9,605 research outputs found
Doppler tomography of relativistic accretion disks
Spectral lines from a source orbiting around a compact object are studied.
Time variations of observed frequency and count rate due to motion of the
source and gravitational lensing are considered. Gravitational field of the
central object is described by the Kerr metric. It is shown that: (i)
simultaneous temporal and frequency resolution enables us to restrict
parameters of the model (inclination angle, position of the source, angular
momentum of the black hole); (ii) techniques of image restoration, familiar
from other fields of astronomy, can be applied to study inner regions of active
galactic nuclei. This contribution is relevant for extremely variable X-ray
sources with high parameter of efficiency, such as Seyfert 1 galaxy PHL 1092
observed by ROSAT.Comment: 12 pages, to appear in Publications of the Astronomical Society of
Japan, Vol. 48 (October 1996), figures available upon request from the
authors, or at
http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht
A time-dependent variational principle for dissipative dynamics
We extend the time-dependent variational principle to the setting of
dissipative dynamics. This provides a locally optimal (in time) approximation
to the dynamics of any Lindblad equation within a given variational manifold of
mixed states. In contrast to the pure-state setting there is no canonical
information geometry for mixed states and this leads to a family of possible
trajectories --- one for each information metric. We focus on the case of the
operationally motivated family of monotone riemannian metrics and show further,
that in the particular case where the variational manifold is given by the set
of fermionic gaussian states all of these possible trajectories coincide. We
illustrate our results in the case of the Hubbard model subject to spin
decoherence.Comment: Published versio
Ground States of Fermionic lattice Hamiltonians with Permutation Symmetry
We study the ground states of lattice Hamiltonians that are invariant under
permutations, in the limit where the number of lattice sites, N -> \infty. For
spin systems, these are product states, a fact that follows directly from the
quantum de Finetti theorem. For fermionic systems, however, the problem is very
different, since mode operators acting on different sites do not commute, but
anti-commute. We construct a family of fermionic states, \cal{F}, from which
such ground states can be easily computed. They are characterized by few
parameters whose number only depends on M, the number of modes per lattice
site. We also give an explicit construction for M=1,2. In the first case,
\cal{F} is contained in the set of Gaussian states, whereas in the second it is
not. Inspired by that constructions, we build a set of fermionic variational
wave functions, and apply it to the Fermi-Hubbard model in two spatial
dimensions, obtaining results that go beyond the generalized Hartree-Fock
theory.Comment: 23 pages, published versio
A Perfect Match: Partnering with Education Faculty for Pedagogical Professional Development
A persistent challenge for many librarians is a lack of formal training in pedagogical techniques. In addition to lacking academic coursework in this area, librarians seldom look beyond their professional community for opportunities to develop these vital skills. Given the obvious parallels in mission and responsibilities, the field of education seems a natural fit. This chapter explores the benefits of crossdisciplinary professional development in the context of a collaboration between a librarian and an educational studies professor. Through alternating points of view, it presents the motivation for the partnership, the challenges it presented, and the positive outcomes for each participant. It also offers an in-depth look at the instructional development itself
Optimization of light collection from crystal scintillators for cryogenic experiments
High light collection efficiency is an important requirement in any
application of scintillation detectors. The purpose of this study is to
investigate the possibility for improving this parameter in cryogenic
scintillation bolometers, which can be considered as a promising detectors in
experiments investigating neutrinoless double beta decay and dark matter.
Energy resolutions and relative pulse amplitudes of scintillation detectors
using ZnWO4 scintillation crystals of different shapes (cylinder 20 mm in
dimater by 20 mm and hexagonal prism with diagonal 20 mm and height 20 mm),
reflector materials and shapes, optical contact and surface properties
(polished and diffused) were measured at room temperature. Propagation of
optical photons in these experimental conditions was simulated using Geant4 and
ZEMAX codes. The results of the simulations are found to be in good agreement
with each other and with direct measurements of the crystals. This could be
applied to optimize the geometry of scintillation detectors used in the
cryogenic experiments.Comment: 12 pages, 8 figures, 1 Tabl
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