Monolayer AlB$_2$ is composed of two atomic layers: honeycomb borophene and triangular aluminum. In contrast with the bulk phase, monolayer AlB$_2$ is predicted to be a superconductor with a high critical temperature. Here, we demonstrate that monolayer AlB$_2$ can be synthesized on Al(111) via molecular beam epitaxy. Our theoretical calculations revealed that the monolayer AlB$_2$ hosts several Dirac cones along the $Γ$--M and $Γ$--K directions; these Dirac cones are protected by crystal symmetries and are thus resistant to external perturbations. The extraordinary electronic structure of the monolayer AlB$_2$ was confirmed via angle-resolved photoemission spectroscopy measurements. These results are likely to stimulate further research interest to explore the exotic properties arising from the interplay of Dirac fermions and superconductivity in two-dimensional materials.
Two-dimensional (2D) half-metallic materials are of great interest for their promising applications in spintronics. Although numerous of 2D half-metals have been proposed theoretically, rarely of them can be synthesized experimentally. Here, exemplified by monolayer FeCl2, we show three mechanisms in such quantum magnets that would cause the metal-insulator transition by using first-principles calculations. In particular, half-metallicity, especially that protected by symmetry-induced degeneracies, predicted by the previous theoretical simulations could be destroyed by electron correlation, sp…
Materials can, in principle, be imaged at the level of individual atoms with aberration corrected transmission electron microscopy. However, such resolution can be attained only with very high electron doses. Consequently, radiation damage is often the limiting factor when characterizing sensitive materials. Here, we demonstrate a simple and effective method to increase the electron radiation tolerance of materials by using graphene as protective coating. This leads to an improvement of three orders of magnitude in the radiation tolerance of monolayer MoS2. Further on, we construct samples in …
Nonsymmorphic space group symmetries can generate exotic band-crossings in topological metals and semimetals. Here, based on symmetry analysis and first-principles calculations, we reveal rich band-crossing features in the existing layered compounds Ta$_3$SiTe$_6$ and Nb$_3$SiTe$_6$, enabled by nonsymmorphic symmetries. We show that in the absence of spin-orbit coupling (SOC), these three-dimensional (3D) bulk materials possess accidental Dirac loops and essential fourfold nodal lines. In the presence of SOC, there emerges an hourglass Dirac loop---a fourfold degenerate nodal loop, on which ea…
The optical selection rules obeyed by two-dimensional materials with spin-valley coupling enable the selective excitation of carriers. We show that six members of the monolayer MA2Z4 (M = Mo and W; A = C, Si, and Ge; Z = N, P, and As) family are direct band-gap semiconductors with protected valley states and that circularly polarized infrared light can induce valley-selective inter-band transitions. Our optovalleytronic device demonstrates a close to 100% valley- and spin-polarized current under in-plane bias and circularly polarized infrared light, which can be exploited to encode, process, a…
The electronic structure of two-dimensional (2D) materials are inherently prone to environmental perturbations, which may pose significant challenges to their applications in electronic or optoelectronic devices. A 2D material couples with its environment through two mechanisms: local chemical coupling and nonlocal dielectric screening effects. The local chemical coupling is often difficult to predict or control experimentally. Nonlocal dielectric screening, on the other hand, can be tuned by choosing the substrates or layer thickness in a controllable manner. Therefore, a compelling 2D electr…
We report the development and implementation of hybrid methods that combine quantum mechanics (QM) with molecular mechanics (MM) to theoretically characterize thiolated gold clusters. We use, as training systems, structures such as Au25(SCH2-R)18 and Au38(SCH2-R)24, which can be readily compared with recent crystallographic data. We envision that such an approach will lead to an accurate description of key structural and electronic signatures at a fraction of the cost of a full quantum chemical treatment. As an example, we demonstrate that calculations of the 1H and 13C NMR shielding constants…
Monolayer WTe$_2$, a centrosymmetric transition metal dichacogenide, has recently been established as a quantum spin Hall insulator and found superconducting upon gating. Here we study the pairing symmetry and topological nature of superconducting WTe$_2$ with a microscopic model at mean-field level. Surprisingly, we find that the spin-triplet phases in our phase diagram all host Majorana modes localized on two opposite corners. Even when the conventional pairing is favored, we find that an intermediate in-plane magnetic field exceeding the Pauli limit stabilizes an unconventional equal-spin p…
Here we report the photoemission studies of intercalation process of Fe underneath graphene layer on Ni(111). The process of intercalation was monitored via XPS of corresponding core levels and UPS of the graphene-derived $π$ states in the valence band. \textit{fcc}-Fe films with thickness of 2-5 monolayers at the interface between graphene and Ni(111) form epitaxial magnetic layer passivated from the reactive environment, like for example oxygen gas.
