Edge detection by nonlinear dynamics
Wong, Yiu-fai
1994-07-01
We demonstrate how the formulation of a nonlinear scale-space filter can be used for edge detection and junction analysis. By casting edge-preserving filtering in terms of maximizing information content subject to an average cost function, the computed cost at each pixel location becomes a local measure of edgeness. This computation depends on a single scale parameter and the given image data. Unlike previous approaches which require careful tuning of the filter kernels for various types of edges, our scheme is general enough to be able to handle different edges, such as lines, step-edges, corners and junctions. Anisotropy in the data is handled automatically by the nonlinear dynamics.
Final-state screening dynamics in resonant Auger decay at the 2p edge of vanadium
NASA Astrophysics Data System (ADS)
Ilakovac, V.; Kralj, M.; Pervan, P.; Richter, M. C.; Goldoni, A.; Larciprete, R.; Petaccia, L.; Hricovini, K.
2005-02-01
We investigated the resonant Auger process near the V 2p3/2 edge in vanadium metal. Attention is centered on the onset of Auger decays and their behavior below the 2p3/2 resonance. The 2p3/23d3d decay has a crossover from the Raman-Auger to the normal Auger regime at the 2p ionization threshold. Meanwhile, Auger decays with core holes in the final state have normal Auger behavior even below the ionization threshold, the 2p3/23p3p process being visible at 2.2 eV lower photon energy. The different resonant behavior of these Auger decays can be understood within the one-step model as final-state screening effects affecting the photoexcitation.
Topological number of edge states
NASA Astrophysics Data System (ADS)
Hashimoto, Koji; Kimura, Taro
2016-05-01
We show that the edge states of the four-dimensional class A system can have topological charges, which are characterized by Abelian/non-Abelian monopoles. The edge topological charges are a new feature of relations among theories with different dimensions. From this novel viewpoint, we provide a non-Abelian analog of the TKNN number as an edge topological charge, which is defined by an SU(2) 't Hooft-Polyakov BPS monopole through an equivalence to Nahm construction. Furthermore, putting a constant magnetic field yields an edge monopole in a noncommutative momentum space, where D-brane methods in string theory facilitate study of edge fermions.
Edge mode dynamics of quenched topological wires
NASA Astrophysics Data System (ADS)
Sacramento, P. D.
2016-06-01
The fermionic and Majorana edge mode dynamics of various topological systems are compared, after a sudden global quench of the Hamiltonian parameters takes place. Attention is focused on the regimes where the survival probability of an edge state has oscillations either due to critical or off-critical quenches. The nature of the wave functions and the overlaps between the eigenstates of different points in parameter space determine the various types of behaviors, and the distinction due to the Majorana nature of the excitations plays a lesser role. Performing a sequence of quenches, it is shown that the edge states, including Majorana modes, may be switched off and on. Also, the generation of Majoranas due to quenching from a trivial phase is discussed.
Topological edge states in pnictides
NASA Astrophysics Data System (ADS)
Youmans, Cody; Ghaemi, Pouyan; Kargarian, Mehdi
In some members of the ferro-pnictides, non-trivial topology in the bulk band-structure is related to potentially observable gapless edge states. We study these states numerically and analytically for a range of parameters, with and without superconductivity and antiferromagnetic SDW ordering, and their relation to the symmetries and topologically non-trivial aspects of our model Hamiltonian. Support was provided by the Doctoral Student Research Grant program at the Graduate Center, CUNY.
Giant edge state splitting at atomically precise graphene zigzag edges.
Wang, Shiyong; Talirz, Leopold; Pignedoli, Carlo A; Feng, Xinliang; Müllen, Klaus; Fasel, Roman; Ruffieux, Pascal
2016-01-01
Zigzag edges of graphene nanostructures host localized electronic states that are predicted to be spin-polarized. However, these edge states are highly susceptible to edge roughness and interaction with a supporting substrate, complicating the study of their intrinsic electronic and magnetic structure. Here, we focus on atomically precise graphene nanoribbons whose two short zigzag edges host exactly one localized electron each. Using the tip of a scanning tunnelling microscope, the graphene nanoribbons are transferred from the metallic growth substrate onto insulating islands of NaCl in order to decouple their electronic structure from the metal. The absence of charge transfer and hybridization with the substrate is confirmed by scanning tunnelling spectroscopy, which reveals a pair of occupied/unoccupied edge states. Their large energy splitting of 1.9 eV is in accordance with ab initio many-body perturbation theory calculations and reflects the dominant role of electron-electron interactions in these localized states. PMID:27181701
Giant edge state splitting at atomically precise graphene zigzag edges
NASA Astrophysics Data System (ADS)
Wang, Shiyong; Talirz, Leopold; Pignedoli, Carlo A.; Feng, Xinliang; Müllen, Klaus; Fasel, Roman; Ruffieux, Pascal
2016-05-01
Zigzag edges of graphene nanostructures host localized electronic states that are predicted to be spin-polarized. However, these edge states are highly susceptible to edge roughness and interaction with a supporting substrate, complicating the study of their intrinsic electronic and magnetic structure. Here, we focus on atomically precise graphene nanoribbons whose two short zigzag edges host exactly one localized electron each. Using the tip of a scanning tunnelling microscope, the graphene nanoribbons are transferred from the metallic growth substrate onto insulating islands of NaCl in order to decouple their electronic structure from the metal. The absence of charge transfer and hybridization with the substrate is confirmed by scanning tunnelling spectroscopy, which reveals a pair of occupied/unoccupied edge states. Their large energy splitting of 1.9 eV is in accordance with ab initio many-body perturbation theory calculations and reflects the dominant role of electron-electron interactions in these localized states.
Giant edge state splitting at atomically precise graphene zigzag edges
Wang, Shiyong; Talirz, Leopold; Pignedoli, Carlo A.; Feng, Xinliang; Müllen, Klaus; Fasel, Roman; Ruffieux, Pascal
2016-01-01
Zigzag edges of graphene nanostructures host localized electronic states that are predicted to be spin-polarized. However, these edge states are highly susceptible to edge roughness and interaction with a supporting substrate, complicating the study of their intrinsic electronic and magnetic structure. Here, we focus on atomically precise graphene nanoribbons whose two short zigzag edges host exactly one localized electron each. Using the tip of a scanning tunnelling microscope, the graphene nanoribbons are transferred from the metallic growth substrate onto insulating islands of NaCl in order to decouple their electronic structure from the metal. The absence of charge transfer and hybridization with the substrate is confirmed by scanning tunnelling spectroscopy, which reveals a pair of occupied/unoccupied edge states. Their large energy splitting of 1.9 eV is in accordance with ab initio many-body perturbation theory calculations and reflects the dominant role of electron–electron interactions in these localized states. PMID:27181701
Bosonic edge states in gapped honeycomb lattices
NASA Astrophysics Data System (ADS)
Guo, Huaiming; Niu, Yuekun; Chen, Shu; Feng, Shiping
2016-03-01
By quantum Monte Carlo simulations of bosons in gapped honeycomb lattices, we show the existence of bosonic edge states. For a single layer honeycomb lattice, bosonic edge states can be controlled to appear, cross the gap, and merge into bulk states by an on-site potential applied on the outermost sites of the boundary. On a bilayer honeycomb lattice, A bosonic edge state traversing the gap at half filling is demonstrated. The topological origin of the bosonic edge states is discussed with pseudo Berry curvature. The results will simulate experimental studies of these exotic bosonic edge states with ultracold bosons trapped in honeycomb optical lattices.
Saddle-node dynamics for edge detection
Wong, Y.F.
1994-09-01
The author demonstrates how the formulation of a nonlinear scale-space filter can be used for edge detection and junction analysis. By casting edge-preserving filtering in terms of maximizing information content subject to an average cost function, the computed cost at each pixel location becomes a local measure of edgeness. This computation depends on a single scale parameter and the given image data. Unlike previous approaches which require careful tuning of the filter kernels for various types of edges, this scheme is general enough to be able to handle different edges, such as lines, step edges, corners and junctions. Anisotropy in the data is handled automatically by the nonlinear dynamics.
Decay patterns of edge states at reconstructed armchair graphene edges
NASA Astrophysics Data System (ADS)
Park, Changwon; Ihm, Jisoon; Kim, Gunn
Density functional theory calculations are used to investigate the electronic structures of localized states at reconstructed armchair graphene edges. We consider graphene nanoribbons with two different edge types and obtain the energy band structures and charge densities of the edge states. By examining the imaginary part of the wave vector in the forbidden energy region, we reveal the decay behavior of the wave functions in graphene. The complex band structures of graphene in the armchair and zigzag directions are presented in the first-principles framework. G.K. acknowledges the support of the Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education (Grant No. 2013R1A1A2009131) and the Priority Research Center Program (Grant No. 2010-0020207).
Edge States in Transitional Pipe Flow
NASA Astrophysics Data System (ADS)
Schneider, Tobias M.; Eckhardt, Bruno
2006-11-01
We study the boundary of the laminar region near the onset of turbulence. Approaching the boundary from the laminar side, the lifetime of perturbations increases, diverges when the boundary is reached, and varies chaotically for larger amplitudes. In the chaotic region, lifetimes vary sensitively with amplitude, consistent with the strange saddle picture of the turbulence proposed earlier. The trajectory on the edge between the laminar and chaotic regions is asymptotic to a single well defined state, essentially independent of the type of perturbation. The edge then becomes the stable manifold of this structure. In the case of a model shear flow, the edge states are simple or period doubled or chaotic trajectories. In the case of pipe flow the edge state seems to remain close to a state with simple vortical structure. Edge of Chaos in a Parallel Shear Flow, Joseph D. Skufca, James A. Yorke, and Bruno Eckhardt, Phys. Rev. Lett. 96, 174101 (2006)
Diagnosing Topological Edge States via Entanglement Monogamy
NASA Astrophysics Data System (ADS)
Meichanetzidis, K.; Eisert, J.; Cirio, M.; Lahtinen, V.; Pachos, J. K.
2016-04-01
Topological phases of matter possess intricate correlation patterns typically probed by entanglement entropies or entanglement spectra. In this Letter, we propose an alternative approach to assessing topologically induced edge states in free and interacting fermionic systems. We do so by focussing on the fermionic covariance matrix. This matrix is often tractable either analytically or numerically, and it precisely captures the relevant correlations of the system. By invoking the concept of monogamy of entanglement, we show that highly entangled states supported across a system bipartition are largely disentangled from the rest of the system, thus, usually appearing as gapless edge states. We then define an entanglement qualifier that identifies the presence of topological edge states based purely on correlations present in the ground states. We demonstrate the versatility of this qualifier by applying it to various free and interacting fermionic topological systems.
Diagnosing Topological Edge States via Entanglement Monogamy.
Meichanetzidis, K; Eisert, J; Cirio, M; Lahtinen, V; Pachos, J K
2016-04-01
Topological phases of matter possess intricate correlation patterns typically probed by entanglement entropies or entanglement spectra. In this Letter, we propose an alternative approach to assessing topologically induced edge states in free and interacting fermionic systems. We do so by focussing on the fermionic covariance matrix. This matrix is often tractable either analytically or numerically, and it precisely captures the relevant correlations of the system. By invoking the concept of monogamy of entanglement, we show that highly entangled states supported across a system bipartition are largely disentangled from the rest of the system, thus, usually appearing as gapless edge states. We then define an entanglement qualifier that identifies the presence of topological edge states based purely on correlations present in the ground states. We demonstrate the versatility of this qualifier by applying it to various free and interacting fermionic topological systems. PMID:27081962
Edge states in polariton honeycomb lattices
NASA Astrophysics Data System (ADS)
Milićević, M.; Ozawa, T.; Andreakou, P.; Carusotto, I.; Jacqmin, T.; Galopin, E.; Lemaître, A.; Le Gratiet, L.; Sagnes, I.; Bloch, J.; Amo, A.
2015-09-01
The experimental study of edge states in atomically thin layered materials remains a challenge due to the difficult control of the geometry of the sample terminations, the stability of dangling bonds, and the need to measure local properties. In the case of graphene, localized edge modes have been predicted in zigzag and bearded edges, characterized by flat dispersions connecting the Dirac points. Polaritons in semiconductor microcavities have recently emerged as an extraordinary photonic platform to emulate 1D and 2D Hamiltonians, allowing the direct visualization of the wavefunctions in both real- and momentum-space as well as of the energy dispersion of eigenstates via photoluminescence experiments. Here we report on the observation of edge states in a honeycomb lattice of coupled micropillars. The lowest two bands of this structure arise from the coupling of the lowest energy modes of the micropillars, and emulate the π and π* bands of graphene. We show the momentum-space dispersion of the edge states associated with the zigzag and bearded edges, holding unidimensional quasi-flat bands. Additionally, we evaluate polarization effects characteristic of polaritons on the properties of these states.
Floquet edge states in germanene nanoribbons.
Tahir, M; Zhang, Q Y; Schwingenschlögl, U
2016-01-01
We theoretically demonstrate versatile electronic properties of germanene monolayers under circularly, linearly, and elliptically polarized light. We show for the high frequency regime that the edge states can be controlled by tuning the amplitude of the light and by applying a static electric field. For circularly polarized light the band gap in one valley is reduced and in the other enhanced, enabling single valley edge states. For linearly polarized light spin-split states are found for both valleys, being connected by time reversal symmetry. The effects of elliptically polarized light are similar to those of circularly polarized light. The transport properties of zigzag nanoribbons in the presence of disorder confirm a nontrivial nature of the edge states under circularly and elliptically polarized light. PMID:27550632
Floquet edge states in germanene nanoribbons
Tahir, M.; Zhang, Q. Y.; Schwingenschlögl, U.
2016-01-01
We theoretically demonstrate versatile electronic properties of germanene monolayers under circularly, linearly, and elliptically polarized light. We show for the high frequency regime that the edge states can be controlled by tuning the amplitude of the light and by applying a static electric field. For circularly polarized light the band gap in one valley is reduced and in the other enhanced, enabling single valley edge states. For linearly polarized light spin-split states are found for both valleys, being connected by time reversal symmetry. The effects of elliptically polarized light are similar to those of circularly polarized light. The transport properties of zigzag nanoribbons in the presence of disorder confirm a nontrivial nature of the edge states under circularly and elliptically polarized light. PMID:27550632
Universal nonequilibrium states at the fractional quantum Hall edge
NASA Astrophysics Data System (ADS)
Levkivskyi, Ivan P.
2016-04-01
Integrability of electron dynamics in one dimension is manifested by the nonequilibrium stationary states. They emerge near a point contact coupling two quantum Hall edges with different chemical potentials. I use the nonequilibrium bosonization technique to show that the effective temperature of such states at the fractional quantum Hall edges has a universal linear dependence on the current through the contact. In contrast, the temperature at eventual equilibrium scales as the square root of the power dissipating at the point contact. I propose to use this distinction to detect these intriguing nonequilibrium states.
Edge states of zigzag bilayer graphite nanoribbons
NASA Astrophysics Data System (ADS)
Rhim, Jun-Won; Moon, Kyungsun
2008-09-01
The electronic structures of zigzag bilayer graphite nanoribbons (Z-BGNRs) with various ribbon widths N are studied within the tight binding approximation. Neglecting the inter-layer hopping amplitude γ4, which is an order of magnitude smaller than the other inter-layer hopping parameters, there exist two fixed Fermi points ± k* independent of the ribbon width with a peculiar energy dispersion near k* as ɛ(k)~ ± (k-k*)N. By investigating the edge states of Z-BGNRs, we notice that the trigonal warping of the bilayer graphene sheets is reflected in the edge state structure. With the inclusion of γ4, the above two Fermi points are not fixed but drift toward the vicinity of the Dirac point with increasing width N, as shown by the finite scaling method, and the peculiar dispersions change to parabolic ones. The edge magnetism of Z-BGNRs is also examined by solving the half-filled Hubbard Hamiltonian for the ribbon using the Hartree-Fock approximation. We have shown that within the same side of the edges, the edge spins are aligned ferromagnetically for the experimentally relevant set of parameters.
Nonlocal edge state transport in topological insulators
NASA Astrophysics Data System (ADS)
Protogenov, Alexander P.; Verbus, Valery A.; Chulkov, Evgueni V.
2013-11-01
We use the N-terminal scheme for studying the edge-state transport in two-dimensional topological insulators. We find the universal nonlocal response in the ballistic transport approach. This macroscopic exhibition of the topological order offers different areas for applications.
Edge Sheared Flows and Blob Dynamics
NASA Astrophysics Data System (ADS)
Myra, J. R.
2012-10-01
The dynamics of blob-filaments [S. I. Krasheninnikov, et al. J. Plasma Phys. 74, 679 (2008); D. A. D'Ippolito, et al., Phys. Plasmas 18, 060501 (2011)] in the strongly radially inhomogeneous edge and scrape-off-layer (SOL) region of a tokamak plasma is considered, with emphasis on sheared flow generation and interaction. The work is motivated by the potential importance of edge sheared flows for turbulence regulation, (e.g. the L-H transition), and the influence of flows on the character of emitted blob-filament structures which ultimately contact plasma-facing components. To study the dynamics of blobs and sheared flows, we employ both numerical simulations and experimental data analysis. The simulations use the fluid-based 2D curvature-interchange model embedded in the SOLT (SOL turbulence) code [D. A. Russell, et al, Phys. Plasmas 16, 122304 (2009)]. A blob-tracking algorithm has also been developed and applied to NSTX and Alcator C-Mod data. The algorithm is based on 2D time-resolved images from the gas puff imaging (GPI) diagnostic [S. J. Zweben, et al. Phys. Plasmas 9, 1981 (2002)]. The algorithm is able to track the blob motion and changes in blob structure, such as elliptical deformations, that can be affected by sheared flows. Results of seeded blob simulations are compared with the experimental data to determine the role of plasma parameters on the blob tracks and to evaluate the exchange of momentum between the blobs and flows. Seeded blob simulations are shown to reproduce many qualitative and quantitative features of the data including size, scale and direction of perpendicular (approximately poloidal) flows and the inferred Reynolds forces, poloidal reversal of blob tracks, and blob trapping and/or ejection. Simulation and experimental data comparisons permit the inference of dynamical mechanisms associated with blob motion and sheared flow generation in these shots, and their relation to previous theoretical work.
Chiral Thermoelectrics with Quantum Hall Edge States
NASA Astrophysics Data System (ADS)
Sánchez, Rafael; Sothmann, Björn; Jordan, Andrew N.
2015-04-01
The thermoelectric properties of a three-terminal quantum Hall conductor are investigated. We identify a contribution to the thermoelectric response that relies on the chirality of the carrier motion rather than on spatial asymmetries. The Onsager matrix becomes maximally asymmetric with configurations where either the Seebeck or the Peltier coefficients are zero while the other one remains finite. Reversing the magnetic field direction exchanges these effects, which originate from the chiral nature of the quantum Hall edge states. The possibility to generate spin-polarized currents in quantum spin Hall samples is discussed.
Edge State and Intrinsic Hole Doping in Bilayer Phosphorene
NASA Astrophysics Data System (ADS)
Osada, Toshihito
2015-01-01
Using a simple LCAO model by Harrison, we have qualitatively studied the edge state of bilayer phosphorene, which is a unit structure of the layered crystal of black phosphorus. This model successfully reproduces the isolated edge state in the bulk gap in monolayer phosphorene. In bilayer phosphorene, however, it shows that edge states are almost buried in the valence band and there is no isolated midgap edge state at the zigzag edge. Since the buried edge state works as acceptor, holes are doped from the edge state into the bulk. This gives a possible explanation for p-type conduction in undoped black phosphorus. Under the vertical electric field, the intrinsic hole doping is reduced because a part of edge states move into the gap. These features of bilayer phosphorene might be better suited for device application.
Observation of unconventional edge states in 'photonic graphene'.
Plotnik, Yonatan; Rechtsman, Mikael C; Song, Daohong; Heinrich, Matthias; Zeuner, Julia M; Nolte, Stefan; Lumer, Yaakov; Malkova, Natalia; Xu, Jingjun; Szameit, Alexander; Chen, Zhigang; Segev, Mordechai
2014-01-01
Graphene, a two-dimensional honeycomb lattice of carbon atoms, has been attracting much interest in recent years. Electrons therein behave as massless relativistic particles, giving rise to strikingly unconventional phenomena. Graphene edge states are essential for understanding the electronic properties of this material. However, the coarse or impure nature of the graphene edges hampers the ability to directly probe the edge states. Perhaps the best example is given by the edge states on the bearded edge that have never been observed-because such an edge is unstable in graphene. Here, we use the optical equivalent of graphene-a photonic honeycomb lattice-to study the edge states and their properties. We directly image the edge states on both the zigzag and bearded edges of this photonic graphene, measure their dispersion properties, and most importantly, find a new type of edge state: one residing on the bearded edge that has never been predicted or observed. This edge state lies near the Van Hove singularity in the edge band structure and can be classified as a Tamm-like state lacking any surface defect. The mechanism underlying its formation may counterintuitively appear in other crystalline systems. PMID:24193661
Anomalous Edge State in a Non-Hermitian Lattice.
Lee, Tony E
2016-04-01
We show that the bulk-boundary correspondence for topological insulators can be modified in the presence of non-Hermiticity. We consider a one-dimensional tight-binding model with gain and loss as well as long-range hopping. The system is described by a non-Hermitian Hamiltonian that encircles an exceptional point in momentum space. The winding number has a fractional value of 1/2. There is only one dynamically stable zero-energy edge state due to the defectiveness of the Hamiltonian. This edge state is robust to disorder due to protection by a chiral symmetry. We also discuss experimental realization with arrays of coupled resonator optical waveguides. PMID:27081980
Anomalous Edge State in a Non-Hermitian Lattice
NASA Astrophysics Data System (ADS)
Lee, Tony E.
2016-04-01
We show that the bulk-boundary correspondence for topological insulators can be modified in the presence of non-Hermiticity. We consider a one-dimensional tight-binding model with gain and loss as well as long-range hopping. The system is described by a non-Hermitian Hamiltonian that encircles an exceptional point in momentum space. The winding number has a fractional value of 1 /2 . There is only one dynamically stable zero-energy edge state due to the defectiveness of the Hamiltonian. This edge state is robust to disorder due to protection by a chiral symmetry. We also discuss experimental realization with arrays of coupled resonator optical waveguides.
Edge states in confined active fluids
NASA Astrophysics Data System (ADS)
Souslov, Anton; Vitelli, Vincenzo
Recently, topologically protected edge modes have been proposed and realized in both mechanical and acoustic metamaterials. In one class of such metamaterials, Time-Reversal Symmetry is broken, and, to achieve this TRS breaking in mechanical and acoustic systems, an external energy input must be used. For example, motors provide a driving force that uses energy and, thus, explicitly break TRS. As a result, motors have been used as an essential component in the design of topological metamaterials. By contrast, we explore the design of topological metamaterials that use a class of far-from-equilibrium liquids, called polar active liquids, that spontaneously break TRS. We thus envision the confinement of a polar active liquid to a prescribed geometry in order to realize topological order with broken time-reversal symmetry. We address the design of the requisite geometries, for example a regular honeycomb lattice composed of annular channels, in which the active liquid may be confined. We also consider the physical character of the active liquid that, when introduced into the prescribed geometry, will spontaneously form the flow pattern of a metamaterial with topologically protected edge states. Finally, we comment on potential experimental realizations of such metamaterials.
Observation of chiral edge states with neutral fermions in synthetic Hall ribbons
NASA Astrophysics Data System (ADS)
Mancini, M.; Pagano, G.; Cappellini, G.; Livi, L.; Rider, M.; Catani, J.; Sias, C.; Zoller, P.; Inguscio, M.; Dalmonte, M.; Fallani, L.
2015-09-01
Chiral edge states are a hallmark of quantum Hall physics. In electronic systems, they appear as a macroscopic consequence of the cyclotron orbits induced by a magnetic field, which are naturally truncated at the physical boundary of the sample. Here we report on the experimental realization of chiral edge states in a ribbon geometry with an ultracold gas of neutral fermions subjected to an artificial gauge field. By imaging individual sites along a synthetic dimension, encoded in the nuclear spin of the atoms, we detect the existence of the edge states and observe the edge-cyclotron orbits induced during quench dynamics. The realization of fermionic chiral edge states opens the door for edge state interferometry and the study of non-Abelian anyons in atomic systems.
Edge states in a honeycomb lattice: effects of anisotropic hopping and mixed edges
Dahal, Hari P; Balatsky, Alexander V; Sinistsyn, N A; Hu, Zi - Xiang; Yang, Kun
2008-01-01
We study the edge states in graphene in the presence of a magnetic field perpendicular to the plane of the lattice. Most of the work done so far discusses the edge states in either zigzag or armchair edge graphene considering an isotropic electron hopping. In practice, graphene can have a mixture of armchair and zigzag edges and the electron hopping can be anisotropic, which is the subject of this article. We predict that the mixed edges smear the enhanced local density of states (LDOS) at E=0 of the zigzag edge and, on the other hand, the anisotropic hopping gives rise to the enhanced LDOS at E=0 in the armchair edge. The behavior of the LDOS can be studied using scanning tunneling microscopy (STM) experiments. We suggest that care must be taken while interpreting the STM data, because the clear distinction between the zigzag edge (enhanced LDOS at E=0) and armchair edge (suppressed LDOS at E=0) can be lost if the hopping is not isotropic and if the edges are mixed.
Green's function approach to edge states in transition metal dichalcogenides
NASA Astrophysics Data System (ADS)
Farmanbar, Mojtaba; Amlaki, Taher; Brocks, Geert
2016-05-01
The semiconducting two-dimensional transition metal dichalcogenides MX 2 show an abundance of one-dimensional metallic edges and grain boundaries. Standard techniques for calculating edge states typically model nanoribbons, and require the use of supercells. In this paper, we formulate a Green's function technique for calculating edge states of (semi-)infinite two-dimensional systems with a single well-defined edge or grain boundary. We express Green's functions in terms of Bloch matrices, constructed from the solutions of a quadratic eigenvalue equation. The technique can be applied to any localized basis representation of the Hamiltonian. Here, we use it to calculate edge states of MX 2 monolayers by means of tight-binding models. Aside from the basic zigzag and armchair edges, we study edges with a more general orientation, structurally modifed edges, and grain boundaries. A simple three-band model captures an important part of the edge electronic structures. An 11-band model comprising all valence orbitals of the M and X atoms is required to obtain all edge states with energies in the MX 2 band gap. Here, states of odd symmetry with respect to a mirror plane through the layer of M atoms have a dangling-bond character, and tend to pin the Fermi level.
Dynamic Stall Characteristics of Drooped Leading Edge Airfoils
NASA Technical Reports Server (NTRS)
Sankar, Lakshmi N.; Sahin, Mehmet; Gopal, Naveen
2000-01-01
Helicopters in high-speed forward flight usually experience large regions of dynamic stall over the retreating side of the rotor disk. The rapid variations in the lift and pitching moments associated with the stall process can result in vibratory loads, and can cause fatigue and failure of pitch links. In some instances, the large time lag between the aerodynamic forces and the blade motion can trigger stall flutter. A number of techniques for the alleviation of dynamic stall have been proposed and studied by researchers. Passive and active control techniques have both been explored. Passive techniques include the use of high solidity rotors that reduce the lift coefficients of individual blades, leading edge slots and leading edge slats. Active control techniques include steady and unsteady blowing, and dynamically deformable leading edge (DDLE) airfoils. Considerable amount of experimental and numerical data has been collected on the effectiveness of these concepts. One concept that has not received as much attention is the drooped-leading edge airfoil idea. It has been observed in wind tunnel studies and flight tests that drooped leading edge airfoils can have a milder dynamic stall, with a significantly milder load hysteresis. Drooped leading edge airfoils may not, however, be suitable at other conditions, e.g. in hover, or in transonic flow. Work needs to be done on the analysis and design of drooped leading edge airfoils for efficient operation in a variety of flight regimes (hover, dynamic stall, and transonic flow). One concept that is worthy of investigation is the dynamically drooping airfoil, where the leading edge shape is changed roughly once-per-rev to mitigate the dynamic stall.
Low temperature edge dynamics of AB-stacked bilayer graphene: naturally favored closed zigzag edges.
Zhan, Da; Liu, Lei; Xu, Ya Nan; Ni, Zhen Hua; Yan, Jia Xu; Zhao, Chun; Shen, Ze Xiang
2011-01-01
Closed edges bilayer graphene (CEBG) is a recent discovered novel form of graphene structures, whose regulated edge states may critically change the overall electronic behaviors. If stacked properly with the AB style, the bilayer graphene with closed zigzag edges may even present amazing electronic properties of bandgap opening and charge separation. Experimentally, the CEBG has been confirmed recently with HRTEM observations after extremely high temperature annealing (2000 °C). From the application point of view, the low temperature closing of the graphene edges would be much more feasible for large-scale graphene-based electronic devices fabrication. Here, we demonstrate that the zigzag edges of AB-stacked bilayer graphene will form curved close structure naturally at low annealing temperature (< 500 °C) based on Raman observation and first principles analysis. Such findings may illuminate a simple and easy way to engineer graphene electronics. PMID:22355531
Edge states and phase diagram for graphene under polarized light
NASA Astrophysics Data System (ADS)
Wang, Yi-Xiang; Li, Fuxiang
2016-07-01
In this work, we investigate the topological phase transitions in graphene under the modulation of circularly polarized light, by analyzing the changes of edge states and its topological structures. A full phase diagram, with several different topological phases, is presented in the parameter space spanned by the driving frequency and light strength. We find that the high-Chern number behavior is very common in the driven system. While the one-photon resonance can create the chiral edge states in the π-gap, the two-photon resonance will induce the counter-propagating edge modes in the zero-energy gap. When the driving light strength is strong, the number and even the chirality of the edge states may change in the π-gap. The robustness of the edge states to disorder potential is also examined. We close by discussing the feasibility of experimental proposals.
Edge states and phase diagram for graphene under polarized light
Wang, Yi -Xiang; Li, Fuxiang
2016-03-22
In this paper, we investigate the topological phase transitions in graphene under the modulation of circularly polarized light, by analyzing the changes of edge states and its topological structures. A full phase diagram, with several different topological phases, is presented in the parameter space spanned by the driving frequency and light strength. We find that the high-Chern number behavior is very common in the driven system. While the one-photon resonance can create the chiral edge states in the π-gap, the two-photon resonance will induce the counter-propagating edge modes in the zero-energy gap. When the driving light strength is strong, themore » number and even the chirality of the edge states may change in the π-gap. The robustness of the edge states to disorder potential is also examined. We close by discussing the feasibility of experimental proposals.« less
Dynamic insight into protein structure utilizing red edge excitation shift.
