We propose four different thermodynamically stable structural phases of arsenic monolayers based on ab-initio density functional theory calculations all of which undergo a topological phase transition on application of a perpendicular electric field. All the four arsenic monolayer allotropes have a wide band gap, varying from 1.21 eV to 3.0 eV (based on GW calculations), and in general they undergo a metal-insulator quantum phase transition on application of uniaxial in-layer strain. Additionally an increasing transverse electric field induces band-inversion at the Γ point in all four monolayer allotropes, leading to a nontrivial topological phase (insulating for three and metallic for one allotrope), characterized by the switching of the Z2 index, from 0 (before band inversion) to 1 (after band inversion). The topological phase tuned by the transverse electric field, should support spin-separated gapless edge states which should manifest in quantum spin Hall effect. II. LATTICE STRUCTURE, STABILITY AND BAND STRUCTUREThe equilibrium structure, stability and electronic properties are analyzed using the ab initio density functional theory (DFT) based calculations, using a plane-wave basis set and ultrasoft pseudopotentials, as implemented in Quantum Espresso 41 . Electron ex-arXiv:1604.03064v3 [cond-mat.mtrl-sci]

Kagome materials often host exotic quantum phases, including spin liquids, Chern gap, charge density wave, and superconductivity. Existing scanning microscopy studies of the kagome charge order have been limited to nonkagome surface layers. Here, we tunnel into the kagome lattice of FeGe to uncover features of the charge order. Our spectroscopic imaging identifies a 2 × 2 charge order in the magnetic kagome lattice, resembling that discovered in kagome superconductors. Spin mapping across steps of unit cell height demonstrates the existence of spin-polarized electrons with an antiferromagnetic stacking order. We further uncover the correlation between antiferromagnetism and charge order anisotropy, highlighting the unusual magnetic coupling of the charge order. Finally, we detect a pronounced edge state within the charge order energy gap, which is robust against the irregular shape fluctuations of the kagome lattice edges. We discuss our results with the theoretically considered topological features of the kagome charge order including unconventional magnetism and bulk-boundary correspondence.

Band-crossings occurring on a mirror plane are compelled to form a nodal loop in the momentum space without spin-orbit coupling (SOC). In the presence of other equivalent mirror planes, multiple such nodal loops can combine to form interesting nodal-link structures. Here, based on firstprinciples calculations and an effective k.p model analysis, we show that CaAuAs hosts a unique starfruit-like nodal-link structure in the bulk electronic dispersion in the absence of SOC. This nodal-link is comprised of three nodal loops, which cross each other at the time-reversal-invariant momentum point A. When the SOC is turned on, the nodal loops are gapped out, resulting in a stable Dirac semimetal state with a pair of Dirac points along the Γ − A direction in the Brillouin zone. The Dirac points are protected by the combination of time reversal, inversion, and C3 rotation symmetries. We show how a systematic elimination of the symmetry constraints yields a Weyl semimetal and eventually a topological insulator state. arXiv:1806.01858v1 [cond-mat.mtrl-sci]

Kagome-nets, appearing in electronic, photonic and cold-atom systems, host frustrated fermionic and bosonic excitations. However, it is rare to find a system to study their fermion–boson many-body interplay. Here we use state-of-the-art scanning tunneling microscopy/spectroscopy to discover unusual electronic coupling to flat-band phonons in a layered kagome paramagnet, CoSn. We image the kagome structure with unprecedented atomic resolution and observe the striking bosonic mode interacting with dispersive kagome electrons near the Fermi surface. At this mode energy, the fermionic quasi-particle dispersion exhibits a pronounced renormalization, signaling a giant coupling to bosons. Through the self-energy analysis, first-principles calculation, and a lattice vibration model, we present evidence that this mode arises from the geometrically frustrated phonon flat-band, which is the lattice bosonic analog of the kagome electron flat-band. Our findings provide the first example of kagome bosonic mode (flat-band phonon) in electronic excitations and its strong interaction with fermionic degrees of freedom in kagome-net materials.

Materials with triply-degenerate nodal points in their low-energy electronic spectrum produce crystalline-symmetry-enforced three-fold fermions, which conceptually lie between the two-fold Weyl and four-fold Dirac fermions. Here we show how a silver-based Dirac semimetal BaAgAs realizes three-fold fermions through our first-principles calculations combined with a low-energy effective k.p model Hamiltonian analysis. BaAgAs is shown to harbor triply-degenerate nodal points, which lie on its C3 rotation axis, and are protected by the C6v(C2 ⊗ C3v) point-group symmetry in the absence of spin-orbit coupling (SOC) effects. When the SOC is turned on, BaAgAs transitions into a nearly-ideal Dirac semimetal state with a pair of Dirac nodes lying on the C3 rotation axis. We show that breaking inversion symmetry in the BaAgAs1−xPx alloy yields a clean and tunable three-fold fermion semimetal. Systematic relaxation of other symmetries in BaAgAs generates a series of other topological phases. BaAgAs materials thus provide an ideal platform for exploring tunable topological properties associated with a variety of different fermionic excitations.Introduction.-Topological semimetals are currently drawing intense interest in condensed matter and materials physics [1][2][3]. In addition to their potential use as platforms for next-generation electronics/spintronics device applications, they provide a fertile ground for exploring relativistic particles and high-energy phenomenology at the far more accessible solid-state physics scale. Well-known examples are Dirac and Weyl semimetals in which electronic states near the band crossings mimic the Dirac [4][5][6][7][8][9][10][11][12][13] and Weyl fermions [14][15][16][17][18][19] familiar in the standard model of high-energy physics. Unlike highenergy physics, however, where particles are subject to the constraints of Poincaré symmetry, the fermions in condensed matter physics are less constrained in that they only need to respect the crystalline space-group symmetries. This easing of symmetry can lead to new fermionic particles at three-, six-, and eight-fold degenerate points in semimetals that have no high-energy counterparts [20][21][22][23][24][25][26]. In particular, three-fold fermions have been predicted in materials with triply-degenerate nodal-points (TPs) in the electronic spectrum. The triple-point semimetals (TPSs) exhibit unique topolog-