Yanyan Geng+, Haoyu Dong+, Renhong Wang+, Jianfeng Guo, Shuo Mi, Le Lei, Yan Li, Li Huang, Fei Pang, Rui Xu, Weiqiang Yu, Hong-Jun Gao, Wei Ji*, Weichang Zhou*, and Zhihai Cheng*
Abstract:
The delicate interplay among the complex intra-/inter-layer electron-electron and electron-lattice interactions is the fundamental prerequisite of these exotic quantum states, such as superconductivity, nematic order, and checkerboard charge order. Here we explore the filling-dependent multiple stable intertwined electronic and atomic orders of flat-band state of 1T-TaS2 encompassing hole order, phase orders, coexisting left- and right-chiral orders and mixed phase/chiral orders via scanning tunneling microscopy (STM). Combining first principles calculations, the novel emergent electronic/ atomic orders can be attributed to the weakening of electron-electron correlations and stacking-dependent interlayer interactions. Moreover, achiral intermediate ring-like clusters and nematic charge density wave (CDW) states are successfully realized in intralayer chiral domain wall and interlayer heterochiral stacking regions through chiral overlap configurations. Our study not only deepens the understanding of filling-dependent electronic/atomic orders in flat-band systems, but also offers new perspectives for exploring exotic quantum states in correlated electronic systems.
Zhongqin Zhang† , Jiaqi Dai† , Cong Wang , Hua Zhu , Fei Pang , Zhihai Cheng, and Wei Ji*
Abstract:
In recent years, kagome materials have attracted significant attention due to their rich emergent phenomena arising from the quantum interplay of geometry, topology, spin, and correlations. However, in the search for kagome materials, it has been found that bulk compounds with electronic properties related to the kagome lattice are relatively scarce, primarily due to the hybridization of kagome layers with adjacent layers. Therefore, researchers have shown increasing interest in the discovery and construction of two-dimensional (2D) kagome materials, aiming to achieve clean kagome bands near the Fermi level in monolayer or few-layer systems. Substantial advancements have already been made in this area. In this review, we summarize the current progress in the construction and development of 2D kagome materials. We begin by introducing the geometric and electronic structures of the kagome lattice model and its variants, followed by discussions on the experimental realizations and electronic structure characterizations of 2D kagome materials. Finally, we provide an outlook on the future developments of 2D kagome materials.
Two-dimensional Janus materials exhibit unique physical properties due to broken inversional symmetries. However, it remains elusive to synthesize Janus monolayer crystals with tailored long-range magnetic orders. Here, we show a 2 ×√𝟑 charge density wave (CDW) transition and regulations of magnetization in a uniform Janus CrTeSe monolayer, selectively selenized from a pristine CrTe2 monolayer using molecular beam epitaxy. Scanning transmission electron microscopy images indicate the high quality and uniformity of the Janus structure. Spin-polarized scanning tunneling microscopy/spectroscopy measurements and density functional theory calculations unveil a robust zigzag antiferromagnetic order and the CDW transition in the CrTeSe monolayer. The one-side selenization breaks the vertical inversion symmetry, rotating the magnetic moment directions to the in-plane direction. The CDW transition opens a gap at the Fermi level and reorients the magnetic moments in tilted directions. Our work demonstrates the construction of large-area Janus structures and the tailoring of electronic and magnetic properties of two-dimensional Janus layers.
The quantum anomalous Hall (QAH) effect in two-dimensional (2D) topological materials has attracted widespread attention due to its potential for dissipationless chiral edge transport without an external magnetic field, which is highly promising for low-power electronic applications. However, identifying materials that exhibit these properties remains particularly challenging, as only a limited number of such materials are known, raising the intriguing question of whether it is possible to induce the QAH effect in materials with ordinary properties through structural modifications. In this work, we grow an unreported 2D titanium selenide (Ti3Se4) on a Cu(111) substrate using molecular beam epitaxy. Low-energy electron diffraction and scanning tunneling microscopy characterizations reveal a brick-like structure. First-principles calculations and X-ray photoelectron spectroscopy measurements confirm its composition to be Ti3Se4. Our calculations further demonstrate that monolayer Ti3Se4, in its grown form on Cu(111), has the potential to host the QAH effect. Interestingly, when we examine its freestanding form, the monolayer transitions from a QAH insulator candidate into a conventional semiconductor, despite only minor differences in their atomic structures. This transition enlightens us that subtle lattice adjustments can induce a transition from semiconductor to QAH properties in freestanding Ti3Se4. This discovery provides a potential route to engineering practical materials that may exhibit the QAH effect.
In conventional electrides, excess electrons are localized in crystal voids to serve as anions. Most of these electrides are metallic and the metal cations are primarily from the s-block, d-block, or rare-earth elements. Here, we report a class of p-block metal-based electrides found in bilayer SnO and PbO, which are semiconducting and feature electride states in both the valence band (VB) and conduction band (CB), as referred to 2D “bipolar” electrides. These bilayers are hybrid electrides where excess electrons are localized in the interlayer region and hybridize with the orbitals of Sn atoms in the VB, exhibiting strong covalent-like interactions with neighboring metal atoms. Compared to previously studied hybrid electrides, the higher electronegativity of Sn and Pb enhances these covalent-like interactions, leading to largely enhanced semiconducting bandgap of up to 2.5 eV. Moreover, the CBM primarily arises from the overlap between metal states and interstitial charges, denoting a potential electride and forming a free-electron-like (FEL) state with small effective mass. This state offers high carrier mobilities for both electron and hole in bilayer SnO, suggesting its potential as a promising p-type semiconductor material.