Chattopadhyay, Amitabha; Haldar, Sourav
2014-01-21
Proteins are considered the workhorses in the cellular machinery. They are often organized in a highly ordered conformation in the crowded cellular environment. These conformations display characteristic dynamics over a range of time scales. An emerging consensus is that protein function is critically dependent on its dynamics. The subtle interplay between structure and dynamics is a hallmark of protein organization and is essential for its function. Depending on the environmental context, proteins can adopt a range of conformations such as native, molten globule, unfolded (denatured), and misfolded states. Although protein crystallography is a well established technique, it is not always possible to characterize various protein conformations by X-ray crystallography due to transient nature of these states. Even in cases where structural characterization is possible, the information obtained lacks dynamic component, which is needed to understand protein function. In this overall scenario, approaches that reveal information on protein dynamics are much appreciated. Dynamics of confined water has interesting implications in protein folding. Interfacial hydration combines the motion of water molecules with the slow moving protein molecules. The red edge excitation shift (REES) approach becomes relevant in this context. REES is defined as the shift in the wavelength of maximum fluorescence emission toward higher wavelengths, caused by a shift in the excitation wavelength toward the red edge of absorption spectrum. REES arises due to slow rates (relative to fluorescence lifetime) of solvent relaxation (reorientation) around an excited state fluorophore in organized assemblies such as proteins. Consequently, REES depends on the environment-induced motional restriction imposed on the solvent molecules in the immediate vicinity of the fluorophore. In the case of a protein, the confined water in the protein creates a dipolar field that acts as the solvent for a fluorophore
Thermoelectric transport of edge/surface states of topological insulators
NASA Astrophysics Data System (ADS)
Murakami, Shuichi; Takahashi, Ryuji
2011-03-01
In my talk we theoretically study thermoelectric properties of topological insulators (TI), where novel properties of edge/surface states are expected to appear. As compared to the number of bulk states, the edge/surface states are very few; we therefore consider a narrow ribbon for 2D and a thin slab for 3D TI to make the edge/surface-state transport larger. By considering edge/surface and bulk transport together, we calculate the charge and heat conductivity, and Seebeck coefficient. We find that in 2D TI the bulk and edge transport compete each other in the thermoelectric transport. By lowering temperature, the thermoelectric figure of merit ZT has a minimum, corresponding to the bulk-to-edge crossover, and then increases again at low temperature where the edge state dominates. The crossover is estimated to be at around 5K-10K for 10nm-width ribbon. We also discuss surface state transport for 3D TI as well.
Roles of edge weights on epidemic spreading dynamics
NASA Astrophysics Data System (ADS)
Zhan, Xiu-Xiu; Liu, Chuang; Zhang, Zi-Ke; Sun, Gui-Quan
2016-08-01
Epidemic spreading on complex networks has attracted much attention in recent years. A large number of studies have focused on investigating the impacts of network topology on spreading dynamics. However, the weighted network is very common in real systems, and we attempt to study the role of edge weights on epidemic spreading. In this work, the spreading process was presented as the SIS model and three edge-breaking strategies according to the weight of the SI links were performed simultaneously, which was used to illustrate the influence of the edge weights. Simulation results on three real networks showed the different spreading patterns of different edge-breaking strategies, which in turn indicated the influence of edge weights on the spreading process. Therefore we can take different measures at different periods according to the edge weights to impede the epidemic. In addition, the detailed analyses of relationship between the edge weight and the network structure was given to interpret the role of edge weights in the epidemic spreading process.
Tunneling into and between helical edge states: Fermionic approach
NASA Astrophysics Data System (ADS)
Aristov, D. N.; Niyazov, R. A.
2016-07-01
We study the four-terminal junction of spinless Luttinger liquid wires, which describes either a corner junction of two helical edge states of topological insulators or the tunneling from the spinful wire into the helical edge state. We use the fermionic representation and the scattering state formalism, in order to compute the renormalization group (RG) equations for the linear response conductances. We establish our approach by considering a junction between two possibly nonequivalent helical edge states and find an agreement with the earlier analysis of this situation. Tunneling from the tip of the spinful wire to the edge state is further analyzed which requires some modification of our formalism. In the latter case we demonstrate (i) the existence of both fixed lines and conventional fixed points of RG equations, and (ii) certain proportionality relations holding for conductances during renormalization. The scaling exponents and phase portraits are obtained in all cases.
Edge mixing dynamics in graphene p-n junctions in the quantum Hall regime
NASA Astrophysics Data System (ADS)
Matsuo, Sadashige; Takeshita, Shunpei; Tanaka, Takahiro; Nakaharai, Shu; Tsukagoshi, Kazuhito; Moriyama, Takahiro; Ono, Teruo; Kobayashi, Kensuke
2015-09-01
Massless Dirac electron systems such as graphene exhibit a distinct half-integer quantum Hall effect, and in the bipolar transport regime co-propagating edge states along the p-n junction are realized. Additionally, these edge states are uniformly mixed at the junction, which makes it a unique structure to partition electrons in these edge states. Although many experimental works have addressed this issue, the microscopic dynamics of electron partition in this peculiar structure remains unclear. Here we performed shot-noise measurements on the junction in the quantum Hall regime as well as at zero magnetic field. We found that, in sharp contrast with the zero-field case, the shot noise in the quantum Hall regime is finite in the bipolar regime, but is strongly suppressed in the unipolar regime. Our observation is consistent with the theoretical prediction and gives microscopic evidence that the edge states are uniquely mixed along the p-n junction.
Edge mixing dynamics in graphene p–n junctions in the quantum Hall regime
Matsuo, Sadashige; Takeshita, Shunpei; Tanaka, Takahiro; Nakaharai, Shu; Tsukagoshi, Kazuhito; Moriyama, Takahiro; Ono, Teruo; Kobayashi, Kensuke
2015-01-01
Massless Dirac electron systems such as graphene exhibit a distinct half-integer quantum Hall effect, and in the bipolar transport regime co-propagating edge states along the p–n junction are realized. Additionally, these edge states are uniformly mixed at the junction, which makes it a unique structure to partition electrons in these edge states. Although many experimental works have addressed this issue, the microscopic dynamics of electron partition in this peculiar structure remains unclear. Here we performed shot-noise measurements on the junction in the quantum Hall regime as well as at zero magnetic field. We found that, in sharp contrast with the zero-field case, the shot noise in the quantum Hall regime is finite in the bipolar regime, but is strongly suppressed in the unipolar regime. Our observation is consistent with the theoretical prediction and gives microscopic evidence that the edge states are uniquely mixed along the p–n junction. PMID:26337445
Dynamic fracture mechanics analysis for an edge delamination crack
NASA Technical Reports Server (NTRS)
Rizzi, Stephen A.; Doyle, James F.
1994-01-01
A global/local analysis is applied to the problem of a panel with an edge delamination crack subject to an impulse loading to ascertain the dynamic J integral. The approach uses the spectral element method to obtain the global dynamic response and local resultants to obtain the J integral. The variation of J integral along the crack front is shown. The crack behavior is mixed mode (Mode 2 and Mode 3), but is dominated by the Mode 2 behavior.
Magnetic edge states and magnetotransport in graphene antidot barriers
NASA Astrophysics Data System (ADS)
Thomsen, M. R.; Power, S. R.; Jauho, A.-P.; Pedersen, T. G.
2016-07-01
Magnetic fields are often used for characterizing transport in nanoscale materials. Recent magnetotransport experiments have demonstrated that ballistic transport is possible in graphene antidot lattices (GALs). These experiments have inspired the present theoretical study of GALs in a perpendicular magnetic field. We calculate magnetotransport through graphene antidot barriers (GABs), which are finite rows of antidots arranged periodically in a pristine graphene sheet, using a tight-binding model and the Landauer-Büttiker formula. We show that GABs behave as ideal Dirac mass barriers for antidots smaller than the magnetic length and demonstrate the presence of magnetic edge states, which are localized states on the periphery of the antidots due to successive reflections on the antidot edge in the presence of a magnetic field. We show that these states are robust against variations in lattice configuration and antidot edge chirality. Moreover, we calculate the transmittance of disordered GABs and find that magnetic edge states survive a moderate degree of disorder. Due to the long phase-coherence length in graphene and the robustness of these states, we expect magnetic edge states to be observable in experiments as well.
Theory of Magnetic Edge States in Chiral Graphene Nanoribbons
NASA Astrophysics Data System (ADS)
Capaz, Rodrigo; Yazyev, Oleg; Louie, Steven
2011-03-01
Using a model Hamiltonian approach including electron Coulomb interactions, we systematically investigate the electronic structure and magnetic properties of chiral graphene nanoribbons. We show that the presence of magnetic edge states is an intrinsic feature of any smooth graphene nanoribbons with chiral edges, and discover a number of structure-property relations. Specifically, we describe how the edge-state energy gap, zone-boundary edge-state energy splitting, and magnetic moment per edge length depend on the nanoribbon width and chiral angle. The role of environmental screening effects is also studied. Our results address a recent experimental observation of signatures of magnetic ordering at smooth edges of chiral graphene nanoribbons and provide an avenue towards tuning their properties via the structural and environmental degrees of freedom. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and the ONR MURI program. RBC acknowledges financial support from Brazilian agencies CNPq, FAPERJ and INCT-Nanomateriais de Carbono.
Computational fluid dynamics: A two-edged sword
Baker, A.J.; Kelso, R.M.; Gordon, E.B.; Roy, S.; Schaub, E.G.
1997-08-01
This article examines computational fluid dynamics (CFD) limitations as a design tool. Two decades have passed since the first paper was published in the ASHRAE Transactions suggesting the use of CFD for quantitative prediction of room air motion. CFD is an emerging methodology, with roots in the defense/aerospace industry, wherein a mathematical model of fluid flow is converted into a digital computational procedure, yielding numbers that approximate the solution of this modeled system, hence the genuine flow state. CFD methodology has indeed brought bright glimmers of an ability to establish firm quantitative data regarding how room air moves. In fact, CFD can predict fluid levels and pressure differences to very low levels, that are essentially impossible to experimentally measure. However, a CFD model constitutes the culmination of a large number of assumptions and approximations, such that the answers produced are essentially never correct. Further, it is the very approximation process in CFD theory that leads to intrinsic error mechanisms that can range from benign to pathological. The ASHRAE professional who seeks to use CFD to assist in system design needs to be fully aware of these two edges of the CFD sword.
Slope Edge Deformation and Permafrost Dynamics Along the Arctic Shelf Edge, Beaufort Sea, Canada
NASA Astrophysics Data System (ADS)
Paull, C. K.; Dallimore, S.; Caress, D. W.; Gwiazda, R.; Lundsten, E. M.; Anderson, K.; Riedel, M.; Melling, H.
2015-12-01
The shelf of the Canadian Beaufort Sea is underlain by relict offshore permafrost that formed in the long intervals of terrestrial exposure during glacial periods. At the shelf edge the permafrost thins rapidly and also warms. This area has a very distinct morphology that we attribute to both the formation and degradation of ice bearing permafrost. Positive relief features include circular to oval shaped topographic mounds, up to 10 m high and ~50 m in diameter which occur at a density of ~6 per km2. Intermixed are circular topographic depressions up to 20 m deep. This topography was investigated using an autonomous underwater vehicle that provides 1 m horizontal resolution bathymetry and chirp profiles, a remotely operated vehicle to document seafloor textures, and sediment cores to sample pore waters. A consistent down-core freshening at rates of 14 to 96 mM Cl- per meter was found in these pore waters near the shelf edge. Downward extrapolation of these trends indicates water with ≤335 mM Cl- should occur at 2.3 to 22.4 m sub-seafloor depths within this shelf edge deformation band. Pore water with 335 mM Cl- or less freezes at -1.4°C. As bottom water temperatures in this area are persistently (<-1.4°C) cold and ground ice was observed in some core samples, we interpret the volume changes associated with mound formation are in part due to pore water freezing. Thermal models (Taylor et al., 2014) predict brackish water along the shelf edge may be sourced in relict permafrost melting under the adjacent continental shelf. Buoyant brackish water is hypothesized to migrate along the base of the relict permafrost, to emerge at the shelf edge and then refreeze when it encounters the colder seafloor. Expansion generated by the formation of ice-bearing permafrost generates the positive relief mounds and ridges. The associated negative relief features may be related to permafrost dynamics also. Permafrost dynamics may have geohazard implications that are unique to the
Quantum pump in quantum spin Hall edge states
NASA Astrophysics Data System (ADS)
Cheng, Fang
2016-09-01
We present a theory for quantum pump in a quantum spin Hall bar with two quantum point contacts (QPCs). The pump currents can be generated by applying harmonically modulating gate voltages at QPCs. The phase difference between the gate voltages introduces an effective gauge field, which breaks the time-reversal symmetry and generates pump currents. The pump currents display very different pump frequency dependence for weak and strong e-e interaction. These unique properties are induced by the helical feature of the edge states, and therefore can be used to detect and control edge state transport.
The effect of leading edge tubercles on dynamic stall
NASA Astrophysics Data System (ADS)
Hrynuk, John
The effect of the leading edge tubercles of humpback whales has been heavily studied for their static benefits. These studies have shown that tubercles inhibit flow separation, limit spanwise flow, and extend the operating angle of a wing beyond the static stall point while maintaining lift, all while having a comparatively low negative impact on drag. The current study extends the prior work to investigating the effect of tubercles on dynamic stall, a fundamental flow phenomenon that occurs when wings undergo dynamic pitching motions. Flow fields around the wing models tested were studied using Laser Induced Fluorescence (LIF) and Molecular Tagging Velocimetry (MTV).Resulting velocity fields show that the dynamics of the formation and separation of the leading edge vortex were fundamentally different between the straight wing and the tubercled wing. Tracking of the Dynamic Stall Vortex (DSV) and Shear Layer Vortices (SLVs), which may have a significant impact on the overall flow behavior, was done along with calculations of vortex circulation. Proximity to the wing surface and total circulation were used to evaluate potential dynamic lift increases provided by the tubercles. The effects of pitch rate on the formation process and benefits of the tubercles were also studied and were generally consistent with prior dynamic stall studies. However, tubercles were shown to affect the SLV formation and the circulation differently at higher pitch rates.
Leading edge vortex dynamics on a pitching delta wing
NASA Technical Reports Server (NTRS)
Lemay, S. P.; Batill, S. M.; Nelson, R. C.
1988-01-01
A study of the dynamic behavior of the leading edge vortices on a delta wing undergoing oscillatory pitching motion is presented. A sharp edge, flat plate, delta wing having a sweep angle of 70 deg was used in this investigation. The wing was sinusoidally pitched about its 1/2 chord position at reduced frequencies ranging from k = 2(pi)fc/u = 0.05 to 0.30 at chord Reynolds numbers between 90,000 and 350,000, for angle of attack ranges of 29 to 39 deg and 0 to 45 deg. During these dynamic motions, visualization of the leading edge vortices was obtained by marking the vortices with TiCl4 introduced through ports located near the model apex. The location of vortex breakdown was recorded using high speed motion picture photography. The motion picture records were analyzed to determine vortex trajectory and breakdown position as a function of angle of attack. When the wing was sinusoidally pitched, a hysteresis was observed in the location of breakdown position. This hysteresis increased with reduced frequency. The velocity of breakdown propagation along the wing, and the phase lag between model motion and breakdown location were also determined. Detailed information was also obtained on the oscillation of breakdown position in both static and dynamic cases.
Stabilization and dynamics of edge flames in narrow channels
NASA Astrophysics Data System (ADS)
Bieri, Joanna A.
The dynamics of edge flames in narrow channels is studied, first within the context of a reactive diffusive (or constant density) model and then in a variable density model which allows for the consideration of thermal expansion effects. Fuel and oxidizer, separated upstream by a thin plate of finite length, flow into a channel with a prescribed upstream velocity. At the end of the plate, the fuel and oxidizer mix and, when ignited, an edge flame is sustained at some distance from the tip of the plate. Typically, the flame, which is stabilized by heat conduction back to the cold plate, has a tribrachial structure. It consists of a leading edge, made up of lean and rich premixed segments, and an attached diffusion flame trailing behind. The flame can also have a hook-like shape, when one of the premixed branches is missing. This often happens for conditions away from stoichiometry and when the mass diffusivities of the fuel and oxidizer are unequal. Earlier work has determined the behavior of an edge flame in a mixing layer that develops downstream of a splitter plate with no boundaries in the lateral direction. This is relevant to the stabilization and liftoff of jet diffusion flames. The confined case has other possible applications, such as flames in mini-combustor systems, that have been recently tested experimentally. The objective in this work is to determine the effect that confinement has on the edge standoff distance, on the flame shape and on the flame stability. In particular, we examine the influence of channel width, wall temperature, and the effects of differential diffusion. We determine conditions under which the edge flame is stabilized near the tip of the splitter plate, is held near the tip but oscillates back and forth, or is blown-off. We consider a wide range of channel widths and boundary conditions at the walls.
Measles on the edge: coastal heterogeneities and infection dynamics.
Bharti, Nita; Xia, Yingcun; Bjornstad, Ottar N; Grenfell, Bryan T
2008-01-01
Mathematical models can help elucidate the spatio-temporal dynamics of epidemics as well as the impact of control measures. The gravity model for directly transmitted diseases is currently one of the most parsimonious models for spatial epidemic spread. This model uses distance-weighted, population size-dependent coupling to estimate host movement and disease incidence in metapopulations. The model captures overall measles dynamics in terms of underlying human movement in pre-vaccination England and Wales (previously established). In spatial models, edges often present a special challenge. Therefore, to test the model's robustness, we analyzed gravity model incidence predictions for coastal cities in England and Wales. Results show that, although predictions are accurate for inland towns, they significantly underestimate coastal persistence. We examine incidence, outbreak seasonality, and public transportation records, to show that the model's inaccuracies stem from an underestimation of total contacts per individual along the coast. We rescue this predicted 'edge effect' by increasing coastal contacts to approximate the number of per capita inland contacts. These results illustrate the impact of 'edge effects' on epidemic metapopulations in general and illustrate directions for the refinement of spatiotemporal epidemic models. PMID:18398467
Majorana fermions and Dirac edge states in topological phases
NASA Astrophysics Data System (ADS)
Shivamoggi, Vasudha Bhimsen
In part 1, we study a realization of a chain of Majorana bound states at the interfaces between alternating ferromagnetic and superconducting regions at a quantum spin Hall insulator edge. In the limit of well separated Majoranas, the system can be mapped to the transverse field Ising model. The disordered critical point can be reached by tuning the relative magnitude or phases of the ferromagnetic and superconducting order parameters. We compute the voltage dependence of the tunneling current from a metallic tip into the Majorana chain as a direct probe of the random critical state. In part 2, we present an analytic prescription for computing the edge dispersion E( k) of a tight-binding Dirac Hamiltonian terminated at an abrupt crystalline edge. Specifically, we consider translationally invariant Dirac Hamiltonians with nearest-layer interaction. We present and prove a geometric formula that relates the existence of surface states as well as their energy dispersion to properties of the bulk Hamiltonian. We further prove the bulk-boundary correspondence between the Chern number and the chiral edge modes for quantum Hall systems within the class of Hamiltonians studied in the paper. Our results can be extended to the case of continuum theories which are quadratic in the momentum, as well as other symmetry classes.
Two-dimensional topological insulator edge state backscattering by dephasing
NASA Astrophysics Data System (ADS)
Essert, Sven; Krueckl, Viktor; Richter, Klaus
2015-11-01
To understand the seemingly absent temperature dependence in the conductance of two-dimensional topological insulator edge states, we perform a numerical study which identifies the quantitative influence of the combined effect of dephasing and elastic scattering in charge puddles close to the edges. We show that this mechanism may be responsible for the experimental signatures in HgTe/CdTe quantum wells if the puddles in the samples are large and weakly coupled to the sample edges. We propose experiments on artificial puddles which allow one to verify this hypothesis and to extract the real dephasing time scale using our predictions. In addition, we present a method to include the effect of dephasing in wave-packet-time-evolution algorithms.
Coexisting Edge States and Gapless Bulk in Topological States of Matter
NASA Astrophysics Data System (ADS)
Baum, Yuval; Posske, Thore; Fulga, Ion Cosma; Trauzettel, Björn; Stern, Ady
2015-04-01
We consider two-dimensional systems in which edge states coexist with a gapless bulk. Such systems may be constructed, for example, by coupling a gapped two-dimensional state of matter that carries edge states to a gapless two-dimensional system in which the spectrum is composed of a number of Dirac cones. We find that, in the absence of disorder, the edge states could be protected even when the two systems are coupled, due to momentum and energy conservation. We distinguish between weak and strong edge states by the level of their mixing with the bulk. In the presence of disorder, the edge states may be stabilized when the bulk is localized or destabilized when the bulk is metallic. We analyze the conditions under which these two cases occur. Finally, we propose a concrete physical realization for one of our models based on bilayer Hg(Cd)Te quantum wells.
Edge states for the Kalmeyer-Laughlin wave function
NASA Astrophysics Data System (ADS)
Herwerth, Benedikt; Sierra, Germán; Tu, Hong-Hao; Cirac, J. Ignacio; Nielsen, Anne E. B.
2015-12-01
We study lattice wave functions obtained from the SU(2)1 Wess-Zumino-Witten conformal field theory. Following Moore and Read's construction, the Kalmeyer-Laughlin fractional quantum Hall state is defined as a correlation function of primary fields. By an additional insertion of Kac-Moody currents, we associate a wave function with each state of the conformal field theory. These wave functions span the complete Hilbert space of the lattice system. On the cylinder, we study global properties of the lattice states analytically and correlation functions numerically using a Metropolis Monte Carlo method. By comparing short-range bulk correlations, numerical evidence is provided that the states with one current operator represent edge states in the thermodynamic limit. We show that the edge states with one Kac-Moody current of lowest order have a good overlap with low-energy excited states of a local Hamiltonian, for which the Kalmeyer-Laughlin state approximates the ground state. For some states, exact parent Hamiltonians are derived on the cylinder. These Hamiltonians are SU(2) invariant and nonlocal with up to four-body interactions.
NASA Astrophysics Data System (ADS)
Li, Xiang-Bing; Huang, Wen-Kai; Lv, Yang-Yang; Zhang, Kai-Wen; Yang, Chao-Long; Zhang, Bin-Bin; Chen, Y. B.; Yao, Shu-Hua; Zhou, Jian; Lu, Ming-Hui; Sheng, Li; Li, Shao-Chun; Jia, Jin-Feng; Xue, Qi-Kun; Chen, Yan-Feng; Xing, Ding-Yu
2016-04-01
We report an atomic-scale characterization of ZrTe5 by using scanning tunneling microscopy. We observe a bulk band gap of ˜80 meV with topological edge states at the step edge and, thus, demonstrate that ZrTe5 is a two-dimensional topological insulator. We also find that an applied magnetic field induces an energetic splitting of the topological edge states, which can be attributed to a strong link between the topological edge states and bulk topology. The relatively large band gap makes ZrTe5 a potential candidate for future fundamental studies and device applications.
A General Theorem Relating the Bulk Topological Number to Edge States in Two-dimensional Insulators
Qi, Xiao-Liang; Wu, Yong-Shi; Zhang, Shou-Cheng; /Stanford U., Phys. Dept. /Tsinghua U., Beijing
2010-01-15
We prove a general theorem on the relation between the bulk topological quantum number and the edge states in two dimensional insulators. It is shown that whenever there is a topological order in bulk, characterized by a non-vanishing Chern number, even if it is defined for a non-conserved quantity such as spin in the case of the spin Hall effect, one can always infer the existence of gapless edge states under certain twisted boundary conditions that allow tunneling between edges. This relation is robust against disorder and interactions, and it provides a unified topological classification of both the quantum (charge) Hall effect and the quantum spin Hall effect. In addition, it reconciles the apparent conflict between the stability of bulk topological order and the instability of gapless edge states in systems with open boundaries (as known happening in the spin Hall case). The consequences of time reversal invariance for bulk topological order and edge state dynamics are further studied in the present framework.
Statistical theory of relaxation of high-energy electrons in quantum Hall edge states
NASA Astrophysics Data System (ADS)
Lunde, Anders Mathias; Nigg, Simon E.
2016-07-01
We investigate theoretically the energy exchange between the electrons of two copropagating, out-of-equilibrium edge states with opposite spin polarization in the integer quantum Hall regime. A quantum dot tunnel coupled to one of the edge states locally injects electrons at high energy. Thereby a narrow peak in the energy distribution is created at high energy above the Fermi level. A second downstream quantum dot performs an energy-resolved measurement of the electronic distribution function. By varying the distance between the two dots, we are able to follow every step of the energy exchange and relaxation between the edge states, even analytically under certain conditions. In the absence of translational invariance along the edge, e.g., due to the presence of disorder, energy can be exchanged by non-momentum-conserving two-particle collisions. For weakly broken translational invariance, we show that the relaxation is described by coupled Fokker-Planck equations. From these we find that relaxation of the injected electrons can be understood statistically as a generalized drift-diffusion process in energy space for which we determine the drift velocity and the dynamical diffusion parameter. Finally, we provide a physically appealing picture in terms of individual edge-state heating as a result of the relaxation of the injected electrons.
Magnon edge states in the hardcore- Bose-Hubbard model.
Owerre, S A
2016-11-01
Quantum Monte Carlo (QMC) simulation has uncovered nonzero Berry curvature and bosonic edge states in the hardcore-Bose-Hubbard model on the gapped honeycomb lattice. The competition between the chemical potential and staggered onsite potential leads to an interesting quantum phase diagram comprising the superfluid phase, Mott insulator, and charge density wave insulator. In this paper, we present a semiclassical perspective of this system by mapping to a spin-1/2 quantum XY model. We give an explicit analytical origin of the quantum phase diagram, the Berry curvatures, and the edge states using semiclassical approximations. We find very good agreement between the semiclassical analyses and the QMC results. Our results show that the topological properties of the hardcore-Bose-Hubbard model are the same as those of magnon in the corresponding quantum spin system. Our results are applicable to systems of ultracold bosonic atoms trapped in honeycomb optical lattices. PMID:27603092
Edge-state blockade of transport in quantum dot arrays
NASA Astrophysics Data System (ADS)
Benito, Mónica; Niklas, Michael; Platero, Gloria; Kohler, Sigmund
2016-03-01
We propose a transport blockade mechanism in quantum dot arrays and conducting molecules based on an interplay of Coulomb repulsion and the formation of edge states. As a model we employ a dimer chain that exhibits a topological phase transition. The connection to a strongly biased electron source and drain enables transport. We show that the related emergence of edge states is manifest in the shot noise properties as it is accompanied by a crossover from bunched electron transport to a Poissonian process. For both regions we develop a scenario that can be captured by a rate equation. The resulting analytical expressions for the Fano factor agree well with the numerical solution of a full quantum master equation.
Optical isolation in topological-edge-state photonic arrays.
El-Ganainy, Ramy; Levy, Miguel
2015-11-15
We introduce a new type of optical isolator based on breaking time reversal symmetry in dissipative finite Su-Schrieffer-Heeger (SSH) waveguide arrays that support topological edge states at one end of the structure. In the forward propagation direction, light is launched into the edge waveguide to excite the localized topological midgap state. As a result, most of the input optical power is transmitted to the output port. On the other hand, backward reflected light encounters a propagation constant mismatch in that same channel which shifts the otherwise midgap state into one of the bands and hence becomes delocalized over the whole array. We show that under these conditions, a judicious spatial distribution of the optical dissipation across the structure can produce an isolation ratio of -50 dB. The required nonreciprocal phase shift is introduced by depositing a magnetic garnet film only on the edge waveguide and, thus, the required magnetic field can be generated by an integrated micromagnet. Similar concepts can also be applied to SSH arrays made from optical resonators. PMID:26565853
Andreev conversion of quantum Hall edge state in graphene
NASA Astrophysics Data System (ADS)
Lee, Gil-Ho; Hart, Sean; Wei, Di; Huang, Katie; Efetov, Dmitri; Taniguchi, Takashi; Watanabe, Kenji; Yacoby, Amir; Kim, Philip
Understanding the interplay between superconductivity (SC) and quantum Hall effect (QHE) has been a long-sought theoretical and experimental problem. SC contacts to QHE systems enable us to study interesting physics, such as Cooper pair injection into ballistic 2D channels, Andreev edge states, and emergent excitations of non-Abelian anyons. We developed an in-situ etching technique for highly transparent superconducting contact (NbN) to hBN encapsulated graphene channels. The high critical field of NbN electrodes (Hc 2 > 30 T) and the high quality of our graphene devices allows us to experimentally access a wide range of magnetic field where SC and QHE coexist. In order to probe the Andreev conversion of QH edge states, we measure the chemical potential of normal electrodes located on the upstream and the downstream QH edge states relative to a narrow grounded superconducting electrode. We observed that the chemical potential in downstream has sign opposite to the one measured in upstream suggesting Andreev conversion of incident electrons to outgoing holes across the narrow superconducting contact. We systematically investigated this phenomena as a function of temperature, magnetic field, bias voltage and the width and length of the superconducting electrode.
Majorana edge states in superconductor-noncollinear magnet interfaces
NASA Astrophysics Data System (ADS)
Chen, Wei; Schnyder, Andreas P.
2015-12-01
Through s -d coupling, a superconducting thin film interfaced to a noncollinear magnetic insulator inherits its magnetic order, which may induce unconventional superconductivity that hosts Majorana edge states. We present a unified formalism that covers the cycloidal, helical, and tilted conical order discovered in multiferroics, as well as Bloch and Neel domain walls of ferromagnetic insulators, and show that they induce (px+py )-wave pairing that supports Majorana edge modes. The advantages over one-dimensional proposals are that the Majorana states can exist without fine tuning of the chemical potential, can be stabilized in a much larger parameter space, and can be separated over the distance of long-range noncollinear order that is known to reach a macroscopic scale. A skyrmion spin texture, on the other hand, induces a nonuniform (pr+i pφ )-wave-like pairing under the influence of an emergent electromagnetic field, yielding a vortex state that displays both a bulk persistent current and a topological edge current.
Superposition State Molecular Dynamics.
Venkatnathan, Arun; Voth, Gregory A
2005-01-01
The ergodic sampling of rough energy landscapes is crucial for understanding phenomena like protein folding, peptide aggregation, polymer dynamics, and the glass transition. These rough energy landscapes are characterized by the presence of many local minima separated by high energy barriers, where Molecular Dynamics (MD) fails to satisfy ergodicity. To enhance ergodic behavior, we have developed the Superposition State Molecular Dynamics (SSMD) method, which uses a superposition of energy states to obtain an effective potential for the MD simulation. In turn, the dynamics on this effective potential can be used to sample the configurational free energy of the real potential. The effectiveness of the SSMD method for a one-dimensional rough potential energy landscape is presented as a test case. PMID:26641113
Thermodynamic signatures of edge states in Topological Insulators
NASA Astrophysics Data System (ADS)
Quelle, Anton; Cobanera, Emilio; Morais Smith, Cristinae
Topological insulators are states of matter distinguished by the presence of symmetry protected metallic boundary modes. These edge modes have been characterised in terms of transport and spectroscopic measurements, but a thermodynamic description has been lacking. The challenge arises because in conventional thermodynamics the potentials are required to scale linearly with extensive variables like volume, which does not allow for a general treatment of boundary effects. In this paper, we overcome this challenge with Hill thermodynamics. In this extension of the thermodynamic formalism, the grand potential is split into an extensive, conventional contribution, and the subdivision potential, which is the central construct of Hill's theory. For topologically non-trivial electronic matter, the subdivision potential captures measurable contributions to the density of states and the heat capacity: it is the thermodynamic manifestation of the topological edge structure. Furthermore, the subdivision potential reveals phase transitions of the edge even when they are not manifested in the bulk, thus opening a variety of new possibilities for investigating, manipulating, and characterizing topological quantum matter solely in terms of equilibrium boundary physics.
Visualizing edge states with an atomic Bose gas in the quantum Hall regime.
