58 research outputs found
A new class of models for surface relaxation with exact mean-field solutions
We introduce a class of discrete models for surface relaxation. By exactly
solving the master equation which governs the microscopic dynamics of the
surface, we determine the steady state of the surface and calculate its
roughness. We will also map our model to a diffusive system of particles on a
ring and reinterpret our results in this new setting.Comment: 12 pages, 3 figures,references adde
Midgap spectrum of the fermion-vortex system
I study the midgap spectrum of the fermion-vortex system in two spatial
dimensions. The existence of bound states, in addition to the zero modes found
by Jackiw and Rossi, is established. For a singly quantized vortex, I present
complete analytical solutions in terms of generalized Laguerre polynomials in
the opposite limits of vanishing and large vortex core size. There is an
infinite number of such bound states, with a spectrum that is, when squared,
given by, respectively, the Coulomb potential and the isotropic harmonic
oscillator. Possible experimental signatures of this spectrum in
condensed-matter realizations of the system are pointed out.Comment: 10 pages, no figure
Fractionalization in a square-lattice model with time-reversal symmetry
We propose a two-dimensional time-reversal invariant system of essentially
non-interacting electrons on a square lattice that exhibits configurations with
fractional charges e/2. These are vortex-like topological defects in the
dimerization order parameter describing spatial modulation in the electron
hopping amplitudes. Charge fractionalization is established by a simple
counting argument, analytical calculation within the effective low-energy
theory, and by an exact numerical diagonalization of the lattice Hamiltonian.
We comment on the exchange statistics of fractional charges and possible
realizations of the system.Comment: 4 pages, 3 figures, RevTex 4. (v2) improved discussion of lattice
effects and confinement; clearer figure
Vortices, zero modes and fractionalization in bilayer-graphene exciton condensate
A real-space formulation is given for the recently discussed exciton
condensate in a symmetrically biased graphene bilayer. We show that in the
continuum limit an oddly-quantized vortex in this condensate binds exactly one
zero mode per valley index of the bilayer. In the full lattice model the zero
modes are split slightly due to intervalley mixing. We support these results by
an exact numerical diagonalization of the lattice Hamiltonian. We also discuss
the effect of the zero modes on the charge content of these vortices and deduce
some of their interesting properties.Comment: (v2) A typo in Fig. 1 and a slight error in Eq. (4) corrected; all
the main results and conclusions remain unchange
Linear Response Theory and Optical Conductivity of Floquet Topological Insulators
Motivated by the quest for experimentally accessible dynamical probes of
Floquet topological insulators, we formulate the linear response theory of a
periodically driven system. We illustrate the applications of this formalism by
giving general expressions for optical conductivity of Floquet systems,
including its homodyne and heterodyne components and beyond. We obtain the
Floquet optical conductivity of specific driven models, including
two-dimensional Dirac material such as the surface of a topological insulator,
graphene, and the Haldane model irradiated with circularly or linearly
polarized laser, as well as semiconductor quantum well driven by an ac
potential. We obtain approximate analytical expressions and perform numerically
exact calculations of the Floquet optical conductivity in different scenarios
of the occupation of the Floquet bands, in particular, the diagonal Floquet
distribution and the distribution obtained after a quench. We comment on
experimental signatures and detection of Floquet topological phases using
optical probes.Comment: 16 pages, 10 figure
Floquet Perturbation Theory: Formalism and Application to Low-Frequency Limit
We develop a low-frequency perturbation theory in the extended Floquet
Hilbert space of a periodically driven quantum systems, which puts the high-
and low-frequency approximations to the Floquet theory on the same footing. It
captures adiabatic perturbation theories recently discussed in the literature
as well as diabatic deviation due to Floquet resonances. For illustration, we
apply our Floquet perturbation theory to a driven two-level system as in the
Schwinger-Rabi and the Landau-Zener-St\"uckelberg-Majorana models. We reproduce
some known expressions for transition probabilities in a simple and systematic
way and clarify and extend their regime of applicability. We then apply the
theory to a periodically-driven system of fermions on the lattice and obtain
the spectral properties and the low-frequency dynamics of the system.Comment: v2: 28 single-column pages, 5 figures; various typos fixed; some
notation and connection to other perturbation schemes clarified; new, more
descriptive title and abstract. Published versio
Quantum noise detects Floquet topological phases
We study quantum noise in a nonequilibrium, periodically driven, open system
attached to static leads. Using a Floquet Green's function formalism we show,
both analytically and numerically, that local voltage noise spectra can detect
the rich structure of Floquet topological phases unambiguously. Remarkably,
both regular and anomalous Floquet topological bound states can be detected,
and distinguished, via peak structures of noise spectra at the edge around
zero-, half-, and full-drive-frequency. We also show that the topological
features of local noise are robust against moderate disorder. Thus, local noise
measurements are sensitive detectors of Floquet topological phases.Comment: 4.5 pages + supplemental material; v2: improved presentation and new
and updated reference
Exciton condensation and charge fractionalization in a topological insulator film
An odd number of gapless Dirac fermions is guaranteed to exist at a surface
of a strong topological insulator. We show that in a thin-film geometry and
under external bias, electron-hole pairs that reside in these surface states
can condense to form a coherent exciton condensate, similar in general terms to
the exciton condensate recently argued to exist in a biased graphene bilayer,
but with different topological properties. Such a `topological' exciton
condensate (TEC) exhibits a host of unusual properties; the most interesting
among them is the fractional charge +-e/2 carried by a singly quantized vortex
in the TEC order parameter.Comment: 4 pages, 1 figure, version to appear in PRL (minor stylistic
changes). For related work and info visit http://www.physics.ubc.ca/~franz
Two-dimensional Dirac fermions in a topological insulator: transport in the quantum limit
Pulsed magnetic fields of up to 55T are used to investigate the transport
properties of the topological insulator Bi_2Se_3 in the extreme quantum limit.
For samples with a bulk carrier density of n = 2.9\times10^16cm^-3, the lowest
Landau level of the bulk 3D Fermi surface is reached by a field of 4T. For
fields well beyond this limit, Shubnikov-de Haas oscillations arising from
quantization of the 2D surface state are observed, with the \nu =1 Landau level
attained by a field of 35T. These measurements reveal the presence of
additional oscillations which occur at fields corresponding to simple rational
fractions of the integer Landau indices.Comment: 5 pages, 4 figure
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