Search

We present the experimental realization of tunable honeycomb superlattice plasma photonic crystals (PPCs) in dielectric barrier discharge by utilizing mesh-liquid electrodes. Fast reconfiguration among the simple honeycomb lattice, honeycomb superlattice, and honeycomb-snowflake superlattice is achieved. A dynamic control on the sizes of center scattering elements in the honeycomb superlattice has been realized. A phenomenological activator-inhibitor reaction diffusion model is established to demonstrate the formation and reconstruction of the honeycomb superlattice. The simulations reproduce well the experimental observations. The photonic band diagrams of different honeycomb PPCs are studied by using the finite element method. The addition of large center elements in honeycomb superlattice yields remarkable omnidirectional band gaps that are about 2.5 times larger than in the simple honeycomb lattice. We propose an effective scheme to fabricate spatiotemporally controllable honeycomb lattices that enable great improvement in band gap size and dynamic control of microwave radiations for wide applications. [ABSTRACT FROM AUTHOR]

Dumbbell-like structures are recently found to be energetically favored in group IV two-dimensional (2D) materials, exhibiting rich physics and many interesting properties. In this paper, using first-principles calculations, we have investigated the oxidized form of the hexagonal honeycomb (ODB-h) and zigzag dumbbell silicene (ODB-z). We confirm that both oxidization processes are energetically favorable, and their phonon spectra further demonstrate the dynamic stability. Contrary to the pristine dumbbell silicene structures (PDB-h and PDB-z silicene), these oxidized products ODB-h and ODB-z silicene are both semimetals with Dirac cones at the Fermi level. The Dirac cones of ODB-h and ODB-z silicene are at the K point and between Y and Γ points respectively, possessing high Fermi velocities of 3.1 × 105 m s−1 (ODB-h) and 2.9–3.4 × 105 m s−1 (ODB-z). The origin of the Dirac cones is further explained by tight-binding models. The semimetallic properties of ODB-h and ODB-z are sensitive to compression due to the self-absorption effect, but quite robust against the tensile strain. These outstanding properties make oxidized dumbbell silicene a promising material for quantum computing and high-speed electronic devices. [ABSTRACT FROM AUTHOR]

There has been growing interest in revealing exotic properties of chiral phonons since they were found in honeycomb AB lattice, and very recently they were experimentally verified in a tungsten diselenide monolayer (2018 Science 359 579). In this work, we manipulate phonon chirality through interface transmission via a one-dimensional atomic junction model by using the scattering boundary method. Due to the difference of phase change between two transverse directions induced by the anisotropy at interface coupling, the phonon polarization can be tuned between circular and linear in the highfrequency range. In a double-junction atomic model with an anisotropic center, we find that the phase change accumulates when the phonon transmits through the interface material thus the phonon can be tuned between different chirality in the medium frequency range. The phase change is found to linearly depend on the width of the interface material, while the transmission coefficient vibrates. To obtain the same value of the transmission coefficients along the two transverse directions and thus to keep the outgoing phonon circularly polarized, we can connect two interface materials with opposite anisotropy, where the phase-change difference for chirality tuning can be adjusted by the difference of widths of the two materials. Therefore, by using the atomic junction model, we find that the phonon chirality can be effectively tuned through interfaces, which is helpful for the manipulation and application of chiral phonons. [ABSTRACT FROM AUTHOR]

