4,290 research outputs found
Surface and Edge States in Topological Semi-metals
We study the topologically non-trivial semi-metals by means of the 6-band
Kane model. Existence of surface states is explicitly demonstrated by
calculating the LDOS on the material surface. In the strain free condition,
surface states are divided into two parts in the energy spectrum, one part is
in the direct gap, the other part including the crossing point of surface state
Dirac cone is submerged in the valence band. We also show how uni-axial strain
induces an insulating band gap and raises the crossing point from the valence
band into the band gap, making the system a true topological insulator. We
predict existence of helical edge states and spin Hall effect in the thin film
topological semi-metals, which could be tested with future experiment. Disorder
is found to significantly enhance the spin Hall effect in the valence band of
the thin films
Non-magnetic impurities and in-gap bound states in topological insulators
In-gap bound states induced by non-magnetic impurities in various dimensional topological insulators are investigated based on a modified Dirac model that considers quadratic corrections to the mass term. Their existence and features greatly rely on the potential form of the impurity as well as the dimensionality of the topological insulator. It is analytically proven that the impurity potential modeled by the delta function can induce the bound states in one dimension (1D), but not in two and three. For a single non-magnetic impurity with a general isotropic potential, formal solutions are obtained and further numerical calculations are performed. In particular, the in-gap bound states induced by a non-magnetic impurity with isotropic Gaussian potentials in two-dimensional (2D) and three-dimensional (3D) topological insulators are numerically investigated. Information on how many in-gap bound states can be trapped by a non-magnetic Gaussian impurity is presented for the parameters from a series of topologically non-trivial materials. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.published_or_final_versio
Vacancy-induced bound states in topological insulators
We present an exact solution of a modified Dirac equation for topological insulator in the presence of a hole or vacancy to demonstrate that vacancies can induce bound states in the band gap of topological insulators. They arise due to the Z 2 classification of time-reversal invariant insulators. Coexistence of the in-gap bound states and the edge or surface states in topological insulators suggests that imperfections may affect transport properties of topological insulators via additional bound states near the system boundary. © 2011 American Physical Society.published_or_final_versio
Massive Dirac fermions and spin physics in an ultrathin film of topological insulator
We study transport and optical properties of the surface states which lie in
the bulk energy gap of a thin-film topological insulator. When the film
thickness is comparable with the surface state decay length into the bulk, the
tunneling between the top and bottom surfaces opens an energy gap and form two
degenerate massive Dirac hyperbolas. Spin dependent physics emerges in the
surface bands which are vastly different from the bulk behavior. These include
the surface spin Hall effects, spin dependent orbital magnetic moment, and spin
dependent optical transition selection rule which allows optical spin
injection. We show a topological quantum phase transition where the Chern
number of the surface bands changes when varying the thickness of the thin
film.Comment: 7 pages, 5 figure
How many supermassive black hole binaries are detectable through tracking relative motions by sub/millimeter VLBI
The sub/millimeter wavelengths (86-690 GHz) very long baseline interferometry
(VLBI) will provide as angular resolution, mJy baseline
sensitivity, and as/yr proper motion precision, which can directly
detect supermassive black hole binary (SMBHB) systems by imaging the two
visible sources and tracking their relative motions. Such a way exhibits an
advantage compared to indirect detect methods of observing periodic signals in
motion and light curves, which are difficult to confirm from competing models.
Moreover, tracking relative motion at sub/millimeter wavelengths is more
reliable, as there is a negligible offset between the emission region and the
black hole center. In this way, it is unnecessary to correct the black hole
location by a prior of jet morphology as it would be required at longer
wavelengths. We extend the formalism developed in D'Orazio & Loeb (2018) to
link the observations with the orbital evolution of SMBHBs from the
10 kpc dynamical friction stages to the pc
gravitational radiation stages, and estimate the detectable numbers of SMBHBs.
By assuming 5\% of AGNs holding SMBHBs, we find that the number of detectable
SMBHBs with redshift and mass is about 20.
Such detection relies heavily on proper motion precision and sensitivity.
Furthermore, we propose that the simultaneous multi-frequency technique plays a
key role in meeting the observational requirements
Effective continuous model for surface states and thin films of three dimensional topological insulators
Two-dimensional effective continuous models are derived for the surface
states and thin films of the three-dimensional topological insulator (3DTI).
Starting from an effective model for 3DTI based on the first principles
calculation [Zhang \emph{et al}, Nat. Phys. 5, 438 (2009)], we present
solutions for both the surface states in a semi-infinite boundary condition and
in the thin film with finite thickness. An effective continuous model was
derived for surface states and the thin film 3DTI. The coupling between
opposite topological surfaces and structure inversion asymmetry (SIA) give rise
to gapped Dirac hyperbolas with Rashba-like splittings in energy spectrum.
Besides, the SIA leads to asymmetric distributions of wavefunctions along the
film growth direction, making some branches in the energy spectra much harder
than others to be probed by light. These features agree well with the recent
angle-resolved photoemission spectra of BiSe films grown on SiC
substrate [Zhang et al, arXiv: 0911.3706]. More importantly, we use the
effective model to fit the experimental data and determine the model
parameters. The result indicates that the thin film BiSe lies in
quantum spin Hall region based on the calculation of the Chern number and the
invariant. In addition, strong SIA always intends to destroy the
quantum spin Hall state.Comment: 12 pages, 7 figures, references are update
Spin-orbit scattering in quantum diffusion of massive Dirac fermions
Effect of spin-orbit scattering on quantum diffusive transport of
two-dimensional massive Dirac fermions is studied by the diagrammatic
technique. The quantum diffusion of massive Dirac fermions can be viewed as a
singlet Cooperon in the massless limit and a triplet Cooperon in the large-mass
limit. The spin-orbit scattering behaves like random magnetic fields only to
the triplet Cooperon, and suppresses the weak localization of Dirac fermions in
the large-mass regime. This behavior suggests an experiment to detect the weak
localization of bulk subbands in topological insulator thin films, in which a
narrowing of the cusp of the negative magnetoconductivity is expected after
doping heavy-element impurities. Finally, a detailed comparison between the
conventional two-dimensional electrons and Dirac fermions is presented for
impurities of orthogonal, symplectic, and unitary symmetries.Comment: 5 pages, 3 figures, 2 tables. To be submitted, comments are welcom
- …