spectroscopy (ARPES), we systematically investigated the electronic structure of Fe5–xGeTe2 crystals and its temperature evolution. Our ARPES measurement reveals two types of band structures from two different terminations with slight kz evolution. Interestingly, across the ferromagnetic transition, we observed the merging of two split bands above the Curie temperature, suggesting the band splitting due to the exchange interaction within the itinerant Stoner model. Our results provide important insights into the electronic and magnetic properties of Fe5–xGeTe2 and the understanding of magnetism in a two-dimensional ferromagnetic system.
