105 research outputs found
Nonexponential Relaxations in a Two-Dimensional Electron System in Silicon
The relaxations of conductivity have been studied in a strongly disordered
two-dimensional (2D) electron system in Si after excitation far from
equilibrium by a rapid change of carrier density n_s at low temperatures T. The
dramatic and precise dependence of the relaxations on n_s and T strongly
suggests (a) the transition to a glassy phase as T->0, and (b) the Coulomb
interactions between 2D electrons play a dominant role in the observed
out-of-equilibrium dynamics.Comment: 5 pages, 5 figure
Prospects for the Determination of Star Orbits Near the Galactic Center
We simulate the observations of proper motion of stars very close to the
Galactic Center. We show that the speckle interferometry done with the Keck II
telescope is accurate enough to obtain orbital parameters for stars with the
period P about 10 y during 10 seasons of astrometric observations made once a
year. The determination of a single orbit will give central mass estimate with
the typical uncertainty of the existing mass determinations based on velocity
dispersion measurements. A much higher precision orbits will be measured in
several years when Keck Interferometer becomes operational, and fainter stars
are discovered even closer to Sgr A*. Astrometry alone will provide accurate
determination of the ratio: M/D^3, where M is the black hole mass and D is the
distance to the Galactic Center. If spectroscopic orbits of the stars are also
measured then both: M and D will be precisely determined.Comment: 13 pages, 5 figures, accepted by Ap
On the stability and evolution of relativistic radiation tori: equations and speculations
Equations describing the general relativistic evolution of hypothetical radiation-supported tori in active galactic nuclei are presented and discussed. Equations describing global, non-axisymmetric perturbations of radiation tori are also given. The role of instabilities in transporting angular momentum and heat is examined and some simple prescriptions for use in numerical calculation of the secular evolution of these structures are suggested. In particular, if a sufficiently optically thick torus evolves to become dynamically unstable then a gyrotropic, marginally stable convection zone with angular momentum constant on isentropes may be established. Goldreich–Schubert–Fricke instability should be adequate to maintain a radiative zone in a barotropic state
Nonequilibrium Relaxations and Aging Effects in a Two-Dimensional Coulomb Glass
The relaxations of conductivity have been studied in the glassy regime of a
strongly disordered two-dimensional electron system in Si after a temporary
change of carrier density during the waiting time t_w. Two types of response
have been observed: a) monotonic, where relaxations exhibit aging, i.e.
dependence on history, determined by t_w and temperature; b) nonmonotonic,
where a memory of the sample history is lost. The conditions that separate the
two regimes have been also determined.Comment: 4 pages; published versioi
Strong deflection limit of black hole gravitational lensing with arbitrary source distances
The gravitational field of supermassive black holes is able to strongly bend
light rays emitted by nearby sources. When the deflection angle exceeds ,
gravitational lensing can be analytically approximated by the so-called strong
deflection limit. In this paper we remove the conventional assumption of
sources very far from the black hole, considering the distance of the source as
an additional parameter in the lensing problem to be treated exactly. We find
expressions for critical curves, caustics and all lensing observables valid for
any position of the source up to the horizon. After analyzing the spherically
symmetric case we focus on the Kerr black hole, for which we present an
analytical 3-dimensional description of the higher order caustic tubes.Comment: 20 pages, 8 figures, appendix added. In press on Physical Review
Quantum Hall states under conditions of vanishing Zeeman energy
We report on magneto-transport measurements of a two-dimensional electron gas
confined in a CdMnTe quantum well structure under
conditions of vanishing Zeeman energy. The electron Zeeman energy has been
tuned via the exchange interaction in order to probe different quantum
Hall states associated with metallic and insulating phases. We have observed
that reducing Zeeman energy to zero does not necessary imply the disappearing
of quantum Hall states, i.e. a closing of the spin gap. The spin gap value
under vanishing Zeeman energy conditions is shown to be dependent on the
filling factor. Numerical simulations support a qualitative description of the
experimental data presented in terms of a crossing or an avoided-crossing of
spin split Landau levels with same orbital quantum number
OGLE-2009-BLG-023/MOA-2009-BLG-028: Characterization of a Binary Microlensing Event Based on Survey Data
We report the result of the analysis of the light curve of a caustic-crossing
binary-lens microlensing event OGLE-2009-BLG-023/MOA-2009-BLG-028. Even though
the event was observed solely by survey experiments, we could uniquely
determine the mass of the lens and distance to it by simultaneously measuring
the Einstein radius and lens parallax. From this, we find that the lens system
is composed of M-type dwarfs with masses and
located in the Galactic disk with a distance of kpc toward the Galactic bulge direction. The event demonstrates that
physical lens parameters of binary-lens events can be routinely determined from
future high-cadence lensing surveys and thus microlensing can provide a new way
to study Galactic binaries.Comment: 8 pages, 5 figure
Ising Quantum Hall Ferromagnet in Magnetically Doped Quantum Wells
We report on the observation of the Ising quantum Hall ferromagnet with Curie
temperature as high as 2 K in a modulation-doped (Cd,Mn)Te
heterostructure. In this system field-induced crossing of Landau levels occurs
due to the giant spin-splitting effect. Magnetoresistance data, collected over
a wide range of temperatures, magnetic fields, tilt angles, and electron
densities, are discussed taking into account both Coulomb electron-electron
interactions and sd coupling to Mn spin fluctuations. The critical behavior
of the resistance ``spikes'' at corroborates theoretical
suggestions that the ferromagnet is destroyed by domain excitations.Comment: revised, 4 pages, 4 figure
Universal Behavior of the Resistance Noise across the Metal-Insulator Transition in Silicon Inversion Layers
Studies of low-frequency resistance noise show that the glassy freezing of
the two-dimensional (2D) electron system in the vicinity of the metal-insulator
transition occurs in all Si inversion layers. The size of the metallic glass
phase, which separates the 2D metal and the (glassy) insulator, depends
strongly on disorder, becoming extremely small in high-mobility samples. The
behavior of the second spectrum, an important fourth-order noise statistic,
indicates the presence of long-range correlations between fluctuators in the
glassy phase, consistent with the hierarchical picture of glassy dynamics.Comment: revtex4; 4+ pages, 5 figure
Multi-patch methods in general relativistic astrophysics - I. Hydrodynamical flows on fixed backgrounds
Many systems of interest in general relativistic astrophysics, including
neutron stars, accreting compact objects in X-ray binaries and active galactic
nuclei, core collapse, and collapsars, are assumed to be approximately
spherically symmetric or axisymmetric. In Newtonian or fixed-background
relativistic approximations it is common practice to use spherical polar
coordinates for computational grids; however, these coordinates have
singularities and are difficult to use in fully relativistic models. We
present, in this series of papers, a numerical technique which is able to use
effectively spherical grids by employing multiple patches. We provide detailed
instructions on how to implement such a scheme, and present a number of code
tests for the fixed background case, including an accretion torus around a
black hole.Comment: 26 pages, 20 figures. A high-resolution version is available at
http://www.cct.lsu.edu/~bzink/papers/multipatch_1.pd
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