17,613 research outputs found
Spin polarized STM spectra of Dirac Fermions on the surface of a topological insulator
We provide a theory for the tunneling conductance of Dirac Fermions on
the surface of a topological insulator as measured by a spin-polarized scanning
tunneling microscope tip for low bias voltages . We show that
exhibits an unconventional dependence on the direction of magnetization of the
tip and can be used to measure the magnitude of the local out-of-plane spin
orientation of the Dirac Fermions on the surface. We also demonstrate that if
the in-plane rotational symmetry on the surface of the topological insulator is
broken by an external field, then acquires a dependence on the azimuthal
angle of the magnetization of the tip. We explain the role of the Dirac
Fermions in this unconventional behavior and suggest experiments to test our
theory.Comment: 7 pages, 5 Fig
Tachyon Kink on non-BPS Dp-brane in the General Background
This paper is devoted to the study of the tachyon kink on the worldvolume of
a non-BPS Dp-brane that is embedded in general background, including NS-NS two
form B and also general Ramond-Ramond field. We will explicitly show that the
dynamics of the kink is described by the equations of motion that arrise from
the DBI and WZ action for D(p-1)-brane.Comment: 28 page
On Orientifold Constructions of Type IIA Dual Pairs
In this paper we analyze the earlier constructions of the type IIA dual pairs
through orientifolding. By an appropriate choice of -matrix basis for
the spinor representations of the -duality group, we give an explicit
relationship between the orientifold models and their dual pairs.Comment: 11pages, Late
A wave driver theory for vortical waves propagating across junctions with application to those between rigid and compliant walls
A theory is described for propagation of vortical waves across alternate rigid and compliant panels. The structure in the fluid side at the junction of panels is a highly vortical narrow viscous structure which is idealized as a wave driver. The wave driver is modelled as a ‘half source cum half sink’. The incoming wave terminates into this structure and the outgoing wave emanates from it. The model is described by half Fourier–Laplace transforms respectively for the upstream and downstream sides of the junction. The cases below cutoff and above cutoff frequencies are studied. The theory completely reproduces the direct numerical simulation results of Davies & Carpenter (J. Fluid Mech., vol. 335, 1997, p. 361). Particularly, the jumps across the junction in the kinetic energy integral, the vorticity integral and other related quantities as obtained in the work of Davies & Carpenter are completely reproduced. Also, some important new concepts emerge, notable amongst which is the concept of the pseudo group velocity
Evolution of Tachyon Kink with Electric Field
We investigate the decay of an inhomogeneous D1-brane wrapped on a with
an electric field. The model that we consider consists of an array of tachyon
kink and anti-kink with a constant electric flux. Beginning with an initially
static configuration, we numerically evolve the tachyon field with some
perturbations under a fixed boundary condition at diametrically opposite points
on the circle . When the electric flux is smaller than the critical value,
the tachyon kink becomes unstable; the tachyon field rolls down the potential,
and the lower dimensional D0- and -brane become thin, which
resembles the caustic formation known for this type of the system in the
literature. For the supercritical values of the electric flux, the tachyon kink
remains stable.Comment: 27 pages, 8 figures, some changes, one reference added, version to
appear in JHE
On the evolution of tachyonic perturbations at super-Hubble scales
In the slow-roll inflationary scenario, the amplitude of the curvature
perturbations approaches a constant value soon after the modes leave the Hubble
radius. However, relatively recently, it was shown that the amplitude of the
curvature perturbations induced by the canonical scalar field can grow at
super-Hubble scales if there is either a transition to fast roll inflation or
if inflation is interrupted for some period of time. In this work, we extend
the earlier analysis to the case of a non-canonical scalar field described by
the Dirac-Born-Infeld action. With the help of a specific example, we show that
the amplitude of the tachyonic perturbations can be enhanced or suppressed at
super-Hubble scales if there is a transition from slow roll to fast roll
inflation. We also illustrate as to how the growth of the entropy perturbations
during the fast roll regime proves to be responsible for the change in the
amplitude of the curvature perturbations at super-Hubble scales. Furthermore,
following the earlier analysis for the canonical scalar field, we show that the
power spectrum evaluated in the long wavelength approximation matches the exact
power spectrum obtained numerically very well. Finally, we briefly comment on
an application of this phenomenon.Comment: v1: 15 pages, 4 figures; v2: 16 pages, 5 figures, power spectrum
included, discussion in section 5 enlarged, references added; v3: 17 pages, 5
figures, enhancement AS WELL AS suppression of modes at super-Hubble scales
pointed out, title changed, discussions enlarged, references added, to appear
in JCA
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