Stuhl, B K; Lu, H-I; Aycock, L M; Genkina, D; Spielman, I B
2015-09-25
Bringing ultracold atomic gases into the quantum Hall regime is challenging. We engineered an effective magnetic field in a two-dimensional lattice with an elongated-strip geometry, consisting of the sites of an optical lattice in the long direction and of three internal atomic spin states in the short direction. We imaged the localized states of atomic Bose-Einstein condensates in this strip; via excitation dynamics, we further observed both the skipping orbits of excited atoms traveling down the system's edges, analogous to edge magnetoplasmons in two-dimensional electron systems, and a dynamical Hall effect for bulk excitations. Our technique involves minimal heating, which will be important for spectroscopic measurements of the Hofstadter butterfly and realizations of Laughlin's charge pump. PMID:26404830
Graphene quantum dots: localized states, edges and bilayer systems
NASA Astrophysics Data System (ADS)
Ensslin, Klaus
2014-03-01
Graphene quantum dots show Coulomb blockade, excited states and their orbital and spin properties have been investigated in high magnetic fields. Most quantum dots fabricated to date are fabricated with electron beam lithography and dry etching which generally leads to uncontrolled and probably rough edges. We demonstrate that devices with reduced bulk disorder fabricated on BN substrates display similar localized states as those fabricated on the more standard SiO2 substrates. For a highly symmetric quantum dot with short tunnel barriers the experimentally detected transport features can be explained by three localized states, 1 in the dot and 2 in the constrictions. A way to overcome edge roughness and the localized states related to this are bilayer devices where a band gap can be induced by suitable top and back gate voltages. By placing bilayer graphene between two BN layers high electronic quality can be achieved as documented by the observation of broken symmetry states in the quantum Hall regime. We discuss how this method can be exploited to achieve smoother and better tunable graphene quantum devices. This work was done in collaboration with D. Bischoff, P. Simonet, A. Varlet, Y. Tian, and T. Ihn.
Artificial gauge fields and chiral edge states for ultracold fermions in synthetic dimensions
NASA Astrophysics Data System (ADS)
Fallani, Leonardo
2015-05-01
I will report on very recent experiments performed at LENS with ultracold 173Yb Fermi gases in artificial gauge fields. We have engineered Raman transitions between different 173Yb nuclear spin states to synthesize an effective lattice dynamics in a finite-sized ``extra dimension,'' which is encoded in the internal degree of freedom of the atoms. By using this innovative approach, we have realized synthetic magnetic fields for effectively-charged fermions in ladder geometries with a variable number of legs. Direct imaging of the individual legs allowed us to demonstrate the emergence of chiral edge currents and to observe edge-cyclotron orbits propagating along the edges of the system, thus providing a direct evidence of a fundamental feature of quantum Hall physics in condensed-matter systems.
Big-data-based edge biomarkers: study on dynamical drug sensitivity and resistance in individuals.
Zeng, Tao; Zhang, Wanwei; Yu, Xiangtian; Liu, Xiaoping; Li, Meiyi; Chen, Luonan
2016-07-01
Big-data-based edge biomarker is a new concept to characterize disease features based on biomedical big data in a dynamical and network manner, which also provides alternative strategies to indicate disease status in single samples. This article gives a comprehensive review on big-data-based edge biomarkers for complex diseases in an individual patient, which are defined as biomarkers based on network information and high-dimensional data. Specifically, we firstly introduce the sources and structures of biomedical big data accessible in public for edge biomarker and disease study. We show that biomedical big data are typically 'small-sample size in high-dimension space', i.e. small samples but with high dimensions on features (e.g. omics data) for each individual, in contrast to traditional big data in many other fields characterized as 'large-sample size in low-dimension space', i.e. big samples but with low dimensions on features. Then, we demonstrate the concept, model and algorithm for edge biomarkers and further big-data-based edge biomarkers. Dissimilar to conventional biomarkers, edge biomarkers, e.g. module biomarkers in module network rewiring-analysis, are able to predict the disease state by learning differential associations between molecules rather than differential expressions of molecules during disease progression or treatment in individual patients. In particular, in contrast to using the information of the common molecules or edges (i.e.molecule-pairs) across a population in traditional biomarkers including network and edge biomarkers, big-data-based edge biomarkers are specific for each individual and thus can accurately evaluate the disease state by considering the individual heterogeneity. Therefore, the measurement of big data in a high-dimensional space is required not only in the learning process but also in the diagnosing or predicting process of the tested individual. Finally, we provide a case study on analyzing the temporal expression
Entanglement and Majorana edge states in the Kitaev model
NASA Astrophysics Data System (ADS)
Mandal, Saptarshi; Maiti, Moitri; Varma, Vipin Kerala
2016-07-01
We investigate the von Neumann entanglement entropy and Schmidt gap in the vortex-free ground state of the Kitaev model on the honeycomb lattice for square/rectangular and cylindrical subsystems. We find that, for both the subsystems, the free-fermionic contribution to the entanglement entropy SE exhibits signatures of the phase transitions between the gapless and gapped phases. However, within the gapless phase, we find that SE does not show an expected monotonic behavior as a function of the coupling Jz between the suitably defined one-dimensional chains for either geometry; moreover, the system generically reaches a point of minimum entanglement within the gapless phase before the entanglement saturates or increases again until the gapped phase is reached. This may be attributed to the onset of gapless modes in the bulk spectrum and the competition between the correlation functions along various bonds. In the gapped phase, on the other hand, SE always monotonically varies with Jz independent of the subregion size or shape. Finally, further confirming the Li-Haldane conjecture, we find that the Schmidt gap Δ defined from the entanglement spectrum also signals the topological transitions but only if there are corresponding zero-energy Majorana edge states that simultaneously appear or disappear across the transitions. We analytically corroborate some of our results on entanglement entropy, the Schmidt gap, and the bulk-edge correspondence using perturbation theory.
Dual Transition Edge Sensor Bolometer for Enhanced Dynamic Range
NASA Technical Reports Server (NTRS)
Chervenak, J. A.; Benford, D. J.; Moseley, S. H.; Irwin, K. D.
2004-01-01
Broadband surveys at the millimeter and submillimeter wavelengths will require bolometers that can reach new limits of sensitivity and also operate under high background conditions. To address this need, we present results on a dual transition edge sensor (TES) device with two operating modes: one for low background, ultrasensitive detection and one for high background, enhanced dynamic range detection. The device consists of a detector element with two transition temperatures (T(sub c)) of 0.25 and 0.51 K located on the same micromachined, thermally isolated membrane structure. It can be biased on either transition, and features phonon-limited noise performance at the lower T(sub c). We measure noise performance on the lower transition 7 x 10(exp -18) W/rt(Hz) and the bias power on the upper transition of 12.5 pW, giving a factor of 10 enhancement of the dynamic range for the device. We discuss the biasable range of this type of device and present a design concept to optimize utility of the device.
The Seasonal Dynamics of Artificial Nest Predation Rates along Edges in a Mosaic Managed Reedbed
Malzer, Iain; Helm, Barbara
2015-01-01
Boundaries between different habitats can be responsible for changes in species interactions, including modified rates of encounter between predators and prey. Such ‘edge effects’ have been reported in nesting birds, where nest predation rates can be increased at habitat edges. The literature concerning edge effects on nest predation rates reveals a wide variation in results, even within single habitats, suggesting edge effects are not fixed, but dynamic throughout space and time. This study demonstrates the importance of considering dynamic mechanisms underlying edge effects and their relevance when undertaking habitat management. In reedbed habitats, management in the form of mosaic winter reed cutting can create extensive edges which change rapidly with reed regrowth during spring. We investigate the seasonal dynamics of reedbed edges using an artificial nest experiment based on the breeding biology of a reedbed specialist. We first demonstrate that nest predation decreases with increasing distance from the edge of cut reed blocks, suggesting edge effects have a pivotal role in this system. Using repeats throughout the breeding season we then confirm that nest predation rates are temporally dynamic and decline with the regrowth of reed. However, effects of edges on nest predation were consistent throughout the season. These results are of practical importance when considering appropriate habitat management, suggesting that reed cutting may heighten nest predation, especially before new growth matures. They also contribute directly to an overall understanding of the dynamic processes underlying edge effects and their potential role as drivers of time-dependent habitat use. PMID:26448338
The Seasonal Dynamics of Artificial Nest Predation Rates along Edges in a Mosaic Managed Reedbed.
Malzer, Iain; Helm, Barbara
2015-01-01
Boundaries between different habitats can be responsible for changes in species interactions, including modified rates of encounter between predators and prey. Such 'edge effects' have been reported in nesting birds, where nest predation rates can be increased at habitat edges. The literature concerning edge effects on nest predation rates reveals a wide variation in results, even within single habitats, suggesting edge effects are not fixed, but dynamic throughout space and time. This study demonstrates the importance of considering dynamic mechanisms underlying edge effects and their relevance when undertaking habitat management. In reedbed habitats, management in the form of mosaic winter reed cutting can create extensive edges which change rapidly with reed regrowth during spring. We investigate the seasonal dynamics of reedbed edges using an artificial nest experiment based on the breeding biology of a reedbed specialist. We first demonstrate that nest predation decreases with increasing distance from the edge of cut reed blocks, suggesting edge effects have a pivotal role in this system. Using repeats throughout the breeding season we then confirm that nest predation rates are temporally dynamic and decline with the regrowth of reed. However, effects of edges on nest predation were consistent throughout the season. These results are of practical importance when considering appropriate habitat management, suggesting that reed cutting may heighten nest predation, especially before new growth matures. They also contribute directly to an overall understanding of the dynamic processes underlying edge effects and their potential role as drivers of time-dependent habitat use. PMID:26448338
Measurement of peeling mode edge current profile dynamics.
Bongard, M W; Fonck, R J; Hegna, C C; Redd, A J; Schlossberg, D J
2011-07-15
Peeling modes, an instability mechanism underlying deleterious edge localized mode (ELM) activity in fusion-grade plasmas, are observed at the edge of limited plasmas in a low aspect ratio tokamak under conditions of high edge current density (J(edge) ∼ 0.1 MA/m2) and low magnetic field (B ∼ 0.1 T). They generate edge-localized, electromagnetic activity with low toroidal mode numbers n≤3 and amplitudes that scale strongly with measured J(edge)/B instability drive, consistent with theory. ELM-like field-aligned, current-carrying filaments form from an initial current-hole J(edge) perturbation that detach and propagate outward. PMID:21838369
Assessing the state of the art in edge detection: 1992
NASA Astrophysics Data System (ADS)
Boyer, Kim L.; Sarkar, S.
1992-03-01
Hoping the reader will not find the title overly pompous, we offer a brief and decidedly informal view of the state of the edge detection art, as we see it, in early 1992. We make no claim to clairvoyance, nor even to being especially insightful. But we have looked over the recent literature and made some attempt to evaluate where we are as a community with respect to this most ubiquitous problem and where we should be headed. We also briefly summarize the work of this session and our own recent contributions to compare the spectrum of philosophies represented to the community at large. This paper should be taken in the spirit in which it was written, which is to say not too seriously. Our aim is by no means frivolous, but we did try to have a little fun while dabbling as futurists. The ultimate goal of this paper is to stimulate some interesting interchange not so much on the `how to' of edge detection as on the `what next.'
Magnetic edge states in Aharonov-Bohm graphene quantum rings
NASA Astrophysics Data System (ADS)
Farghadan, R.; Saffarzadeh, A.; Heidari Semiromi, E.
2013-12-01
The effect of electron-electron interaction on the electronic structure of Aharonov-Bohm (AB) graphene quantum rings (GQRs) is explored theoretically using the single-band tight-binding Hamiltonian and the mean-field Hubbard model. The electronic states and magnetic properties of hexagonal, triangular, and circular GQRs with different sizes and zigzag edge terminations are studied. The results show that, although the AB oscillations in the all types of nanoring are affected by the interaction, the spin splitting in the AB oscillations strongly depends on the geometry and the size of graphene nanorings. We found that the total spin of hexagonal and circular rings is zero and therefore, no spin splitting can be observed in the AB oscillations. However, the non-zero magnetization of the triangular rings breaks the degeneracy between spin-up and spin-down electrons, which produces spin-polarized AB oscillations.
Spatio-Temporal Analysis of Forest Edge Dynamics in South Western Amazonia
NASA Astrophysics Data System (ADS)
Numata, I.; Cochrane, M. A.; Roberts, D. A.; Soares, J. V.
2008-12-01
Beyond removing forest, deforestation in the Amazon creates a lot of forest edges. These edges change the microclimate and ecosystem dynamics of the remaining tropical rain forests, contributing directly to forest degradation in the Amazon. Edge-induced changes such as tree mortality and fire vulnerability occur as a function of distance from edges and time since forest fragmentation. New edges are created and older edges are eliminated constantly as deforestation advances. However, Amazon forest edge dynamics over time and space are not well understood. We need to improve our knowledge about forest edge dynamics in order to estimate the actual amount of forest degradation caused by forest fragmentation. In this study, we performed deep spatio-temporal analyses of forest fragmentation for Rondônia, in the southwestern Amazon, using a multitemporal Landsat dataset (1984-2005). Our goals were to: 1) calculate erosion/persistence of forest edges; 2) detect edge age-composition of all forest edges and; 3) estimate total degraded forest area due to forest edge effects. Two counties of different stages of deforestation were selected. Campo Novo de Rondônia (early stage) and Ouro Preto (final stage). Overall, more than 50% of forest edges were eliminated in the first four years, while only 20% of edges survived more than 10 years after edge creation. The composition of edge-ages differs according to the stage of deforestation. Between 2001 and 2005, nearly 60% of forest edges in recently developed Campo Novo de Rondônia were 0-4 years old, with only 20% > 10 years old. Conversely, in the old frontier Ouro Preto region, only 23% of forest edges were 0-4 years old and 50% were > 10 years old. These results suggest that high edge erosion rates in the years following edge creation may cause many edges disappear before they experience the complete process of edge-induced changes such as biomass collapse, potentially reducing the estimated impact of existing forest edges on
Boundary-induced dynamics in one-dimensional topological systems and memory effects of edge modes
NASA Astrophysics Data System (ADS)
He, Yan; Chien, Chih-Chun
2016-07-01
Dynamics induced by a change of boundary conditions reveals rate-dependent signatures associated with topological properties in one-dimensional Kitaev chain and SSH model. While the perturbation from a change of the boundary propagates into the bulk, the density of topological edge modes in the case of transforming to open boundary condition reaches steady states. The steady-state density depends on the transformation rate of the boundary and serves as an illustration of quantum memory effects in topological systems. Moreover, while a link is physically broken as the boundary condition changes, some correlation functions can remain finite across the broken link and keep a record of the initial condition. By testing those phenomena in the nontopological regimes of the two models, none of the interesting signatures of memory effects can be observed. Our results thus contrast the importance of topological properties in boundary-induced dynamics.
Qubit transient dynamics at tunneling Fermi-edge singularity
NASA Astrophysics Data System (ADS)
Ponomarenko, V. V.; Larkin, I. A.
2016-03-01
We consider tunneling of spinless electrons from a single-channel emitter into an empty collector through an interacting resonant level of the quantum dot. When all Coulomb screening of sudden charge variations of the dot during the tunneling is realized by the emitter channel, the system is described with an exactly solvable model of a dissipative qubit. We derive the corresponding Bloch equation for its quantum evolution. We further use it to specify the qubit transient dynamics towards its stationary quantum state after a sudden change of the level position. We demonstrate that the time-dependent tunneling current characterizing this dynamics exhibits an oscillating behavior for a wide range of the model parameters.
Dynamics of edge dislocations in a sheared lamellar mesophase
NASA Astrophysics Data System (ADS)
Kumaran, V.
2013-10-01
The dynamics and interactions of edge dislocations in a nearly aligned sheared lamellar mesophase is analysed to provide insights into the relationship between disorder and rheology. First, the mesoscale permeation and momentum equations for the displacement field in the presence of external forces are derived from the model H equations for the concentration and momentum field. The secondary flow generated due to the mean shear around an isolated defect is calculated, and the excess viscosity due to the presence of the defect is determined from the excess energy dissipation due to the secondary flow. The excess viscosity for an isolated defect is found to increase with system size in the cross-stream direction as L3/2 for an isolated defect, though this divergence is cut-off due to interactions in a defect suspension. As the defects are sheared past each other due to the mean flow, the Peach-Koehler force due to elastic interaction between pairs of defects is found to cause no net displacement relative to each other as they approach from large separation to the distance of closest approach. The equivalent force due to viscous interactions is found to increase the separation for defects of opposite sign, and decrease the separation for defects of same sign. During defect interactions, we find that there is no buckling instability due to dilation of layers for systems of realistic size. However, there is another mechanism, which is the velocity difference generated across a slightly deformed bilayer due to the mean shear, which could result in the creation of new defects.
Effect of Trailing Edge Shape on the Unsteady Aerodynamics of Reverse Flow Dynamic Stall
NASA Astrophysics Data System (ADS)
Lind, Andrew; Jones, Anya
2015-11-01
This work considers dynamic stall in reverse flow, where flow travels over an oscillating airfoil from the geometric trailing edge towards the leading edge. An airfoil with a sharp geometric trailing edge causes early formation of a primary dynamic stall vortex since the sharp edge acts as the aerodynamic leading edge in reverse flow. The present work experimentally examines the potential merits of using an airfoil with a blunt geometric trailing edge to delay flow separation and dynamic stall vortex formation while undergoing oscillations in reverse flow. Time-resolved and phase-averaged flow fields and pressure distributions are compared for airfoils with different trailing edge shapes. Specifically, the evolution of unsteady flow features such as primary, secondary, and trailing edge vortices is examined. The influence of these flow features on the unsteady pressure distributions and integrated unsteady airloads provide insight on the torsional loading of rotor blades as they oscillate in reverse flow. The airfoil with a blunt trailing edge delays reverse flow dynamic stall, but this leads to greater downward-acting lift and pitching moment. These results are fundamental to alleviating vibrations of high-speed helicopters, where much of the rotor operates in reverse flow.
Exogenous antioxidants—Double-edged swords in cellular redox state
Bohn, Torsten
2010-01-01
The balance between oxidation and antioxidation is believed to be critical in maintaining healthy biological systems. Under physiological conditions, the human antioxidative defense system including e.g., superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione (GSH) and others, allows the elimination of excess reactive oxygen species (ROS) including, among others superoxide anions (O2.-), hydroxyl radicals (OH.), alkoxyl radicals (RO.) and peroxyradicals (ROO.). However, our endogenous antioxidant defense systems are incomplete without exogenous originating reducing compounds such as vitamin C, vitamin E, carotenoids and polyphenols, playing an essential role in many antioxidant mechanisms in living organisms. Therefore, there is continuous demand for exogenous antioxidants in order to prevent oxidative stress, representing a disequilibrium redox state in favor of oxidation. However, high doses of isolated compounds may be toxic, owing to prooxidative effects at high concentrations or their potential to react with beneficial concentrations of ROS normally present at physiological conditions that are required for optimal cellular functioning. This review aims to examine the double-edged effects of dietary originating antioxidants with a focus on the most abundant compounds, especially polyphenols, vitamin C, vitamin E and carotenoids. Different approaches to enrich our body with exogenous antioxidants such as via synthetic antioxidants, diets rich in fruits and vegetables and taking supplements will be reviewed and experimental and epidemiological evidences discussed, highlighting that antioxidants at physiological doses are generally safe, exhibiting interesting health beneficial effects. PMID:20972369
High dynamic range infrared images detail enhancement based on local edge preserving filter
NASA Astrophysics Data System (ADS)
Song, Qiong; Wang, Yuehuan; Bai, Kun
2016-07-01
In the field of infrared (IR) image processing, displaying a high dynamic range (HDR) image on a low dynamic range display equipment with a natural visual effect, clear details on local areas and less artifacts is an important issue. In this paper, we present a new approach to display HDR IR images with contrast enhancement. First, the local edge-preserving filter (LEPF) is utilized to separate the image into a base layer and detail layer(s). After the filtering procedure, we use an adaptive Gamma transformation to adjust the gray distribution of the base layer, and stretch the detail layer based on a human visual effect principle. Then, we recombine the detail layer and base layer to obtain the enhance output. Finally, we adjust the luminance of output by applying multiple exposure fusion method. The experimental results demonstrate that our proposed method can provide a significant performance in terms of enhancing details and less artifacts than the state of the arts.
Geometric phase in p -n junctions of helical edge states
NASA Astrophysics Data System (ADS)
Wadhawan, Disha; Mehta, Poonam; Das, Sourin
2016-02-01
The quantum spin Hall effect is endowed with topologically protected edge modes with a gapless Dirac spectrum. Applying a magnetic field locally along the edge leads to a gapped edge spectrum with the opposite parity for winding of spin texture for conduction and valence bands. Using Pancharatnam's prescription for the geometric phase it is shown that mismatch of this parity across a p -n junction, which could be engineered into the edge by electrical gate induced doping, leads to a phase dependence in the two-terminal conductance which is quantized to either zero or π . It is further shown that application of a nonuniform magnetic field across the junction could lead to a nonquantized value of this geometric phase which is tunable between zero and π . A current asymmetry measurement which is shown to be robust against electron-electron interactions is proposed to infer the appearance of this Pancharatnam's geometric phase in transport across such junctions.
Edge State Structure of the ν = 0 quantum Hall State in monolayer Graphene
NASA Astrophysics Data System (ADS)
Knothe, Angelika; Jolicoeur, Thierry
Single-layer graphene at neutrality under a magnetic field is a many-body insulator whose phase structure is under intense scrutiny. When tilting the applied magnetic field, there is a phase transition towards a conducting state. A plausible description is to start from a SU(4) spin-valley symmetric quantum Hall ferromagnet and add some lattice-scale anisotropies in valley space. In the manifold of ground states captured by this approach, it has been proposed that graphene undergoes a transition between a canted antiferromagnetic state and a ferromagnetic state. While this picture is clear in the bulk of the system, it remains to understand the effect of this phase change on the current-carrying edge states that are formed a the physical boundaries of a real sample. We use an extended Hartree-Fock approach to describe a finite-size system with a simple model for the edge and extract the one-body spectrum. We then describe the current-carrying edge textures.
Leading edge vortex dynamics on a pitching delta wing. M.S. Thesis
NASA Technical Reports Server (NTRS)
Lemay, Scott P.
1988-01-01
The leading edge flow structure was investigated on a 70 deg flat plate delta wing which was pitched about its 1/2 chord position, to increase understanding of the high angle of attack aerodynamics on an unsteady delta wing. The wing was sinusoidally pitched at reduced frequencies ranging from k being identical with 2pi fc/u = 0.05 to 0.30 at chord Reynolds numbers between 90,000 and 350,000, for angle of attack ranges of alpha = 29 to 39 deg and alpha = 0 to 45 deg. The wing was also impulsively pitched at an approximate rate of 0.7 rad/s. During these dynamic motions, visualization of the leading edge vorticies was obtained by entraining titanium tetrachloride into the flow at the model apex. The location of vortex breakdown was recorded using 16mm high speed motion picture photography. When the wing was sinusoidally pitched, a hysteresis was observed in the location of breakdown position. This hysteresis increased with reduced frequency. The velocity of breakdown propagation along the wing, and the phase lag between model motion and breakdown location were also determined. When the wing was impulsively pitched, several convective times were required for the vortex flow to reach a steady state. Detailed information was also obtained on the oscillation of breakdown position in both static and dynamic cases.
Self-induced topological transitions and edge states supported by nonlinear staggered potentials
NASA Astrophysics Data System (ADS)
Hadad, Yakir; Khanikaev, Alexander B.; Alò, Andrea
2016-04-01
The canonical Su-Schrieffer-Heeger (SSH) array is one of the basic geometries that have spurred significant interest in topological band-gap modes. Here, we show that the judicious inclusion of third-order Kerr nonlinearities in SSH arrays opens rich physics in topological insulators, including the possibility of supporting self-induced topological transitions, as a function of the applied intensity. We highlight the emergence of a class of topological solutions in nonlinear SSH arrays localized at the array edges and with unusual properties. As opposed to their linear counterparts, these nonlinear states decay to a plateau of nonzero amplitude inside the array, highlighting the local nature of topologically nontrivial band gaps in nonlinear systems. We study the conditions under which these states can be excited and their temporal dynamics as a function of the applied excitation, paving the way to interesting directions in the physics of topological edge states with robust propagation properties based on nonlinear interactions in suitably designed periodic arrays.
Edge effect on a vacancy state in semi-infinite graphene
NASA Astrophysics Data System (ADS)
Deng, Hai-Yao; Wakabayashi, Katsunori
2014-09-01
The edge effect on a single vacancy state of semi-infinite graphene (SIG) has been studied using Green's function method within the tight-binding model. In the case of infinite graphene, it is known that a vacancy induces a zero-energy resonance state, whose wave function decays inversely with distance (R) from the vacancy and is not normalizable. However, for SIG with an armchair edge, we find that the corresponding wave function decays as R-2 and hence becomes normalizable owing to the intervalley interference caused by the armchair edge. For SIG with a zigzag edge, the vacancy state depends on the sublattice of the vacancy. When the vacancy and the edge belong to different sublattices, the vacancy has no effect on the zero-energy vacancy state. In contrast, when the vacancy is located on the same sublattice as the edge, the resonance state disappears but the wave function at zero energy is strongly distorted near the vacancy. Our results reveal that the presence of edges crucially changes the vacancy state in graphene, and thus such a state can be used to probe the edge structure.
Construction of edge states in fractional quantum Hall systems by Jack polynomials
NASA Astrophysics Data System (ADS)
Lee, Ki Hoon; Hu, Zi-Xiang; Wan, Xin
2014-04-01
We study the edge-mode excitations of a fractional quantum Hall droplet by expressing the edge-state wave functions as linear combinations of Jack polynomials with a negative parameter. We show that the exact diagonalization within a subspace of Jack polynomials can be used to generate the chiral edge-mode excitation spectrum in the Laughlin phase and the Moore-Read phase with realistic Coulomb interaction. The truncation technique for the edge excitations simplifies the procedure to reliably extract the edge-mode velocities, which avoids the otherwise complicated analysis of the full spectrum that contains both edge and bulk excitations. Generalization to the Read-Rezayi state is also discussed.
Instability of bosonic topological edge states in the presence of interactions
NASA Astrophysics Data System (ADS)
Lumer, Yaakov; Rechtsman, Mikael C.; Plotnik, Yonatan; Segev, Mordechai
2016-08-01
We analyze the stability of extended edge modes in a nonlinear (i.e., interacting) bosonic topological insulator. We see that these nonlinear modes are always unstable, despite the topological protection of edge modes in the linear system. For concreteness we use a photonic platform, but the results generalize to other bosonic systems. We give a detailed description of the system in two extreme cases, low nonlinearity and high nonlinearity, and discuss the breakup of the nonlinear edge states into solitons.
Edge-states ferromagnetism of WS{sub 2} nanosheets
Huo, Nengjie; Li, Yan; Kang, Jun; Li, Renxiong; Xia, Qinglin; Li, Jingbo
2014-05-19
The multilayer WS{sub 2} nanosheets prepared from WO{sub 3} nanowires exhibit strong ferromagnetic behavior with saturation magnetization (M{sub S}) of 0.0058 emu/g and coercive field (H{sub C}) of 92 Oe at room temperature. By decreasing the temperature down to 3 K the H{sub c} is increased up to 1115 Oe, revealing the existence of long-range magnetic ordering. Density functional theory spin-polarized calculations predict that strong ferromagnetic moments in WS{sub 2} nanosheets are attributed to the zigzag edge sulphur S and tungsten W atoms. Our findings also suggest that the WS{sub 2} nanosheets with a high density of edge spins could be used to fabricate spintronics devices, which are circuits utilizing the spin of the electron to process and store information.
Edge localized mode rotation and the nonlinear dynamics of filaments
NASA Astrophysics Data System (ADS)
Morales, J. A.; Bécoulet, M.; Garbet, X.; Orain, F.; Dif-Pradalier, G.; Hoelzl, M.; Pamela, S.; Huijsmans, G. T. A.; Cahyna, P.; Fil, A.; Nardon, E.; Passeron, C.; Latu, G.
2016-04-01
Edge Localized Modes (ELMs) rotating precursors were reported few milliseconds before an ELM crash in several tokamak experiments. Also, the reversal of the filaments rotation at the ELM crash is commonly observed. In this article, we present a mathematical model that reproduces the rotation of the ELM precursors as well as the reversal of the filaments rotation at the ELM crash. Linear ballooning theory is used to establish a formula estimating the rotation velocity of ELM precursors. The linear study together with nonlinear magnetohydrodynamic simulations give an explanation to the rotations observed experimentally. Unstable ballooning modes, localized at the pedestal, grow and rotate in the electron diamagnetic direction in the laboratory reference frame. Approaching the ELM crash, this rotation decreases corresponding to the moment when the magnetic reconnection occurs. During the highly nonlinear ELM crash, the ELM filaments are cut from the main plasma due to the strong sheared mean flow that is nonlinearly generated via the Maxwell stress tensor.
One-dimensional edge state of Bi thin film grown on Si(111)
Kawakami, Naoya; Lin, Chun-Liang; Kawai, Maki; Takagi, Noriaki; Arafune, Ryuichi
2015-07-20
The geometric and electronic structures of the Bi thin film grown on Si(111) were investigated by using scanning tunneling microscopy and spectroscopy. We have found two types of edges, one of which hosts an electronic state localized one-dimensionally. We also revealed the energy dispersion of the localized edge state from the evolution of quasiparticle interference patterns as a function of energy. These spectroscopic findings well reproduce those acquired for the cleaved surface of the bulk Bi crystal [I. K. Drozdov et al., Nat. Phys. 10, 664 (2014)]. The present results indicate that the deposited Bi film provides a tractable stage for further scrutiny of the one-dimensional edge state.
Direct imaging of topological edge states at a bilayer graphene domain wall.
Yin, Long-Jing; Jiang, Hua; Qiao, Jia-Bin; He, Lin
2016-01-01
The AB-BA domain wall in gapped graphene bilayers is a rare naked structure hosting topological electronic states. Although it has been extensively studied in theory, a direct imaging of its topological edge states is still missing. Here we image the topological edge states at the graphene bilayer domain wall by using scanning tunnelling microscope. The simultaneously obtained atomic-resolution images of the domain wall provide us unprecedented opportunities to measure the spatially varying edge states within it. The one-dimensional conducting channels are observed to be mainly located around the two edges of the domain wall, which is reproduced quite well by our theoretical calculations. Our experiment further demonstrates that the one-dimensional topological states are quite robust even in the presence of high magnetic fields. The result reported here may raise hopes of graphene-based electronics with ultra-low dissipation. PMID:27312315
Direct imaging of topological edge states at a bilayer graphene domain wall
NASA Astrophysics Data System (ADS)
Yin, Long-Jing; Jiang, Hua; Qiao, Jia-Bin; He, Lin
2016-06-01
The AB-BA domain wall in gapped graphene bilayers is a rare naked structure hosting topological electronic states. Although it has been extensively studied in theory, a direct imaging of its topological edge states is still missing. Here we image the topological edge states at the graphene bilayer domain wall by using scanning tunnelling microscope. The simultaneously obtained atomic-resolution images of the domain wall provide us unprecedented opportunities to measure the spatially varying edge states within it. The one-dimensional conducting channels are observed to be mainly located around the two edges of the domain wall, which is reproduced quite well by our theoretical calculations. Our experiment further demonstrates that the one-dimensional topological states are quite robust even in the presence of high magnetic fields. The result reported here may raise hopes of graphene-based electronics with ultra-low dissipation.
Direct imaging of topological edge states at a bilayer graphene domain wall
Yin, Long-Jing; Jiang, Hua; Qiao, Jia-Bin; He, Lin
2016-01-01
The AB–BA domain wall in gapped graphene bilayers is a rare naked structure hosting topological electronic states. Although it has been extensively studied in theory, a direct imaging of its topological edge states is still missing. Here we image the topological edge states at the graphene bilayer domain wall by using scanning tunnelling microscope. The simultaneously obtained atomic-resolution images of the domain wall provide us unprecedented opportunities to measure the spatially varying edge states within it. The one-dimensional conducting channels are observed to be mainly located around the two edges of the domain wall, which is reproduced quite well by our theoretical calculations. Our experiment further demonstrates that the one-dimensional topological states are quite robust even in the presence of high magnetic fields. The result reported here may raise hopes of graphene-based electronics with ultra-low dissipation. PMID:27312315
Identifying topological edge states in 2D optical lattices using light scattering
NASA Astrophysics Data System (ADS)
Goldman, Nathan; Beugnon, Jérôme; Gerbier, Fabrice
2013-02-01
We recently proposed in a Letter [Phys. Rev. Lett. 108, 255303] a novel scheme to detect topological edge states in an optical lattice, based on a generalization of Bragg spectroscopy. The scope of the present article is to provide a more detailed and pedagogical description of the system - the Hofstadter optical lattice - and probing method. We first show the existence of topological edge states, in an ultra-cold gas trapped in a 2D optical lattice and subjected to a synthetic magnetic field. The remarkable robustness of the edge states is verified for a variety of external confining potentials. Then, we describe a specific laser probe, made from two lasers in Laguerre-Gaussian modes, which captures unambiguous signatures of these edge states. In particular, the resulting Bragg spectra provide the dispersion relation of the edge states, establishing their chiral nature. In order to make the Bragg signal experimentally detectable, we introduce a "shelving method", which simultaneously transfers angular momentum and changes the internal atomic state. This scheme allows to directly visualize the selected edge states on a dark background, offering an instructive view on topological insulating phases, not accessible in solid-state experiments.