We evidence the possibility for coherent electrical manipulation of the spin orientation of topologically protected edge states in a low-symmetry quantum spin Hall insulator. By using a combination of ab-initio simulations, symmetry-based modeling, and large-scale calculations of the spin Hall conductivity, it is shown that small electric fields can efficiently vary the spin textures of edge currents in monolayer 1T'-WTe2 by up to a 90-degree spin rotation, without jeopardizing their topological character. These findings suggest a new kind of gate-controllable spin-based device, topologically …
Epitaxial graphene on SiC possesses, quite remarkably, an electron spectrum similar to that of freestanding samples. Yet, the coupling to the substrate, albeit small, affects the quasiparticle properties. Combining \emph{ab initio} calculations with symmetry analysis, we derive a modified Dirac-Weyl Hamiltonian for graphene epilayers. While for the epilayer on the C-face the Dirac cone remains almost intact, for epilayers on the Si-face the band splitting is about 30\,meV. At certain energies, the Dirac bands are significantly distorted by the resonant interaction with interface states, which …
In this letter, we report the visualization of topologically protected spin textures, in the form of magnetic skyrmions, in recently discovered monoatomic-thin two-dimensional CrI$_3$. By combining density functional theory and atomistic spin dynamic simulation, we demonstrate that an application of out-of-plane electric field to CrI$_3$ lattice favors the formation of sub-10 nm skyrmions at 0 K temperature. The spin texture arises due to a strong correlation between magneto-crystalline anisotropy, Dzyaloshinskii-Moriya interaction and the vertical electric field, whose shape and size could be…
Based on first-principles calculations and effective model analysis, a Dirac nodal-net semimetal state is recognized in AlB$_2$-type TiB$_2$ and ZrB$_2$ when spin-orbit coupling (SOC) is ignored. Taking TiB$_2$ as an example, there are several topological excitations in this nodal-net structure including triple point, nexus, and nodal link, which are protected by coexistence of spatial-inversion symmetry and time reversal symmetry. This nodal-net state is remarkably different from that of IrF$_4$, which requires sublattice chiral symmetry. In addition, linearly and quadratically dispersed two-…
Ultrathick MA$_2$N$_4$(M'N) Intercalated Monolayers with Sublayer-Protected Fermi Surface Conduction States: Interconnect and Metal Contact Applications
Topological nodal line semimetals, hosting one-dimensional Fermi lines with symmetry protection, has become a hot topic in topological quantum matter. Due to the breaking of time reversal symmetry in magnetic system, nodal lines require protection by additional symmetries. Here, we report the discovery of antiferromagnetic type-I and type-II nodal lines coexist in the monolayer CrAs$_{2}$ based on a systematic first-principles calculation. Remarkably, the type-I nodal line in CrAs$_{2}$ form a concentric loop centered around the $Γ$ point is filling-enforced by nonsymmorphic analogue symmetry …
Two-dimensional (2D) topological crystalline insulators (TCIs) were recently predicted in thin films of the SnTe class of IV-VI semiconductors, which can host metallic edge states protected by mirror symmetry. As thickness decreases, quantum confinement effect will increase and surpass the inverted gap below a critical thickness, turning TCIs into normal insulators. Surprisingly, based on first-principles calculations, here we demonstrate that (001) monolayers of rocksalt IV-VI semiconductors XY (X=Ge, Sn, Pb and Y= S, Se, Te) are 2D TCIs with the fundamental band gap as large as 260 meV in mo…
The intriguing properties, especially Dirac physics in graphene, have inspired the pursuit of two-dimensional materials in honeycomb structure. Here we achieved a monolayer transition metal monochalcogenide AgTe on Ag(111) by tellurization of the substrate. High-resolution scanning tunneling microscopy, combined with low-energy electron diffraction, angle-resolved photoemission spectroscopy, and density functional theory calculations, demonstrates the planar honeycomb structure of AgTe. The first principle calculations further reveal that, protected by the in-plane mirror reflection symmetry, …
