STM imaging of electronic waves on the surface of Bi2_2Te3_3: topologically protected surface states and hexagonal warping effects

Scanning tunneling spectroscopy studies on high-quality Bi$_2$Te$_3$ 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 $\cal H$, apply only to those regions which exclude $\cal H$.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 ($P$) and a tilt angle ($\Theta$). 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 $\Theta$ and $P$. 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 $\Theta$. We also find that of the mass inflow rate across the inner boundary does not change significantly with increasing $\Theta$. (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