Electronic states of zigzag graphene nanoribbons with edges reconstructed with topological defects
NASA Astrophysics Data System (ADS)
Pincak, R.; Smotlacha, J.; Osipov, V. A.
2015-10-01
The energy spectrum and electronic density of states (DOS) of zigzag graphene nanoribbons with edges reconstructed with topological defects are investigated within the tight-binding method. In case of the Stone-Wales zz(57) edge the low-energy spectrum is markedly changed in comparison to the pristine zz edge. We found that the electronic DOS at the Fermi level is different from zero at any width of graphene nanoribbons. In contrast, for ribbons with heptagons only at one side and pentagons at another one the energy gap at the Fermi level is open and the DOS is equal to zero. The reason is the influence of uncompensated topological charges on the localized edge states, which are topological in nature. This behavior is similar to that found for the structured external electric potentials along the edges.
Edge states in the transition to turbulence in pipe and other shear flows
NASA Astrophysics Data System (ADS)
Eckhardt, Bruno; Skufca, Joseph D.; Yorke, James A.
2005-11-01
We study the boundary of the laminar region in pipe and other shear flows near the onset of turbulence. Approaching the boundary from the laminar side, the lifetime of perturbations increases, and it diverges when the boundary is reached. Once this critical amplitude is exceeded the trajectory swings up to the turbulent regime, but its lifetime varies sensitively with amplitude, consistent with the strange saddle picture of the turbulence proposed earlier. The edge trajectory is asymptotic to a single well defined state, independent of the type of perturbation. The edge then becomes the stable manifold of this structure. In the case of a model shear flow, the edge states are simple or period doubled or chaotic trajectories. The case of pipe flow shows less variability and the edge state seems to remain close to a state with simple vortex structure.
Bounds on probability of state transfer with respect to readout time and edge weight
NASA Astrophysics Data System (ADS)
Gordon, Whitney; Kirkland, Steve; Li, Chi-Kwong; Plosker, Sarah; Zhang, Xiaohong
2016-02-01
We analyze the sensitivity of a spin chain modeled by an undirected weighted connected graph exhibiting perfect state transfer to small perturbations in readout time and edge weight in order to obtain physically relevant bounds on the probability of state transfer. At the heart of our analysis is the concept of the numerical range of a matrix; our analysis of edge weight errors additionally makes use of the spectral and Frobenius norms.
Finite size effects on the helical edge states on the Lieb lattice
NASA Astrophysics Data System (ADS)
Rui, Chen; Bin, Zhou
2016-06-01
For a two-dimensional Lieb lattice, that is, a line-centered square lattice, the inclusion of the intrinsic spin–orbit (ISO) coupling opens a topologically nontrivial gap, and gives rise to the quantum spin Hall (QSH) effect characterized by two pairs of gapless helical edge states within the bulk gap. Generally, due to the finite size effect in QSH systems, the edge states on the two sides of a strip of finite width can couple together to open a gap in the spectrum. In this paper, we investigate the finite size effect of helical edge states on the Lieb lattice with ISO coupling under three different kinds of boundary conditions, i.e., the straight, bearded and asymmetry edges. The spectrum and wave function of edge modes are derived analytically for a tight-binding model on the Lieb lattice. For a strip Lieb lattice with two straight edges, the ISO coupling induces the Dirac-like bulk states to localize at the edges to become the helical edge states with the same Dirac-like spectrum. Moreover, it is found that in the case with two straight edges the gapless Dirac-like spectrum remains unchanged with decreasing the width of the strip Lieb lattice, and no gap is opened in the edge band. It is concluded that the finite size effect of QSH states is absent in the case with the straight edges. However, in the other two cases with the bearded and asymmetry edges, the energy gap induced by the finite size effect is still opened with decreasing the width of the strip. It is also proposed that the edge band dispersion can be controlled by applying an on-site potential energy on the outermost atoms. Project supported by the National Natural Science Foundation of China (Grant No. 11274102), the Program for New Century Excellent Talents in University of the Ministry of Education of China (Grant No. NCET-11-0960), and the Specialized Research Fund for the Doctoral Program of the Higher Education of China (Grant No. 20134208110001).
Quantum interferences and edge states in Bismuth based Josephson junctions
NASA Astrophysics Data System (ADS)
Murani, Anil; Sengupta, Shamashis; Kasumov, Alik; Gueron, Sophie; Bouchiat, Hélène; MESO group Team
We have investigated proximity induced superconductivity in single crystal bismuth nanowires connected to superconducting electrodes with a high critical field. I will specially report recent results on nanowires whose crystalline orientation could be determined by electron diffraction. At low temperature a supercurrent is measured which persists up to the critical field of the electrodes and exhibits sample dependent fast squid-like oscillations (period one to few hundred gauss) modulated by slower (few thousand Gauss) oscillations. We attribute this striking result to the appearance of 1D topological edge channels on special surfaces of Bi due to its strong spin-orbit coupling, in addition to a strong Zeeman effect caused by an unusually high g-factor.
Analytical approach to the edge state of the Kane-Mele model
NASA Astrophysics Data System (ADS)
Doh, Hyeonjin; Jeon, Gun Sang; Choi, Hyoung Joon
2014-03-01
We investigate the edge state of a two-dimensional topological insulator based on the Kane- Mele model. We consider the two semi-infinite honeycomb lattices with a zig-zag and an armchair boundary, respectively. We construct the effective Hamiltonians for the edge states assuming exponentially decaying wave functions. With the boundary conditions for the both types of the boundaries, we derive the self-consistent equations for the energies and the decaying factors of the edge states. The numerical solutions of the self-consistent equations exhibit intriguing spatial behaviors of the edge states with respect to the spin-orbit coupling and the sub-lattice potential. We found the bifurcation behavior of the edge state width with respect to the sub-lattice potential in zigzag boundary. The bifurcation behavior discriminates the boundary dependencies of the edge state properties. We also discuss the relation between the sample size and the interaction parameters in the phase transition from normal insulator to topological insulator. This work was supported by NRF of Korea (Grant No. 2011-0018306).
Experimental Investigation of Dynamic Stall on an Airfoil with Leading Edge Tubercles
NASA Astrophysics Data System (ADS)
Hrynuk, John; Bohl, Douglas
2013-11-01
Humpback whales are unique in that their flippers have leading edge ``bumps'' or tubercles. Past work on airfoils modeled after whale flippers has centered on the static aerodynamic characteristics of these airfoils. In the current work, NACA 0012 airfoils modified with leading edge tubercles are investigated to determine the effect of the tubercles on the dynamic characteristics, specifically on dynamic stall vortex formation, of the airfoils. Molecular Tagging Velocimetry (MTV) is used to measure the flow field around the modified airfoils at nondimensional pitch rates of Ω = 0.1, 0.2, and 0.4. The results show that the characteristics of the dynamics stall vortex are dependent on the location relative to the peak or valley of the leading edge bumps. These characteristics are also found to be different than those observed in dynamic stall on a smooth leading edge airfoil. In specific, the location of the dynamic stall vortex appears to form further aft on the airfoil for the tubercle case versus the smooth case. This work supported by NSF Grant # 0845882.
NASA Astrophysics Data System (ADS)
Gall, Peter D.
The aircraft industry, as a whole, has been deeply concerned with improving the aerodynamic efficiency of current and future flight vehicles, particularly in the commercial and military markets. However, of particular interest to the field of aerodynamics is the elusive concept of a workable flow control mechanism. Effective flow control is a concept which if properly applied can increase aerodynamic efficiency. Various concepts and ideas to obtain successful flow control have been studied in an attempt to reap these rewards. Some examples include boundary layer blowing (steady and periodic), suction, and compliant walls for laminar flow control. The overall goal of flow control is to increase performance by increasing lift, reducing drag, and delaying or eliminating leading edge separation. The specific objectives of flow control are to (1) delay or eliminate flow separation, (2) delay boundary layer transition, and (3) and reduce skin friction drag. The purpose of this research is to investigate dynamic roughness as a novel method of flow control technology for external boundary layer flows. As opposed to standard surface roughness, dynamic roughness incorporates small time dependent perturbations to the surface of the airfoil. These surface perturbations are actual humps and/or ridges on the surface of the airfoil that are on the scale of the laminar boundary, and oscillate with an unsteady motion. Research has shown that this can provide a means to modify the instantaneous and mean velocity profile near the wall and favorably control the existing state of the boundary layer. Several flow control parameters were studied including dynamic roughness frequency, amplitude, and geometry. The results of this study have shown, both numerically and experimentally, that dynamic roughness can provide an effective means for eliminating both a short and long laminar separation bubble and possibly prove a viable alternative in effective flow control, hence reaping some of
Stability of graphene edges under electron beam: equilibrium energetics versus dynamic effects.
Kotakoski, Jani; Santos-Cottin, David; Krasheninnikov, Arkady V
2012-01-24
Electron beam of a transmission electron microscope can be used to alter the morphology of graphene nanoribbons and create atomically sharp edges required for applications of graphene in nanoelectronics. Using density-functional-theory-based simulations, we study the radiation hardness of graphene edges and show that the response of the ribbons to irradiation is not determined by the equilibrium energetics as assumed in previous experiments, but by kinetic effects associated with the dynamics of the edge atoms after impacts of energetic electrons. We report an unexpectedly high stability of armchair edges, comparable to that of pristine graphene, and demonstrate that the electron energy should be below ~50 keV to minimize the knock-on damage. PMID:22188561
Time-of-Flight Measurements of Single-Electron Wave Packets in Quantum Hall Edge States
NASA Astrophysics Data System (ADS)
Kataoka, M.; Johnson, N.; Emary, C.; See, P.; Griffiths, J. P.; Jones, G. A. C.; Farrer, I.; Ritchie, D. A.; Pepper, M.; Janssen, T. J. B. M.
2016-03-01
We report time-of-flight measurements on electrons traveling in quantum Hall edge states. Hot-electron wave packets are emitted one per cycle into edge states formed along a depleted sample boundary. The electron arrival time is detected by driving a detector barrier with a square wave that acts as a shutter. By adding an extra path using a deflection barrier, we measure a delay in the arrival time, from which the edge-state velocity v is deduced. We find that v follows 1 /B dependence, in good agreement with the E →×B → drift. The edge potential is estimated from the energy dependence of v using a harmonic approximation.
Transport of massless Dirac fermions in non-topological type edge states.
Latyshev, Yu I; Orlov, A P; Volkov, V A; Enaldiev, V V; Zagorodnev, I V; Vyvenko, O F; Petrov, Yu V; Monceau, P
2014-01-01
There are two types of intrinsic surface states in solids. The first type is formed on the surface of topological insulators. Recently, transport of massless Dirac fermions in the band of "topological" states has been demonstrated. States of the second type were predicted by Tamm and Shockley long ago. They do not have a topological background and are therefore strongly dependent on the properties of the surface. We study the problem of the conductivity of Tamm-Shockley edge states through direct transport experiments. Aharonov-Bohm magneto-oscillations of resistance are found on graphene samples that contain a single nanohole. The effect is explained by the conductivity of the massless Dirac fermions in the edge states cycling around the nanohole. The results demonstrate the deep connection between topological and non-topological edge states in 2D systems of massless Dirac fermions. PMID:25524881
Transport of Massless Dirac Fermions in Non-topological Type Edge States
Latyshev, Yu I.; Orlov, A. P.; Volkov, V. A.; Enaldiev, V. V.; Zagorodnev, I. V.; Vyvenko, O. F.; Petrov, Yu V.; Monceau, P.
2014-01-01
There are two types of intrinsic surface states in solids. The first type is formed on the surface of topological insulators. Recently, transport of massless Dirac fermions in the band of “topological” states has been demonstrated. States of the second type were predicted by Tamm and Shockley long ago. They do not have a topological background and are therefore strongly dependent on the properties of the surface. We study the problem of the conductivity of Tamm-Shockley edge states through direct transport experiments. Aharonov-Bohm magneto-oscillations of resistance are found on graphene samples that contain a single nanohole. The effect is explained by the conductivity of the massless Dirac fermions in the edge states cycling around the nanohole. The results demonstrate the deep connection between topological and non-topological edge states in 2D systems of massless Dirac fermions. PMID:25524881
Confinement effect on spin-polarized edge states in graphene nanostructures
NASA Astrophysics Data System (ADS)
Ramos-Castillo, Carlos; de Coss, Romeo
2014-03-01
One of the most intriguing phenomena in condensed matter physics is the existence of edge states on the boundary of a 2D system. In graphene, the edge states have distinct properties from the bulk states and play important roles in the physicochemical properties of the material. In this work, we show ab-initio results of spin-polarized electronic edge states in graphene quantum dots of different sizes and shape. We found a critical size at which the singlet nonmagnetic ground state becomes singlet open-shell with antiferromagnetic order. We found that the critical size is strongly influenced by the shape of the quantum dot. We discuss this behavior based on energetics and electronic structure of the system under study. The calculations are base on the Density functional Theory (DFT). The Linear Combination of Atomic Orbital (LCAO) method for bases functions it was used. For exchange-correlation functional has been used the Generalized Gradient Approximation (GGA).
Generic helical edge states due to Rashba spin-orbit coupling in a topological insulator
NASA Astrophysics Data System (ADS)
Ortiz, Laura; Molina, Rafael A.; Platero, Gloria; Lunde, Anders Mathias
2016-05-01
We study the helical edge states of a two-dimensional topological insulator without axial spin symmetry due to the Rashba spin-orbit interaction. Lack of axial spin symmetry can lead to so-called generic helical edge states, which have energy-dependent spin orientation. This opens the possibility of inelastic backscattering and thereby nonquantized transport. Here we find analytically the new dispersion relations and the energy dependent spin orientation of the generic helical edge states in the presence of Rashba spin-orbit coupling within the Bernevig-Hughes-Zhang model, for both a single isolated edge and for a finite width ribbon. In the single-edge case, we analytically quantify the energy dependence of the spin orientation, which turns out to be weak for a realistic HgTe quantum well. Nevertheless, finite size effects combined with Rashba spin-orbit coupling result in two avoided crossings in the energy dispersions, where the spin orientation variation of the edge states is very significantly increased for realistic parameters. Finally, our analytical results are found to compare well to a numerical tight-binding regularization of the model.
Electric field control of spin-resolved edge states in graphene quantum nanorings
Farghadan, R.; Saffarzadeh, A.
2014-05-07
The electric-field effect on the electronic and magnetic properties of triangular and hexagonal graphene quantum rings with zigzag edge termination is investigated by means of the single-band tight-binding Hamiltonian and the mean-field Hubbard model. It is shown how the electron and spin states in the nanoring structures can be manipulated by applying an electric field. We find different spin-depolarization behaviors with variation of electric field strength due to the dependence of spin densities on the shapes and edges of this kind of nanorings. In the case of triangular quantum rings, the magnetization on the inner and outer edges can be selectively tuned and the spin states depolarize gradually as the field strength is increased, while in the case of hexagonal nanorings, the transverse electric field reduces the magnetic moments on both inner and outer edges symmetrically and rapidly.
Fendley, Paul; Fisher, Matthew P.A.; Nayak, Chetan
2009-07-15
We explain how (perturbed) boundary conformal field theory allows us to understand the tunneling of edge quasiparticles in non-Abelian topological states. The coupling between a bulk non-Abelian quasiparticle and the edge is due to resonant tunneling to a zero mode on the quasiparticle, which causes the zero mode to hybridize with the edge. This can be reformulated as the flow from one conformally invariant boundary condition to another in an associated critical statistical mechanical model. Tunneling from one edge to another at a point contact can split the system in two, either partially or completely. This can be reformulated in the critical statistical mechanical model as the flow from one type of defect line to another. We illustrate these two phenomena in detail in the context of the {nu}=5/2 quantum Hall state and the critical Ising model. We briefly discuss the case of Fibonacci anyons and conclude by explaining the general formulation and its physical interpretation.
The poleward edge of the mid-latitude trough - Its formation, orientation and dynamics
NASA Astrophysics Data System (ADS)
Rodger, A. S.; Brace, L. H.; Hoegy, W. R.; Winningham, J. D.
1986-08-01
Data from the Advanced Ionospheric Sounder (AIS) deployed at Halley, Antarctica (76-deg S, 27-deg W; L = 4.2) and the Dynamics Explorer-2 spacecraft (DE-2) are used to investigate several aspects of the formation processes and dynamics of the poleward edge of the midlatitude electron density trough. These include a study of the flux and energy of charged particles precipitating into the F-region as a function of Magnetic Local Time. It is found that local energetic electron precipitation is a major source of ionization of the poleward edge in the evening sector, but only after magnetic midnight transport processes become more important. Occasionally a significant increase in the flux of conjugate photoelectrons is colocated with the poleward edge of the trough in the morning sector. The combination of AIS and DE-2 data has allowed identification of significant longitudinal structure on the poleward edge of the trough that may be the result of substorm activity. It is found that the orientation of the poleward edge of the trough and the locus of the plasmapause predicted from the 'tear-drop' model vary in rather a similar manner with local time, though no close physical link between the two features is inferred from this coincidence.
The poleward edge of the mid-latitude trough - Its formation, orientation and dynamics
NASA Technical Reports Server (NTRS)
Rodger, A. S.; Brace, L. H.; Hoegy, W. R.; Winningham, J. D.
1986-01-01
Data from the Advanced Ionospheric Sounder (AIS) deployed at Halley, Antarctica (76-deg S, 27-deg W; L = 4.2) and the Dynamics Explorer-2 spacecraft (DE-2) are used to investigate several aspects of the formation processes and dynamics of the poleward edge of the midlatitude electron density trough. These include a study of the flux and energy of charged particles precipitating into the F-region as a function of Magnetic Local Time. It is found that local energetic electron precipitation is a major source of ionization of the poleward edge in the evening sector, but only after magnetic midnight transport processes become more important. Occasionally a significant increase in the flux of conjugate photoelectrons is colocated with the poleward edge of the trough in the morning sector. The combination of AIS and DE-2 data has allowed identification of significant longitudinal structure on the poleward edge of the trough that may be the result of substorm activity. It is found that the orientation of the poleward edge of the trough and the locus of the plasmapause predicted from the 'tear-drop' model vary in rather a similar manner with local time, though no close physical link between the two features is inferred from this coincidence.
Dynamic vortex interactions with flexible fibers and edges for prediction of owl noise suppression
NASA Astrophysics Data System (ADS)
Korykora, Sarah; Jaworski, Justin
2015-11-01
The compliant trailing-edge fringe of owls and the soft downy material on their upper wing surfaces are thought to enable their silent flight by weakening the interaction of boundary layer turbulence with these flexible structures. Previous analysis of turbulence noise generation by wave-bearing elastic edges have shown that the far-field acoustic power scaling can be weakened by up to the square of the Mach number relative to a rigid edge. However, it is unclear whether or not the wave-bearing feature or simply the flexible nature of the edge scatterer produces this noise suppression. To assess this distinction, a dynamic vortex interaction model is developed whereby the motion of a line vortex round a rigid but elastically-restrained wall-mounted fiber or trailing edge is determined numerically. Special attention is paid to the dynamic interaction between the flexible structure and vortex, which is accomplished via a conformal mapping relationship determined in closed form. Results from this analysis seek to develop a vortex sound model to discern the effect of flexible versus wave-bearing scatterers on turbulence noise suppression and help explain the mechanisms of silent owl flight.
Pannuzzo, Martina; Raudino, Antonio; Böckmann, Rainer A
2014-07-14
Peptide- or protein-induced curvatures of lipid membranes may be studied in molecular dynamics (MD) simulations. In these, membranes are usually modeled as infinitely extended bilayers by using periodic boundary conditions. However, the enforced periodicity results in an underestimation of the bending power of peptides, unless the patch size is much larger than the induced curvature radii. In this letter, we propose a novel approach to evaluate the bending power of a given distribution and/or density of peptides based on the use of flat open-edged lipid patches. To ensure long-lived metastable structures, the patch rim is stabilized in MD simulations by a local enrichment with short-chain lipids. By combining the theory of continuum elastic media with MD simulations, we prove that open-edged patches evolve from a planar state to a closed vesicle, with a transition rate that strongly depends on the concentration of lipid soluble peptides. For close-to-critical values for the patch size and edge energy, the response to even small changes in peptide concentration adopts a transition-like behavior (buckling instability). The usage of open-edged membrane patches amplifies the bending power of peptides, thereby enabling the analysis of the structural properties of membrane-peptide systems. We applied the presented method to investigate the curvature induced by aggregating β -amyloid peptides, unraveling a strong sensitivity of membrane deformation to the peptide concentration. PMID:25028040
NASA Astrophysics Data System (ADS)
Pannuzzo, Martina; Raudino, Antonio; Böckmann, Rainer A.
2014-07-01
Peptide- or protein-induced curvatures of lipid membranes may be studied in molecular dynamics (MD) simulations. In these, membranes are usually modeled as infinitely extended bilayers by using periodic boundary conditions. However, the enforced periodicity results in an underestimation of the bending power of peptides, unless the patch size is much larger than the induced curvature radii. In this letter, we propose a novel approach to evaluate the bending power of a given distribution and/or density of peptides based on the use of flat open-edged lipid patches. To ensure long-lived metastable structures, the patch rim is stabilized in MD simulations by a local enrichment with short-chain lipids. By combining the theory of continuum elastic media with MD simulations, we prove that open-edged patches evolve from a planar state to a closed vesicle, with a transition rate that strongly depends on the concentration of lipid soluble peptides. For close-to-critical values for the patch size and edge energy, the response to even small changes in peptide concentration adopts a transition-like behavior (buckling instability). The usage of open-edged membrane patches amplifies the bending power of peptides, thereby enabling the analysis of the structural properties of membrane-peptide systems. We applied the presented method to investigate the curvature induced by aggregating β -amyloid peptides, unraveling a strong sensitivity of membrane deformation to the peptide concentration.
Zero-field Dissipationless Chiral Edge Current in Quantum Anomalous Hall State
NASA Astrophysics Data System (ADS)
Chang, Cui-Zu; Zhao, Weiwei; Kim, Duk Y.; Wei, Peng; Jain, J. K.; Liu, Chaoxing; Chan, Moses H. W.; Moodera, Jagadeesh S.
The quantum anomalous Hall (QAH) state is predicted to possess, at zero magnetic field, chiral edge channels that conduct spin polarized current without dissipation, and thus holds great promise for future high-performance information processing. In this talk, we will discuss our transport experiments that probe the QAH state with gate bias and temperature dependences, by local and nonlocal magnetoresistance measurements. This allows us to unambiguously distinguish the dissipationless edge transport from transport via other dissipative channels in the QAH system. Our experiments confirm a fundamental feature of the QAH state, namely the dissipationless transport by edge channels in zero applied fields, which will be crucial for future chiral interconnected electric and spintronic applications. This research is supported by the NSF Grants (DMR-1420620, Penn State MRSEC; in MIT by DMR-1207469 and the STC Center for Integrated Quantum Materials under NSF Grant DMR-1231319) and by ONR Grant N00014-13-1-0301.
Topological edge states in two-gap unitary systems: a transfer matrix approach
NASA Astrophysics Data System (ADS)
Tauber, Clément; Delplace, Pierre
2015-11-01
We construct and investigate a family of two-band unitary systems living on a cylinder geometry and presenting localized edge states. Using the transfer matrix formalism, we solve and investigate in detail such states in the thermodynamic limit. Analytic considerations then suggest the construction of a family of Riemann surfaces associated to the band structure of the system. In this picture, the corresponding edge states naturally wind around non-contractile loops, defining a topological invariant associated to each gap of the system.
Shot noise in the edge states of two-dimensional topological insulators
NASA Astrophysics Data System (ADS)
Aseev, P. P.; Nagaev, K. E.
2016-07-01
We calculate the resistance and shot noise in the edge states of a two-dimensional topological insulator that result from the exchange of electrons between these states and conducting puddles in the bulk of the insulator. The two limiting cases where the energy relaxation is either absent or very strong are considered. A finite time of spin relaxation in the puddles is introduced phenomenologically. Depending on this time and on the strength of coupling between the edge states and the puddles, the Fano factor F =SI/2 e I ranges from 0 to 1/3, which is in an agreement with the available experimental data.
Properties of Edge States at the Graphene P-N Junction Interface
NASA Astrophysics Data System (ADS)
Le, Son; Klimov, Nikolai; Newell, David; Yan, Jun; Lee, Ji Ung; Richter, Curt
The Landau level edge states from the p- and the n-section of a graphene P/N junction (pnJ) interact with each other differently across the junction depending upon the properties of the junction and the graphene. Full equilibration was reported for a two terminal graphene pnJ device in Williams et al.. In our four-terminal device, however, only the lowest Landau level edge state is equilibrated across the pnJ. When the two devices are compared, the LL energy spacings, the length of the edge states along the pnJ interface, and the carrier mobility are similar. Electrostatic simulations for our device geometry and that of contrast the rate of change of the electrostatic potential across the pnJs. Edge states at an electrostatically smooth junction are spatially further apart than those at a relatively abrupt junction, which decreases the probability of edge states mixing. Thus, we attribute the difference in equilibration in our device and that of to the dramatic difference in the shape of the electrostatic junction.
Image analysis tools to quantify cell shape and protein dynamics near the leading edge.
Ryan, Gillian L; Watanabe, Naoki; Vavylonis, Dimitrios
2013-01-01
We present a set of flexible image analysis tools to analyze dynamics of cell shape and protein concentrations near the leading edge of cells adhered to glass coverslips. Plugins for ImageJ streamline common analyses of microscopic images of cells, including the calculation of leading edge speeds, total and average intensities of fluorescent markers, and retrograde flow rate measurements of fluorescent single-molecule speckles. We also provide automated calculations of auto- and cross-correlation functions between velocity and intensity measurements. The application of the methods is illustrated on images of XTC cells. PMID:23165752
Xu, X. Q.; Ma, J. F.; Li, G. Q.
2014-12-29
The latest BOUT++ studies show an emerging understanding of dynamics of edge localized mode(ELM) crashes and the consistent collisionality scaling of ELMenergy losses with the world multi-tokamak database. A series of BOUT++ simulations are conducted to investigate the scaling characteristics of the ELMenergy losses vs collisionality via a density scan. Moreover, the linear results demonstrate that as the pedestal collisionality decreases, the growth rate of the peeling-ballooning modes decreases for high n but increases for low n (1 < n < 5), therefore the width of the growth rate spectrum γ(n) becomes narrower and the peak growth shifts to lowermore » n. For nonlinear BOUT++ simulations show a two-stage process of ELM crash evolution of (i) initial bursts of pressure blob and void creation and (ii) inward void propagation. The inward void propagation stirs the top of pedestal plasma and yields an increasing ELM size with decreasing collisionality after a series of micro-bursts. The pedestal plasma density plays a major role in determining the ELMenergy loss through its effect on the edge bootstrap current and ion diamagnetic stabilization. Finally, the critical trend emerges as a transition (1) linearly from ballooning-dominated states at high collisionality to peeling-dominated states at low collisionality with decreasing density and (2) nonlinearly from turbulence spreading dynamics at high collisionality into avalanche-like dynamics at low collisionality.« less
Xu, X. Q.; Ma, J. F.; Li, G. Q.
2014-12-29
The latest BOUT++ studies show an emerging understanding of dynamics of edge localized mode(ELM) crashes and the consistent collisionality scaling of ELMenergy losses with the world multi-tokamak database. A series of BOUT++ simulations are conducted to investigate the scaling characteristics of the ELMenergy losses vs collisionality via a density scan. Moreover, the linear results demonstrate that as the pedestal collisionality decreases, the growth rate of the peeling-ballooning modes decreases for high n but increases for low n (1 < n < 5), therefore the width of the growth rate spectrum γ(n) becomes narrower and the peak growth shifts to lower n. For nonlinear BOUT++ simulations show a two-stage process of ELM crash evolution of (i) initial bursts of pressure blob and void creation and (ii) inward void propagation. The inward void propagation stirs the top of pedestal plasma and yields an increasing ELM size with decreasing collisionality after a series of micro-bursts. The pedestal plasma density plays a major role in determining the ELMenergy loss through its effect on the edge bootstrap current and ion diamagnetic stabilization. Finally, the critical trend emerges as a transition (1) linearly from ballooning-dominated states at high collisionality to peeling-dominated states at low collisionality with decreasing density and (2) nonlinearly from turbulence spreading dynamics at high collisionality into avalanche-like dynamics at low collisionality.
Xu, X. Q.; Ma, J. F.; Li, G. Q.
2014-12-15
The latest BOUT++ studies show an emerging understanding of dynamics of edge localized mode (ELM) crashes and the consistent collisionality scaling of ELM energy losses with the world multi-tokamak database. A series of BOUT++ simulations are conducted to investigate the scaling characteristics of the ELM energy losses vs collisionality via a density scan. Linear results demonstrate that as the pedestal collisionality decreases, the growth rate of the peeling-ballooning modes decreases for high n but increases for low n (1 < n < 5), therefore the width of the growth rate spectrum γ(n) becomes narrower and the peak growth shifts to lower n. Nonlinear BOUT++ simulations show a two-stage process of ELM crash evolution of (i) initial bursts of pressure blob and void creation and (ii) inward void propagation. The inward void propagation stirs the top of pedestal plasma and yields an increasing ELM size with decreasing collisionality after a series of micro-bursts. The pedestal plasma density plays a major role in determining the ELM energy loss through its effect on the edge bootstrap current and ion diamagnetic stabilization. The critical trend emerges as a transition (1) linearly from ballooning-dominated states at high collisionality to peeling-dominated states at low collisionality with decreasing density and (2) nonlinearly from turbulence spreading dynamics at high collisionality into avalanche-like dynamics at low collisionality.
Topological Edge States in the One-Dimensional Superlattice Bose-Hubbard Model
NASA Astrophysics Data System (ADS)
Grusdt, Fabian; Höning, Michael; Fleischhauer, Michael
2013-06-01
We analyze interacting ultracold bosonic atoms in a one-dimensional superlattice potential with alternating tunneling rates t1 and t2 and inversion symmetry, which is the bosonic analogue of the Su-Schrieffer-Heeger model. A Z2 topological order parameter is introduced which is quantized for the Mott insulating (MI) phases. Depending on the ratio t1/t2 the n=1/2 MI phase is topologically nontrivial, which results in many-body edge states at open boundaries. In contrast to the Su-Schrieffer-Heeger model the bosonic counterpart lacks chiral symmetry and the edge states are no longer midgap. This leads to a generalization of the bulk-edge correspondence, which we discuss in detail. The edge states can be observed in cold atom experiments by creating a step in the effective confining potential, e.g., by a second heavy atom species, which leads to an interface between two MI regions with filling n=1 and n=1/2. The shape and energy of the edge states as well as the conditions for their occupation are determined analytically in the strong coupling limit and in general by density-matrix renormalization group simulations.
