146 research outputs found
Kondo lattice on the edge of a two-dimensional topological insulator
We revisit the problem of a single quantum impurity on the edge of a
two-dimensional time-reversal invariant topological insulator and show that the
zero temperature phase diagram contains a large local moment region for
antiferromagnetic Kondo coupling which was missed by previous poor man's
scaling treatments. The combination of an exact solution at the so-called
decoupling point and a renormalization group analysis \`a la
Anderson-Yuval-Hamann allows us to access the regime of strong
electron-electron interactions on the edge and strong Kondo coupling. We apply
similar methods to the problem of a regular one-dimensional array of quantum
impurities interacting with the edge liquid. When the edge electrons are at
half-filling with respect to the impurity lattice, the system remains gapless
unless the Luttinger parameter of the edge is less than 1/2, in which case
two-particle backscattering effects drive the system to a gapped phase with
long-range Ising antiferromagnetic order. This is in marked contrast with the
gapped disordered ground state of the ordinary half-filled one-dimensional
Kondo lattice.Comment: 18 pages, 3 figures; fixed typos, updated reference
Symmetry and topology of hyperbolic Haldane models
Particles hopping on a two-dimensional hyperbolic lattice feature
unconventional energy spectra and wave functions that provide a largely
uncharted platform for topological phases of matter beyond the Euclidean
paradigm. Using real-space topological markers as well as Chern numbers defined
in the higher-dimensional momentum space of hyperbolic band theory, we
construct and investigate hyperbolic Haldane models, which are generalizations
of Haldane's honeycomb-lattice model to various hyperbolic lattices. We present
a general framework to characterize point-group symmetries in hyperbolic
tight-binding models, and use this framework to constrain the multiple first
and second Chern numbers in momentum space. We observe several topological gaps
characterized by first Chern numbers of value and . The momentum-space
Chern numbers respect the predicted symmetry constraints and agree with
real-space topological markers, indicating a direct connection to observables
such as the number of chiral edge modes. With our large repertoire of models,
we further demonstrate that the topology of hyperbolic Haldane models is
trivialized for lattices with strong negative curvature.Comment: main text (14 pages with 7 figures and 2 tables) + appendices (28
pages with 10 figures and 2 tables) + bibliography (2 pages
Disorder-Induced Multiple Transition involving Z2 Topological Insulator
Effects of disorder on two-dimensional Z2 topological insulator are studied
numerically by the transfer matrix method. Based on the scaling analysis, the
phase diagram is derived for a model of HgTe quantum well as a function of
disorder strength and magnitude of the energy gap. In the presence of sz
non-conserving spin-orbit coupling, a finite metallic region is found that
partitions the two topologically distinct insulating phases. As disorder
increases, a narrow-gap topologically trivial insulator undergoes a series of
transitions; first to metal, second to topological insulator, third to metal,
and finally back to trivial insulator. We show that this multiple transition is
a consequence of two disorder effects; renormalization of the band gap, and
Anderson localization. The metallic region found in the scaling analysis
corresponds roughly to the region of finite density of states at the Fermi
level evaluated in the self-consistent Born approximation.Comment: 5 pages, 5 figure
Spin polarization of the quantum spin Hall edge states
While the helical character of the edge channels responsible for charge
transport in the quantum spin Hall regime of a two-dimensional topological
insulator is by now well established, an experimental confirmation that the
transport in the edge channels is spin-polarized is still outstanding. We
report experiments on nanostructures fabricated from HgTe quantum wells with an
inverted band structure, in which a split gate technique allows us to combine
both quantum spin Hall and metallic spin Hall transport in a single device. In
these devices, the quantum spin Hall effect can be used as a spin current
injector and detector for the metallic spin Hall effect, and vice versa,
allowing for an all-electrical detection of spin polarization.Comment: version 2: supplementary material with additional three figures
added. In total 27 pages, 8 figure
Green's function formalism for spin transport in metal-insulator-metal heterostructures
Second Order Speckle Statistics in Optical Coherence Tomography
Peer reviewed: NoNRC publication: Ye
Faraday rotation in graphene
We study magneto--optical properties of monolayer graphene by means of
quantum field theory methods in the framework of the Dirac model. We reveal a
good agreement between the Dirac model and a recent experiment on giant Faraday
rotation in cyclotron resonance. We also predict other regimes when the effects
are well pronounced. The general dependence of the Faraday rotation and
absorption on various parameters of samples is revealed both for suspended and
epitaxial graphene.Comment: 10 pp; v2: typos corrected and references added, v3, v4: small
changes and more reference
Photonic Analogue of Two-dimensional Topological Insulators and Helical One-Way Edge Transport in Bi-Anisotropic Metamaterials
Recent progress in understanding the topological properties of condensed
matter has led to the discovery of time-reversal invariant topological
insulators. Because of limitations imposed by nature, topologically non-trivial
electronic order seems to be uncommon except in small-band-gap semiconductors
with strong spin-orbit interactions. In this Article we show that artificial
electromagnetic structures, known as metamaterials, provide an attractive
platform for designing photonic analogues of topological insulators. We
demonstrate that a judicious choice of the metamaterial parameters can create
photonic phases that support a pair of helical edge states, and that these edge
states enable one-way photonic transport that is robust against disorder.Comment: 13 pages, 3 figure
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