Silicene consists of a monolayer of silicon atoms in a buckled honeycomb structure. It was recently discovered that the symmetry of such a system allows for interesting Rashba spin–orbit effects. A perpendicular electric field is able to couple to the sublattice pseudospin, making it possible to electrically tune and close the band gap. Therefore, external electric fields may generate a topological phase transition from a topological insulator to a normal insulator (or semimetal) and vice versa. The contribution of the present paper to the study of silicene is twofold. Firstly, we perform a group theoretical analysis to systematically construct the Hamiltonian in the vicinity of the K points of the Brillouin zone and find an additional, electric field induced spin–orbit term, that is allowed by symmetry. Subsequently, we identify a tight-binding model that corresponds to the group theoretically derived Hamiltonian near the K points. Secondly, we start from this tight-binding model to analyze the topological phase diagram of silicene by an explicit calculation of the ℤ2 topological invariant of the band structure. To this end, we calculate the ℤ2 topological invariant of the honeycomb lattice in a manifestly gauge invariant way which allows us to include Sz symmetry breaking terms—like Rashba spin–orbit interaction—into the topological analysis. Interestingly, we find that the interplay of a Rashba and an intrinsic spin–orbit term can generate a non-trivial quantum spin Hall phase in silicene. This is in sharp contrast to the more extensively studied honeycomb system graphene where Rashba spin–orbit interaction is known to compete with the quantum spin Hall effect in a detrimental way. [ABSTRACT FROM AUTHOR]

We propose a scheme of band engineering by means of staggered electric potential, antiferromagnetic exchange field and intrinsic spin-orbit coupling for electrons on a honeycomb lattice. With fine control on the degrees of freedom of spin, sublattice and valley, one can achieve a topological state with simultaneous non-zero charge and spin Chern numbers. With first principles calculations, we confirm that the scheme can be realized by material modification in perovskite G-type antiferromagnetic insulators grown along the [111] direction, where Dirac electrons from d orbits are achieved on an atomic sheet of a buckled honeycomb lattice. In a finite sample, this state provides a spin-polarized quantum edge current, robust to both non-magnetic and magnetic defects, with the spin polarization reversible by electric field, and is thus ideal for spintronics applications. [ABSTRACT FROM AUTHOR]

In 1935, Pauling estimated the residual entropy of water ice with remarkable accuracy by considering the degeneracy of the ice rule solely at the vertex level. Indeed, his estimate works well for both the three-dimensional pyrochlore lattice and the two-dimensional six-vertex model, solved by Lieb in 1967. A similar estimate can be done for the honeycomb artificial spin. Indeed, its pseudo-ice rule, like the ice rule in Pauling and Lieb's systems, simply extends to the global ground state a degeneracy which is already present in the vertices. Unfortunately, the anisotropy of the magnetic interaction limits the design of inherently degenerate vertices in artificial spin ice, and the honeycomb is the only degenerate array produced so far. In this paper we show how to engineer artificial spin ice in a virtually infinite variety of degenerate geometries built out of non-degenerate vertices. In this new class of vertex models, the residual entropy follows not from a freedom of choice at the vertex level, but from the nontrivial relative arrangement of the vertices themselves. In such arrays not all of the vertices can be chosen in their lowest energy configuration. They are therefore vertex-frustrated and contain unhappy vertices. This can lead to residual entropy and to a variety of exotic states, such as sliding phases, smectic phases and emerging chirality. These new geometries will finally allow for the fabrication of many novel, extensively degenerate versions of artificial spin ice. [ABSTRACT FROM AUTHOR]

Ultracold fermions trapped in a honeycomb optical lattice constitute a versatile setup to experimentally realize the Haldane model (1988 Phys. Rev. Lett. 61 2015). In this system, a non-uniform synthetic magnetic flux can be engineered through laser-induced methods, explicitly breaking time-reversal symmetry. This potentially opens a bulk gap in the energy spectrum, which is associated with a non-trivial topological order, i.e. a non-zero Chern number. In this paper, we consider the possibility of producing and identifying such a robust Chern insulator in the laser-coupled honeycomb lattice. We explore a large parameter space spanned by experimentally controllable parameters and obtain a variety of phase diagrams, clearly identifying the accessible topologically non-trivial regimes. We discuss the signatures of Chern insulators in coldatom systems, considering available detection methods. We also highlight the existence of topological semi-metals in this system, which are gapless phases characterized by non-zero winding numbers, not present in Haldane's original model. [ABSTRACT FROM AUTHOR]