The search for new quantum spin Hall (QSH) phase and effective manipulations of their edge states are very important for both fundamental sciences and practical applications. Here, we use first-principles calculations to study the strain-driven topological phase transition of two-dimensional (2D) arsenene monolayer. We find that the band gap of arsenene decreases with increasing strain and changes from indirect to direct, and then the s-p band inversion takes place at Γ point as the tensile strain is larger than 11.14%, which lead to a nontrivially topological state. A single pair of topologic…
Designing two-dimensional (2D) materials with magnetic and topological properties has continuously attracted intense interest in fundamental science and potential applications. Here, on the basis of first-principles calculations, we predict the coexistence of antiferromagnetism and Dirac nodal loop (NL) in the monolayer MnB, where the band crossing points are very close to the Fermi Level. Remarkably, a moderate strain can induce an antiferromagnetic to ferromagnetic phase transition, driving the monolayer MnB to a ferromagnetic metal with Weyl NLs. Such a type of topological quantum phase tra…
Two dimensional electron gases (2DEGs) at a oxide heterostructures are attracting considerable attention, as these might substitute conventional semiconductors for novel electronic devices [1]. Here we present a minimal set-up for such a 2DEG -the SrTiO3(110)-(4 x 1) surface, natively terminated with one monolayer of chemically-inert titania. Oxygen vacancies induced by synchrotron radiation migrate under- neath this overlayer, this leads to a confining potential and electron doping such that a 2DEG develops. Our angular resolved photoemission spectroscopy (ARPES) and theoretical results show …
Topological nodal line semimetals, a novel quantum state of materials, possess topologically nontrivial valence and conduction bands that touch at a line near the Fermi level. The exotic band structure can lead to various novel properties, such as long-range Coulomb interaction and flat Landau levels. Recently, topological nodal lines have been observed in several bulk materials, such as PtSn4, ZrSiS, TlTaSe2 and PbTaSe2. However, in two-dimensional materials, experimental research on nodal line fermions is still lacking. Here, we report the discovery of two-dimensional Dirac nodal line fermio…
Xenes, graphene-like two-dimensional (2D) monoelemental crystals with a honeycomb symmetry, have been the focus of numerous experimental and theoretical studies. In comparison, single-element 2D materials with a triangular lattice symmetry have not received due attention. Here, taking Pb as an example, we investigate the triangular-lattice monolayer made of group-IV atoms employing first-principles density functional theory calculations. The flat Pb monolayer supports a mirror-symmetry-protected spinless nodal line in the absence spin-orbit coupling (SOC). The introduction of an out-of-plane b…
Topological materials with both insulating and semimetal phases can be protected by crystalline (e.g. mirror) symmetry. The insulating phase, called topological crystalline insulator (TCI), has been intensively investigated and observed in three-dimensional materials. However, the predicted two-dimensional (2D) materials with TCI phase are explored much less than 3D TCIs and 2D topological insulator, while so far considered 2D TCIs almost exclusively possess a square lattice structure with the mirror Chern number $\mathcal C_{M} =-2$. Here, we predict theoretically that hexagonal monolayer of …
Recently, a 2D orthorhombic silver-organic framework, Ag3C20 monolayer, was synthesized by assembling organic molecules linked with multiple aryl-metal bonds. Herein, via first-principles study, we demonstrate that owing to the unique bonding feature, Ag3C20 monolayer not only exhibits strong mechanical anisotropy, but also possesses various tunable direction-dependent Dirac states. Around the Fermi level blow, the intrinsic Dirac points form two antiparallel quasi type-III nodal lines protected by mirror symmetry, which can further evolve into hybrid nodal loops under tiny strains. Intriguing…
Quadratic band crossing points (QBCPs) and quantum anomalous Hall effect (QAHE) have attracted the attention of both theoretical and experimental researchers in recent years. Based on first-principle calculations, we find that the FeB$_2$ monolayer is a nonmagnetic semimetal with QBCPs at $K$. Through symmetry analysis and $\mathbf{k}\cdot\mathbf{p}$ invariant theory, we find that the QBCP is not protected by rotation symmetry and consists of two Dirac points with same chirality (Berry phase of $2π$). Once introducing Coulomb interactions, we find that there is a spontaneous-time-reversal-brea…
