299 research outputs found
STM imaging of electronic waves on the surface of BiTe: topologically protected surface states and hexagonal warping effects
Scanning tunneling spectroscopy studies on high-quality BiTe crystals
exhibit perfect correspondence to ARPES data, hence enabling identification of
different regimes measured in the local density of states (LDOS). Oscillations
of LDOS near a step are analyzed. Within the main part of the surface band
oscillations are strongly damped, supporting the hypothesis of topological
protection. At higher energies, as the surface band becomes concave,
oscillations appear which disperse with a particular wave-vector that may
result from an unconventional hexagonal warping term.Comment: 4 pages, 4 figures. Revised manuscript with improved analysis and
figure
Structure and Stability of Keplerian MHD Jets
MHD jet equilibria that depend on source properties are obtained using a
simplified model for stationary, axisymmetric and rotating magnetized outflows.
The present rotation laws are more complex than previously considered and
include a Keplerian disc. The ensuing jets have a dense, current-carrying
central core surrounded by an outer collar with a return current. The
intermediate part of the jet is almost current-free and is magnetically
dominated. Most of the momentum is located around the axis in the dense core
and this region is likely to dominate the dynamics of the jet. We address the
linear stability and the non-linear development of instabilities for our models
using both analytical and 2.5-D numerical simulation's. The instabilities seen
in the simulations develop with a wavelength and growth time that are well
matched by the stability analysis. The modes explored in this work may provide
a natural explanation for knots observed in astrophysical jets.Comment: 35 pages, accepted by the Ap
Waves and Instabilities in Accretion Disks: MHD Spectroscopic Analysis
A complete analytical and numerical treatment of all magnetohydrodynamic
waves and instabilities for radially stratified, magnetized accretion disks is
presented. The instabilities are a possible source of anomalous transport.
While recovering results on known hydrodynamicand both weak- and strong-field
magnetohydrodynamic perturbations, the full magnetohydrodynamic spectra for a
realistic accretion disk model demonstrates a much richer variety of
instabilities accessible to the plasma than previously realized. We show that
both weakly and strongly magnetized accretion disks are prone to strong
non-axisymmetric instabilities.The ability to characterize all waves arising in
accretion disks holds great promise for magnetohydrodynamic spectroscopic
analysis.Comment: FOM-Institute for plasma physics "Rijnhuizen", Nieuwegein, the
Netherlands 12 pages, 3 figures, Accepted for publication in ApJ
Imaging nonequilibrium atomic vibrations with x-ray diffuse scattering
For over a century, x-ray scattering has been the most powerful tool for
determining the equilibrium structure of crystalline materials. Deviations from
perfect periodicity, for example due to thermal motion of the atoms, reduces
the intensity of the Bragg peaks as well as produces structure in the diffuse
scattering background. Analysis of the thermal diffuse scattering (TDS) had
been used to determine interatomic force constants and phonon dispersion in
relatively simple cases before inelastic neutron scattering became the
preferred technique to study lattice dynamics. With the advent of intense
synchrotron x-ray sources, there was a renewed interest in TDS for measuring
phonon dispersion. The relatively short x-ray pulses emanating from these
sources also enables the measurement of phonon dynamics in the time domain.