Topological edge States in the one-dimensional superlattice Bose-Hubbard model.
Grusdt, Fabian; Höning, Michael; Fleischhauer, Michael
2013-06-28
We analyze interacting ultracold bosonic atoms in a one-dimensional superlattice potential with alternating tunneling rates t1 and t2 and inversion symmetry, which is the bosonic analogue of the Su-Schrieffer-Heeger model. A Z2 topological order parameter is introduced which is quantized for the Mott insulating (MI) phases. Depending on the ratio t1/t2 the n=1/2 MI phase is topologically nontrivial, which results in many-body edge states at open boundaries. In contrast to the Su-Schrieffer-Heeger model the bosonic counterpart lacks chiral symmetry and the edge states are no longer midgap. This leads to a generalization of the bulk-edge correspondence, which we discuss in detail. The edge states can be observed in cold atom experiments by creating a step in the effective confining potential, e.g., by a second heavy atom species, which leads to an interface between two MI regions with filling n=1 and n=1/2. The shape and energy of the edge states as well as the conditions for their occupation are determined analytically in the strong coupling limit and in general by density-matrix renormalization group simulations. PMID:23848851
Low Temperature STM Experiments on Helical Edge States in InAs/GaSb
NASA Astrophysics Data System (ADS)
Du, Rui-Rui; Li, Tingxin; Mou, Xiaoyang; Du, Lingjie; Sullivan, Gerald
2014-03-01
Inverted InAs/GaSb quantum wells have been recently shown to be a 2D topological insulator hosting robust helical edge states. Attributing to the fact that the hybridized minigap in this system opens at a finite wavevector, the edge states here have a low Fermi velocity VF, and consequently their transport properties may reveal interesting interaction effects. Moreover, the VF in this system can be continuously tuned by electrostatic gates, providing an experimental knob for tuning the interactions. We report work in progress for STM/STS measurements of edge states in the tunneling regime, where the edge states are exposed at the cleaved edge/UHV interface. Experiments are performed in a 400 mK STM/vector magnet system with in situ sample cleavage and thin film deposition capabilities. Ref. I. Knez, R.-R. Du and G. Sullivan, Phys. Rev. Lett. 107, 136603 (2011); L-.J. Du, I. Knez, G. Sullivan, R-.R. Du, ArXiv:1306.1925 (2013). The work in PKU is supported by Basic Research Program of MOST; work in Rice is supported by NSF and DOE.
Edge-states in graphene nanoribbons: a combined spectroscopy and transport study
NASA Astrophysics Data System (ADS)
Baringhaus, Jens; Edler, Frederik; Tegenkamp, Christoph
2013-10-01
Graphene structures of finite size are expected to reveal exceptional electronic and magnetic properties which are highly attractive for future nano-technological applications. In this study we have looked at the edge-states in graphene nanoribbons (GNR) grown by self-assembly on mesa structured SiC(0001) templates. By means of a 4-tip STM/SEM system, both local spectroscopy and lateral transport have been performed in situ on the same nanostructures. The conductance in these structures was found to be e2/h for temperatures up to 400 K. Scanning tunneling spectroscopy clearly reveals edge-localized states on these ribbons. The local bonding of these ribbons to their support turns out to be essential in order to preserve the metallicity of the edge-states.
Edge-state transport in graphene p -n junctions in the quantum Hall regime
NASA Astrophysics Data System (ADS)
Klimov, Nikolai N.; Le, Son T.; Yan, J.; Agnihotri, Pratik; Comfort, Everett; Lee, Ji Ung; Newell, David B.; Richter, Curt A.
2015-12-01
We experimentally investigate charge carrier transport in a graphene p -n junction device by using independent p -type and n -type electrostatic gating which allow full characterization of the junction interface in the quantum Hall regime covering a wide range of filling factors [-10 ≤(ν1,ν2) ≤10 ] . Recent charge transport measurements across a graphene p -n junction in this quantized regime presume that equilibration of all of the Landauer-Büttiker edge states occurs across the p -n junction interface. Here we show that, in our devices, only the edge state associated with the lowest Landau level fully equilibrate across the p -n junction, while none of the other edge states equilibrate to transmit current across the junction.
Energy Bandgap and Edge States in an Epitaxially Grown Graphene/h-BN Heterostructure
Hwang, Beomyong; Hwang, Jeongwoon; Yoon, Jong Keon; Lim, Sungjun; Kim, Sungmin; Lee, Minjun; Kwon, Jeong Hoon; Baek, Hongwoo; Sung, Dongchul; Kim, Gunn; Hong, Suklyun; Ihm, Jisoon; Stroscio, Joseph A.; Kuk, Young
2016-01-01
Securing a semiconducting bandgap is essential for applying graphene layers in switching devices. Theoretical studies have suggested a created bulk bandgap in a graphene layer by introducing an asymmetry between the A and B sub-lattice sites. A recent transport measurement demonstrated the presence of a bandgap in a graphene layer where the asymmetry was introduced by placing a graphene layer on a hexagonal boron nitride (h-BN) substrate. Similar bandgap has been observed in graphene layers on metal substrates by local probe measurements; however, this phenomenon has not been observed in graphene layers on a near-insulating substrate. Here, we present bulk bandgap-like features in a graphene layer epitaxially grown on an h-BN substrate using scanning tunneling spectroscopy. We observed edge states at zigzag edges, edge resonances at armchair edges, and bandgap-like features in the bulk. PMID:27503427
Energy Bandgap and Edge States in an Epitaxially Grown Graphene/h-BN Heterostructure.
Hwang, Beomyong; Hwang, Jeongwoon; Yoon, Jong Keon; Lim, Sungjun; Kim, Sungmin; Lee, Minjun; Kwon, Jeong Hoon; Baek, Hongwoo; Sung, Dongchul; Kim, Gunn; Hong, Suklyun; Ihm, Jisoon; Stroscio, Joseph A; Kuk, Young
2016-01-01
Securing a semiconducting bandgap is essential for applying graphene layers in switching devices. Theoretical studies have suggested a created bulk bandgap in a graphene layer by introducing an asymmetry between the A and B sub-lattice sites. A recent transport measurement demonstrated the presence of a bandgap in a graphene layer where the asymmetry was introduced by placing a graphene layer on a hexagonal boron nitride (h-BN) substrate. Similar bandgap has been observed in graphene layers on metal substrates by local probe measurements; however, this phenomenon has not been observed in graphene layers on a near-insulating substrate. Here, we present bulk bandgap-like features in a graphene layer epitaxially grown on an h-BN substrate using scanning tunneling spectroscopy. We observed edge states at zigzag edges, edge resonances at armchair edges, and bandgap-like features in the bulk. PMID:27503427
Energy Bandgap and Edge States in an Epitaxially Grown Graphene/h-BN Heterostructure
NASA Astrophysics Data System (ADS)
Hwang, Beomyong; Hwang, Jeongwoon; Yoon, Jong Keon; Lim, Sungjun; Kim, Sungmin; Lee, Minjun; Kwon, Jeong Hoon; Baek, Hongwoo; Sung, Dongchul; Kim, Gunn; Hong, Suklyun; Ihm, Jisoon; Stroscio, Joseph A.; Kuk, Young
2016-08-01
Securing a semiconducting bandgap is essential for applying graphene layers in switching devices. Theoretical studies have suggested a created bulk bandgap in a graphene layer by introducing an asymmetry between the A and B sub-lattice sites. A recent transport measurement demonstrated the presence of a bandgap in a graphene layer where the asymmetry was introduced by placing a graphene layer on a hexagonal boron nitride (h-BN) substrate. Similar bandgap has been observed in graphene layers on metal substrates by local probe measurements; however, this phenomenon has not been observed in graphene layers on a near-insulating substrate. Here, we present bulk bandgap-like features in a graphene layer epitaxially grown on an h-BN substrate using scanning tunneling spectroscopy. We observed edge states at zigzag edges, edge resonances at armchair edges, and bandgap-like features in the bulk.
Comparative dynamics of avian communities across edges and interiors of North American ecoregions
Karanth, K.K.; Nichols, J.D.; Sauer, J.R.; Hines, J.E.
2006-01-01
Aim Based on a priori hypotheses, we developed predictions about how avian communities might differ at the edges vs. interiors of ecoregions. Specifically, we predicted lower species richness and greater local turnover and extinction probabilities for regional edges. We tested these predictions using North American Breeding Bird Survey (BBS) data across nine ecoregions over a 20-year time period. Location Data from 2238 BBS routes within nine ecoregions of the United States were used. Methods The estimation methods used accounted for species detection probabilities < 1. Parameter estimates for species richness, local turnover and extinction probabilities were obtained using the program COMDYN. We examined the difference in community-level parameters estimated from within exterior edges (the habitat interface between ecoregions), interior edges (the habitat interface between two bird conservation regions within the same ecoregion) and interior (habitat excluding interfaces). General linear models were constructed to examine sources of variation in community parameters for five ecoregions (containing all three habitat types) and all nine ecoregions (containing two habitat types). Results Analyses provided evidence that interior habitats and interior edges had on average higher bird species richness than exterior edges, providing some evidence of reduced species richness near habitat edges. Lower average extinction probabilities and turnover rates in interior habitats (five-region analysis) provided some support for our predictions about these quantities. However, analyses directed at all three response variables, i.e. species richness, local turnover, and local extinction probability, provided evidence of an interaction between habitat and region, indicating that the relationships did not hold in all regions. Main conclusions The overall predictions of lower species richness, higher local turnover and extinction probabilities in regional edge habitats, as opposed to
LOGISTIC NETWORK REGRESSION FOR SCALABLE ANALYSIS OF NETWORKS WITH JOINT EDGE/VERTEX DYNAMICS
Almquist, Zack W.; Butts, Carter T.
2015-01-01
Change in group size and composition has long been an important area of research in the social sciences. Similarly, interest in interaction dynamics has a long history in sociology and social psychology. However, the effects of endogenous group change on interaction dynamics are a surprisingly understudied area. One way to explore these relationships is through social network models. Network dynamics may be viewed as a process of change in the edge structure of a network, in the vertex set on which edges are defined, or in both simultaneously. Although early studies of such processes were primarily descriptive, recent work on this topic has increasingly turned to formal statistical models. Although showing great promise, many of these modern dynamic models are computationally intensive and scale very poorly in the size of the network under study and/or the number of time points considered. Likewise, currently used models focus on edge dynamics, with little support for endogenously changing vertex sets. Here, the authors show how an existing approach based on logistic network regression can be extended to serve as a highly scalable framework for modeling large networks with dynamic vertex sets. The authors place this approach within a general dynamic exponential family (exponential-family random graph modeling) context, clarifying the assumptions underlying the framework (and providing a clear path for extensions), and they show how model assessment methods for cross-sectional networks can be extended to the dynamic case. Finally, the authors illustrate this approach on a classic data set involving interactions among windsurfers on a California beach. PMID:26120218
NASA Astrophysics Data System (ADS)
Churakov, Sergey V.
2007-03-01
Edge sites of clay minerals play a key role for pH dependent sorption of ions from solutions of electrolytes. Pyrophyllite, Al 2[Si 4O 10](OH) 2, is an important structural prototype for a variety of 2:1 dioctahedral phyllosilicates but in contrast to the other clays has no permanent structural charge. The structure of thin water films confined between most common edges of 1Tc pyrophyllite: (0 1 0), (1 1 0) and (1 0 0), was analyzed by means of ab initio molecular dynamic simulations. The system setup allowed for a full flexibility of the interfaces and a proton exchange between the edges of pyrophyllite and water molecules in solution. The structure of hydrated surfaces is compared with the recent predictions of static geometry optimizations for edge-vacuum interfaces. All surfaces studied reveal a strong hydrophilic character of edge similar to the hydrated silica surface and the facets of simple layered hydroxides. Spontaneous proton transfer between different surface sites were observed in molecular dynamics simulations of the (0 1 0) interface. The proton bound to the tbnd Si sbnd OH site was found to exchange with the tbnd Al sbnd OH group by the mechanism tbnd Si sbnd OH +tbnd Al sbnd OH ↔tbnd Si sbnd O+tbnd Al sbnd OH 2+. The direction of the proton transfer agrees with the scale of relative proton affinities for surface sites obtained from the static calculations. Alternatively, the proton attached to the tbnd Al sbnd OH 2 site exchanges with the tbnd Al sbnd OH group. In both reactions, the protons are transferred through the chains of hydrogen bonds formed between water molecules in the solution and the surface sites. The observed mechanisms might be one of the basic schemes for the surface proton diffusion in compacted clays. Kinetics of the proton transfer at edge sites is limited by the rate of rearrangements of the water molecules near interface.
NASA Astrophysics Data System (ADS)
Lü, Xiaoling; Jiang, Liwei; Zheng, Yisong
2016-04-01
Graphene has intrinsic edge states localized at zigzag edge or lattice defect. Helical boundary states can also be established in such a two-dimensional carbon material at the boundary of topological insulator (TI) phase realized by the extrinsic Rashba spin-orbital coupling (SOC) in gated bilayer graphene. We theoretically investigate the interaction between these two kinds of edge (boundary) states when they coexist in a bilayer graphene. We find that this interaction gives rise to some very interesting results. In a zigzag edged nanoribbon of bilayer graphene, it is possible that the TI helical state does not localize at the TI phase boundary. Instead it moves to the nanoribbon edge even though the SOC is absent therein. In a bulk lattice of bilayer graphene embedded with two line defects, the numbers of helical state subbands at the two line defects are not equal to each other. In such a case, the backscattering lacking is still forbidden since the Kramers pairs are valley polarized.
NASA Astrophysics Data System (ADS)
Dehghani, Hossein; Mitra, Aditi
2016-05-01
Results are presented for the occupation probabilities and current densities of bulk and edge states of half-filled graphene in a cylindrical geometry and irradiated by a circularly polarized laser. It is assumed that the system is closed and that the laser has been switched on as a quench. Laser parameters corresponding to some representative topological phases are studied: one where the Chern number of the Floquet bands equals the number of chiral edge modes, a second where anomalous edge states appear in the Floquet Brillouin zone boundaries, and a third where the Chern number is zero, yet topological edge states appear at the center and boundaries of the Floquet Brillouin zone. Qualitative differences are found for the high-frequency off-resonant and low-frequency on-resonant laser with edge states arising due to resonant processes occupied with a high effective temperature on the one hand, while edge states arising due to off-resonant processes occupied with a low effective temperature on the other. For an ideal half-filled system where only one of the bands in the Floquet Brillouin zone is occupied and the other empty, particle-hole and inversion symmetry of the Floquet Hamiltonian implies zero current density. However the laser switch-on protocol breaks the inversion symmetry, resulting in a net cylindrical sheet of current density at steady state. Due to the underlying chirality of the system, this current density profile is associated with a net charge imbalance between the top and bottom of the cylinders.
Nanoscale Mach-Zehnder interferometer with spin-resolved quantum Hall edge states
NASA Astrophysics Data System (ADS)
Karmakar, Biswajit; Venturelli, Davide; Chirolli, Luca; Giovannetti, Vittorio; Fazio, Rosario; Roddaro, Stefano; Pfeiffer, Loren N.; West, Ken W.; Taddei, Fabio; Pellegrini, Vittorio
2015-11-01
We realize a nanoscale-area Mach-Zehnder interferometer with co-propagating quantum Hall spin-resolved edge states and demonstrate the persistence of gate-controlled quantum interference oscillations, as a function of an applied magnetic field, at relatively large temperatures. Arrays of top-gate magnetic nanofingers are used to induce a resonant charge transfer between the pair of spin-resolved edge states. To account for the pattern of oscillations measured as a function of magnetic field and gate voltage, we have developed a simple theoretical model which satisfactorily reproduces the data.
NASA Astrophysics Data System (ADS)
Dubey, Sudipta; Deshmukh, Mandar M.
2016-07-01
We probe quantum Hall effect in a tunable 1-D lateral superlattice (SL) in graphene created using electrostatic gates. Lack of equilibration is observed along edge states formed by electrostatic gates inside the superlattice. We create strong local electric field at the interface of regions of different charge densities. Crossed electric and magnetic fields modify the wavefunction of the Landau Levels (LLs) - a phenomenon unique to graphene. In the region of copropagating electrons and holes at the interface, the electric field is high enough to modify the Landau levels resulting in increased scattering that tunes equilibration of edge states and this results in large longitudinal resistance.
Spatially Resolving Spin-split Edge States of Chiral Graphene Nanoribbons
NASA Astrophysics Data System (ADS)
Crommie, M. F.
2011-03-01
A central question in the field of graphene-related research is how graphene behaves when it is patterned at the nanometer scale with different edge geometries. The most fundamental shape in this regard is the graphene nanoribbon (GNR), a narrow strip of graphene that is characterized by its width and chirality. GNRs have been predicted to exhibit a wide range of behavior that includes tunable energy gaps and unique 1D edge states with unusual magnetic structure. I will discuss a scanning tunneling microscopy and spectroscopy (STS) study of GNRs that allows us to examine how GNR electronic structure depends on the chirality of atomically well-defined GNR edges. Our STS measurements reveal the presence of 1D GNR edge states that closely match theoretical expectations for GNRs of similar width and chirality. We additionally observe width-dependent energy splitting in GNR edge states, providing compelling evidence of their magnetic nature. This work performed in collaboration with Chenggang Tao, Liying Jiao, Oleg V. Yazyev, Yen-Chia Chen, Juanjuan Feng, Xiaowei Zhang, Rodrigo B. Capaz, James M. Tour, Alex Zettl, Steven G. Louie, and Hongjie Dai.
Code of Federal Regulations, 2010 CFR
2010-01-01
... 12 Banks and Banking 3 2010-01-01 2010-01-01 false Acquisition of Edge corporation affiliate by... Acquisition of Edge corporation affiliate by State member banks of registered bank holding company. (a) The... of the holding company's Edge corporation subsidiary organized under section 25(a) of the...
Topological phase transition and quantum spin Hall edge states of antimony few layers.
Kim, Sung Hwan; Jin, Kyung-Hwan; Park, Joonbum; Kim, Jun Sung; Jhi, Seung-Hoon; Yeom, Han Woong
2016-01-01
While two-dimensional (2D) topological insulators (TI's) initiated the field of topological materials, only very few materials were discovered to date and the direct access to their quantum spin Hall edge states has been challenging due to material issues. Here, we introduce a new 2D TI material, Sb few layer films. Electronic structures of ultrathin Sb islands grown on Bi2Te2Se are investigated by scanning tunneling microscopy. The maps of local density of states clearly identify robust edge electronic states over the thickness of three bilayers in clear contrast to thinner islands. This indicates that topological edge states emerge through a 2D topological phase transition predicted between three and four bilayer films in recent theory. The non-trivial phase transition and edge states are confirmed for epitaxial films by extensive density-functional-theory calculations. This work provides an important material platform to exploit microscopic aspects of the quantum spin Hall phase and its quantum phase transition. PMID:27624972
Selective Equilibration of Spin-Polarized Quantum Hall Edge States in Graphene
NASA Astrophysics Data System (ADS)
Amet, F.; Williams, J. R.; Watanabe, K.; Taniguchi, T.; Goldhaber-Gordon, D.
2014-05-01
We report on transport measurements of dual-gated, single-layer graphene devices in the quantum Hall regime, allowing for independent control of the filling factors in adjoining regions. Progress in device quality allows us to study scattering between edge states when the fourfold degeneracy of the Landau level is lifted by electron correlations, causing edge states to be spin and/or valley polarized. In this new regime, we observe a dramatic departure from the equilibration seen in more disordered devices: edge states with opposite spins propagate without mixing. As a result, the degree of equilibration inferred from transport can reveal the spin polarization of the ground state at each filling factor. In particular, the first Landau level is shown to be spin polarized at half filling, providing an independent confirmation of a conclusion of Young et al. [Nat. Phys. 8, 550 (2012)]. The conductance in the bipolar regime is strongly suppressed, indicating that copropagating edge states, even with the same spin, do not equilibrate along PN interfaces. We attribute this behavior to the formation of an insulating ν =0 stripe at the PN interface.
Edge-state-induced Andreev oscillation in quantum anomalous Hall insulator-superconductor junctions
NASA Astrophysics Data System (ADS)
Lian, Biao; Wang, Jing; Zhang, Shou-Cheng
2016-04-01
We study the quantum Andreev oscillation induced by interference of the edge chiral Majorana fermions in junctions made of quantum anomalous Hall (QAH) insulators and superconductors (SCs). We show two chiral Majorana fermions on a QAH edge with SC proximity generically have a momentum difference Δ k , which depends on the chemical potentials of both the QAH insulator and the SC. Due to the spatial interference induced by Δ k , the longitudinal conductance of QAH-SC junctions oscillates with respect to the edge lengths and the chemical potentials, which can be probed via charge transport. Furthermore, we show the dynamical SC phase fluctuation will give rise to a geometrical correction to the longitudinal conductance of the junctions.
Direct imaging of topological edge states in cold-atom systems
Goldman, Nathan; Dalibard, Jean; Dauphin, Alexandre; Gerbier, Fabrice; Lewenstein, Maciej; Zoller, Peter; Spielman, Ian B.
2013-01-01
Detecting topological order in cold-atom experiments is an ongoing challenge, the resolution of which offers novel perspectives on topological matter. In material systems, unambiguous signatures of topological order exist for topological insulators and quantum Hall devices. In quantum Hall systems, the quantized conductivity and the associated robust propagating edge modes—guaranteed by the existence of nontrivial topological invariants—have been observed through transport and spectroscopy measurements. Here, we show that optical-lattice-based experiments can be tailored to directly visualize the propagation of topological edge modes. Our method is rooted in the unique capability for initially shaping the atomic gas and imaging its time evolution after suddenly removing the shaping potentials. Our scheme, applicable to an assortment of atomic topological phases, provides a method for imaging the dynamics of topological edge modes, directly revealing their angular velocity and spin structure. PMID:23569266
Direct imaging of topological edge states in cold-atom systems.
Goldman, Nathan; Dalibard, Jean; Dauphin, Alexandre; Gerbier, Fabrice; Lewenstein, Maciej; Zoller, Peter; Spielman, Ian B
2013-04-23
Detecting topological order in cold-atom experiments is an ongoing challenge, the resolution of which offers novel perspectives on topological matter. In material systems, unambiguous signatures of topological order exist for topological insulators and quantum Hall devices. In quantum Hall systems, the quantized conductivity and the associated robust propagating edge modes--guaranteed by the existence of nontrivial topological invariants--have been observed through transport and spectroscopy measurements. Here, we show that optical-lattice-based experiments can be tailored to directly visualize the propagation of topological edge modes. Our method is rooted in the unique capability for initially shaping the atomic gas and imaging its time evolution after suddenly removing the shaping potentials. Our scheme, applicable to an assortment of atomic topological phases, provides a method for imaging the dynamics of topological edge modes, directly revealing their angular velocity and spin structure. PMID:23569266
Shot-noise at a Fermi-edge singularity: Non-Markovian dynamics
Ubbelohde, N.; Maire, N.; Haug, R. J.; Roszak, K.; Hohls, F.; Novotný, T.
2013-12-04
For an InAs quantum dot we study the current shot noise at a Fermi-edge singularity in low temperature cross-correlation measurements. In the regime of the interaction effect the strong suppression of noise observed at zero magnetic field and the sequence of enhancement and suppression in magnetic field go beyond a Markovian master equation model. Qualitative and quantitative agreement can however be achieved by a generalized master equation model taking non-Markovian dynamics into account.
NASA Astrophysics Data System (ADS)
Werner, Sebastian; Lehnertz, Klaus
2015-07-01
We study the impact of dynamical and structural heterogeneity on the collective dynamics of large small-world networks of pulse-coupled integrate-and-fire oscillators endowed with refractory periods and time delay. Depending on the choice of homogeneous control parameters (here, refractoriness and coupling strength), these networks exhibit a large spectrum of dynamical behaviors, including asynchronous, partially synchronous, and fully synchronous states. Networks exhibit transitions between these dynamical behaviors upon introducing heterogeneity. We show that the probability for a network to exhibit a certain dynamical behavior (network susceptibility) is affected differently by dynamical and structural heterogeneity and depends on the respective homogeneous dynamics.
Werner, Sebastian; Lehnertz, Klaus
2015-07-01
We study the impact of dynamical and structural heterogeneity on the collective dynamics of large small-world networks of pulse-coupled integrate-and-fire oscillators endowed with refractory periods and time delay. Depending on the choice of homogeneous control parameters (here, refractoriness and coupling strength), these networks exhibit a large spectrum of dynamical behaviors, including asynchronous, partially synchronous, and fully synchronous states. Networks exhibit transitions between these dynamical behaviors upon introducing heterogeneity. We show that the probability for a network to exhibit a certain dynamical behavior (network susceptibility) is affected differently by dynamical and structural heterogeneity and depends on the respective homogeneous dynamics. PMID:26232952
Hierarchy of Floquet gaps and edge states for driven honeycomb lattices
NASA Astrophysics Data System (ADS)
Perez-Piskunow, P. M.; Foa Torres, L. E. F.; Usaj, Gonzalo
2015-04-01
Electromagnetic driving in a honeycomb lattice can induce gaps and topological edge states with a structure of increasing complexity as the frequency of the driving lowers. While the high-frequency case is the most simple to analyze we focus on the multiple photon processes allowed in the low-frequency regime to unveil the hierarchy of Floquet edge states. In the case of low intensities an analytical approach allows us to derive effective Hamiltonians and address the topological character of each gap in a constructive manner. At high intensities we obtain the net number of edge states, given by the winding number, with a numerical calculation of the Chern numbers of each Floquet band. Using these methods, we find a hierarchy that resembles that of a Russian nesting doll. This hierarchy classifies the gaps and the associated edge states in different orders according to the electron-photon coupling strength. For large driving intensities, we rely on the numerical calculation of the winding number, illustrated in a map of topological phase transitions. The hierarchy unveiled with the low-energy effective Hamiltonians, along with the map of topological phase transitions, discloses the complexity of the Floquet band structure in the low-frequency regime. The proposed method for obtaining the effective Hamiltonian can be easily adapted to other Dirac Hamiltonians of two-dimensional materials and even the surface of a three-dimensional topological insulator.
Photonic simulation of topological superconductor edge state and zero-energy mode at a vortex
Tan, Wei; Chen, Liang; Ji, Xia; Lin, Hai-Qing
2014-01-01
Photonic simulations of quantum Hall edge states and topological insulators have inspired considerable interest in recent years. Interestingly, there are theoretical predictions for another type of topological states in topological superconductors, but debates over their experimental observations still remain. Here we investigate the photonic analogue of the px + ipy model of topological superconductor. Two essential characteristics of topological superconductor, particle-hole symmetry and px + ipy pairing potentials, are well emulated in photonic systems. Its topological features are presented by chiral edge state and zero-energy mode at a vortex. This work may fertilize the study of photonic topological states, and open up the possibility for emulating wave behaviors in superconductors. PMID:25488408
Edge states and integer quantum Hall effect in topological insulator thin films.
Zhang, Song-Bo; Lu, Hai-Zhou; Shen, Shun-Qing
2015-01-01
The integer quantum Hall effect is a topological state of quantum matter in two dimensions, and has recently been observed in three-dimensional topological insulator thin films. Here we study the Landau levels and edge states of surface Dirac fermions in topological insulators under strong magnetic field. We examine the formation of the quantum plateaux of the Hall conductance and find two different patterns, in one pattern the filling number covers all integers while only odd integers in the other. We focus on the quantum plateau closest to zero energy and demonstrate the breakdown of the quantum spin Hall effect resulting from structure inversion asymmetry. The phase diagrams of the quantum Hall states are presented as functions of magnetic field, gate voltage and chemical potential. This work establishes an intuitive picture of the edge states to understand the integer quantum Hall effect for Dirac electrons in topological insulator thin films. PMID:26304795
Initial Investigations of H-mode Edge Dynamics in the PEGASUS Toroidal Experiment
NASA Astrophysics Data System (ADS)
Bongard, M. W.; Fonck, R. J.; Thome, K. E.; Thompson, D. S.
2013-10-01
Experiments with ultra-low aspect ratio (A < 1 . 2) H-mode plasmas in PEGASUS enable unique measurements of Edge Localized Mode (ELM) phenomena of import to next-step fusion devices. The modest temperatures and pulse lengths in PEGASUS allow the use of insertable probes to diagnose the edge plasma with high spatial and temporal resolution. In particular, the compatibility of the Hall probe Jedge diagnostic with the H-mode edge to date affords the opportunity to study current profile dynamics throughout the ELM cycle. A pedestal in Jedge is formed following the L-H transition that is transiently destroyed during ELMs. Presently, Type I and Type III ELMs are accessible. Both types generate field-aligned filaments during the ELM. A prominent current-hole Jedge perturbation and low- n MHD signature is evident during Type III ELM crash events, similar to that seen in prior peeling mode studies conducted in L-mode with strong edge current drive. In contrast, Type I ELMs are found to have a complex MHD signature comprised of multiple intermediate toroidal mode numbers (5 < n < 15) , a steepening of the Jedge gradient scale length as well as a slight hump in Jedge , which is consistent with a peeling-ballooning nature and the presence of bootstrap current drive. Particle trapping and associated neoclassical effects are expected to be large in PEGASUS plasmas at A ~ 1, even with modest pedestal parameters. Work supported by US DOE Grant DE-FG02-96ER54375.
Martin, Katrin; Reimann, Andreas; Fritz, Rafael D.; Ryu, Hyunryul; Jeon, Noo Li; Pertz, Olivier
2016-01-01
The three canonical Rho GTPases RhoA, Rac1 and Cdc42 co-ordinate cytoskeletal dynamics. Recent studies indicate that all three Rho GTPases are activated at the leading edge of motile fibroblasts, where their activity fluctuates at subminute time and micrometer length scales. Here, we use a microfluidic chip to acutely manipulate fibroblast edge dynamics by applying pulses of platelet-derived growth factor (PDGF) or the Rho kinase inhibitor Y-27632 (which lowers contractility). This induces acute and robust membrane protrusion and retraction events, that exhibit stereotyped cytoskeletal dynamics, allowing us to fairly compare specific morphodynamic states across experiments. Using a novel Cdc42, as well as previously described, second generation RhoA and Rac1 biosensors, we observe distinct spatio-temporal signaling programs that involve all three Rho GTPases, during protrusion/retraction edge dynamics. Our results suggest that Rac1, Cdc42 and RhoA regulate different cytoskeletal and adhesion processes to fine tune the highly plastic edge protrusion/retraction dynamics that power cell motility. PMID:26912264
Nonequilibrium noise correlations in a point contact of helical edge states
NASA Astrophysics Data System (ADS)
Lee, Yu-Wen; Lee, Yu-Li; Chung, Chung-Hou
2012-12-01
We investigate theoretically the nonequilibrium finite-frequency current noise in a four-terminal quantum point contact of interacting helical edge states at a finite bias voltage. Special focus is put on the effects of the single-particle and two-particle scattering between the two helical edge states on the fractional charge quasiparticle excitations shown in the nonequilibrium current noise spectra. Via the Keldysh perturbative approach, we find that the effects of the single-particle and the two-particle scattering processes on the current noise depend sensitively on the Luttinger liquid parameter. Moreover, the Fano factors for the auto- and cross correlations of the currents in the terminals are distinct from the ones for tunneling between the chiral edge states in the quantum Hall liquid. The current noise spectra in the single-particle-scattering-dominated and the two-particle-scattering-dominated regime are shown. Experimental implications of our results on the transport through the helical edges in two-dimensional topological insulators are discussed.
