2,364 research outputs found
Quasi-Normal Modes of a Natural AdS Wormhole in Einstein-Born-Infeld Gravity
We study the matter perturbations of a new AdS wormhole in (3+1)-dimensional
Einstein-Born-Infeld gravity, called "natural wormhole", which does not require
exotic matters. We discuss the stability of the perturbations by numerically
computing the quasi-normal modes (QNMs) of a massive scalar field in the
wormhole background. We investigate the dependence of quasi-normal frequencies
on the mass of scalar field as well as other parameters of the wormhole. It is
found that the perturbations are always stable for the wormhole geometry which
has the general relativity (GR) limit when the scalar field mass m satisfies a
certain, tachyonic mass bound m^2 > m^2_* with m^2_* < 0, analogous to the
Breitenlohner-Freedman (BF) bound in the global-AdS space, m^2_BF = 3 Lambda/4.
It is also found that the BF-like bound m^2_* shifts by the changes of the
cosmological constant Lambda or angular-momentum number l, with a level
crossing between the lowest complex and pure-imaginary modes for zero angular
momentum l = 0. Furthermore, it is found that the unstable modes can also have
oscillatory parts as well as non-oscillatory parts depending on whether the
real and imaginary parts of frequencies are dependent on each other or not,
contrary to arguments in the literature. For wormhole geometries which do not
have the GR limit, the BF-like bound does not occur and the perturbations are
stable for arbitrary tachyonic and non-tachyonic masses, up to a critical mass
m^2_c > 0 where the perturbations are completely frozen.Comment: Added comments and references, Accepted in EPJ
Optochemically Responsive 2D Nanosheets of a 3D Metal-Organic Framework Material
Outstanding functional tunability underpinning metal-organic framework (MOF)
confers a versatile platform to contrive next-generation chemical sensors,
optoelectronics, energy harvesters and converters. We report a rare exemplar of
a porous 2D nanosheet material, constructed from an extended 3D MOF structure.
We develop a rapid supramolecular self-assembly methodology at ambient
conditions, to synthesize readily-exfoliatable MOF nanosheets, functionalized
in situ by adopting the Guest@MOF (Host) strategy. Nanoscale confinement of
light-emitting molecules (as functional guest) inside the MOF pores generates
unusual combination of optical, electronic, and chemical properties, arising
from the strong host-guest coupling effects. We show highly promising photonics
based chemical sensing opened up by the new Guest@MOF composite systems. By
harnessing host-guest optochemical interactions of functionalized MOF
nanosheets, we have accomplished detection of an extensive range of volatile
organic compounds (VOCs) and small molecules important for many practical
applications.Comment: 21 pages, 5 figures, 1 schem
Quasi-Normal Modes and Stability of Einstein-Born-Infeld Black Holes in de Sitter Space
We study gravitational perturbations of electrically charged black holes in
(3+1)-dimensional Einstein-Born-Infeld gravity with a positive cosmological
constant. For the axial perturbations, we obtain a set of decoupled
Schrodinger-type equations, whose formal expressions, in terms of metric
functions, are the same as those without cosmological constant, corresponding
to the Regge-Wheeler equation in the proper limit. We compute the quasi-normal
modes (QNMs) of the decoupled perturbations using the Schutz-Iyer-Will's WKB
method. We discuss the stability of the charged black holes by investigating
the dependence of quasi-normal frequencies on the parameters of the theory,
correcting some errors in the literature. It is found that all the axial
perturbations are stable for the cases where the WKB method applies. There are
cases where the conventional WKB method does not apply, like the
three-turning-points problem, so that a more generalized formalism is necessary
for studying their QNMs and stabilities. We find that, for the degenerate
horizons with the "point-like" horizons at the origin, the QNMs are quite
long-lived, close to the quasi-resonance modes, in addition to the "frozen"
QNMs for the Nariai-type horizons and the usual (short-lived) QNMs for the
extremal black hole horizons. This is a genuine effect of the branch which does
not have the general relativity limit. We also study the exact solution near
the (charged) Nariai limit and find good agreements even far beyond the limit
for the imaginary frequency parts.Comment: Matches published versio
Retrieval of NO2 Column Amounts from Ground-Based Hyperspectral Imaging Sensor Measurements
Total column amounts of NO2 (TCN) were estimated from ground-based hyperspectral imaging sensor (HIS) measurements in a polluted urban area (Seoul, Korea) by applying the radiance ratio fitting method with five wavelength pairs from 400 to 460 nm. We quantified the uncertainty of the retrieved TCN based on several factors. The estimated TCN uncertainty was up to 0.09 Dobson unit (DU), equivalent to 2.687 ?? 1020 molecules m???2) given a 1?? error for the observation geometries, including the solar zenith angle, viewing zenith angle, and relative azimuth angle. About 0.1 DU (6.8%) was estimated for an aerosol optical depth (AOD) uncertainty of 0.01. In addition, the uncertainty due to the NO2 vertical profile was 14% to 22%. Compared with the co-located Pandora spectrophotometer measurements, the HIS captured the temporal variation of the TCN during the intensive observation period. The correlation between the TCN from the HIS and Pandora also showed good agreement, with a slight positive bias (bias: 0.6 DU, root mean square error: 0.7 DU)
Group-Based Key Management Protocol for Energy Efficiency in Long-Lived and Large-Scale Distributed Sensor Networks
As wireless sensor networks grow, so does the need for effective security mechanisms. We propose a cryptographic key-management protocol, called energy-efficient key-management (EEKM) protocol. Using a location-based group key scheme, the protocol supports the revocation of compromised nodes and energy-efficient rekeying. The design is motivated by the observation that unicast-based rekeying does not meet the security requirements of periodic rekeying in long-lived wireless sensor networks. EEKM supports broadcast-based rekeying for low-energy key management and high resilience. In addition, to match the increasing complexity of encryption keys, the protocol uses a dynamic composition key scheme. EEKM also provides group-management protocols for secure group communication. We analyzed the energy efficiency and security of EEKM and compared it to other key-management protocols using a network simulator
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