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.
Qingyang Wang, Mengmeng Niu, Weikang Zhou, Yicheng Ma, Chun Huang, Gege Yang, Yan Shao, Xu Wu, Cong Wang, Wei Ji*, Yeliang Wang*, Jingsi Qiao*
Abstract:
Two-dimensional (2D) multiferroic materials have significant application potential for novel storage devices due to their tunable magnetic and ferroelectric properties. Transition metal phosphorus chalcogenides MPX3 (X = S, Se, and Te) were found to be magnetic and multiferroic with excellent tunability, promising for multifunctionalized applications. In this study, we investigated the antiferromagnetic and antiferroelectric properties of two-dimensional FePX3 and CuFeP2X6 by density functional theory. Monolayer FePS3/FePSe3 and FePTe3 take intralayer zigzag and Neel antiferromagnetic ground states, respectively. This tunability of intralayer magnetism results from the competition between the spin-exchange interactions of the first and second nearest Fe atoms. Bilayer FePX3 shows weak interlayer interactions and keeps electronic and magnetic characteristics similar to those of the monolayer. Moreover, by introducing the nonmagnetic Cu atom into FePX3, the inversion symmetry broken induces CuFeP2X6 to be multiferroic materials. The transition barrier between ferroelectric (FE) and antiferroelectric (AFE) phases in CuFeP2S6 and CuFeP2Se6 is 0.09 and 0.04 eV/f.u., similar to well-known multiferroic CuCrP2S6. FE-to-AFE phase transition is expected to be achieved by applying an electric field and uniaxial strain. CuFeP2Te6 shows the ground state with a distorted paraelectric phase. Our results show the fundamental properties and in-depth understanding of 2D FePX3 and CuFeP2X6, guiding further investigation of 2D multifunctionalized magnetoelectric devices.
The structure and dynamics of ferroelectric domain walls are essential for polarization switching in ferroelectrics, which remains relatively unexplored in two-dimensional ferroelectric α-In2Se3. Interlayer interactions engineering via selecting the stacking order in two-dimensional materials allows modulation of ferroelectric properties. Here, we report stacking-dependent ferroelectric domain walls in 2H and 3R stacked α-In2Se3, elucidating the resistance switching mechanism in ferroelectric semiconductor-metal junction devices. In 3R α-In2Se3, the in-plane movement of out-of-plane ferroelectric domain walls yield a large hysteresis window. Conversely, 2H α-In2Se3 devices favor in-plane domain walls and out-of-plane domain wall motion, producing a small hysteresis window. High electric fields induce a ferro-paraelectric phase transition of In2Se3, where 3R In2Se3 reaches the transition through intralayer atomic gliding, while 2H In2Se3 undergoes a complex process comprising intralayer bond dissociation and interlayer bond reconstruction. Our findings demonstrate tunable ferroelectric properties via stacking configurations, offering an expanded dimension for material engineering in ferroelectric devices.
Charge carrier densities in electronic heterostructures are typically responsive to external electric fields or chemical doping but rarely to their magnetization history. Here, we demonstrate that magnetization acts as a non-volatile control parameter for the density of states in bilayer graphene (BLG) interfaced with the antiferromagnetic insulator chromium oxychloride (COC). Using capacitance measurements, we observe a hysteretic behavior in the density of states of BLG on a COC substrate in response to an external magnetic field, which is unrelated to the history of electrostatic gating. First-principles calculations revealed that such hysteresis arises from the magnetic-field-controlled charge transfer between BLG and COC during the antiferromagnetic (AFM) to ferrimagnetic-like (FiM) state phase transition of COC. Our work demonstrates that interfacial charging states can be effectively controlled magnetically, and it also shows that capacitance measurement is a suitable technique for detecting subtle changes not detectable via conventional resistivity measurements. These findings broaden the scope of proximity effects and open new possibilities for nanoelectronics applications.
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.