We perform both lattice dynamics analysis and molecular dynamics simulations to demonstrate the existence of topologically protected phonon modes in a two-dimensional, monolayer hexagonal boron nitride sheet. The topological phonon modes are found to be localized at an in-plane interface that divides the system into two regions of distinct valley Chern numbers. The dispersion of this topological phonon mode crosses over the frequency gap [1123, 1278] cm^{-1}, which is opened through analogy with the quantum valley Hall effect by breaking inversion symmetry of the boron and nitride atoms in the…
Based on first-principles calculations and tight-binding model analysis, we propose monolayer graphdiyne as a candidate material for a two-dimensional higher-order topological insulator protected by inversion symmetry. Despite the absence of chiral symmetry, the higher-order topology of monolayer graphdiyne is manifested in the filling anomaly and charge accumulation at two corners. Although its low energy band structure can be properly described by the tight-binding Hamiltonian constructed by using only the $p_z$ orbital of each atom, the corresponding bulk band topology is trivial. The nontr…
Strain-tunable multipiezo effects in Janus monolayer Cr2SSe: Selective reversal of valley polarization and single-spin-channel anomalous valley Hall effect
Using density functional theory calculations and the Greens's function formalism, we report the existence of magnetic edge states with a non-collinear spin texture present on different edges of the 1T' phase of the three monolayer transition metal dichalcogenides (TMDs): MoS$_2$, MoTe$_2$ and WTe$_2$. The magnetic states are gapless and accompanied by a spontaneous breaking of the time-reversal symmetry. This may have an impact on the prospects of utilizing WTe$_2$ as a quantum spin Hall insulator. It has previously been suggested that the topologically protected edge states of the 1T' TMDs co…
Symmetry-Protected Phonon Topology and Low Lattice Thermal Conductivity in Square-Octagonal Chalcogenides
Khalifa University（哈利法大学）；Seoul National University（首尔国立大学）；Indian Institute of Technology, Bombay（印度理工学院孟买分校）；Research and Innovation Center for Graphene and 2D materials (RIC2D)（石墨烯与二维材料研究与创新中心）
Exciton polaritons in atomically-thin transition metal dichalcogenide crystals (monolayer TMDCs) have emerged as a promising candidate to enable topological transport, ultra-efficient laser technologies, and collective quantum phenomena such as polariton condensation and superfluidity at room temperature. However, integrating monolayer TMDCs into high-quality planar microcavities to achieve the required strong coupling between the cavity photons and the TMDC excitons (bound electron-hole pairs) has proven challenging. Previous approaches to integration had to compromise between various adverse…
The hourglass fermions in solid-state materials have been attracting significant interest recently. However, realistic two-dimensional (2D) materials with hourglass-shaped band structures are still very scarce. Here, through the first-principles calculations, we identify the monolayer Pb(ClO$_{2}$)$_{2}$ and Sr(ClO$_{2}$)$_{2}$ materials as the new realistic materials platform to realize 2D hourglass Weyl nodal loop. We show that these monolayer materials possess an hourglass Weyl nodal loop circling around the $Γ$ point and Weyl nodal line on the Brillouin zone (BZ) boundary in the absence of…
We propose a new topological quantum state of matter---the two-dimensional (2D) Weyl half semimetal (WHS), which features 2D Weyl points at Fermi level belonging to a single spin channel, such that the low-energy electrons are described by fully spin-polarized 2D Weyl fermions. We predict its realization in the ground state of monolayer PtCl$_3$. We show that the material is a half metal with an in-plane magnetization, and its Fermi surface consists of a pair of fully spin-polarized Weyl points protected by a mirror symmetry, which are robust against spin-orbit coupling. Remarkably, we show th…
The topological edge states of two-dimensional topological insulators with large energy gap furnish ideal conduction channels for dissipationless current transport. Transition metal tellurides XTe5 (X=Zr, Hf) are theoretically predicted to be large-gap two-dimensional topological insulators and the experimental observations of their bulk insulating gap and in-gap edge states have been reported, but the topological nature of these edge states still remains to be further elucidated. Here, we report our low temperature scanning tunneling microscopy/spectroscopy study on single crystals of HfTe5. …
