1,715 research outputs found
Collapse of Vacuum Bubbles in a Vacuum
Motivated by the discovery of a plenitude of metastable vacua in a string
landscape and the possibility of rapid tunneling between these vacua, we
revisit the dynamics of a false vacuum bubble in a background de Sitter
spacetime. We find that there exists a large parameter space that allows the
bubble to collapse into a black hole or to form a wormhole. This may have
interesting implications to inflationary physics.Comment: 8 pages including 6 figures, LaTex; references adde
A Proposal to Detect Dark Matter Using Axionic Topological Antiferromagnets
Antiferromagnetically doped topological insulators (A-TI) are among the
candidates to host dynamical axion fields and axion-polaritons; weakly
interacting quasiparticles that are analogous to the dark axion, a long sought
after candidate dark matter particle. Here we demonstrate that using the axion
quasiparticle antiferromagnetic resonance in A-TI's in conjunction with
low-noise methods of detecting THz photons presents a viable route to detect
axion dark matter with mass 0.7 to 3.5 meV, a range currently inaccessible to
other dark matter detection experiments and proposals. The benefits of this
method at high frequency are the tunability of the resonance with applied
magnetic field, and the use of A-TI samples with volumes much larger than 1
mm.Comment: 6 pages, 4 figures. v2 accepted for publication in Physical Review
Letters. Many points clarified, some parameter estimates revise
The role of electron-electron interactions in two-dimensional Dirac fermions
The role of electron-electron interactions on two-dimensional Dirac fermions
remains enigmatic. Using a combination of nonperturbative numerical and
analytical techniques that incorporate both the contact and long-range parts of
the Coulomb interaction, we identify the two previously discussed regimes: a
Gross-Neveu transition to a strongly correlated Mott insulator, and a
semi-metallic state with a logarithmically diverging Fermi velocity accurately
described by the random phase approximation. Most interestingly, experimental
realizations of Dirac fermions span the crossover between these two regimes
providing the physical mechanism that masks this velocity divergence. We
explain several long-standing mysteries including why the observed Fermi
velocity in graphene is consistently about 20 percent larger than the best
values calculated using ab initio and why graphene on different substrates show
different behavior.Comment: 11 pages, 4 figure
The Evolution of Cas A at Low Radio Frequencies
We have used archival 74 MHz VLA data spanning the last 15 years in
combination with new data from the Long Wavelength Demonstrator Array (LWDA)
and data from the literature covering the last 50 years to explore the
evolution of Cas A at low radio frequencies. We find that the secular decrease
of the flux density of Cas A at ~80 MHz is rather stable over five decades of
time, decreasing at a rate of 0.7-0.8% yr^-1. This is entirely consistent with
previous estimates at frequencies as low as 38 MHz, indicating that the secular
decrease is roughly the same at low frequencies, at least between 38 and 80
MHz. We also find strong evidence for as many as four modes of flux density
oscillation about the slower secular decrease with periods of 3.10+/-0.02$ yr,
5.1+/-0.3 yr, 9.0+/-0.2 yr, and 24+/-2 yr. These are also consistent with
fluctuations seen previously to occur on scales of a few years. These results
provide compelling motivation for a thorough low frequency monitoring campaign
of Cas A to constrain the nature and physical origins of these fluctuations,
and to be able to better predict the flux density of Cas A at any given epoch
so that it may be used as a reliable low frequency calibrator.Comment: accepted for publication in A
Interaction driven metal-insulator transition in strained graphene
The question of whether electron-electron interactions can drive a metal to
insulator transition in graphene under realistic experimental conditions is
addressed. Using three representative methods to calculate the effective
long-range Coulomb interaction between -electrons in graphene and solving
for the ground state using quantum Monte Carlo methods, we argue that without
strain, graphene remains metallic and changing the substrate from SiO to
suspended samples hardly makes any difference. In contrast, applying a rather
large -- but experimentally realistic -- uniform and isotropic strain of about
seems to be a promising route to making graphene an antiferromagnetic
Mott insulator.Comment: Updated version: 6 pages, 3 figure
Intermittency in Two-Dimensional Turbulence with Drag
We consider the enstrophy cascade in forced two-dimensional turbulence with a
linear drag force. In the presence of linear drag, the energy wavenumber
spectrum drops with a power law faster than in the case without drag, and the
vorticity field becomes intermittent, as shown by the anomalous scaling of the
vorticity structure functions. Using a previous theory, we compare numerical
simulation results with predictions for the power law exponent of the energy
wavenumber spectrum and the scaling exponents of the vorticity structure
functions obtained in terms of the distribution of finite time
Lyapunov exponents. We also study, both by numerical experiment and theoretical
analysis, the multifractal structure of the viscous enstrophy dissipation in
terms of its R\'{e}nyi dimension spectrum and singularity spectrum
. We derive a relation between and , and discuss
its relevance to a version of the refined similarity hypothesis. In addition,
we obtain and compare theoretically and numerically derived results for the
dependence on separation of the probability distribution of
\delta_{\V{r}}\omega, the difference between the vorticity at two points
separated by a distance . Our numerical simulations are done on a grid.Comment: 18 pages, 17 figure
Red Data Updates for Orenburg Oblast
One of the ways to study and protect flora is keeping and publishing regional Red Data Books. A floristic study resulted in a series of data on the flora of Orenburg Oblast, which made it possible to make a set of criteria for updating the list of regional species for conservation. In early 2018, the authors proposed 6 species of vascular plants to be registered in the Red Data Book of Orenburg Oblast including Stipa zalesskii Wilensky, Iris aphylla L., Botrychium lunaria (L.) Sw., Pedicularis palustris L., Saxifraga sibirica L., Salvia glutinosa L. The registration of some of them in the Red Data Book was recommended in view of an increased anthropogenic adverse impact on their habitats and a growing threat of extinction. On September 2018, the proposed Red Data updates for Orenburg Oblast outlined in this article were adopted and approved by the Government of the Orenburg Oblast
Interviewing Criminal Justice Populations without Electronic Recording Devices: A Guide
We outline a guide for facilitating face-to-face in-depth interviews without the use of electronic recording devices in criminal justice research. It is designed to provide researchers with step-by-step directions they can follow to conduct interviews when recording equipment is not available, not allowed, or not used due to other reasons. In-depth interviews are common in qualitative criminal justice research but require researchers to be highly flexible and adaptive. When interviews are conducted on sensitive issues or carried out in high security environments, recording devices may not be permitted or welcomed. This protocol aims to make the interviews more structured, systematic and organized when electronic recording devices are not used in an attempt to enhance the accuracy and transparency. These guidelines were developed based on practical and theoretical foundations
Emotional intelligence and organisational citizenship behaviour of manufacturing sector employees
Possible Constraints on the Duration of Inflationary Expansion from Quantum Stress Tensor Fluctuations
We discuss the effect of quantum stress tensor fluctuations in deSitter
spacetime upon the expansion of a congruence of timelike geodesics. We treat a
model in which the expansion fluctuations begin on a given hypersurface in
deSitter spacetime, and find that this effect tends to grow, in contrast to the
situation in flat spacetime. This growth potentially leads to observable
consequences in inflationary cosmology in the form of density perturbations
which depend upon the duration of the inflationary period. In the context of
our model, the effect may be used to place upper bounds on this duration.Comment: 21 pages, no figures; Sect. IV rewritten and expanded, several
comments and references adde
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