Functionalized Tellurene; a candidate large-gap 2D Topological Insulator

The discovery of group IV and V elemental Xene’s with topologically non-trivial characters in their honeycomb lattice structure (HLS) has led to extensive efforts in realising analogous behaviour in group VI elemental monolayers. Theoretically; it was concluded that, group VI elemental monolayers cannot exist in HLS. However, some recent experimental evidence suggests that group VI elemental monolayers can be realised in HLS. In this letter, we report HLS of group VI elemental monolayer (such as, Tellurene) can be realised to be dynamically stable when functionzalised with Oxygen. The functionalization leads to, peculiar orbital filtering effects (OFE) and broken spatial inversion symmetry which gives rise to the non-trivial topological character. The exotic quantum behaviour of this system is characterized by, spin-orbit coupling induced large-gap (≈ 0.36 eV) with isolated Dirac cone along the edges indicating potential room temperature spin-transport applications. Further investigations of spin Hall conductivity and the Berry curvatures unravel high conductivity as compared to previously explored Xene’s alongside the potential valley Hall effects. The non-trivial topological character is quantified in terms of the Z2 invaraint as ν = 1 and Chern number C = 1. Also, for practical purposes, we report that, hBN/TeO/hBN quantum-wells can be strain engineered to realize a sizable nontrivial gap (≈ 0.11 eV). We finally conclude that, functionalization of group VI elemental monolayer with Oxygen gives rise to, exotic quantum properties which are robust against surface oxidation and degradations while providing viable electronic degrees of freedom for spintronic applications.

Influence of anisotropy, tilt and pairing of Weyl nodes: the Weyl semimetals TaAs, TaP, NbAs and NbP

By means of ab initio band structure methods and model Hamiltonians we investigate the electronic, spin and topological properties of four monopnictides crystallizing in bct structure. We show that the Weyl bands around a WP W1 or W2 possess a strong anisotropy and tilt of the accompanying Dirac cones. These effects are larger for W2 nodes than for W1 ones. The node tilts and positions in energy space significantly influence the DOS of single-particle Weyl excitations. The node anisotropies destroy the conventional picture of (anti)parallel spin and wave vector of a Weyl fermion. This also holds for the Berry curvature around a node, while the monopole charges are independent as integrated quantities. The pairing of the nodes strongly modifies the spin texture and the Berry curvature for wave vectors in between the two nodes. Spin components may change their orientation. Integrals over planes perpendicular to the connection line yield finite Zak phases and winding numbers for planes between the two nodes, thereby indicating the topological character. Graphical abstract

Homotopy Phases of FQHE with Long-Range Quantum Entanglement in Monolayer and Bilayer Hall Systems

Correlated phases in Hall systems have topological character. Multilayer configurations of planar electron systems create the opportunity to change topological phases on demand using macroscopic factors, such as vertical voltage. We present an analysis of such phenomena in close relation to recent experiments with multilayer Hall setups including GaAs and graphene multi-layers. The consequences of the blocking or not of the inter-layer electron tunneling in stacked Hall configurations are analyzed and presented in detail. Multilayer Hall systems are thus tunable topological composite nanomaterials, in the case of graphene-stacked systems by both intra- and inter-layer voltage.

Local Berry curvature signatures in dichroic angle-resolved photoelectron spectroscopy from two-dimensional materials

Topologically nontrivial two-dimensional materials hold great promise for next-generation optoelectronic applications. However, measuring the Hall or spin-Hall response is often a challenge and practically limited to the ground state. An experimental technique for tracing the topological character in a differential fashion would provide useful insights. In this work, we show that circular dichroism angle-resolved photoelectron spectroscopy provides a powerful tool that can resolve the topological and quantum-geometrical character in momentum space. In particular, we investigate how to map out the signatures of the momentum-resolved Berry curvature in two-dimensional materials by exploiting its intimate connection to the orbital polarization. A spin-resolved detection of the photoelectrons allows one to extend the approach to spin-Chern insulators. The present proposal can be extended to address topological properties in materials out of equilibrium in a time-resolved fashion.

Quantum spin liquids

Spin liquids are quantum phases of matter with a variety of unusual features arising from their topological character, including “fractionalization”—elementary excitations that behave as fractions of an electron. Although there is not yet universally accepted experimental evidence that establishes that any single material has a spin liquid ground state, in the past few years a number of materials have been shown to exhibit distinctive properties that are expected of a quantum spin liquid. Here, we review theoretical and experimental progress in this area.

Do We Really Understand Graphene Nanoribbons? A New Understanding of the 3n, 3n±1 Rule, Edge “Magnetism” and Much More

Using the inherent shell structure of graphene and geometrical/topological constrains, we verify that there are only three families of armchair graphene nanoribbons (AGNR) with Z zigzag edge-rings, categorized by Z=3n, 3n±1, n=1,2,… each with unique aromatic, electronic and topological properties. The Z=3n+1, 3n AGNR-families are aromatic with large bandgaps,characteristic aromaticity-patterns, and unique “active” frontier orbitals, in contrast to the ordinary ones. Such AGNRs due to sublattice/molecular-group symmetry-conflict develop 2n zigzag-edge-localized “gapless” frontier-states, which are “pseudospin-polarized” (not real-spin-polarized) with total pseudospin S=n, effectively optimizing sublattice “balance” and total energy. The “active” frontier orbitals, obtained after neglecting such gapless non-bonding states, have large “active” bandgaps, which are in very good agreement with experiment. The Z=3n-1 AGNRs have mixt aromatic, electronic and topological character, with vanishingly small bandgaps. Zigzag GNRs, contrary to opposite reports, have no magnetic zigzag edges, unless magnetization due to polarization of atomic pz orbital angular momentum is operative<br>

Do We Really Understand Graphene Nanoribbons? A New Understanding of the 3n, 3n±1 Rule, Edge “Magnetism” and Much More

Using the inherent shell structure of graphene and geometrical/topological constrains, we verify that there are only three families of armchair graphene nanoribbons (AGNR) with Z zigzag edge-rings, categorized by Z=3n, 3n±1, n=1,2,… each with unique aromatic, electronic and topological properties. The Z=3n+1, 3n AGNR-families are aromatic with large bandgaps,characteristic aromaticity-patterns, and unique “active” frontier orbitals, in contrast to the ordinary ones. Such AGNRs due to sublattice/molecular-group symmetry-conflict develop 2n zigzag-edge-localized “gapless” frontier-states, which are “pseudospin-polarized” (not real-spin-polarized) with total pseudospin S=n, effectively optimizing sublattice “balance” and total energy. The “active” frontier orbitals, obtained after neglecting such gapless non-bonding states, have large “active” bandgaps, which are in very good agreement with experiment. The Z=3n-1 AGNRs have mixt aromatic, electronic and topological character, with vanishingly small bandgaps. Zigzag GNRs, contrary to opposite reports, have no magnetic zigzag edges, unless magnetization due to polarization of atomic pz orbital angular momentum is operative<br>