The effect of spin-orbit coupling in band structure and edge states of bilayer graphene
Sahdan, Muhammad Fauzi; Darma, Yudi
2015-04-16
Topological insulators are predicted to be useful ranging from spintronics to quantum computation. Graphene was first predicted to be the precursor of topological insulator by Kane-Mele. They developed a Hamiltonian model to describe the gap opening in graphene. In this work, we investigate the band structure of bilayer grapheme and also its edge states by using this model with analytical approach. The results of our calculation show that the gap opening occurs at K and K’ point in bilayer graphene.In addition, a pair of gapless edge modes occurs both in the zigzag and arm-chair configurations are no longer exist. There are gap created at the edge even though thery are very small.
Beggi, Andrea; Bordone, Paolo; Buscemi, Fabrizio; Bertoni, Andrea
2015-12-01
We compute the exact single-particle time-resolved dynamics of electronic Mach-Zehnder interferometers based on Landau edge-states transport, and assess the effect of the spatial localization of carriers on the interference pattern. The exact carrier dynamics is obtained by solving numerically the time-dependent Schrödinger equation with a suitable 2D potential profile reproducing the interferometer design. An external magnetic field, driving the system to the quantum Hall regime with filling factor one, is included. The injected carriers are represented by a superposition of edge states, and their interference pattern-controlled via magnetic field and/or area variation-reproduces the one of (Ji et al 2003 Nature 422 415). By tuning the system towards different regimes, we find two additional features in the transmission spectra, both related to carrier localization, namely a damping of the Aharonov-Bohm oscillations with increasing difference in the arms length, and an increased mean transmission that we trace to the energy-dependent transmittance of quantum point contacts. Finally, we present an analytical model, also accounting for the finite spatial dispersion of the carriers, able to reproduce the above effects. PMID:26548374
NASA Astrophysics Data System (ADS)
Beggi, Andrea; Bordone, Paolo; Buscemi, Fabrizio; Bertoni, Andrea
2015-12-01
We compute the exact single-particle time-resolved dynamics of electronic Mach-Zehnder interferometers based on Landau edge-states transport, and assess the effect of the spatial localization of carriers on the interference pattern. The exact carrier dynamics is obtained by solving numerically the time-dependent Schrödinger equation with a suitable 2D potential profile reproducing the interferometer design. An external magnetic field, driving the system to the quantum Hall regime with filling factor one, is included. The injected carriers are represented by a superposition of edge states, and their interference pattern—controlled via magnetic field and/or area variation—reproduces the one of (Ji et al 2003 Nature 422 415). By tuning the system towards different regimes, we find two additional features in the transmission spectra, both related to carrier localization, namely a damping of the Aharonov-Bohm oscillations with increasing difference in the arms length, and an increased mean transmission that we trace to the energy-dependent transmittance of quantum point contacts. Finally, we present an analytical model, also accounting for the finite spatial dispersion of the carriers, able to reproduce the above effects.
Formation and Development of the Dynamic Stall Vortex on a Wing with Leading Edge Tubercles
NASA Astrophysics Data System (ADS)
Hrynuk, John; Bohl, Douglas
2015-11-01
Humpback whales are unique in that their flippers have leading edge ``bumps'' or tubercles. Past work on airfoils inspired by whale flippers has centered on the static aerodynamic characteristics of these airfoils. The current study uses Molecular Tagging Velocimetry (MTV) to investigate the effects of tubercles on dynamically pitching NACA 0012 airfoils. A baseline (i.e. straight leading edge) wing and one modified with leading edge tubercles are investigated. Tracking of the Dynamic Stall Vortex (DSV) is performed to quantitatively compare the DSV formation location, path, and convective velocity for tubercled and baseline wings. The results show that there is a spanwise variation in the initial formation location and motion of the DSV on the modified wing. Once formed, the DSV aligns into a more uniform spanwise structure. As the pitching motion progresses, the DSV on the modified wing convects away from the airfoil surface later and slower than is observed for the baseline airfoil. The results indicate that the tubercles may delay stall when compared to the baseline airfoil. This work was supported by NSF Grant # 0845882.
Humidity variations across the edge of trade wind cumuli: Observations and dynamical implications
NASA Astrophysics Data System (ADS)
Wang, Yonggang; Geerts, Bart
2010-07-01
Aircraft data are used to analyze the composite horizontal structure of shallow tropical maritime cumulus clouds across the cloud edge into the ambient clear air. The emphasis is on humidity variations, and their implications for cumulus dynamics. The Lyman-α humidity probe has the required fast response and is unaffected by wetting in-cloud. On average the water vapor mixing ratio increases gradually from the clear air towards the cloud edge, and air is often sub-saturated in the outer fringe of the cloud, implying that droplets are evaporating. Similarly, conserved variables such as the total water concentration and the wet equivalent potential temperature gradually transition in the "margin" of cumulus clouds. The gradual change of water vapor mixing ratio and conserved variables across the cloud edge highlights the significance of lateral entrainment and detrainment, and it reveals a characteristic penetration depth of mixing eddies of 10 to 15% of the cloud diameter, or about 50 m. An ˜ 100 m wide region just outside the cloud is generally characterized by negatively buoyant, sinking air. The excess water vapor in this region, also documented in several recent studies, confirms that the negative buoyancy is caused by evaporative cooling in the cloud margin. Although rather weak, this cooling appears strong enough to evoke a dynamical response, even in the relative small trade wind cumuli.
Topological Edge States with Zero Hall Conductivity in a Dimerized Hofstadter Model
NASA Astrophysics Data System (ADS)
Lau, Alexander; Ortix, Carmine; van den Brink, Jeroen
2015-11-01
The Hofstadter model is a simple yet powerful Hamiltonian to study quantum Hall physics in a lattice system, manifesting its essential topological states. Lattice dimerization in the Hofstadter model opens an energy gap at half filling. Here we show that even if the ensuing insulator has a Chern number equal to zero, concomitantly a doublet of edge states appear that are pinned at specific momenta. We demonstrate that these states are topologically protected by inversion symmetry in specific one-dimensional cuts in momentum space, define and calculate the corresponding invariants, and identify a platform for the experimental detection of these novel topological states.
Spectrally narrowed leaky waveguide edge emission and transient electrluminescent dynamics of OLEDs
Zhengqing, Gan
2010-01-01
In summary, there are two major research works presented in this dissertation. The first research project (Chapter 4) is spectrally narrowed edge emission from Organic Light Emitting Diodes. The second project (Chapter 5) is about transient electroluminescent dynamics in OLEDs. Chapter 1 is a general introduction of OLEDs. Chapter 2 is a general introduction of organic semiconductor lasers. Chapter 3 is a description of the thermal evaporation method for OLED fabrication. The detail of the first project was presented in Chapter 4. Extremely narrowed spectrum was observed from the edge of OLED devices. A threshold thickness exists, above which the spectrum is narrow, and below which the spectrum is broad. The FWHM of spectrum depends on the material of the organic thin films, the thickness of the organic layers, and length of the OLED device. A superlinear relationship between the output intensity of the edge emission and the length of the device was observed, which is probably due to the misalignment of the device edge and the optical fiber detector. The original motivation of this research is for organic semiconductor laser that hasn't been realized due to the extremely high photon absorption in OLED devices. Although we didn't succeed in fabricating an electrically pumped organic laser diode, we made a comprehensive research in edge emission of OLEDs which provides valuable results in understanding light distribution and propagation in OLED devices. Chapter 5 focuses on the second project. A strong spike was observed at the falling edge of a pulse, and a long tail followed. The spike was due to the recombination of correlated charge pair (CCP) created by trapped carriers in guest molecules of the recombination zone. When the bias was turned off, along with the decreasing of electric field in the device, the electric field induced quenching decreases and the recombination rate of the CCP increases which result in the spike. This research project provides a
Topological transition and edge states in HgTe quantum wells from first principles
NASA Astrophysics Data System (ADS)
Küfner, Sebastian; Bechstedt, Friedhelm
2014-05-01
(HgTe)N(CdTe)M(110) and (001) superlattices are studied by means of ab initio calculations versus the thickness of the HgTe quantum wells (QWs). The used approximate quasiparticle theory including spin-orbit coupling (SOC) gives the correct band ordering, band gap, and SOC splitting for bulk HgTe and CdTe. The resulting band discontinuities indicate confinement also for occupied states. In agreement with earlier k .p calculations and experiments we find a topological transition from the topological nontrivial quantum spin Hall state into a trivial insulator with decreasing QW thickness. The spatial localization near the interfaces and the spin polarization are demonstrated for the edge states for QWs with thicknesses near the critical one. They do not depend on the QW orientation and are therefore topologically protected. Below the critical QW thickness, the trivial insulator exhibits drastic confinement effects with a significant gap opening. We show that the inclusion of inversion symmetry, the nonaxial rotation symmetry of the QWs, and the real QW barriers lead to some agreement but also significant deviations from the predictions within toy models. The deviations concern the critical thickness, the number and localization of edge states, and the possibility to find QW subbands between edge states.
Long-distance entanglement of spin qubits via quantum Hall edge states
NASA Astrophysics Data System (ADS)
Yang, Guang; Hsu, Chen-Hsuan; Stano, Peter; Klinovaja, Jelena; Loss, Daniel
2016-02-01
The implementation of a functional quantum computer involves entangling and coherent manipulation of a large number of qubits. For qubits based on electron spins confined in quantum dots, which are among the most investigated solid-state qubits at present, architectural challenges are often encountered in the design of quantum circuits attempting to assemble the qubits within the very limited space available. Here, we provide a solution to such challenges based on an approach to realizing entanglement of spin qubits over long distances. We show that long-range Ruderman-Kittel-Kasuya-Yosida interaction of confined electron spins can be established by quantum Hall edge states, leading to an exchange coupling of spin qubits. The coupling is anisotropic and can be either Ising type or XY type, depending on the spin polarization of the edge state. Such a property, combined with the dependence of the electron spin susceptibility on the chirality of the edge state, can be utilized to gain valuable insights into the topological nature of various quantum Hall states.
Long-distance entanglement of spin qubits via quantum Hall edge states
NASA Astrophysics Data System (ADS)
Yang, Guang; Hsu, Chen-Hsuan; Stano, Peter; Klinovaja, Jelena; Loss, Daniel
The implementation of a functional quantum computer involves entangling and coherent manipulation of a large number of qubits. For qubits based on electron spins confined in quantum dots, which are among the most investigated solid-state qubits at present, architectural challenges are often encountered in the design of quantum circuits attempting to assemble the qubits within the very limited space available. Here, we provide a solution to such challenges based on an approach to realizing entanglement of spin qubits over long distances. We show that long-range Ruderman-Kittel-Kasuya-Yosida interaction of confined electron spins can be established by quantum Hall edge states, leading to an exchange coupling of spin qubits. The coupling is anisotropic and can be either Ising-type or XY-type, depending on the spin polarization of the edge state. Such a property, combined with the dependence of the electron-spin susceptibility on the chirality of the edge state, can be utilized to gain valuable insights into the topological nature of various quantum Hall states.
Dynamic Stall Measurements and Computations for a VR-12 Airfoil with a Variable Droop Leading Edge
NASA Technical Reports Server (NTRS)
Martin, P. B.; McAlister, K. W.; Chandrasekhara, M. S.; Geissler, W.
2003-01-01
High density-altitude operations of helicopters with advanced performance and maneuver capabilities have lead to fundamental research on active high-lift system concepts for rotor blades. The requirement for this type of system was to improve the sectional lift-to-drag ratio by alleviating dynamic stall on the retreating blade while simultaneously reducing the transonic drag rise of the advancing blade. Both measured and computational results showed that a Variable Droop Leading Edge (VDLE) airfoil is a viable concept for application to a rotor high-lift system. Results are presented for a series of 2D compressible dynamic stall wind tunnel tests with supporting CFD results for selected test cases. These measurements and computations show a dramatic decrease in the drag and pitching moment associated with severe dynamic stall when the VDLE concept is applied to the Boeing VR-12 airfoil. Test results also show an elimination of the negative pitch damping observed in the baseline moment hysteresis curves.
Pseudo-time-reversal symmetry and topological edge states in two-dimensional acoustic crystals
Mei, Jun; Chen, Zeguo; Wu, Ying
2016-01-01
We propose a simple two-dimensional acoustic crystal to realize topologically protected edge states for acoustic waves. The acoustic crystal is composed of a triangular array of core-shell cylinders embedded in a water host. By utilizing the point group symmetry of two doubly degenerate eigenstates at the Γ point, we can construct pseudo-time-reversal symmetry as well as pseudo-spin states in this classical system. We develop an effective Hamiltonian for the associated dispersion bands around the Brillouin zone center, and find the inherent link between the band inversion and the topological phase transition. With numerical simulations, we unambiguously demonstrate the unidirectional propagation of acoustic edge states along the interface between a topologically nontrivial acoustic crystal and a trivial one, and the robustness of the edge states against defects with sharp bends. Our work provides a new design paradigm for manipulating and transporting acoustic waves in a topologically protected manner. Technological applications and devices based on our design are expected in various frequency ranges of interest, spanning from infrasound to ultrasound. PMID:27587311
Pseudo-time-reversal symmetry and topological edge states in two-dimensional acoustic crystals.
Mei, Jun; Chen, Zeguo; Wu, Ying
2016-01-01
We propose a simple two-dimensional acoustic crystal to realize topologically protected edge states for acoustic waves. The acoustic crystal is composed of a triangular array of core-shell cylinders embedded in a water host. By utilizing the point group symmetry of two doubly degenerate eigenstates at the Γ point, we can construct pseudo-time-reversal symmetry as well as pseudo-spin states in this classical system. We develop an effective Hamiltonian for the associated dispersion bands around the Brillouin zone center, and find the inherent link between the band inversion and the topological phase transition. With numerical simulations, we unambiguously demonstrate the unidirectional propagation of acoustic edge states along the interface between a topologically nontrivial acoustic crystal and a trivial one, and the robustness of the edge states against defects with sharp bends. Our work provides a new design paradigm for manipulating and transporting acoustic waves in a topologically protected manner. Technological applications and devices based on our design are expected in various frequency ranges of interest, spanning from infrasound to ultrasound. PMID:27587311
The use of bulk states to accelerate the band edge statecalculation of a semiconductor quantum dot
Vomel, Christof; Tomov, Stanimire Z.; Wang, Lin-Wang; Marques,Osni A.; Dongarra, Jack J.
2006-05-10
We present a new technique to accelerate the convergence of the folded spectrum method in empirical pseudopotential band edge state calculations for colloidal quantum dots. We use bulk band states of the materials constituent of the quantum dot to construct initial vectors and a preconditioner. We apply these to accelerate the convergence of the folded spectrum method for the interior states at the top of the valence and the bottom of the conduction band. For large CdSe quantum dots, the number of iteration steps until convergence decreases by about a factor of 4 compared to previous calculations.
Edge cracks in nickel and aluminium single crystals: A molecular dynamics study
NASA Astrophysics Data System (ADS)
Chandra, Sagar; Samal, M. K.; Chavan, V. M.; Patel, R. J.
2016-05-01
A molecular dynamics study of edge cracks in Ni and Al single crystals under mode-I loading conditions is presented. Simulations are performed using embedded-atom method potentials for Ni and Al at a temperature of 0.5K. The results reveal that Ni and Al show different fracture mechanisms. Overall failure behavior of Ni is brittle, while fracture in Al proceeds through void nucleation and coalescence with a zig-zag pattern of crack growth. The qualitative nature of results is discussed in the context of vacancy-formation energies and surface energies of the two FCC metals.
Chiral and nonchiral edge states in quantum Hall systems with charge density modulation
NASA Astrophysics Data System (ADS)
Szumniak, Paweł; Klinovaja, Jelena; Loss, Daniel
2016-06-01
We consider a system of weakly coupled wires with quantum Hall effect (QHE) and in the presence of a spatially periodic modulation of the chemical potential along the wire, equivalent to a charge density wave (CDW). We investigate the competition between the two effects which both open a gap. We show that by changing the ratio between the amplitudes of the CDW modulation and the tunneling between wires, one can switch between nontopological CDW-dominated phase to topological QHE-dominated phase. Both phases host edge states of chiral and nonchiral nature robust to on-site disorder. However, only in the topological phase, the edge states are immune to disorder in the phase shifts of the CDWs. We provide analytical solutions for filling factor ν =1 and study numerically effects of disorder as well as present numerical results for higher filling factors.
Edge-state-enhanced transport in a two-dimensional quantum walk
NASA Astrophysics Data System (ADS)
Asboth, Janos K.; Edge, Jonathan M.
2015-02-01
Quantum walks on translation-invariant regular graphs spread quadratically faster than their classical counterparts. The same coherence that gives them this quantum speedup inhibits or even stops their spread in the presence of disorder. We ask how to create an efficient transport channel from a fixed source site (A ) to fixed target site (B ) in a disordered two-dimensional discrete-time quantum walk by cutting some of the links. We show that the somewhat counterintuitive strategy of cutting links along a single line connecting A to B creates such a channel. The efficient transport along the cut is due to topologically protected chiral edge states, which exist even though the bulk Chern number in this system vanishes. We give a realization of the walk as a periodically driven lattice Hamiltonian and identify the bulk topological invariant responsible for the edge states as the quasienergy winding of this Hamiltonian.
A New Dynamic Edge Detection toward Better Human-Robot Interaction
NASA Astrophysics Data System (ADS)
Hafiz, Abdul Rahman; Alnajjar, Fady; Murase, Kazuyuki
Robot’s vision plays a significant role in human-robot interaction, e.g., face recognition, expression understanding, motion tracking, etc. Building a strong vision system for the robot, therefore, is one of the fundamental issues behind the success of such an interaction. Edge detection, which is known as the basic units for measuring the strength of any vision system, has recently been taken attention from many groups of robotic researchers. Most of the reported works surrounding this issue have been based on designing a static mask, which sequentially move through the pixels in the image to extract edges. Despite the success of these works, such statically could restrict the model’s performance in some domains. Designing a dynamic mask by the inspiration from the basic principle of “retina”, and which supported by a unique distribution of photoreceptor, therefore, could overcome this problem. A human-like robot (RobovieR-2) has been used to examine the validity of the proposed model. The experimental results show the validity of the model, and it is ability to offer a number of advantages to the robot, such as: accurate edge detection and better attention to the front user, which is a step towards human-robot interaction.
Dynamic representation of spectral edges in guinea pig primary auditory cortex
Montejo, Noelia
2015-01-01
The central representation of a given acoustic motif is thought to be strongly context dependent, i.e., to rely on the spectrotemporal past and present of the acoustic mixture in which it is embedded. The present study investigated the cortical representation of spectral edges (i.e., where stimulus energy changes abruptly over frequency) and its dependence on stimulus duration and depth of the spectral contrast in guinea pig. We devised a stimulus ensemble composed of random tone pips with or without an attenuated frequency band (AFB) of variable depth. Additionally, the multitone ensemble with AFB was interleaved with periods of silence or with multitone ensembles without AFB. We have shown that the representation of the frequencies near but outside the AFB is greatly enhanced, whereas the representation of frequencies near and inside the AFB is strongly suppressed. These cortical changes depend on the depth of the AFB: although they are maximal for the largest depth of the AFB, they are also statistically significant for depths as small as 10 dB. Finally, the cortical changes are quick, occurring within a few seconds of stimulus ensemble presentation with AFB, and are very labile, disappearing within a few seconds after the presentation without AFB. Overall, this study demonstrates that the representation of spectral edges is dynamically enhanced in the auditory centers. These central changes may have important functional implications, particularly in noisy environments where they could contribute to preserving the central representation of spectral edges. PMID:25744885
On the Connection Between Flap Side-Edge Noise and Tip Vortex Dynamics
NASA Technical Reports Server (NTRS)
Casalino, D.; Hazir, A.; Fares, E.; Duda, B.; Khorrami, M. R.
2015-01-01
The goal of the present work is to investigate how the dynamics of the vortical flow about the flap side edge of an aircraft determine the acoustic radiation. A validated lattice- Boltzmann CFD solution of the unsteady flow about a detailed business jet configuration in approach conditions is used for the present analysis. Evidence of the connection between the noise generated by several segments of the inboard flap tip and the aerodynamic forces acting on the same segments is given, proving that the noise generation mechanism has a spatially coherent and acoustically compact character on the scale of the flap chord, and that the edge-scattering effects are of secondary importance. Subsequently, evidence of the connection between the kinematics of the tip vortex system and the aerodynamic force is provided. The kinematics of the dual vortex system are investigated via a core detection technique. Emphasis is placed on the mutual induction effects between the two main vortices rolling up from the pressure and suction sides of the flap edge. A simple heuristic formula that relates the far-field noise spectrum and the cross-spectrum of the unsteady vortical positions is developed.
Komnik, A; Saleur, H
2011-09-01
We verify the validity of the Cohen-Gallavotti fluctuation theorem for the strongly correlated problem of charge transfer through an impurity in a chiral Luttinger liquid, which is realizable experimentally as a quantum point contact in a fractional quantum Hall edge state device. This is accomplished via the development of an analytical method to calculate the full counting statistics of the problem in all the parameter regimes involving the temperature, the Hall voltage, and the gate voltage. PMID:21981487
Edge-mediated skyrmion chain and its collective dynamics in a confined geometry
NASA Astrophysics Data System (ADS)
Du, Haifeng; Che, Renchao; Kong, Lingyao; Zhao, Xuebing; Jin, Chiming; Wang, Chao; Yang, Jiyong; Ning, Wei; Li, Runwei; Jin, Changqing; Chen, Xianhui; Zang, Jiadong; Zhang, Yuheng; Tian, Mingliang
2015-10-01
The emergence of a topologically nontrivial vortex-like magnetic structure, the magnetic skyrmion, has launched new concepts for memory devices. Extensive studies have theoretically demonstrated the ability to encode information bits by using a chain of skyrmions in one-dimensional nanostripes. Here, we report experimental observation of the skyrmion chain in FeGe nanostripes by using high-resolution Lorentz transmission electron microscopy. Under an applied magnetic field, we observe that the helical ground states with distorted edge spins evolve into individual skyrmions, which assemble in the form of a chain at low field and move collectively into the interior of the nanostripes at elevated fields. Such a skyrmion chain survives even when the width of the nanostripe is much larger than the size of single skyrmion. This discovery demonstrates a way of skyrmion formation through the edge effect, and might, in the long term, shed light on potential applications.
Edge-mediated skyrmion chain and its collective dynamics in a confined geometry
Du, Haifeng; Che, Renchao; Kong, Lingyao; Zhao, Xuebing; Jin, Chiming; Wang, Chao; Yang, Jiyong; Ning, Wei; Li, Runwei; Jin, Changqing; Chen, Xianhui; Zang, Jiadong; Zhang, Yuheng; Tian, Mingliang
2015-01-01
The emergence of a topologically nontrivial vortex-like magnetic structure, the magnetic skyrmion, has launched new concepts for memory devices. Extensive studies have theoretically demonstrated the ability to encode information bits by using a chain of skyrmions in one-dimensional nanostripes. Here, we report experimental observation of the skyrmion chain in FeGe nanostripes by using high-resolution Lorentz transmission electron microscopy. Under an applied magnetic field, we observe that the helical ground states with distorted edge spins evolve into individual skyrmions, which assemble in the form of a chain at low field and move collectively into the interior of the nanostripes at elevated fields. Such a skyrmion chain survives even when the width of the nanostripe is much larger than the size of single skyrmion. This discovery demonstrates a way of skyrmion formation through the edge effect, and might, in the long term, shed light on potential applications. PMID:26446692
Magnetic edge states and mixed-parity pairing in spin-triplet superconductors
NASA Astrophysics Data System (ADS)
Cuoco, Mario; Gentile, Paola; Noce, Canio; Vekhter, Ilya; Romano, Alfonso
2014-03-01
We show that a spontaneous magnetic moment may appear at the edge of a spin-triplet superconductor if the system allows for pairing in a subdominant channel and non-uniform spatial profile. To unveil the microscopic mechanism behind such effect we combine numerical solution of the Bogoliubov-De Gennes equations for a tight-binding model with nearest-neighbor attraction, and the symmetry based Ginzburg-Landau approach. We find that a modulation of the electronic density near the edge of the system leads to a non-unitary superconducting state where spin-singlet pairing coexists with the dominant triplet superconducting order. We demonstrate that the spin polarization at the edge appears due to the inhomogeneity of the non-unitary state and originates in the lifting of the spin-degeneracy of the Andreev bound-states. For chiral spin-triplet superconductors spin current flows along the interface and surface charge currents exhibit anomalous dependence on the magnetization. - A. Romano, P. Gentile, C. Noce, I. Vekhter, M. Cuoco, Phys. Rev. Lett. 110, 267002 (2013). This research has received funding from the EU -FP7/2007-2013 under grant agreement N. 264098 - MAMA, and was supported in part by US NSF via Grant No. DMR-1105339
Snyder, P.B.; Wilson, H.R.; Xu, X.Q.
2005-05-15
Nonlinear three-dimensional electromagnetic simulations are employed to study the dynamics of edge localized modes (ELMs) driven by intermediate wavelength peeling-ballooning modes. It is found that the early behavior of the modes is similar to expectations from linear, ideal peeling-ballooning mode theory, with the modes growing linearly at a fraction of the Alfven frequency. In the nonlinear phase, the modes grow explosively, forming a number of extended filaments which propagate rapidly from the outer closed flux region into the open flux region toward the outboard wall. Similarities to nonlinear ballooning theory as well as additional complexities are observed. Comparison to observations reveals a number of similarities. Implications of the simulations and proposals for the dynamics of the full ELM crash are discussed.
Ito, H.; Shibata, Y.; Mamyoda, S.; Ootuka, Y.; Nomura, S.; Kashiwaya, S.; Yamaguchi, M.; Akazaki, T.; Tamura, H.
2013-12-04
A high resolution mapping of quantum Hall edge states has been performed by locally creating electrons with small excess energies with a near-field scanning optical microscope in a dilution refrigerator. We have observed fine structures parallel to the edge in photovoltage signals, which appear only at low temperature. The observed fine structures near sample edges have been seen to shift inward with increase in magnetic field in accordance with Chklovskii Shklovskii, and Glazman model.
Transverse Mode Dynamics of Broad-Area Edge- and Surface-Emitting Lasers
NASA Technical Reports Server (NTRS)
Ning, Cun-Zheng; Goorjian, Peter; Saini, Subhash (Technical Monitor)
1998-01-01
This paper reports new results of our recent theoretical and simulational research in broad-area diode lasers. In a broad-area edge- or surface-emitting laser, the large space dimension in the direction transverse to the propagation direction requires an adequate treatment of inhomogeneities of the relevant physical quantities, such as laser field intensity and electron-hole carrier densities. The density inhomogeneity requires gain and refractive index nonlinearities across the laser structure to be included. All these features can be captured by a set of space-time resolved partial differential equations, the so-called effective Bloch equations established recently. We have solved this set of equations for both edge-emitting and surface-emitting lasers. This allows us to investigate temporal dynamics of transverse mode structures in these lasers. The influence of the transverse pumping profile and geometrical structure of the devices will be reported for VCSELs, as well as the complex temporal competition dynamics of different modes.
Noiseless manipulation of helical edge state transport by a quantum magnet
NASA Astrophysics Data System (ADS)
Silvestrov, P. G.; Recher, P.; Brouwer, P. W.
2016-05-01
The current through a helical edge state of a quantum spin Hall insulator may be fully transmitted through a magnetically gapped region due to a combination of spin-transfer torque and spin pumping [Meng et al., Phys. Rev. B 90, 205403 (2014), 10.1103/PhysRevB.90.205403]. Using a scattering approach, we here argue that in such a system the current is effectively carried by electrons with energies below the magnet-induced gap and well below the Fermi energy. This has striking consequences, such as the absence of shot noise, an exponential suppression of thermal noise, and an obstruction of thermal transport. For two helical edges covered by the same quantum magnet, the device can act as a robust noiseless current splitter.
Edge structure of graphene monolayers in the ν =0 quantum Hall state
NASA Astrophysics Data System (ADS)
Knothe, Angelika; Jolicoeur, Thierry
2015-10-01
Monolayer graphene at neutrality in the quantum Hall regime has many competing ground states with various types of ordering. The outcome of this competition is modified by the presence of the sample boundaries. In this paper we use a Hartree-Fock treatment of the electronic correlations allowing for space-dependent ordering. The armchair edge influence is modeled by a simple perturbative effective magnetic field in valley space. We find that all phases found in the bulk of the sample, ferromagnetic, canted antiferromagnetic, charge-density wave, and Kekulé distortion, are smoothly connected to a Kekulé-distorted edge. The single-particle excitations are computed taking into account the spatial variation of the order parameters. An eventual metal-insulator transition as a function of the Zeeman energy is not simply related to the type of bulk order.
Nithianandam, J.; Rife, J.C. ); Windischmann, H. )
1992-01-06
We have made carbon {ital K} edge reflectivity and absorption measurements using synchrotron radiation on diamond crystals and chemical vapor deposited diamond films to determine their electronic structures. Our spectra of diamond films show that both {ital sp}{sup 2} and {ital sp}{sup 3} bonded carbon atoms are formed during initial nucleation and growth. Transmission spectra of a diamond film with 30 nm diameter cystallites show striking features below the carbon {ital sp}{sup 3} {ital K} edge due to internal interface states and/or defects. We compare these absorption features to x-ray absorption spectra of clean diamond (111) surface, graphite, and hydrocarbon gases to understand surface chemistry involved in the deposition process.
GMF promotes leading edge dynamics and collective cell migration in vivo
Poukkula, Minna; Hakala, Markku; Pentinmikko, Nalle; Sweeney, Meredith O.; Jansen, Silvia; Mattila, Jaakko; Hietakangas, Ville; Goode, Bruce L.; Lappalainen, Pekka
2014-01-01
SUMMARY Lamellipodia are dynamic actin-rich cellular extensions, which drive advancement of the leading edge during cell migration [1–3]. Lamellipodia undergo periodic extension/retraction cycles [4–8], but the molecular mechanisms underlying these dynamics and their role in cell migration have remained obscure. We show that gliamaturation factor (GMF), which is an Arp2/3 complex inhibitor and actin filament debranching factor [9, 10], regulates lamellipodial protrusion dynamics in living cells. In cultured S2R+ cells, GMF silencing resulted in an increase in the width of lamellipodial actin filament arrays. Importantly, live-imaging of mutant Drosophila egg chambers revealed that the dynamics of actin-rich protrusions in migrating border cells are diminished in the absence of GMF. Consequently, velocity of border cell clusters undergoing guided migration was reduced in GMF mutant flies. Furthermore, genetic studies demonstrated that GMF cooperates with the Drosophila homologue of Aip1 (flare) in promoting disassembly of Arp2/3-nucleated actin filament networks and driving border cell migration. These data suggest that GMF functions in vivo to promote the disassembly of Arp2/3-nucleated actin filament arrays, making an important contribution to cell migration within a three-dimensional tissue environment. PMID:25308079
Kiyama, H. Fujita, T.; Teraoka, S.; Oiwa, A.; Tarucha, S.
2014-06-30
Spin filtering with electrically tunable efficiency is achieved for electron tunneling between a quantum dot and spin-resolved quantum Hall edge states by locally gating the two-dimensional electron gas (2DEG) leads near the tunnel junction to the dot. The local gating can change the potential gradient in the 2DEG and consequently the edge state separation. We use this technique to electrically control the ratio of the dot–edge state tunnel coupling between opposite spins and finally increase spin filtering efficiency up to 91%, the highest ever reported, by optimizing the local gating.
Dynamics of the ballooning mode and the relation to edge-localized modes in a spherical tokamak
Khan, R.; Mizuguchi, N.; Nakajima, N.; Hayashi, T.
