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.
Abstract: Polymorphic structures of transition metal dichalcogenides (TMDs) host exotic electronic states, like charge density wave and superconductivity. However, the number of these structures is limited by crystal symmetries, which poses a challenge to achieving tailored lattices and properties both theoretically and experimentally. Here, we report a coloring triangle (CT) latticed MoTe2 monolayer, termed CT-MoTe2, constructed by controllably introducing uniform and ordered mirror-twin-boundaries into a pristine monolayer in molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) together with theoretical calculations reveal that the monolayer has an electronic Janus lattice, i.e., an energy-dependent atomic-lattice and a Te pseudo-sublattice, and shares the identical geometry with the Mo5Te8 layer. Dirac-like and flat electronic bands inherently existing in the CT lattice are identified by two broad and two prominent peaks in STS spectra, respectively, and verified with density-functional-theory calculations. Two types of intrinsic domain boundaries were observed, one of which the electronic-Janus-lattice feature maintains, implying potential applications as an energy-tunable electron-tunneling barrier in future functional devices.
Correlated 2D layers, like 1T-phases of TaS2, TaSe2, and NbSe2, exhibit rich tunability through varying interlayer couplings, which promotes the understanding of electron correlation in the 2D limit. However, the coupling mechanism is, so far, poorly understood and is tentatively ascribed to interactions among the dz2 orbitals of Ta or Nb atoms. Here, it is theoretically shown that the interlayer hybridization and localization strength of interfacial Se pz orbitals, rather than Nb dz2 orbitals, govern the variation of electron-correlated properties upon interlayer sliding or twisting in correlated magnetic 1T-NbSe2 bilayers. Each of the layers is in a star-of-David (SOD) charge-density-wave phase. Geometric and electronic structures and magnetic properties of 28 different stacking configurations are examined and analyzed using density-functional-theory calculations. It is found that the SOD contains a localized region, in which interlayer Se pz hybridization plays a paramount role in varying the energy levels of the two Hubbard bands. These variations lead to three electronic transitions among four insulating states, which demonstrate the effectiveness of interlayer interactions to modulate correlated magnetic properties in a prototypical correlated magnetic insulator.
Liwei Liu* Xuan Song Jiaqi Dai Han Yang Yaoyao Chen Xinyu Huang Zeping Huang Hongyan Ji Yu Zhang Xu Wu Jia-Tao Sun Quanzhen Zhang Jiadong Zhou Yuan Huang Jingsi Qiao* Wei Ji Hong-Jun Gao Yeliang Wang*
Abstract:
Layered charge-density-wave (CDW) materials have gained increasing interest due to their CDW stacking-dependent electronic properties for practical applications. Among the large family of CDW materials, those with star of David (SOD) patterns are very important due to the potentials for quantum spin liquid and related device applications. However, the spatial extension and the spin coupling information down to the nanoscale remain elusive. Here, we report the study of heterochiral CDW stackings in bilayer (BL) NbSe2 with high spatial resolution. We reveal that there exist well-defined heterochiral stackings, which have inhomogeneous electronic states among neighboring CDW units (star of David, SOD), significantly different from the homogeneous electronic states in the homochiral stackings. Intriguingly, the different electronic behaviors are spatially localized within each SOD with a unit size of 1.25 nm, and the gap sizes are determined by the different types of SOD stackings. Density functional theory (DFT) calculations match the experimental measurements well and reveal the SOD-stacking-dependent correlated electronic states and antiferromagnetic/ferromagnetic couplings. Our findings give a deep understanding of the spatial distribution of interlayer stacking and the delicate modulation of the spintronic states, which is very helpful for CDW-based nanoelectronic devices.