The nature of the effective spin Hamiltonian and magnetic order in the honeycomb iridates is explored by considering a trigonal crystal field effect and spin-orbit (SO) coupling. Starting from a Hubbard model, an effective spin Hamiltonian is derived in terms of an emergent pseudo-spin-1/2 moment in the limit of large trigonal distortions and SO coupling. The present pseudo-spins arise from a spin-orbital locking and are different from the jeff = 1/2 moments that are obtained when the SO coupling dominates and trigonal distortions are neglected. The resulting spin Hamiltonian is anisotropic and frustrated by further neighbour interactions. Mean-field theory suggests a ground state with foursublattice zigzag magnetic order in a parameter regime that can be relevant to the honeycomb iridate compound Na2IrO3, where a similar magnetic ground state has recently been observed. Various properties of the phase, the spin-wave spectrum and experimental consequences are discussed. The present approach contrasts with the recent proposals to understand iridate compounds starting from the strong SO coupling limit and neglecting non-cubic lattice distortions. [ABSTRACT FROM AUTHOR]

Pristine, single-crystalline graphene displays a unique collection of remarkable electronic properties that arise from its two-dimensional, honeycomb structure. Using in situ low-energy electron microscopy, we show that when deposited on the (111) surface of Au carbon forms such a structure. The resulting monolayer, epitaxial film is formed by the coalescence of dendritic graphene islands that nucleate at a high density. Over 95% of these islands can be identically aligned with respect to each other and to the Au substrate. Remarkably, the dominant island orientation is not the better lattice-matched 30° rotated orientation but instead one in which the graphene [01] and Au [011] inplane directions are parallel. The epitaxial graphene film is only weakly coupled to the Au surface, which maintains its reconstruction under the slightly p-type doped graphene. The linear electronic dispersion characteristic of free-standing graphene is retained regardless of orientation. That a weakly interacting, nonlattice matched substrate is able to lock graphene into a particular orientation is surprising. This ability, however, makes Au(111) a promising substrate for the growth of single crystalline graphene films. [ABSTRACT FROM AUTHOR]

We model the dynamics of magnetization in an artificial analogue of spin ice specializing to the case of a honeycomb network of connected magnetic nanowires. The inherently dissipative dynamics is mediated by the emission and absorption of domain walls in the sites of the lattice, and their propagation in its links. These domain walls carry two natural units of magnetic charge, whereas sites of the lattice contain a unit magnetic charge. Magnetostatic Coulomb forces between these charges play a major role in the physics of the system, as does quenched disorder caused by imperfections of the lattice. We identify and describe different regimes of magnetization reversal in an applied magnetic field determined by the orientation of the applied field with respect to the initial magnetization. One of the regimes is characterized by magnetic avalanches with a 1/n distribution of lengths. [ABSTRACT FROM AUTHOR]

We propose a model to predict and control the statistical ensemble of magnetic degrees of freedom in artificial spin ice (ASI) during thermalized adiabatic growth (Wang et al 2006 Nature 439 303; Morgan et al Nature Phys. 7 75). We predict that as-grown arrays are controlled by the temperature at fabrication and by their lattice constant, and that they can be described by an effective temperature. If the geometry is conducive to a phase transition, then the lowest-temperature phase is accessed in arrays of lattice constant smaller than a critical value, which depends on the temperature at deposition. Alternatively, for arrays of equal lattice constant, there is a temperature threshold at deposition and the lowest-temperature phase is accessed for fabrication temperatures larger rather than smaller than this temperature threshold. Finally, we show how to define and control the effective temperature of the as-grown array and how to measure critical exponents directly. We discuss the role of kinetics at the critical point, and applications to experiments, in particular to as-grown thermalized square ASI and to magnetic monopole crystallization in as-grown honeycomb ASI. [ABSTRACT FROM AUTHOR]