2007-06-15
Nonlinear simulations based on the magnetohydrodynamic model have been executed to reveal the dynamics of the ballooning mode in the spherical tokamak plasma. The simulation results have reproduced the characteristic features of the edge-localized mode crash phase, where the filamentary structures are formed along the magnetic field in the edge region, and separated from the core plasma. Moreover, the finite Larmor radius effect is addressed.
Emergence of Helical Edge Conduction in Graphene in the ν = 0 Quantum Hall State
NASA Astrophysics Data System (ADS)
Fertig, Herbert; Tikhonov, Pavel; Shimshoni, Efrat; Murthy, Ganpathy
The conductance of graphene subject to a strong, tilted magnetic field exhibits a dramatic change with tilt-angle, interpreted as an evidence for the transition from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) ν = 0 quantum Hall state. We develop a theory for the electric transport in this system based on the spin-charge connection, whereby the evolution in the nature of collective spin excitations throughout this quantum phase transition is reflected in the charge-carrying modes. To this end we study quantum fluctuations of the spin-valley configuration in a system with an edge, and derive an effective theory describing collective charge edge excitations coupled to neutral bulk excitations. Focusing particularly on the FM phase, naively expected to exhibit perfect conductance due to the emergence helical edge modes, we analyze the mechanism whereby the coupling to bulk excitations assists in generating back-scattering. Finally, we calculate the conductance as a function of temperature and the Zeeman energy à€`` the parameter that tunes the transition between the two phases. Support provided by the US-Israel BSF, ISF, and NSF.
Topological phase and edge states dependence of the RKKY interaction in zigzag silicene nanoribbon
NASA Astrophysics Data System (ADS)
Zare, Moslem; Parhizgar, Fariborz; Asgari, Reza
2016-07-01
We propose versatile materials based on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in a zigzag silicene nanoribbon (ZSNR) on half filling in the presence of an out-of-plane electric field. We show that the topological phase transition in the band dispersion of ZSNR can be probed by using the RKKY interaction. We find that, due to the zero-energy edge states of the ZSNR, the exchange coupling is significantly enhanced when the impurities are located on the zigzag edges, and also explore that the strength of the interaction in the topological insulator phase is much greater than that when the system is in the band insulator region. We present a model to investigate the phase of a system of two magnetic impurities located on the edge of the ZSNR and find that three different magnetic phases, spiral, ferromagnetic, and antiferromagnetic, are possible for different values of the electric field. This electrical tunability of the magnetic phases in silicene can be explored by using current experimental techniques and can be of interest in the field of spintronics.
Uncovering edge states and electrical inhomogeneity in MoS2 field-effect transistors.
Wu, Di; Li, Xiao; Luan, Lan; Wu, Xiaoyu; Li, Wei; Yogeesh, Maruthi N; Ghosh, Rudresh; Chu, Zhaodong; Akinwande, Deji; Niu, Qian; Lai, Keji
2016-08-01
The understanding of various types of disorders in atomically thin transition metal dichalcogenides (TMDs), including dangling bonds at the edges, chalcogen deficiencies in the bulk, and charges in the substrate, is of fundamental importance for TMD applications in electronics and photonics. Because of the imperfections, electrons moving on these 2D crystals experience a spatially nonuniform Coulomb environment, whose effect on the charge transport has not been microscopically studied. Here, we report the mesoscopic conductance mapping in monolayer and few-layer MoS2 field-effect transistors by microwave impedance microscopy (MIM). The spatial evolution of the insulator-to-metal transition is clearly resolved. Interestingly, as the transistors are gradually turned on, electrical conduction emerges initially at the edges before appearing in the bulk of MoS2 flakes, which can be explained by our first-principles calculations. The results unambiguously confirm that the contribution of edge states to the channel conductance is significant under the threshold voltage but negligible once the bulk of the TMD device becomes conductive. Strong conductance inhomogeneity, which is associated with the fluctuations of disorder potential in the 2D sheets, is also observed in the MIM images, providing a guideline for future improvement of the device performance. PMID:27444021
Edge effects in game-theoretic dynamics of spatially structured tumours.
Kaznatcheev, Artem; Scott, Jacob G; Basanta, David
2015-07-01
Cancer dynamics are an evolutionary game between cellular phenotypes. A typical assumption in this modelling paradigm is that the probability of a given phenotypic strategy interacting with another depends exclusively on the abundance of those strategies without regard for local neighbourhood structure. We address this limitation by using the Ohtsuki-Nowak transform to introduce spatial structure to the go versus grow game. We show that spatial structure can promote the invasive (go) strategy. By considering the change in neighbourhood size at a static boundary--such as a blood vessel, organ capsule or basement membrane--we show an edge effect that allows a tumour without invasive phenotypes in the bulk to have a polyclonal boundary with invasive cells. We present an example of this promotion of invasive (epithelial-mesenchymal transition-positive) cells in a metastatic colony of prostate adenocarcinoma in bone marrow. Our results caution that pathologic analyses that do not distinguish between cells in the bulk and cells at a static edge of a tumour can underestimate the number of invasive cells. Although we concentrate on applications in mathematical oncology, we expect our approach to extend to other evolutionary game models where interaction neighbourhoods change at fixed system boundaries. PMID:26040596
Edge effects in game-theoretic dynamics of spatially structured tumours
Kaznatcheev, Artem; Scott, Jacob G.; Basanta, David
2015-01-01
Cancer dynamics are an evolutionary game between cellular phenotypes. A typical assumption in this modelling paradigm is that the probability of a given phenotypic strategy interacting with another depends exclusively on the abundance of those strategies without regard for local neighbourhood structure. We address this limitation by using the Ohtsuki–Nowak transform to introduce spatial structure to the go versus grow game. We show that spatial structure can promote the invasive (go) strategy. By considering the change in neighbourhood size at a static boundary—such as a blood vessel, organ capsule or basement membrane—we show an edge effect that allows a tumour without invasive phenotypes in the bulk to have a polyclonal boundary with invasive cells. We present an example of this promotion of invasive (epithelial–mesenchymal transition-positive) cells in a metastatic colony of prostate adenocarcinoma in bone marrow. Our results caution that pathologic analyses that do not distinguish between cells in the bulk and cells at a static edge of a tumour can underestimate the number of invasive cells. Although we concentrate on applications in mathematical oncology, we expect our approach to extend to other evolutionary game models where interaction neighbourhoods change at fixed system boundaries. PMID:26040596
Dynamical System Approach for Edge Detection Using Coupled FitzHugh-Nagumo Neurons.
Li, Shaobai; Dasmahapatra, Srinandan; Maharatna, Koushik
2015-12-01
The prospect of emulating the impressive computational capabilities of biological systems has led to considerable interest in the design of analog circuits that are potentially implementable in very large scale integration CMOS technology and are guided by biologically motivated models. For example, simple image processing tasks, such as the detection of edges in binary and grayscale images, have been performed by networks of FitzHugh-Nagumo-type neurons using the reaction-diffusion models. However, in these studies, the one-to-one mapping of image pixels to component neurons makes the size of the network a critical factor in any such implementation. In this paper, we develop a simplified version of the employed reaction-diffusion model in three steps. In the first step, we perform a detailed study to locate this threshold using continuous Lyapunov exponents from dynamical system theory. Furthermore, we render the diffusion in the system to be anisotropic, with the degree of anisotropy being set by the gradients of grayscale values in each image. The final step involves a simplification of the model that is achieved by eliminating the terms that couple the membrane potentials of adjacent neurons. We apply our technique to detect edges in data sets of artificially generated and real images, and we demonstrate that the performance is as good if not better than that of the previous methods without increasing the size of the network. PMID:26276989
Edge plasma dynamics during L-H transition in the JFT-2M tokamak
NASA Astrophysics Data System (ADS)
Kobayashi, T.; Itoh, K.; Ido, T.; Kamiya, K.; Itoh, S.-I.; Miura, Y.; Nagashima, Y.; Fujisawa, A.; Inagaki, S.; Ida, K.; Hoshino, K.
2015-06-01
This article presents a radial electric field measurement by a heavy ion beam probe in the JFT-2M tokamak, during the L-H transition. An abrupt increase (time scale of O(100 µs)) of the strong edge radial electric field (localized in the radius with FWHM ∼7 mm) results in the increase of density gradient and turbulence reduction. Rapid inward propagation of the turbulence suppression front is observed at the transition. After the transition, the electric field structure in the tiny edge localized modes (ELMs) is analyzed. Transport self-regulation events observed in the vicinity of the L-H transition, i.e. the limit cycle oscillation (LCO) in the L-mode, the tiny ELM in the H-mode, as well as the L-H transition itself, are summarized in a single Lissajous diagram in the electric field-density gradient space, which provides a comprehensive explanation of the transition dynamics. This article is dedicated to the memory of Professor Tihiro Ohkawa.
Dynamic Impact Tolerance of Shuttle RCC Leading Edge Panels using LS-DYNA
NASA Technical Reports Server (NTRS)
Fasanella, Edwin; Jackson, Karen E.; Lyle, Karen H.; Jones, Lisa E.; Hardy, Robin C.; Spellman, Regina L.; Carney, Kelly S.; Melis, Matthew E.; Stockwell, Alan E.
2008-01-01
This paper describes a research program conducted to enable accurate prediction of the impact tolerance of the shuttle Orbiter leading-edge wing panels using 'physics-based- codes such as LS-DYNA, a nonlinear, explicit transient dynamic finite element code. The shuttle leading-edge panels are constructed of Reinforced-Carbon-Carbon (RCC) composite material, which issued because of its thermal properties to protect the shuttle during re-entry into the Earth's atmosphere. Accurate predictions of impact damage from insulating foam and other debris strikes that occur during launch required materials characterization of expected debris, including strain-rate effects. First, analytical models of individual foam and RCC materials were validated. Next, analytical models of individual foam cylinders impacting 6-in. x 6-in. RCC flat plates were developed and validated. LS-DYNA pre-test models of the RCC flat plate specimens established the impact velocity of the test for three damage levels: no-detectable damage, non-destructive evaluation (NDE) detectable damage, or visible damage such as a through crack or hole. Finally, the threshold of impact damage for RCC on representative Orbiter wing panels was predicted for both a small through crack and for NDE-detectable damage.
Dynamics Impact Tolerance of Shuttle RCC Leading Edge Panels Using LS-DYNA
NASA Technical Reports Server (NTRS)
Fasanella, Edwin L.; Jackson, Karen E.; Lyle, Karen H.; Jones, Lisa E.; Hardy, Robin C.; Spellman, Regina L.; Carney, Kelly S.; Melis, Matthew E.; Stockwell, Alan E.
2005-01-01
This paper describes a research program conducted to enable accurate prediction of the impact tolerance of the shuttle Orbiter leading-edge wing panels using physics-based codes such as LS-DYNA, a nonlinear, explicit transient dynamic finite element code. The shuttle leading-edge panels are constructed of Reinforced-Carbon-Carbon (RCC) composite material, which is used because of its thermal properties to protect the shuttle during reentry into the Earth's atmosphere. Accurate predictions of impact damage from insulating foam and other debris strikes that occur during launch required materials characterization of expected debris, including strain-rate effects. First, analytical models of individual foam and RCC materials were validated. Next, analytical models of foam cylinders impacting 6- in. x 6-in. RCC flat plates were developed and validated. LS-DYNA pre-test models of the RCC flat plate specimens established the impact velocity of the test for three damage levels: no-detectable damage, non-destructive evaluation (NDE) detectable damage, or visible damage such as a through crack or hole. Finally, the threshold of impact damage for RCC on representative Orbiter wing panels was predicted for both a small through crack and for NDE-detectable damage.
Nelson, G.J.; Chu, Y.; Harris, W.M.; Izzo, J.R.; Grew, K.N., Chiu, W.K.S.; Yi, J.; Andrews, J.C.; Liu, Y., Pierro, P.
2011-04-28
The reduction-oxidation cycling of the nickel-based oxides in composite solid oxide fuel cells and battery electrodes is directly related to cell performance. A greater understanding of nickel redox mechanisms at the microstructural level can be achieved in part using transmission x-ray microscopy (TXM) to explore material oxidation states. X-ray nanotomography combined with x-ray absorption near edge structure (XANES) spectroscopy has been applied to study samples containing distinct regions of nickel and nickel oxide (NiO) compositions. Digitally processed images obtained using TXM demonstrate the three-dimensional chemical mapping and microstructural distribution capabilities of full-field XANES nanotomography.
Simulations of 4D edge transport and dynamics using the TEMPEST gyro-kinetic code
NASA Astrophysics Data System (ADS)
Rognlien, T. D.; Cohen, B. I.; Cohen, R. H.; Dorr, M. R.; Hittinger, J. A. F.; Kerbel, G. D.; Nevins, W. M.; Xiong, Z.; Xu, X. Q.
2006-10-01
Simulation results are presented for tokamak edge plasmas with a focus on the 4D (2r,2v) option of the TEMPEST continuum gyro-kinetic code. A detailed description of a variety of kinetic simulations is reported, including neoclassical radial transport from Coulomb collisions, electric field generation, dynamic response to perturbations by geodesic acoustic modes, and parallel transport on open magnetic-field lines. Comparison is made between the characteristics of the plasma solutions on closed and open magnetic-field line regions separated by a magnetic separatrix, and simple physical models are used to qualitatively explain the differences observed in mean flow and electric-field generation. The status of extending the simulations to 5D turbulence will be summarized. The code structure used in this ongoing project is also briefly described, together with future plans.
Dynamic spectra of radio frequency bursts associated with edge-localized modes
NASA Astrophysics Data System (ADS)
Thatipamula, Shekar G.; Yun, G. S.; Leem, J.; Park, H. K.; Kim, K. W.; Akiyama, T.; Lee, S. G.
2016-06-01
Electromagnetic emissions in the radio frequency (RF) range are detected in the high-confinement-mode (H-mode) plasma using a fast RF spectrometer on the KSTAR tokamak. The emissions at the crash events of edge-localized modes (ELMs) are found to occur as strong RF bursts with dynamic features in intensity and spectrum. The RF burst spectra (obtained with frequency resolution better than 10 MHz) exhibit diverse spectral features and evolve in multiple steps before the onset and through the ELM crash: (1) a narrow-band spectral line around 200 MHz persistent for extended duration in the pre-ELM crash times, (2) harmonic spectral lines with spacing comparable to deuterium or hydrogen ion cyclotron frequency at the pedestal, (3) rapid onset (faster than ~1 μs) of intense RF burst with wide-band continuum in frequency which coincides with the onset of ELM crash, and (4) a few additional intense RF bursts with chirping-down narrow-band spectrum during the crash. These observations indicate plasma waves are excited in the pedestal region and strongly correlated with the ELM dynamics such as the onset of the explosive crash. Thus the investigation of RF burst occurrence and their dynamic spectral features potentially offers the possibility of exploring H-mode physics in great detail.
NASA Astrophysics Data System (ADS)
Wu, R.; Ma, J.-Z.; Nie, S.-M.; Zhao, L.-X.; Huang, X.; Yin, J.-X.; Fu, B.-B.; Richard, P.; Chen, G.-F.; Fang, Z.; Dai, X.; Weng, H.-M.; Qian, T.; Ding, H.; Pan, S. H.
2016-04-01
Two-dimensional topological insulators with a large bulk band gap are promising for experimental studies of quantum spin Hall effect and for spintronic device applications. Despite considerable theoretical efforts in predicting large-gap two-dimensional topological insulator candidates, none of them have been experimentally demonstrated to have a full gap, which is crucial for quantum spin Hall effect. Here, by combining scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy, we reveal that ZrTe5 crystal hosts a large full gap of ˜100 meV on the surface and a nearly constant density of states within the entire gap at the monolayer step edge. These features are well reproduced by our first-principles calculations, which point to the topologically nontrivial nature of the edge states.
Interplay between snake and quantum edge states in a graphene Hall bar with a pn-junction
Milovanović, S. P. Peeters, F. M.; Ramezani Masir, M.
2014-09-22
The magneto- and Hall resistance of a locally gated cross shaped graphene Hall bar is calculated. The edge of the top gate is placed diagonally across the center of the Hall cross. Four-probe resistance is calculated using the Landauer-Büttiker formalism, while the transmission coefficients are obtained using the non-equilibrium Green's function approach. The interplay between transport due to edge channels and snake states is investigated. When two edge channels are occupied, we predict oscillations in the Hall and the bend resistance as function of the magnetic field, which are a consequence of quantum interference between the occupied snake states.
Ultrafast carrier dynamics in band edge and broad deep defect emission ZnSe nanowires
NASA Astrophysics Data System (ADS)
Othonos, Andreas; Lioudakis, Emmanouil; Philipose, U.; Ruda, Harry E.
2007-12-01
Ultrafast carrier dynamics of ZnSe nanowires grown under different growth conditions have been studied. Transient absorption measurements reveal the dependence of the competing effects of state filling and photoinduced absorption on the probed energy states. The relaxation of the photogenerated carriers occupying defect states in the stoichiometric and Se-rich samples are single exponentials with time constants of 3-4ps. State filling is the main contribution for probe energies below 1.85eV in the Zn-rich grown sample. This ultrafast carrier dynamics study provides an important insight into the role that intrinsic point defects play in the observed photoluminescence from ZnSe nanowires.
One-dimensional edge state transport in a topological Kondo insulator
NASA Astrophysics Data System (ADS)
Nakajima, Yasuyuki; Syers, Paul; Wang, Xiangfeng; Wang, Renxiong; Paglione, Johnpierre
2016-03-01
Topological insulators, with metallic boundary states protected against time-reversal-invariant perturbations, are a promising avenue for realizing exotic quantum states of matter, including various excitations of collective modes predicted in particle physics, such as Majorana fermions and axions. According to theoretical predictions, a topological insulating state can emerge from not only a weakly interacting system with strong spin-orbit coupling, but also in insulators driven by strong electron correlations. The Kondo insulator compound SmB6 is an ideal candidate for realizing this exotic state of matter, with hybridization between itinerant conduction electrons and localized f-electrons driving an insulating gap and metallic surface states at low temperatures. Here we exploit the existence of surface ferromagnetism in SmB6 to investigate the topological nature of metallic surface states by studying magnetotransport properties at very low temperatures. We find evidence of one-dimensional surface transport with a quantized conductance value of e2/h originating from the chiral edge channels of ferromagnetic domain walls, providing strong evidence that topologically non-trivial surface states exist in SmB6.
Kim, Chang-Wan; Dai, Mai Duc
2016-01-01
Summary We have studied the finite-size effect on the dynamic behavior of graphene resonators and their applications in atomic mass detection using a continuum elastic model such as modified plate theory. In particular, we developed a model based on von Karman plate theory with including the edge stress, which arises from the imbalance between the coordination numbers of bulk atoms and edge atoms of graphene. It is shown that as the size of a graphene resonator decreases, the edge stress depending on the edge structure of a graphene resonator plays a critical role on both its dynamic and sensing performances. We found that the resonance behavior of graphene can be tuned not only through edge stress but also through nonlinear vibration, and that the detection sensitivity of a graphene resonator can be controlled by using the edge stress. Our study sheds light on the important role of the finite-size effect in the effective design of graphene resonators for their mass sensing applications. PMID:27335758
Kim, Chang-Wan; Dai, Mai Duc; Eom, Kilho
2016-01-01
We have studied the finite-size effect on the dynamic behavior of graphene resonators and their applications in atomic mass detection using a continuum elastic model such as modified plate theory. In particular, we developed a model based on von Karman plate theory with including the edge stress, which arises from the imbalance between the coordination numbers of bulk atoms and edge atoms of graphene. It is shown that as the size of a graphene resonator decreases, the edge stress depending on the edge structure of a graphene resonator plays a critical role on both its dynamic and sensing performances. We found that the resonance behavior of graphene can be tuned not only through edge stress but also through nonlinear vibration, and that the detection sensitivity of a graphene resonator can be controlled by using the edge stress. Our study sheds light on the important role of the finite-size effect in the effective design of graphene resonators for their mass sensing applications. PMID:27335758
Emergence of helical edge conduction in graphene at the ν =0 quantum Hall state
NASA Astrophysics Data System (ADS)
Tikhonov, Pavel; Shimshoni, Efrat; Fertig, H. A.; Murthy, Ganpathy
2016-03-01
The conductance of graphene subject to a strong, tilted magnetic field exhibits a dramatic change from insulating to conducting behavior with tilt angle, regarded as evidence for the transition from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) ν =0 quantum Hall state. We develop a theory for the electric transport in this system based on the spin-charge connection, whereby the evolution in the nature of collective spin excitations is reflected in the charge-carrying modes. To this end, we derive an effective field-theoretical description of the low-energy excitations, associated with quantum fluctuations of the spin-valley domain-wall ground-state configuration which characterizes the two-dimensional (2D) system with an edge. This analysis yields a model describing a one-dimensional charged edge mode coupled to charge-neutral spin-wave excitations in the 2D bulk. Focusing particularly on the FM phase, naively expected to exhibit perfect conductance, we study a mechanism whereby the coupling to these bulk excitations assists in generating backscattering. Our theory yields the conductance as a function of temperature and the Zeeman energy—the parameter that tunes the transition between the FM and CAF phases—with behavior in qualitative agreement with experiment.
Hartree simulations of coupled quantum Hall edge states in corner-overgrown heterostructures
NASA Astrophysics Data System (ADS)
Steinke, L.; Cantwell, P.; Stach, E.; Schuh, D.; Fontcuberta i Morral, A.; Bichler, M.; Abstreiter, G.; Grayson, M.
2013-04-01
The electronic states in a corner-overgrown bent GaAs/AlGaAs quantum well heterostructure are studied with numerical Hartree simulations. Transmission electron microscope pictures of the junction sharpness are shown to justify the sharp-corner potential assumed for these calculations. In a tilted magnetic field, both facets of the bent quantum well are brought to a quantum Hall (QH) state, and the corner hosts an unconventional hybrid system of coupled counter-propagating quantum Hall edges and an additional 1D accumulation wire. We show how, in contrast to coplanar barrier-junctions of QH systems, the coupling between the three subsystems increases as a function of the applied magnetic field, and discuss the implications of the numerical results for the interpretation of experimental data on bent quantum Hall systems reported elsewhere.
Dynamics of a pneumatic artificial muscle actuation system driving a trailing edge flap
NASA Astrophysics Data System (ADS)
Woods, Benjamin K. S.; Kothera, Curt S.; Wang, Gang; Wereley, Norman M.
2014-09-01
This study presents a time domain dynamic model of an antagonistic pneumatic artificial muscle (PAM) driven trailing edge flap (TEF) system for next generation active helicopter rotors. Active rotor concepts are currently being widely researched in the rotorcraft community as a means to provide a significant leap forward in performance through primary aircraft control, vibration mitigation and noise reduction. Recent work has shown PAMs to be a promising candidate for active rotor actuation due to their combination of high force, large stroke, light weight, and suitable bandwidth. When arranged into biologically inspired agonist/antagonist muscle pairs they can produce bidirectional torques for effectively driving a TEF. However, there are no analytical dynamic models in the literature that can accurately capture the behavior of such systems across the broad range of frequencies required for this demanding application. This work combines mechanical, pneumatic, and aerodynamic component models into a global flap system model developed for the Bell 407 rotor system. This model can accurately predict pressure, force, and flap angle response to pneumatic control valve inputs over a range of operating frequencies from 7 to 35 Hz (1/rev to 5/rev for the Bell 407) and operating pressures from 30 to 90 psi.
NASA Astrophysics Data System (ADS)
Chôné, L.; Beyer, P.; Sarazin, Y.; Fuhr, G.; Bourdelle, C.; Benkadda, S.
2015-07-01
L-H transition features are reproduced using three-dimensional first-principles plasma edge turbulence simulations. A transport barrier is observed to form spontaneously above a threshold of the input power. The physical mechanism relies on the coupling between the equilibrium pressure gradient and the poloidal flow, through both the radial force balance and the neoclassical friction. Accounting for the actual radial profile and time evolution of the latter is key to the barrier formation. It is found that neoclassical friction acts as an energy source for the flow, which largely overcomes the sink due to the turbulent Reynolds stress during the whole barrier lifetime. Importantly, experimentally reported dynamical features are recovered during the formation and lifetime of the barrier. This includes dithering of the radial electric field, which is reminiscent of experimentally observed limit-cycle oscillations and quasi-periodic relaxation oscillations showing similarities with type-III ELMs. These rich dynamics emerge from interplay between turbulence, turbulence-driven flows and the equilibrium flow governed by force balance.
Kirby, J. A.; Goodin, D. B.; Wydrzynski, T.; Robertson, A. S.; Klein, M. P.
1980-11-01
X-ray absorption spectra at the Manganese K-edge are presented for spinach chloroplasts, and chloroplasts which have been Tris-treated and hence unable to evolve oxygen. A significant change in the electronic environment of manganese is observed and is attributed to the release of manganese from the thylakoid membranes with a concomitant change in oxidation state. A correlation of the K-edge energy, defined as the energy at the first inflection point, with coordination charge has been established for a number of manganese compounds of known structure and oxidation state. Comparison of the manganese K-edge energies of the chloroplast samples with the reference compounds places the average oxidation state of the chloroplasts between +2 and +3. Using the edge spectra for Tris-treated membranes which were osmotically shocked to remove the released manganese, difference edge spectra were synthesized to approximate the active pool of manganese. Coordination charge predictions for this fraction are consistent with an average resting oxidation state higher than +2. The shape at the edge is also indicative of heterogeneity of the manganese site, of low symmetry, or both.
Solitary state at the edge of synchrony in ensembles with attractive and repulsive interactions.
Maistrenko, Yuri; Penkovsky, Bogdan; Rosenblum, Michael
2014-06-01
We discuss the desynchronization transition in networks of globally coupled identical oscillators with attractive and repulsive interactions. We show that, if attractive and repulsive groups act in antiphase or close to that, a solitary state emerges with a single repulsive oscillator split up from the others fully synchronized. With further increase of the repulsing strength, the synchronized cluster becomes fuzzy and the dynamics is given by a variety of stationary states with zero common forcing. Intriguingly, solitary states represent the natural link between coherence and incoherence. The phenomenon is described analytically for phase oscillators with sine coupling and demonstrated numerically for more general amplitude models. PMID:25019710
Lin, S.; Zhang, G.; Li, C.; Song, Z.
2016-01-01
We study the tight-binding model for a graphene tube with perimeter N threaded by a magnetic field. We show exactly that this model has different nontrivial topological phases as the flux changes. The winding number, as an indicator of topological quantum phase transition (QPT) fixes at N/3 if N/3 equals to its integer part [N/3], otherwise it jumps between [N/3] and [N/3] + 1 periodically as the flux varies a flux quantum. For an open tube with zigzag boundary condition, exact edge states are obtained. There exist two perfect midgap edge states, in which the particle is completely located at the boundary, even for a tube with finite length. The threading flux can be employed to control the quantum states: transferring the perfect edge state from one end to the other, or generating maximal entanglement between them. PMID:27554930
Klein tunneling of helical edge states in narrow strips of a two-dimensional topological insulator
NASA Astrophysics Data System (ADS)
Takagaki, Y.
2016-01-01
The quantum transmission of helical edge states across a square potential barrier is numerically investigated in narrow channels of a two-dimensional topological insulator. Although the transmission probability in general decreases when a potential offset is introduced in the middle of the channels, the transmission remains almost perfect regardless of the amplitude and length of the potential offset when the hybridization energy gap is closed by tuning the off-diagonal spin-orbit terms in the effective four-band Hamiltonian. The approximate absence of scattering resembling the Klein tunneling, where the transmission is unimpeded as an electron propagates relativistically as a hole in the barrier without decay, improves further when an interference condition is satisfied within the barrier. The dependence of the residual reflection on the Fermi level reveals anomalous characteristics in the Klein tunneling regime.
Suppression of dynamic stall with a leading-edge slat on a VR-7 airfoil
NASA Technical Reports Server (NTRS)
Mcalister, K. W.; Tung, C.
1993-01-01
The VR-7 airfoil was experimentally studied with and without a leading-edge slat at fixed angles of attack from 0 deg to 30 deg at Re = 200,000 and for unsteady pitching motions described by alpha equals alpha(sub m) + 10 deg(sin(wt)). The models were two dimensional, and the test was performed in a water tunnel at Ames Research Center. The unsteady conditions ranged over Re equals 100,000 to 250,000, k equals 0.001 to 0.2, and alpha(sub m) = 10 deg to 20 deg. Unsteady lift, drag, and pitching-moment measurements were obtained along with fluorescent-dye flow visualizations. The addition of the slat was found to delay the static-drag and static-moment stall by about 5 degrees and to eliminate completely the development of a dynamic-stall vortex during unsteady motions that reached angles as high as 25 degrees. In all of the unsteady cases studied, the slat caused a significant reduction in the force and moment hysteresis amplitudes. The reduced frequency was found to have the greatest effect on the results, whereas the Reynolds number had little effect on the behavior of either the basic or the slatted airfoil. The slat caused a slight drag penalty at low angles of attack, but generally increased the lift/drag ratio when averaged over the full cycle of oscillation.
Peer pressure is a double-edged sword in vaccination dynamics
NASA Astrophysics Data System (ADS)
Wu, Zhi-Xi; Zhang, Hai-Feng
2013-10-01
Whether or not to change behavior depends not only on the personal success of each individual, but also on the success and/or behavior of others. Using this as motivation, we incorporate the impact of peer pressure into a susceptible-vaccinated-infected-recovered (SVIR) epidemiological model, where the propensity to adopt a particular vaccination strategy depends both on individual success as well as on the strategies of neighbors. We show that plugging into the peer pressure is a double-edged sword, which, on the one hand, strongly promotes vaccination when its cost is below a critical value, but, on the other hand, it can also strongly impede it if the critical value is exceeded. We explain this by revealing a facilitated cluster formation process that is induced by the peer pressure. Due to this, the vaccinated individuals are inclined to cluster together and therefore become unable to efficiently inhibit the spread of the infectious disease if the vaccination is costly. If vaccination is cheap, however, they reinforce each other in using it. Our results are robust to variations of the SVIR dynamics on different population structures.
NASA Technical Reports Server (NTRS)
Boyer, K. L.; Wuescher, D. M.; Sarkar, S.
1991-01-01
Dynamic edge warping (DEW), a technique for recovering reasonably accurate disparity maps from uncalibrated stereo image pairs, is presented. No precise knowledge of the epipolar camera geometry is assumed. The technique is embedded in a system including structural stereopsis on the front end and robust estimation in digital photogrammetry on the other for the purpose of self-calibrating stereo image pairs. Once the relative camera orientation is known, the epipolar geometry is computed and the system can use this information to refine its representation of the object space. Such a system will find application in the autonomous extraction of terrain maps from stereo aerial photographs, for which camera position and orientation are unknown a priori, and for online autonomous calibration maintenance for robotic vision applications, in which the cameras are subject to vibration and other physical disturbances after calibration. This work thus forms a component of an intelligent system that begins with a pair of images and, having only vague knowledge of the conditions under which they were acquired, produces an accurate, dense, relative depth map. The resulting disparity map can also be used directly in some high-level applications involving qualitative scene analysis, spatial reasoning, and perceptual organization of the object space. The system as a whole substitutes high-level information and constraints for precise geometric knowledge in driving and constraining the early correspondence process.
State-to-state dynamics of molecular energy transfer
Gentry, W.R.; Giese, C.F.
1993-12-01
The goal of this research program is to elucidate the elementary dynamical mechanisms of vibrational and rotational energy transfer between molecules, at a quantum-state resolved level of detail. Molecular beam techniques are used to isolate individual molecular collisions, and to control the kinetic energy of collision. Lasers are used both to prepare specific quantum states prior to collision by stimulated-emission pumping (SEP), and to measure the distribution of quantum states in the collision products by laser-induced fluorescence (LIF). The results are interpreted in terms of dynamical models, which may be cast in a classical, semiclassical or quantum mechanical framework, as appropriate.
