15,058 research outputs found
Caching and Auditing in the RPPM Model
Crampton and Sellwood recently introduced a variant of relationship-based
access control based on the concepts of relationships, paths and principal
matching, to which we will refer as the RPPM model. In this paper, we show that
the RPPM model can be extended to provide support for caching of authorization
decisions and enforcement of separation of duty policies. We show that these
extensions are natural and powerful. Indeed, caching provides far greater
advantages in RPPM than it does in most other access control models and we are
able to support a wide range of separation of duty policies.Comment: Accepted for publication at STM 2014 (without proofs, which are
included in this longer version
Quasiparticle Interference on the Surface of the Topological Insulator BiTe
The quasiparticle interference of the spectroscopic imaging scanning
tunneling microscopy has been investigated for the surface states of the large
gap topological insulator BiTe through the T-matrix formalism. Both the
scalar potential scattering and the spin-orbit scattering on the warped
hexagonal isoenergy contour are considered. While backscatterings are forbidden
by time-reversal symmetry, other scatterings are allowed and exhibit strong
dependence on the spin configurations of the eigenfunctions at k points over
the isoenergy contour. The characteristic scattering wavevectors found in our
analysis agree well with recent experiment results.Comment: 5 pages, 2 figures, Some typos are correcte
Phase soliton and pairing symmetry of a two-band superconductor: Role of the proximity effect
We suggest a mechanism which promotes the existence of a phase soliton --
topological defect formed in the relative phase of superconducting gaps of a
two-band superconductor with s+- type of pairing. This mechanism exploits the
proximity effect with a conventional s-wave superconductor which favors the
alignment of the phases of the two-band superconductor which, in the case of
s+- pairing, are pi-shifted in the absence of proximity. In the case of a
strong proximity such effect can be used to reduce soliton's energy below the
energy of a soliton-free state thus making the soliton thermodynamically
stable. Based on this observation we consider an experimental setup, applicable
both for stable and metastable solitons, which can be used to distinguish
between s+- and s++ types of pairing in the iron-based multiband
superconductors.Comment: New references, added discussion about self-consistency of the GL
description of a phase soliton in the presence of a proximity patc
Microscopic investigation of phonon modes in SiGe alloy nanocrystals
Phonon modes in spherical silicon germanium alloy (SiGe) nanocrystals containing up to 1147 atoms (3.6 nm) have been investigated as a function of the Si concentration. Microscopic details of phonon modes, including phonon frequencies and vibrational amplitudes, phonon density-of-states are calculated directly from the dynamic matrices. In particular, the dependence of phonon frequency on the configuration (such as a different ratio of Si to Ge atoms), and location (surface or interior) of clusters of atoms in SiGe alloy nanocrystals have been investigated. Low frequency surface phonons that are related to the spheroidal and torsional modes of a continuum sphere are identified and their frequency dependence on alloy concentration elucidated. The calculated results are compared with measured Raman spectra in bulk, thin films, and superlattices of SiGe alloy reported in the literature. Insights into the behavior of Raman peaks usually identified as Ge-Ge, Si-Si, and Ge-Si optical phonon modes are presented
Theoretical investigation of the surface vibrational modes in germanium nanocrystals
We have used a microscopic lattice dynamical model to study phonon modes in germanium (Ge) NC with size varying between 47 to 7289 atoms (diametersimilar to6.8 nm). By separating these atoms into bulk and surface atoms we have found that surface modes can exist in Ge NC both at low frequencies (\u3c50\u3ecm(-1)) and at high frequency (similar to260 cm(-1)). The latter mode is a resonant mode which occurs in the pseudogap between the acoustic and optical phonon branches in bulk Ge. From the low frequency surface modes we have been able to reconstruct the spheroidal and torsional Lamb modes which have been used to interpret experimental results. Finally, we found that the Lamb model starts to deviate from the lattice dynamical results for Ge NC with diameternm
Modeling the Optical Afterglow of GRB 030329
The best-sampled afterglow light curves are available for GRB 030329. A
distinguishing feature of this event is the obvious rebrightening at around 1.6
days after the burst. Proposed explanations for the rebrightening mainly
include the two-component jet model and the refreshed shock model, although a
sudden density-jump in the circumburst environment is also a potential choice.
Here we re-examine the optical afterglow of GRB 030329 numerically in light of
the three models. In the density-jump model, no obvious rebrightening can be
produced at the jump moment. Additionally, after the density jump, the
predicted flux density decreases rapidly to a level that is significantly below
observations. A simple density-jump model thus can be excluded. In the
two-component jet model, although the observed late afterglow (after 1.6 days)
can potentially be explained as emission from the wide-component, the emergence
of this emission actually is too slow and it does not manifest as a
rebrightening as previously expected. The energy-injection model seems to be
the most preferred choice. By engaging a sequence of energy-injection events,
it provides an acceptable fit to the rebrightening at d, as well as
the whole observed light curve that extends to d. Further studies on
these multiple energy-injection processes may provide a valuable insight into
the nature of the central engines of gamma-ray bursts.Comment: 18 pages, 3 figures; a few references added and minor word changes;
now accepted for publication in Ap
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