A formalism is presented for treating strongly-correlated graphene quantum Hall states in terms of an SO(8) fermion dynamical symmetry that includes pairing as well as particle--hole generators. The graphene SO(8) algebra is isomorphic to an SO(8) algebra that has found broad application in nuclear physics, albeit with physically very different generators, and exhibits a strong formal similarity to SU(4) symmetries that have been proposed to describe high-temperature superconductors. The well-known SU(4) symmetry of quantum Hall ferromagnetism for single-layer graphene is recovered as one subg…
Majorana zero-energy bound states (ZEBSs) have been proposed to exist at the ends of one-dimensional Rashba nanowires proximity-coupled to an s-wave superconductor in an external magnetic field induced Zeeman field. Such hybrid structures have been a central platform in the search for non-Abelian Majorana zero modes (MZMs) toward fault-tolerant topological quantum computing. Here we report the discovery of ZEBSs simultaneously appearing at each end of a one-dimensional atomic line defect in monolayer iron-based high-temperature superconductor FeTe0.5Se0.5 films grown on SrTiO3(001) substrates.…
Recently, the crystal symmetry-protected topological semimetals have aroused extensive interests, especially for the nonsymmorphic symmetry-protected one. We list the possible nonmagnetic topological semimetals and develop their k.p Hamiltonian in all layer groups with multiple screw axes in the absence of spin-orbital coupling. We find a novel cat's cradle-like topological semimetal phase, which looks like multiple hourglass-like band structures staggered together. Furthermore, we propose the monolayer borophene and borophane with pmmn layer group as the first material class to realize such n…
Based on first-principles calculations, we predict that the superconducting MgB$_2$ with a AlB$_2$-type centrosymmetric lattice host the so-called phononic topological Weyl nodal lines (PTWNLs) on its bulk phonon spectrum. These PTWNLs can be viewed as countless Weyl points (WPs) closely aligned along the straight lines in the $-$H-K-H direction within the three-dimensional Brillouin zone (BZ). Their topological non-trivial natures are confirmed by the calculated Berry curvature distributions on the planes perpendicular to these lines. These lines are highly unique, because they exactly locate…
The electronic band topology of monolayer $β$-Sb (antimonene) is studied from the flat honeycomb to the equilibrium buckled structure using first-principles calculations and analyzed using a tight-binding model and low energy Hamiltonians. In flat monolayer Sb, the Fermi level occurs near the intersection of two warped Dirac cones, one associated with the $p_z$-orbitals, and one with the $\{p_x,p_y\}$-orbitals. The differently oriented threefold warping of these two cones leads to an unusually shaped nodal line, which leads to anisotropic in-plane transport properties and goniopolarity. A slig…
Two-dimensional transition metal dichalcogenides with strong spin-orbit interactions and valley-dependent Berry curvature effects have attracted tremendous recent interests. Although novel single-particle and excitonic phenomena related to spin-valley coupling have been extensively studied, effects of spin-momentum locking on collective quantum phenomena remain unexplored. Here we report an observation of superconducting monolayer NbSe$_2$ with an in-plane upper critical field over six times of the Pauli paramagnetic limit by magneto-transport measurements. The effect can be understood in term…
Probing the Polarization of Low-Energy Excitations in 2D Materials from Atomic Crystals to Nanophotonic Arrays using Momentum-Resolved Electron Energy Loss Spectroscopy
Floquet-Engineered Odd-Parity Altermagnetic Higher-Order Topology in a Two-Dimensional Antiferromagnet Cr$_2$CH$_2$
Institute for Basic Science (IBS)（韩国基础科学研究院）；Center for 2D Quantum Heterostructures (2DQH)（二维量子异质结中心）；Sungkyunkwan University（成均馆大学）；Shandong University（山东大学）；Chungnam National University（忠南国立大学）；Institute for Sciences of the Universe, Chungnam National University（忠南国立大学宇宙科学研究所）
Based on first-principles calculations and symmetry analysis, we predict atomically thin ($1-N$ layers) 2H group-VIB TMDs $MX_2$ ($M$ = Mo, W; $X$ = S, Se, Te) are large-gap higher-order topological crystalline insulators protected by $C_3$ rotation symmetry. We explicitly demonstrate the nontrivial topological indices and existence of the hallmark corner states with quantized fractional charge for these familiar TMDs with large bulk optical band gaps ($1.64-1.95$ eV for the monolayers), which would facilitate the experimental detection by STM. We find that the well-defined corner states exist…