Dipolar spin ice has attracted much attention because of its intriguing ground state ordering and elementary excitation properties. We present experimental realizations of magnetic dipolar spin ice on periodic lattices with honeycomb symmetry. We have analyzed in particular the evolution and distribution of excitations with magnetic charges ±3 per vertex as a function of magnetic field and the distance b between the dipoles ranging from b = 0.4 to 1.7μm. In all the dipole patterns investigated, we observe a surprisingly high abundance of ±3 magnetic charges at coercivity in the descending and ascending branches of the magnetic hysteresis. At the same time, these ±3 vertices form a charge ordered state with large domains, resembling an ionic crystal. Monte Carlo simulations of the magnetization reversal confirm in the framework of a macrospin model an enhanced abundance of ±3 magnetic charges at the coercive field. But much better agreement is achieved by taking into account the micromagnetic reversal mechanism, which proceeds via nucleation and domain wall propagation for dipoles aligned with the field and via coherent rotation for all others. [ABSTRACT FROM AUTHOR]

Magnetic monopoles have stimulated a great amount of theoretical and experimental interest since their prediction by Dirac in 1931. To date, their presence has evaded detection in high energy experiments despite intensive efforts. Recently, entities that mimic magnetic monopoles have been observed in bulk and planar frustrated materials known as spin-ice materials, and artificial spin-ice materials, respectively. In this paper we discuss the formation of these so-called monopole defects within a cobalt honeycomb artificial spin-ice lattice. Experimental results and micromagnetic simulations show that monopole defects of opposite sign are created at the boundaries of the lattice, and move in opposing directions. Discrepancies between simulations and experimental results demonstrate the importance of quenched disorder. Furthermore, we show that controlled edge nucleated monopole defect formation can be realized with the use of soft magnetic injection pads, which is a very promising development for technological applications based upon magnetic charge. [ABSTRACT FROM AUTHOR]

We study the phase diagram of ultra-cold bosonic polar molecules loaded on a two-dimensional optical lattice of hexagonal symmetry controlled by external electric and microwave fields. Following a recent proposal in (Büchler et al 2007 Nat. Phys. 3 726), such a system is described by an extended Bose-Hubbard model of hard-core bosons that includes both extended two- and three-body repulsions. Using quantum Monte-Carlo simulations, exact finite cluster calculations and the tensor network renormalization group, we explore the rich phase diagram of this system, resulting from the strongly competing nature of the three-body repulsions on the honeycomb lattice. Already in the classical limit, they induce complex solid states with large unit cells and macroscopic ground-state degeneracies at different fractional lattice fillings. For the quantum regime, we obtain effective descriptions of the various phases in terms of emerging valence bond crystal states and quantum dimer models. Furthermore, we access the experimentally relevant parameter regime and determine the stability of the crystalline phases towards strong two-body interactions. [ABSTRACT FROM AUTHOR]

We propose a mean to obtain computationally useful resource states also known as cluster states, for measurement-based quantum computation, via transitionless quantum driving algorithm. The idea is to cool the system to its unique ground state and tune some control parameters to arrive at computationally useful resource state, which is in one of the degenerate ground states. Even though there is set of conserved quantities already present in the model Hamiltonian, which prevents the instantaneous state to go to any other eigenstate subspaces, one cannot quench the control parameters to get the desired state. In that case, the state will not evolve. With involvement of the shortcut Hamiltonian, we obtain cluster states in fast-forward manner. We elaborate our proposal in the one-dimensional Kitaev honeycomb model, and show that the auxiliary Hamiltonian needed for the counterdiabatic driving is of M-body interaction. [ABSTRACT FROM AUTHOR]