NASA Astrophysics Data System (ADS)
Bocharov, Dmitry; Chollet, Melanie; Krack, Matthias; Bertsch, Johannes; Grolimund, Daniel; Martin, Matthias; Kuzmin, Alexei; Purans, Juris; Kotomin, Eugene
2016-05-01
X-ray absorption spectroscopy is employed to study the local structure of pure and Cr-doped UO2 at 300 K. The U L3-edge EXAFS spectrum is interpreted within the multiplescattering (MS) theory using the results of the classical and ab initio molecular dynamics simulations, allowing us to validate the accuracy of theoretical models. The Cr K-edge XANES is simulated within the full-multiple-scattering formalism considering a substitutional model (Cr at U site). It is shown that both unrelaxed and relaxed structures, produced by ab initio density functional theory (DFT) calculations, fail to describe the experiment.
Dynamics of Affective States during Complex Learning
ERIC Educational Resources Information Center
D'Mello, Sidney; Graesser, Art
2012-01-01
We propose a model to explain the dynamics of affective states that emerge during deep learning activities. The model predicts that learners in a state of engagement/flow will experience cognitive disequilibrium and confusion when they face contradictions, incongruities, anomalies, obstacles to goals, and other impasses. Learners revert into the…
NASA Astrophysics Data System (ADS)
Tai, Yuan-Yen; Choi, Hongchul; Ahmed, Towfiq; Ting, C. S.; Zhu, Jian-Xin
2015-11-01
Recently, topological superconducting states have attracted much interest. In this paper, we consider a topological superconductor with Z2 topological mirror order [Y.-Y. Tai et al., Phys. Rev. B 91, 041111(R) (2015), 10.1103/PhysRevB.91.041111] and s±-wave superconducting pairing symmetry, within a two-orbital model originally designed for iron-based superconductivity [Y.-Y. Tai et al., Europhys. Lett. 103, 67001 (2013), 10.1209/0295-5075/103/67001]. We predict the existence of gapless edge states. We also study the local electronic structure around an adsorbed interstitial magnetic impurity in the system, and find the existence of low-energy in-gap bound states even with a weak spin polarization on the impurity. We also discuss the relevance of our results to a recent scanning tunneling microscopy experiment on a Fe(Te,Se) compound with an adsorbed Fe impurity [J.-X. Yin et al., Nat. Phys. 11, 543 (2015), 10.1038/nphys3371], for which our density functional calculations show the Fe impurity is spin polarized.
NASA Astrophysics Data System (ADS)
Sánchez-Reales, J. M.; Andersen, O. B.; Vigo, M. I.
2016-03-01
With increased geoid resolution provided by the gravity and steady-state ocean circulation explorer (GOCE) mission, the ocean's mean dynamic topography (MDT) can be now estimated with an accuracy not available prior to using geodetic methods. However, an altimetric-derived MDT still needs filtering in order to remove short wavelength noise unless integrated methods are used in which the three quantities are determined simultaneously using appropriate covariance functions. We studied nonlinear anisotropic diffusive filtering applied to the oceańs MDT and a new approach based on edge-enhancing diffusion (EED) filtering is presented. EED filters enable controlling the direction and magnitude of the filtering, with subsequent enhancement of computations of the associated surface geostrophic currents (SGCs). Applying this method to a smooth MDT and to a noisy MDT, both for a region in the Northwestern Pacific Ocean, we found that EED filtering provides similar estimation of the current velocities in both cases, whereas a non-linear isotropic filter (the Perona and Malik filter) returns results influenced by local residual noise when a difficult case is tested. We found that EED filtering preserves all the advantages that the Perona and Malik filter have over the standard linear isotropic Gaussian filters. Moreover, EED is shown to be more stable and less influenced by outliers. This suggests that the EED filtering strategy would be preferred given its capabilities in controlling/preserving the SGCs.
NASA Astrophysics Data System (ADS)
Aytac, Y.; Olson, B. V.; Kim, J. K.; Shaner, E. A.; Hawkins, S. D.; Klem, J. F.; Flatté, M. E.; Boggess, T. F.
2016-05-01
A set of seven InAs /In (As ,Sb ) type-II superlattices (T2SLs) are designed to have specific band-gap energies between 290 meV (4.3 μ m ) and 135 meV (9.2 μ m ) in order to study the effects of the T2SL band-gap energy on the minority-carrier lifetime. A temperature-dependent optical pump-probe technique is used to measure the carrier lifetimes, and the effect of a midgap defect level on the carrier-recombination dynamics is reported. The Shockley-Read-Hall (SRH) defect state is found to be at energy of approximately -250 ±12 meV relative to the valence-band edge of bulk GaSb for the entire set of T2SL structures, even though the T2SL valence-band edge shifts by 155 meV on the same scale. These results indicate that the SRH defect state in InAs /In (As ,Sb ) T2SLs is singular and is nearly independent of the exact position of the T2SL band-gap or band-edge energies. They also suggest the possibility of engineering the T2SL structure such that the SRH state is removed completely from the band gap, a result that should significantly increase the minority-carrier lifetime.
Aytac, Y.; Olson, B. V.; Kim, J. K.; Shaner, E. A.; Hawkins, S. D.; Klem, J. F.; Flatté, M. E.; Boggess, T. F.
2016-05-24
A set of seven InAs/InAsSb type-II superlattices (T2SLs) were designed to have speci c bandgap energies between 290 meV (4.3 m) and 135 meV (9.2 m) in order to study the e ects of the T2SL bandgap energy on the minority carrier lifetime. A temperature dependent optical pump-probe technique is used to measure the carrier lifetimes, and the e ect of a mid-gap defect level on the carrier recombination dynamics is reported. The Shockley-Read-Hall (SRH) defect state is found to be at energy of approximately -250 12 meV relative to the valence band edge of bulk GaSb for the entiremore » set of T2SL structures, even though the T2SL valence band edge shifts by 155 meV on the same scale. These results indicate that the SRH defect state in InAs/InAsSb T2SLs is singular and is nearly independent of the exact position of the T2SL bandgap or band edge energies. They also suggest the possibility of engineering the T2SL structure such that the SRH state is removed completely from the bandgap, a result that should signi cantly increase the minority carrier lifetime.« less
Screening and edge states in two-dimensional metals in a magnetic field
Shikin, V. B. Nazin, S. S.
2011-08-15
The length {lambda}{sub 0} at which the lateral electric-field component E{sub Up-Tack} perpendicular to the boundary is conserved near the boundary of two-dimensional (2D) samples, which is covered by 2D electrons, has been determined. The existence of the finite such length follows from the self-consistent process of the screening of the external fields forming the boundaries of real 2D systems by the electrons of the metal. The effect of E{sub Up-Tack} on the structure of magnetic edge states has been taken into account in the mean field approximation in a wide range of the external field from the semiclassical limit ({epsilon}{sub F} Much-Greater-Than h{omega}{sub c}), where {epsilon}{sub F} is the Fermi energy of the 2D system and h{omega}{sub c} is the cyclotron energy to the quantum Hall effect (QHE) region ({epsilon}{sub F} Much-Less-Than h{omega}{sub c}). The positions of the magnetic edge state peaks against the background of their ideal distribution along the perimeter of the 2D circle in the known problem of transverse magnetic focusing have been determined in the semiclassical limit. The systematic description of the structure of the skin layer with {lambda}{sub H} {>=} {lambda}{sub 0}, consisting of the set of the so-called integer strips (overlapping or independent), which are carriers of the universal quantum conductance, has been proposed in the QHE regime. A relatively large probability of the overlapping of the fields of adjacent strips, as well as the possibility of describing coupled integer cascades, is remarkable. The existing data on the tunneling current through integer strips in the {lambda}{sub H} layer providing suitable information on the actual state of the boundary of the 2D system have been commented. A natural analogy between the properties of magnetic edge states and a well-known problem of the details of the ballistic conductance {sigma}{sub Double-Vertical-Line }(H) of narrow electron channels in the magnetic field H has been
A Bayesian state-space formulation of dynamic occupancy models
Royle, J. Andrew; Kery, M.
2007-01-01
Species occurrence and its dynamic components, extinction and colonization probabilities, are focal quantities in biogeography and metapopulation biology, and for species conservation assessments. It has been increasingly appreciated that these parameters must be estimated separately from detection probability to avoid the biases induced by nondetection error. Hence, there is now considerable theoretical and practical interest in dynamic occupancy models that contain explicit representations of metapopulation dynamics such as extinction, colonization, and turnover as well as growth rates. We describe a hierarchical parameterization of these models that is analogous to the state-space formulation of models in time series, where the model is represented by two components, one for the partially observable occupancy process and another for the observations conditional on that process. This parameterization naturally allows estimation of all parameters of the conventional approach to occupancy models, but in addition, yields great flexibility and extensibility, e.g., to modeling heterogeneity or latent structure in model parameters. We also highlight the important distinction between population and finite sample inference; the latter yields much more precise estimates for the particular sample at hand. Finite sample estimates can easily be obtained using the state-space representation of the model but are difficult to obtain under the conventional approach of likelihood-based estimation. We use R and Win BUGS to apply the model to two examples. In a standard analysis for the European Crossbill in a large Swiss monitoring program, we fit a model with year-specific parameters. Estimates of the dynamic parameters varied greatly among years, highlighting the irruptive population dynamics of that species. In the second example, we analyze route occupancy of Cerulean Warblers in the North American Breeding Bird Survey (BBS) using a model allowing for site
A Bayesian state-space formulation of dynamic occupancy models.
Royle, J Andrew; Kéry, Marc
2007-07-01
Species occurrence and its dynamic components, extinction and colonization probabilities, are focal quantities in biogeography and metapopulation biology, and for species conservation assessments. It has been increasingly appreciated that these parameters must be estimated separately from detection probability to avoid the biases induced by non-detection error. Hence, there is now considerable theoretical and practical interest in dynamic occupancy models that contain explicit representations of metapopulation dynamics such as extinction, colonization, and turnover as well as growth rates. We describe a hierarchical parameterization of these models that is analogous to the state-space formulation of models in time series, where the model is represented by two components, one for the partially observable occupancy process and another for the observations conditional on that process. This parameterization naturally allows estimation of all parameters of the conventional approach to occupancy models, but in addition, yields great flexibility and extensibility, e.g., to modeling heterogeneity or latent structure in model parameters. We also highlight the important distinction between population and finite sample inference; the latter yields much more precise estimates for the particular sample at hand. Finite sample estimates can easily be obtained using the state-space representation of the model but are difficult to obtain under the conventional approach of likelihood-based estimation. We use R and WinBUGS to apply the model to two examples. In a standard analysis for the European Crossbill in a large Swiss monitoring program, we fit a model with year-specific parameters. Estimates of the dynamic parameters varied greatly among years, highlighting the irruptive population dynamics of that species. In the second example, we analyze route occupancy of Cerulean Warblers in the North American Breeding Bird Survey (BBS) using a model allowing for site
Mapping quantum state dynamics in spontaneous emission
Naghiloo, M.; Foroozani, N.; Tan, D.; Jadbabaie, A.; Murch, K. W.
2016-01-01
The evolution of a quantum state undergoing radiative decay depends on how its emission is detected. If the emission is detected in the form of energy quanta, the evolution is characterized by a quantum jump to a lower energy state. In contrast, detection of the wave nature of the emitted radiation leads to different dynamics. Here, we investigate the diffusive dynamics of a superconducting artificial atom under continuous homodyne detection of its spontaneous emission. Using quantum state tomography, we characterize the correlation between the detected homodyne signal and the emitter's state, and map out the conditional back-action of homodyne measurement. By tracking the diffusive quantum trajectories of the state as it decays, we characterize selective stochastic excitation induced by the choice of measurement basis. Our results demonstrate dramatic differences from the quantum jump evolution associated with photodetection and highlight how continuous field detection can be harnessed to control quantum evolution. PMID:27167893
Mapping quantum state dynamics in spontaneous emission.
Naghiloo, M; Foroozani, N; Tan, D; Jadbabaie, A; Murch, K W
2016-01-01
The evolution of a quantum state undergoing radiative decay depends on how its emission is detected. If the emission is detected in the form of energy quanta, the evolution is characterized by a quantum jump to a lower energy state. In contrast, detection of the wave nature of the emitted radiation leads to different dynamics. Here, we investigate the diffusive dynamics of a superconducting artificial atom under continuous homodyne detection of its spontaneous emission. Using quantum state tomography, we characterize the correlation between the detected homodyne signal and the emitter's state, and map out the conditional back-action of homodyne measurement. By tracking the diffusive quantum trajectories of the state as it decays, we characterize selective stochastic excitation induced by the choice of measurement basis. Our results demonstrate dramatic differences from the quantum jump evolution associated with photodetection and highlight how continuous field detection can be harnessed to control quantum evolution. PMID:27167893
Mapping quantum state dynamics in spontaneous emission
NASA Astrophysics Data System (ADS)
Naghiloo, M.; Foroozani, N.; Tan, D.; Jadbabaie, A.; Murch, K. W.
2016-05-01
The evolution of a quantum state undergoing radiative decay depends on how its emission is detected. If the emission is detected in the form of energy quanta, the evolution is characterized by a quantum jump to a lower energy state. In contrast, detection of the wave nature of the emitted radiation leads to different dynamics. Here, we investigate the diffusive dynamics of a superconducting artificial atom under continuous homodyne detection of its spontaneous emission. Using quantum state tomography, we characterize the correlation between the detected homodyne signal and the emitter's state, and map out the conditional back-action of homodyne measurement. By tracking the diffusive quantum trajectories of the state as it decays, we characterize selective stochastic excitation induced by the choice of measurement basis. Our results demonstrate dramatic differences from the quantum jump evolution associated with photodetection and highlight how continuous field detection can be harnessed to control quantum evolution.
ACCRETION THROUGH THE INNER EDGES OF PROTOPLANETARY DISKS BY A GIANT SOLID STATE PUMP
Kelling, T.; Wurm, G.
2013-09-01
At the inner edge of a protoplanetary disk, solids are illuminated by stellar light. This illumination heats the solids and creates temperature gradients along their surfaces. Interactions with ambient gas molecules lead to a radial net gas flow. Every illuminated solid particle within the edge is an individual small gas pump transporting gas inward. In total, the inner edge can provide local mass flow rates as high as M-dot = 10{sup -5} M{sub Sun} yr{sup -1}.
Markov state models of biomolecular conformational dynamics
Chodera, John D.; Noé, Frank
2014-01-01
It has recently become practical to construct Markov state models (MSMs) that reproduce the long-time statistical conformational dynamics of biomolecules using data from molecular dynamics simulations. MSMs can predict both stationary and kinetic quantities on long timescales (e.g. milliseconds) using a set of atomistic molecular dynamics simulations that are individually much shorter, thus addressing the well-known sampling problem in molecular dynamics simulation. In addition to providing predictive quantitative models, MSMs greatly facilitate both the extraction of insight into biomolecular mechanism (such as folding and functional dynamics) and quantitative comparison with single-molecule and ensemble kinetics experiments. A variety of methodological advances and software packages now bring the construction of these models closer to routine practice. Here, we review recent progress in this field, considering theoretical and methodological advances, new software tools, and recent applications of these approaches in several domains of biochemistry and biophysics, commenting on remaining challenges. PMID:24836551
Spin-polarised edge states in atomic Mn chains supported on Cu2N/Cu (100)
NASA Astrophysics Data System (ADS)
Choi, Deung-Jang; Robles, Roberto; Gauyacq, Jean-Pierre; Rubio-Verdú, Carmen; Lorente, Nicolás; Pascual, José Ignacio
2016-06-01
Scanning tunnelling microscopy and density functional theory studies of manganese chains adsorbed on Cu2N/Cu (100) reveal an unsuspected electronic edge state at ∼ 1 eV above the Fermi energy. This Tamm-like state is strongly localised to the terminal Mn atoms of the chain and fully spin polarised. However, no equivalence is found for occupied states, and the electronic structure at ∼ ‑1 eV is mainly spin unpolarised due to the extended p-states of the N atoms that mediate the coupling between the Mn atoms in the chain. The spin polarisation of the edge state is affected by the antiferromagnetic ordering of the chains leading to non-trivial consequences.
Spin-polarised edge states in atomic Mn chains supported on Cu2N/Cu (100).
Choi, Deung-Jang; Robles, Roberto; Gauyacq, Jean-Pierre; Rubio-Verdú, Carmen; Lorente, Nicolás; Ignacio Pascual, José
2016-06-15
Scanning tunnelling microscopy and density functional theory studies of manganese chains adsorbed on Cu2N/Cu (100) reveal an unsuspected electronic edge state at [Formula: see text] eV above the Fermi energy. This Tamm-like state is strongly localised to the terminal Mn atoms of the chain and fully spin polarised. However, no equivalence is found for occupied states, and the electronic structure at [Formula: see text] -1 eV is mainly spin unpolarised due to the extended p-states of the N atoms that mediate the coupling between the Mn atoms in the chain. The spin polarisation of the edge state is affected by the antiferromagnetic ordering of the chains leading to non-trivial consequences. PMID:27158116
Dynamics of molecules in extreme rotational states
Yuan, Liwei; Teitelbaum, Samuel W.; Robinson, Allison; Mullin, Amy S.
2011-01-01
We have constructed an optical centrifuge with a pulse energy that is more than 2 orders of magnitude larger than previously reported instruments. This high pulse energy enables us to create large enough number densities of molecules in extreme rotational states to perform high-resolution state-resolved transient IR absorption measurements. Here we report the first studies of energy transfer dynamics involving molecules in extreme rotational states. In these studies, the optical centrifuge drives CO2 molecules into states with J ∼ 220 and we use transient IR probing to monitor the subsequent rotational, translational, and vibrational energy flow dynamics. The results reported here provide the first molecular insights into the relaxation of molecules with rotational energy that is comparable to that of a chemical bond.
Ultrafast excited-state dynamics of isocytosine.
Szabla, Rafał; Góra, Robert W; Šponer, Jiří
2016-07-27
The alternative nucleobase isocytosine has long been considered as a plausible component of hypothetical primordial informational polymers. To examine this hypothesis we investigated the excited-state dynamics of the two most abundant forms of isocytosine in the gas phase (keto and enol). Our surface-hopping nonadiabatic molecular dynamics simulations employing the algebraic diagrammatic construction to the second order [ADC(2)] method for the electronic structure calculations suggest that both tautomers undergo efficient radiationless deactivation to the electronic ground state with time constants which amount to τketo = 182 fs and τenol = 533 fs. The dominant photorelaxation pathways correspond to ring-puckering (ππ* surface) and C[double bond, length as m-dash]O stretching/N-H tilting (nπ* surface) for the enol and keto forms respectively. Based on these findings, we infer that isocytosine is a relatively photostable compound in the gas phase and in these terms resembles biologically relevant nucleobases. The estimated S1 [radiolysis arrow - arrow with voltage kink] T1 intersystem crossing rate constant of 8.02 × 10(10) s(-1) suggests that triplet states might also play an important role in the overall excited-state dynamics of the keto tautomer. The reliability of ADC(2)-based surface-hopping molecular dynamics simulations was tested against multireference quantum-chemical calculations and the potential limitations of the employed ADC(2) approach are briefly discussed. PMID:27346684
NASA Astrophysics Data System (ADS)
Durnev, M. V.; Tarasenko, S. A.
2016-02-01
We present a theory of the electron structure and the Zeeman effect for the helical edge states emerging in two-dimensional topological insulators based on HgTe/HgCdTe quantum wells with strong natural interface inversion asymmetry. The interface inversion asymmetry, reflecting the real atomistic structure of the quantum well, drastically modifies both bulk and edge states. For the in-plane magnetic field, this asymmetry leads to a strong anisotropy of the edge-state effective g factor, which becomes dependent on the edge orientation. The interface inversion asymmetry also couples the counterpropagating edge states in the out-of-plane magnetic field leading to the opening of the gap in the edge-state spectrum by arbitrary small fields.
NASA Astrophysics Data System (ADS)
Migiyama, Go; Sugimura, Atsuhiko; Osa, Atsushi; Miike, Hidetoshi
Recently, digital cameras are offering technical advantages rapidly. However, the shot image is different from the sight image generated when that scenery is seen with the naked eye. There are blown-out highlights and crushed blacks in the image that photographed the scenery of wide dynamic range. The problems are hardly generated in the sight image. These are contributory cause of difference between the shot image and the sight image. Blown-out highlights and crushed blacks are caused by the difference of dynamic range between the image sensor installed in a digital camera such as CCD and CMOS and the human visual system. Dynamic range of the shot image is narrower than dynamic range of the sight image. In order to solve the problem, we propose an automatic method to decide an effective exposure range in superposition of edges. We integrate multi-step exposure images using the method. In addition, we try to erase pseudo-edges using the process to blend exposure values. Afterwards, we get a pseudo wide dynamic range image automatically.
NASA Astrophysics Data System (ADS)
Cai, C.; Zuo, Z. G.; Liu, S. H.; Wu, Y. L.; Wang, F. B.
2013-12-01
Wavy leading edge modifications of airfoils through imitating humpback whale flippers has been considered as a viable passive way to control flow separation. In this paper, flows around a baseline 634-021 airfoil and one with leading-edge sinusoidal protuberances were simulated using S-A turbulence model. When studying the static stall characteristics, it is found that the modified airfoil does not stall in the traditional manner, with increasing poststall lift coefficients. At high angles of attack, the flows past the wavy leading edge stayed attached for a distance, while the baseline foil is in a totally separated flow condition. On this basis, the simulations of pitch characteristic were carried out for both foils. At high angles of attack mild variations in lift and drag coefficients of the modified foil can be found, leading to a smaller area of hysteresis loop. The special structure of wavy leading edge can help maintain high consistency of the flow field in dynamic pitching station within a particular range of angles of attack.
Cirit, Murat; Krajcovic, Matej; Choi, Colin K.; Welf, Erik S.; Horwitz, Alan F.; Haugh, Jason M.
2010-01-01
Productive cell migration requires the spatiotemporal coordination of cell adhesion, membrane protrusion, and actomyosin-mediated contraction. Integrins, engaged by the extracellular matrix (ECM), nucleate the formation of adhesive contacts at the cell's leading edge(s), and maturation of nascent adhesions to form stable focal adhesions constitutes a functional switch between protrusive and contractile activities. To shed additional light on the coupling between integrin-mediated adhesion and membrane protrusion, we have formulated a quantitative model of leading edge dynamics combining mechanistic and phenomenological elements and studied its features through classical bifurcation analysis and stochastic simulation. The model describes in mathematical terms the feedback loops driving, on the one hand, Rac-mediated membrane protrusion and rapid turnover of nascent adhesions, and on the other, myosin-dependent maturation of adhesions that inhibit protrusion at high ECM density. Our results show that the qualitative behavior of the model is most sensitive to parameters characterizing the influence of stable adhesions and myosin. The major predictions of the model, which we subsequently confirmed, are that persistent leading edge protrusion is optimal at an intermediate ECM density, whereas depletion of myosin IIA relieves the repression of protrusion at higher ECM density. PMID:20195494
NASA Astrophysics Data System (ADS)
Seshadri, Ranjani; Sengupta, K.; Sen, Diptiman
2016-01-01
We study graphene, which has both spin-orbit coupling (SOC), taken to be of the Kane-Mele form, and a Zeeman field induced due to proximity to a ferromagnetic material. We show that a zigzag interface of graphene having SOC with its pristine counterpart hosts robust chiral edge modes in spite of the gapless nature of the pristine graphene; such modes do not occur for armchair interfaces. Next we study the change in the local density of states (LDOS) due to the presence of an impurity in graphene with SOC and Zeeman field, and demonstrate that the Fourier transform of the LDOS close to the Dirac points can act as a measure of the strength of the spin-orbit coupling; in addition, for a specific distribution of impurity atoms, the LDOS is controlled by a destructive interference effect of graphene electrons which is a direct consequence of their Dirac nature. Finally, we study transport across junctions, which separates spin-orbit coupled graphene with Kane-Mele and Rashba terms from pristine graphene both in the presence and absence of a Zeeman field. We demonstrate that such junctions are generally spin active, namely, they can rotate the spin so that an incident electron that is spin polarized along some direction has a finite probability of being transmitted with the opposite spin. This leads to a finite, electrically controllable, spin current in such graphene junctions. We discuss possible experiments that can probe our theoretical predictions.
Dynamics of helical states in MST
NASA Astrophysics Data System (ADS)
Munaretto, Stefano; Auriemma, F.; Brower, D.; Chapman, B. E.; den Hartog, D. J.; Ding, W. X.; Duff, J.; Franz, P.; Goetz, J. A.; Holly, D.; Lin, L.; McCollam, K. J.; McGarry, M.; Morton, L.; Nornberg, M. D.; Parke, E.; Sarff, J. S.
2014-10-01
The thermal and the magnetic dynamics of quasi-single-helicity (QSH) plasmas evolve independently during the formation and sustainment of the core helical structure. At higher plasma current (and Lundquist number) MST plasmas transition from an axisymmetric multi-helicity state to a QSH state characterized by a strong core helical mode and reduced secondary mode amplitudes. Plasmas in the QSH state tend to wall-lock, often in an orientation that is unfavorable for optimized measurements of the 3D structure using MST's advanced diagnostics. Recently a technique to control the locking position through an applied resonant magnetic perturbation has been developed. Using this technique it is possible to adjust the 3D phase more optimally for specific diagnostics, to study the dynamics of the QSH structure and thermal features. The multi-chord FIR interferometer shows the presence of a density structure for the duration of the QSH state. Measurements of the time evolution of the electron temperature profile using the Thomson Scattering diagnostic reveal that the transition to QSH allows the presence of a 3D thermal structure, but this structure is intermittent. Understanding the mechanism(s) driving these dynamics is the goal of this work. Work supported by the US DOE and NSF.
Investigation of ELM [edge localized mode] Dynamics with the Resonant Magnetic Perturbation Effects
Pankin, Alexei Y.; Kritz, Arnold H.
2011-07-19
Topics covered are: anomalous transport and E x B flow shear effects in the H-mode pedestal; RMP (resonant magnetic perturbation) effects in NSTX discharges; development of a scaling of H-mode pedestal in tokamak plasmas with type I ELMs (edge localized modes); and divertor heat load studies.
State-Resolved Dynamics of Photofragmentation
NASA Astrophysics Data System (ADS)
Lee, Yuan-Pern
2003-10-01
We discuss experiments on the dynamics of photodissociation that employ methods to select the energy, sometimes quantum states, of the reactant and to determine the quantum states and energy, sometimes also the orientation and alignment, of products. A summary of new advances of experimental methods is followed by applications to photodissociation of various types. Representative examples of simple bond fission, molecular elimination, and three-body dissociation with determined electronic states-sometimes the orientation of their angular momentum-of product atoms or distributions of electronic and internal states of product molecules illustrate the detailed information and insight that one can derive from such experiments. Photodissociation of van der Waals complexes, ions, species adsorbed on surfaces, and species in solution is excluded from this review.
Motorcycle state estimation for lateral dynamics
NASA Astrophysics Data System (ADS)
Teerhuis, A. P.; Jansen, S. T. H.
2012-08-01
The motorcycle lean (or roll) angle development is one of the main characteristics of motorcycle lateral dynamics. Control of motorcycle motions requires an accurate assessment of this quantity and for safety applications also the risk of sliding needs to be considered. Direct measurement of the roll angle and tyre slip is not available; therefore, a method of model-based estimation is developed to estimate the state of a motorcycle. This paper investigates the feasibility of such a motorcycle state estimator (MCSE). A simplified analytic model of a motorcycle is developed by comparison to an extended multi-body model of the motorcycle, designed in Matlab/SimMechanics. The analytic model is used inside an extended Kalman filter. Experimental results of an instrumented Yamaha FJR1300 motorcycle show that the MCSE is a feasible concept for obtaining signals related to the lateral dynamics of the motorcycle.
Dynamically defined measures and equilibrium states
NASA Astrophysics Data System (ADS)
Werner, Ivan
2011-12-01
A technique of dynamically defined measures is developed and its relation to the theory of equilibrium states is shown. The technique uses Carathéodory's method and the outer measure introduced in a previous work by I. Werner [Math. Proc. Camb. Phil. Soc. 140(2), 333-347 (2006), 10.1017/S0305004105009072]. As an application, equilibrium states for contractive Markov systems [I. Werner, J. London Math. Soc. 71(1), 236-258 (2005), 10.1112/S0024610704006088] are obtained.
NASA Astrophysics Data System (ADS)
Madduri, Bharath
In this thesis, the longitudinal dynamic stability modes, namely Phugoid and Short-period of delta kite with single tether are examined, for different aspect ratios (A) and flow conditions. The equations of motion, of kite are solved in polar-inertial wind frame and the tether is approximated by straight line elements. The vortex lift and induced drag due to leading edge vortices are calculated using Polhamus leading edge suction analogy. The Polhamus proportionality constants (Kp, Kv) are used to estimate the overall coefficient of lift and drag (C L, CD) and are computed using Multhopp lifting surface theory. The values of total coefficient of lift and drag (CL, CD) are examined for a wide variety of aspect ratio of delta kite and are validated by comparing with the experimental data. Linear stability analysis is performed for the chosen design variables to ensure the nominal design has stable longitudinal dynamics. A plot of the root locus of the system matrix for longitudinal dynamics as a function of geometry and flight conditions, provided an intuitive understanding of the flight modes of the kite, with respect to design parameters of interest.
Slope-scale dynamic states of rockfalls
NASA Astrophysics Data System (ADS)
Agliardi, F.; Crosta, G. B.
2009-04-01
Rockfalls are common earth surface phenomena characterised by complex dynamics at the slope scale, depending on local block kinematics and slope geometry. We investigated the nature of this slope-scale dynamics by parametric 3D numerical modelling of rockfalls over synthetic slopes with different inclination, roughness and spatial resolution. Simulations were performed through an original code specifically designed for rockfall modeling, incorporating kinematic and hybrid algorithms with different damping functions available to model local energy loss by impact and pure rolling. Modelling results in terms of average velocity profiles suggest that three dynamic regimes (i.e. decelerating, steady-state and accelerating), previously recognized in the literature through laboratory experiments on granular flows, can set up at the slope scale depending on slope average inclination and roughness. Sharp changes in rock fall kinematics, including motion type and lateral dispersion of trajectories, are associated to the transition among different regimes. Associated threshold conditions, portrayed in "phase diagrams" as slope-roughness critical lines, were analysed depending on block size, impact/rebound angles, velocity and energy, and model spatial resolution. Motion in regime B (i.e. steady state) is governed by a slope-scale "viscous friction" with average velocity linearly related to the sine of slope inclination. This suggest an analogy between rockfall motion in regime B and newtonian flow, whereas in regime C (i.e. accelerating) an analogy with a dilatant flow was observed. Thus, although local behavior of single falling blocks is well described by rigid body dynamics, the slope scale dynamics of rockfalls seem to statistically approach that of granular media. Possible outcomes of these findings include a discussion of the transition from rockfall to granular flow, the evaluation of the reliability of predictive models, and the implementation of criteria for a
State-to-state reaction dynamics: A selective review
NASA Astrophysics Data System (ADS)
Teslja, Alexey; Valentini, James J.
2006-10-01
A selective review of state-to-state reaction dynamics experiments is presented. The review focuses on three classes of reactions that exemplify the rich history and illustrate the current state of the art in such work. These three reactions are (1) the hydrogen exchange reaction, H +H2→H2+H and its isotopomers; (2) the H +RH→H2+R reactions, where RH is an alkane, beginning with H +CH4→H2+CH3 and extending to much larger alkanes; and (3) the Cl +RH→HCl+R reactions, principally Cl +CH4→HCl+CH3. We describe the experiments, discuss their results, present comparisons with theory, and introduce heuristic models.