Density functional theory calculations are carried out to study the electronic and topological properties of $M$P$X_3$ ($M$ = Mn, Fe, Co, Ni, and $X$ = S, Se) monolayers in the ferromagnetic (FM) metastable magnetic state. We find that FM MnPSe$_3$ monolayers host topological semimetal signatures that are gapped out when spin-orbit coupling (SOC) is included. These findings are supported by explicit calculations of the Berry curvature and the Chern number. The choice of the Hubbard-$U$ parameter to describe the $d$-electrons is thoroughly discussed, as well as the influence of using a hybrid-functional approach. The presence of band inversions and the associated topological features are found to be formalism-dependent. Nevertheless, routes to achieve the topological phase via the application of external biaxial strain are demonstrated. Within the hybrid-functional picture, topological band structures are recovered under a pressure of 15% (17 GPa). The present work provides a potential avenue for uncovering new topological phases in metastable ferromagnetic phases.
Layered transition metal trichalcogenides MPX$_3$ (M: transition metal; X: S, Se) demonstrate a wide spectrum of properties and are widely proposed as effective materials for the water splitting reactions. Among these materials, NiPX$_3$ are the most promising ones due to the match their electronic structures for the oxygen and hydrogen evolution reactions. Here, we present first steps of a detailed theoretical description on the adsorption of water molecules on pristine and defected (chalcogen vacancies) surfaces of NiPX$_3$ and show that in all cases a physisorption takes the place with adso…
Two-dimensional (2D) transition metal dichalcogenides MX2 (M = Mo, W, X = S, Se, Te) attracts enormous research interests in recent years. Its 2H phase possesses an indirect to direct bandgap transition in 2D limit, and thus shows great application potentials in optoelectronic devices [1]. The 1T' crystalline phase transition can drive the monolayer MX2 to be a 2D topological insulator. Here we realized the molecular beam epitaxial (MBE) growth of both the 1T' and 2H phase monolayer WSe2 on bilayer graphene (BLG) substrate. The crystalline structures of these two phases were characterized usin…
Twelve two-dimensional group-IV monochalcogenide monolayers (SiS, SiSe, SiTe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe) with a buckled honeycomb atomistic structure--belonging to symmetry group P3m1--and an out-of-plane intrinsic electric polarization are shown to be metastable by three independendent methods. First, we uncover a coordination-preserving structural transformation from the low-buckled honeycomb structure onto the lower-energy Pnm2$_1$ (or Pmmn for PbS, PbSe, and PbTe) phase to estimate {\em energy barriers} $E_B$ that must be overcome during such structural transfor…
From ferromagnetic semiconductor to anti-ferromagnetic metal in epitaxial Cr$_x$Te$_y$ monolayers
University of St Andrews（圣安德鲁斯大学）；STFC Central Laser Facility, Research Complex at Harwell（STFC中央激光设施，哈韦尔研究综合体）；Diamond Light Source（钻石光源）；Universität Bonn（波恩大学）
We report results of density-functional theory calculations on the structural, magnetic, and electronic properties of (1x1)-structures of Co on Cu(001) for coverages up to two monolayers. In particular we discuss the tendency towards phase separation in Co islands and the possibility of segregation of Cu on top of the Co-film. A sandwich structure consisting of a bilayer Co-film covered by 1ML of Cu is found to be the lowest-energy configuration. We also discuss a bilayer c(2x2)-alloy which may form due to kinetic reasons, or be stabilized at strained surface regions. Furthermore, we study the…
Density-functional calculations are used to identify one-atom-thick metallic In overlayers on the Si(111) surface, which have long been sought in quest of the ultimate two-dimensional (2D) limit of free-electron-like metallic properties. We predict two metastable single-layer In phases, one $\sqrt{7}\times\sqrt{3}$ phase with a coverage of 1.4 monolayer (ML; here 1 ML refers to one In atom per top Si atom) and the other $\sqrt{7}\times\sqrt{7}$ phase with 1.43 ML, which indeed match well with experimental evidences. Both phases reveal quasi-1D arrangements of protruded In atoms, leading to 2D-…