We show that spin–orbit interaction and elastic spin-Hall effect can exist in a classical mechanical system consisting of a two-dimensional honeycomb lattice of masses and springs. The band structure shows the presence of splitting at K point induced by the difference of longitudinal and transverse elastic constant, and this splitting can be regarded as an effective Dresselhaus-type spin–orbit coupling. Interestingly, as an initial displacement away from the equilibrium is applied, the time evolution simulation shows that waves of different spin polarization propagates along different directions at the Γ and K point, which is characteristic of spin-Hall effect. Several cases for spin-Hall effect are also discussed. [ABSTRACT FROM AUTHOR]

The article presents a reprint of the article "Graphene: The Magic of Flat Carbon," by Konstantin S. Novoselov, which appeared in the 2009 issue of "ECS Transactions." It explores graphene as a two-dimensional atomic crystal and a mono-atomically thin two-dimensional (2D) film of carbon atoms organized laterally in a honeycomb benzene ring-like structure. It says that graphene is used as the base material for an electronic circuitry envisaged to comprise of nanometer-sized quantum dots.

Quantum spin models with spatially dependent interactions, known as compass models, play an important role in the study of frustrated quantum magnetism. One example is the Kitaev model on the honeycomb lattice with spin-liquid (SL) ground states and anyonic excitations. Another example is the geometrically frustrated quantum 120° model on the same lattice whose ground state has not been unambiguously established. To generalize the Kitaev model beyond the exactly solvable limit and connect it with other compass models, we propose a new model, dubbed ‘the tripod model’, which contains a continuum of compass-type models. It smoothly interpolates the Ising model, the Kitaev model, and the quantum 120° model by tuning a single parameter , the angle between the three legs of a tripod in the spin space. Hence it not only unifies three paradigmatic spin models, but also enables the study of their quantum phase transitions. We obtain the phase diagram of the tripod model numerically by tensor networks in the thermodynamic limit. We show that the ground state of the quantum 120° model has long-range dimer order. Moreover, we find an extended spin-disordered (SL) phase between the dimer phase and an antiferromagnetic phase. The unification and solution of a continuum of frustrated spin models as outline here may be useful to exploring new domains of other quantum spin or orbital models. [ABSTRACT FROM AUTHOR]

We discuss two-dimensional (2D) topological insulators (TIs) based on planar Bi/Sb honeycombs on a SiC(0001) substrate using first-principles computations. The Bi/Sb planar honeycombs on SiC(0001) are shown to support a nontrivial band gap as large as 0.56 eV, which harbors a Dirac cone lying within the band gap. Effects of hydrogen atoms placed on either just one side or on both sides of the planar honeycombs are examined. The hydrogenated honeycombs are found to exhibit topologically protected edge states for zigzag as well as armchair edges, with a wide band gap of 1.03 and 0.41 eV in bismuth and antimony films, respectively. Our findings pave the way for using planar bismuth and antimony honeycombs as potential new 2D-TI platforms for room-temperature applications. [ABSTRACT FROM AUTHOR]

We have investigated topological electronic properties of freestanding bilayers of group IV (C, Si, Ge, Sn, and, Pb) and V (As, Sb, and, Bi) elements of the periodic table in the buckled and planar honeycomb structures under isotropic strain using first-principles calculations. Our focus is on mapping strain driven phase diagrams and identifying topological phase transitions therein as a pathway for guiding search for suitable substrates to grow two-dimensional (2D) topological insulators (TIs) films. Bilayers of group IV elements, excepting Pb, generally transform from trivial metal topological metal TI topological metal trivial metal phase with increasing strain from negative (compressive) to positive (tensile) values. Similarly, among the group V elements, As and Sb bilayers transform from trivial metal trivial insulator TI phase, while Bi transforms from a topological metal to TI phase. The band gap of 0.5 eV in the TI phase of Bi is the largest we found among all bilayers studied, with the band gap increasing further under tensile strain. Differences in the topological characteristics of bilayers of group V elements reflect associated differences in the strength of the spin–orbit coupling (SOC). We show, in particular, that the topological band structure of Sb bilayer becomes similar to that of a Bi bilayer when the strength of the SOC in Sb is artificially enhanced by a factor of 4. This study provides the first report that As can be a 2D TI under tensile strain. Notably, we found the existence of TI phases in all elemental bilayers we studied, except Pb. [ABSTRACT FROM AUTHOR]

