3,185 research outputs found
Shockley model description of surface states in topological insulators
We show that the surface states in topological insulators can be understood
based on a well-known Shockley model, a one-dimensional tight-binding model
with two atoms per elementary cell, connected via alternating tunneling
amplitudes. We generalize the one-dimensional model to the three-dimensional
case corresponding to the sequence of layers connected via the amplitudes,
which depend on the in-plane momentum p = (p_x,p_y). The Hamiltonian of the
model is described a (2 x 2) Hamiltonian with the off-diagonal element t(k,p)
depending also on the out-of-plane momentum k. We show that the complex
function t(k,p) defines the properties of the surface states. The surface
states exist for the in-plane momenta p, where the winding number of the
function t(k,p) is non-zero as k is changed from 0 to 2pi. The sign of the
winding number defines the sublattice on which the surface states are
localized. The equation t(k,p)=0 defines a vortex line in the three-dimensional
momentum space. The projection of the vortex line on the two-dimensional
momentum p space encircles the domain where the surface states exist. We
illustrate how our approach works for a well-known TI model on a diamond
lattice. We find that different configurations of the vortex lines are
responsible for the "weak" and "strong" topological insulator phases. The phase
transition occurs when the vortex lines reconnect from spiral to circular form.
We discuss the Shockley model description of Bi_2Se_3 and the applicability of
the continuous approximation for the description of the topological edge
states. We conclude that the tight-binding model gives a better description of
the surface states.Comment: 18 pages, 17 figures; version 3: Sections I-IV revised, Section VII
added, Refs. [33]-[35] added; Corresponds to the published versio
Magnetic effects in a holographic Fermi-like liquid
We explore the magnetic properties of the Fermi-like liquid represented by
the D3-D7' system. The system exhibits interesting magnetic properties such as
ferromagnetism and an anomalous Hall effect, which are due to the Chern-Simons
term in the effective gravitational action. We investigate the spectrum of
quasi-normal modes in the presence of a magnetic field and show that the
magnetic field mitigates the instability towards a striped phase. In addition,
we find a critical magnetic field above which the zero sound mode becomes
massive.Comment: 18 pages, 15 figure
Rotation Measure Synthesis of Galactic Polarized Emission with the DRAO 26-m Telescope
Radio polarimetry at decimetre wavelengths is the principal source of
information on the Galactic magnetic field. The diffuse polarized emission is
strongly influenced by Faraday rotation in the magneto-ionic medium and
rotation measure is the prime quantity of interest, implying that all Stokes
parameters must be measured over wide frequency bands with many frequency
channels. The DRAO 26-m Telescope has been equipped with a wideband feed, a
polarization transducer to deliver both hands of circular polarization, and a
receiver, all operating from 1277 to 1762 MHz. Half-power beamwidth is between
40 and 30 arcminutes. A digital FPGA spectrometer, based on commercially
available components, produces all Stokes parameters in 2048 frequency channels
over a 485-MHz bandwidth. Signals are digitized to 8 bits and a Fast Fourier
Transform is applied to each data stream. Stokes parameters are then generated
in each frequency channel. This instrument is in use at DRAO for a Northern sky
polarization survey. Observations consist of scans up and down the Meridian at
a drive rate of 0.9 degree per minute to give complete coverage of the sky
between declinations -30 degree and 90 degree. This paper presents a complete
description of the receiver and data acquisition system. Only a small fraction
of the frequency band of operation is allocated for radio astronomy, and about
20 percent of the data are lost to interference. The first 8 percent of data
from the survey are used for a proof-of-concept study, which has led to the
first application of Rotation Measure Synthesis to the diffuse Galactic
emission obtained with a single-antenna telescope. We find rotation measure
values for the diffuse emission as high as approximately 100 rad per square
metre, much higher than recorded in earlier work.Comment: Accepted for publication in The Astronomical Journa
Photonic realization of the relativistic Kronig-Penney model and relativistic Tamm surface states
Photonic analogues of the relativistic Kronig-Penney model and of
relativistic surface Tamm states are proposed for light propagation in fibre