Monolayer (ML) transition-metal dichalcogenides exist in different phases, such as the hexagonal (2H) and distorted octahedral or monoclinic (1T') phases. The different structures show vastly different properties. For example, the 2H MoTe2 ML is a direct-gap semiconductor while 1T' MoTe2 is a semi-metal. It has been suggested that the formation energies between 2H and 1T' MoTe2 differ very little, so there is a high chance to tune the structures of MoTe2 and thereby to bring in new applications such as phase-change electronics. In this work, we report growth of both 2H and 1T' MoTe2 ML by mole…
Bottom-up Synthesis of Metastable 2D Hexagonal Copper(I) Iodide on Monolayer and within Bilayer Graphene
Friedrich Schiller University Jena（耶拿弗里德里希·席勒大学）；Leibniz Institute of Photonic Technology (Leibniz-IPHT)（莱布尼茨光子技术研究所）；Ulm University（乌尔姆大学）；University of Nottingham（诺丁汉大学）；Freie Universität Berlin（柏林自由大学）；Leibniz Institute of Surface Engineering (IOM)（莱布尼茨表面工程研究所）
Using Kohn-Sham wave functions and their energy levels obtained by density-functional-theory total-energy calculations, the electronic structure of the two c(2x2) phases of Na on Al(001) are analysed; namely, the metastable hollow-site structure formed when adsorption takes place at low temperature, and the stable substitutional structure appearing when the substrate is heated thereafter above ca. 180K or when adsorption takes place at room temperature from the beginning. The experimentally obtained two-dimensional band structures of the surface states or resonances are well reproduced by the …
Two-dimensional materials that exhibit spontaneous electric polarization are of notable interest for functional materials. However, despite many two-dimensional polar materials are predicted in theory, the number of experimentally confirmed two-dimensional ferroelectrics are still far less than bulk ferroelectrics. We provide strong evidence that the Pmn2$_1$ phase of arsenic chalcogenides As$_2$X$_3$ (X=S, Se, and Te), which include the recently isolated monolayer orpiment, are intrinsic ferroelectrics and demonstrate strong in-plane piezoelectricity. We found the calculated energy barriers f…
Bimetal transition iodides in two-dimensional scale provide an interesting idea to combine a set of single-transition-metal ferromagnetic semiconductors together. Motivated by structural engineering on bilayer CrI$_3$ to tune its magnetism and works that realize ideal properties by stacking van der Waals transitional metal dichalcogenides in a certain order. Here we stack monolayer VI$_3$ onto monolayer CrI$_3$ with a middle-layer I atoms discarded to construct monolayer V$_2$Cr$_2$I$_9$. Based on this crystal model, the stable and metastable phases are determined among 7 possible phases by fi…
Twist-induced Out-of-plane Ferroelectricity in Bilayer Hafnia
Institute of Materials Science and Engineering, Washington University in St. Louis（华盛顿大学圣路易斯分校材料科学与工程研究所）；Department of Mechanical Engineering and Material Science, Washington University in St. Louis（华盛顿大学圣路易斯分校机械工程和材料科学系）；Department of Physics and Astronomy, University of Nebraska, Lincoln（内布拉斯加大学林肯分校物理与天文学系）；Department of Physics, Washington University in St. Louis（华盛顿大学圣路易斯分校物理系）
Room-temperature multistage metastability in a moiré superstructure
Tsung-Dao Lee Institute, School of Physics and Astronomy, Shanghai Jiao Tong University（上海交大李政道理论与计算科学中心，物理与天文学院）；Massachusetts Institute of Technology（麻省理工学院）；Stanford University（斯坦福大学）；Peking University（北京大学）；Beijing Academy of Quantum Information Sciences（北京量子信息科学研究院）；Tianjin University of Technology（天津理工大学）
Based on the fact that the underpotential electrochemical deposition of Ag atoms on the $Au(100)$ surface exhibits sharp first-order phase transitions at well-defined values of the (coexistence) chemical potential ($μ_{coex}$), we performed extensive simulations aimed at investigating the hysteretic dynamic behavior of the system close to coexistence upon the application of a periodic signal of the form $μ(t) = μ_{coex} + μ_{o}sin(2Πt/τ) $, where $μ_{o}$ and $τ$ are the amplitude and the period of the sweep, respectively. For relatively short periods and small enough amplitudes the system beco…