Silicene, a monolayer of silicon atoms arranged in honeycomb lattices, can be synthesized on the Ag(111) surface, where it forms several superstructures with different buckling patterns and periodicity. Using scanning tunneling microscopy (STM), we obtained high-resolution images of silicene grown on Ag(111) and revealed its five phases, i.e., 4 × 4 − α, 4 × 4 − β, − α, − β and − γ, some observed for the first time. For each of the phases, we have determined its atomic structure by comparing the atomic-resolution STM images with theoretical simulation results previously reported. We thus eliminate the contradictions of previous studies on the structural models of various silicene phases supported by the Ag(111) surface. [ABSTRACT FROM AUTHOR]

We propose and construct a numerical algorithm to calculate the Berry conductivity in topological band insulators. The method is applicable to cold atom systems as well as solid state setups, both for the insulating case where the Fermi energy lies in the gap between two bulk bands as well as in the metallic regime. This method interpolates smoothly between both regimes. The algorithm is gauge-invariant by construction, efficient, and yields the Berry conductivity with known and controllable statistical error bars. We apply the algorithm to several paradigmatic models in the field of topological insulators, including Haldaneʼs model on the honeycomb lattice, the multi-band Hofstadter model, and the BHZ model, which describes the 2D spin Hall effect observed in CdTe/HgTe/CdTe quantum well heterostructures. [ABSTRACT FROM AUTHOR]

We study the structural and electronic properties of silicene on solid Ar(111) substrate using ab initio calculations. We demonstrate that due to weak interaction, quasi-freestanding silicene is realized in this system. The small binding energy of only meV per Si atom also indicates the possibility to separate silicene from the solid Ar(111) substrate. In addition, a band gap of 11 meV and a significant splitting of the energy levels due to spin-orbit coupling are observed. [ABSTRACT FROM AUTHOR]

We theoretically studied an exactly solvable gamma matrix generalization of the Kitaev spin model on the ruby lattice, which is a honeycomb lattice with ‘expanded’ vertices and links. We find that this model displays an exceptionally rich phase diagram that includes (i) gapless phases with stable spin Fermi surfaces, (ii) gapless phases with low-energy Dirac cones and quadratic band touching points and (iii) gapped phases with finite Chern numbers possessing the values ±4,±3,±2 and ±1. The model is then generalized to include Ising-like interactions that break the exact solvability of the model in a controlled manner. When these terms are dominant, they lead to a trivial Ising ordered phase which is shown to be adiabatically connected to a large coupling limit of the exactly solvable phase. In the limit where these interactions are weak, we treat them within mean-field theory and present the resulting phase diagrams. We discuss the nature of the transitions between various phases. Our results show the richness of possible ground states in closely related magnetic systems. [ABSTRACT FROM AUTHOR]

Motivated by the recent discovery of a spin-liquid phase for the Hubbard model on the honeycomb lattice at half-filling (Meng et al 2010 Nature88 487), we apply both perturbative and non-perturbative techniques to derive effective spin Hamiltonians describing the low-energy physics of the Mott-insulating phase of the system. Exact diagonalizations of the so-derived models on small clusters are performed, in order to assess the quality of the effective low-energy theory in the spin-liquid regime. We show that six-spin interactions on the elementary loop of the honeycomb lattice are the dominant sub-leading effective couplings. A minimal spin model is shown to reproduce most of the energetic properties of the Hubbard model on the honeycomb lattice in its spin-liquid phase. Surprisingly, a more elaborate effective low-energy spin model obtained by a systematic graph expansion rather disagrees beyond a certain point with the numerical results for the Hubbard model at intermediate couplings. [ABSTRACT FROM AUTHOR]