Bragg gratings (FBGs) with phase defects. A periodic sequence of phase slips in
the FBG realizes the relativistic Kronig-Penney model, the band structure of
which being mapped into the spectral response of the FBG. For the semi-infinite
FBG Tamm surface states can appear and can be visualized as narrow resonance
peaks in the transmission spectrum of the grating
The Neutrino Magnetic Moment Induced by Leptoquarks
Allowing leptoquarks to interact with both right-handed and left-handed
neutrinos (i.e., ``non-chiral'' leptoquarks), we show that a non-zero neutrino
magnetic moment can arise naturally. Although the mass of the non-chiral vector
leptoquark that couples to the first generation fermions is constrained
severely by universality of the leptonic decays and is found to be
greater than 50 TeV, the masses of the second and third generation non-chiral
vector leptoquarks may evade such constraint and may in general be in the range
of TeV. With reasonable input mass and coupling values, we find
that the neutrino magnetic moment due to the second generation leptoquarks is
of the order of while that caused by the
third generation leptoquarks, being enhanced significantly by the large top
quark mass, is in the range of .Comment: 11 pages, 3 eps figures, uses revte
Physical Response Functions of Strongly Coupled Massive Quantum Liquids
We study physical properties of strongly coupled massive quantum liquids from
their spectral functions using the AdS/CFT correspondence. The generic model
that we consider is dense, heavy fundamental matter coupled to SU(N_c) super
Yang-Mills theory at finite temperature above the deconfinement phase
transition but below the scale set by the baryon number density. In this setup,
we study the current-current correlators of the baryon number density using new
techniques that employ a scaling behavior in the dual geometry. Our results,
the AC conductivity, the quasi-particle spectrum and the Drude-limit parameters
like the relaxation time are simple temperature-independent expressions that
depend only on the mass-squared to density ratio and display a crossover
between a baryon- and meson-dominated regime. We concentrated on the
(2+1)-dimensional defect case, but in principle our results can also be
generalized straightforwardly to other cases.Comment: 21 pages, 10 figures, extra paragraph and figure are added in
response to referee's comment
Determination of the Strong Coupling \boldmath{\as} from hadronic Event Shapes and NNLO QCD predictions using JADE Data
Event Shape Data from annihilation into hadrons collected by the
JADE experiment at centre-of-mass energies between 14 GeV and 44 GeV are used
to determine the strong coupling . QCD predictions complete to
next-to-next-to-leading order (NNLO), alternatively combined with resummed
next-to-leading-log-approximation (NNLO+NLLA) calculations, are used. The
combined value from six different event shape observables at the six JADE
centre-of-mass energies using the NNLO calculations is
= 0.1210 +/- 0.0007(stat.) +/- 0.0021(expt.) +/- 0.0044(had.)
+/- 0.0036(theo.) and with the NNLO+NLLA calculations the combined value is
= 0.1172 +/- 0.0006(stat.) +/- 0.0020(expt.) +/- 0.0035(had.) +/-
0.0030(theo.) . The stability of the NNLO and NNLO+NLLA results with respect to
missing higher order contributions, studied by variations of the
renormalisation scale, is improved compared to previous results obtained with
NLO+NLLA or with NLO predictions only. The observed energy dependence of
agrees with the QCD prediction of asymptotic freedom and excludes
absence of running with 99% confidence level.Comment: 9 pages, EPHJA style, 4 figures, corresponds to published version
with JADE author lis
Holographic zero sound at finite temperature in the Sakai-Sugimoto model
In this paper, we study the fate of the holographic zero sound mode at finite
temperature and non-zero baryon density in the deconfined phase of the
Sakai-Sugimoto model of holographic QCD. We establish the existence of such a
mode for a wide range of temperatures and investigate the dispersion relation,
quasi-normal modes, and spectral functions of the collective excitations in
four different regimes, namely, the collisionless quantum, collisionless
thermal, and two distinct hydrodynamic regimes. For sufficiently high
temperatures, the zero sound completely disappears, and the low energy physics
is dominated by an emergent diffusive mode. We compare our findings to
Landau-Fermi liquid theory and to other holographic models.Comment: 1+24 pages, 19 figures, PDFTeX, v2: some comments and references
added, v3: some clarifications relating to the different regimes added,
matches version accepted for publication in JHEP, v4: corrected typo in eq.
(3.18
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