Prior experiments on nonequilibrium phonons were either limited by
time-resolution and/or to relatively long wavelength excitations. Here we
present the first images of nonequilibrium phonons throughout the Brillouin
zone in photoexcited III-V semiconductors, indium-phosphide and
indium-antimonide, using picosecond time-resolved diffuse scattering. In each
case, we find that the lattice remain out of equilibrium for several hundred
picoseconds up to nanoseconds after laser excitation. The non-equilibrium
population is dominated by transverse acoustic phonons which in InP are
directed along high-symmetry directions. The results have wide implications for
the detailed study of electron-phonon and phonon-phonon coupling in solids.Comment: 10 pages, 3 figure
The structure of black hole magnetospheres. I. Schwarzschild black holes
We introduce a multipolar scheme for describing the structure of stationary,
axisymmetric, force-free black-hole magnetospheres in the ``3+1'' formalism. We
focus here on Schwarzschild spacetime, giving a complete classification of the
separable solutions of the stream equation. We show a transparent term-by-term
analogy of our solutions with the familiar multipoles of flat-space
electrodynamics. We discuss electrodynamic processes around disk-fed black
holes in which our solutions find natural applications: (a) ``interior''
solutions in studies of the Blandford-Znajek process of extracting the hole's
rotational energy, and of the formation of relativistic jets in active galactic
nuclei and ``microquasars'', and, (b) ``exterior'' solutions in studies of
accretion disk dynamos, disk-driven winds and jets. On the strength of existing
numerical studies, we argue that the poloidal field structures found here are
also expected to hold with good accuracy for rotating black holes, except for
maximum possible rotation rates. We show that the closed-loop exterior
solutions found here are not in contradiction with the Macdonald-Thorne
theorem, since these solutions, which diverge logarithmically on the hole's
horizon , apply only to those regions which exclude .Comment: 6 figures. Accepted for publication by MNRA
Comptonization of Infrared Radiation from Hot Dust by Relativistic Jets in Quasars
We demonstrate the importance of near-infrared radiation from hot dust for
Compton cooling of electrons/positrons in quasar jets. In our model, we assume
that the non-thermal radiation spectra observed in OVV quasars are produced by
relativistic electrons/positrons accelerated in thin shells which propagate
down the jet with relativistic speeds. We show that the Comptonization of the
near-IR flux is likely to dominate the radiative output of OVV quasars in the
energy range from tens of keV up to hundreds of MeV, where it exceeds that
produced by Comptonization of the UV radiation reprocessed and rescattered in
the Broad Emission Line region. The main reason for this lies in the fact that
the jet encounters the ambient IR radiation over a relatively large distance as
compared to the distance where the energy density of the broad emission line
light peaks. In the soft - to mid energy X-ray band, the spectral component
resulting from Comptonization of the near-IR radiation joins smoothly with the
synchrotron-self-Compton component, which may be responsible for the soft X-ray
flux. At the highest observed gamma-ray energies, in the GeV range,
Comptonization of broad emission lines dominates over other components.Comment: 23 pages, including 5 Postscript figures and 3 tables, uses aastex.
Astrophysical Journal, accepted for publication in the December 20, 2000
issu
Three-Dimensional Simulations of Inflows Irradiated by a Precessing Accretion Disk in Active Galactic Nuclei: Formation of Outflows
We present three-dimensional (3-D) hydrodynamical simulations of gas flows in
the vicinity of an active galactic nucleus (AGN) powered by a precessing
accretion disk. We consider the effects of the radiation force from such a disk
on its environment on a relatively large scale (up to ~10 pc. We implicitly
include the precessing disk by forcing the disk radiation field to precess
around a symmetry axis with a given period () and a tilt angle ().
We study time evolution of the flows irradiated by the disk, and investigate
basic dependencies of the flow morphology, mass flux, angular momentum on
different combinations of and . We find the gas flow settles into a
configuration with two components, (1) an equatorial inflow and (2) a bipolar
inflow/outflow with the outflow leaving the system along the poles (the
directions of disk normals). However, the flow does not always reach a steady
state. We find that the maximum outflow velocity and the kinetic outflow power
at the outer boundary can be reduced significantly with increasing . We
also find that of the mass inflow rate across the inner boundary does not
change significantly with increasing . (Abbreviated)Comment: Accepted for publication in ApJ. 15 pages, 7 figures. A version with
full resolution figures can be downloaded from
http://www.physics.unlv.edu/~rk/preprint/precess.pd
Ohm's Law for a Relativistic Pair Plasma
We derive the fully relativistic Ohm's law for an electron-positron plasma.
The absence of non-resistive terms in Ohm's law and the natural substitution of
the 4-velocity for the velocity flux in the relativistic bulk plasma equations
do not require the field gradient length scale to be much larger than the
lepton inertial lengths, or the existence of a frame in which the distribution
functions are isotropic.Comment: 12 pages, plain TeX, Phys. Rev. Lett. 71 3481 (1993
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