25,965 research outputs found
Pinned Bilayer Wigner Crystals with Pseudospin Magnetism
We study a model of \textit{pinned} bilayer Wigner crystals (WC) and focus on
the effects of interlayer coherence (IC) on pinning. We consider both a
pseudospin ferromagnetic WC (FMWC) with IC and a pseudospin antiferromagnetic
WC (AFMWC) without IC. Our central finding is that a FMWC can be pinned more
strongly due to the presence of IC. One specific mechanism is through the
disorder induced interlayer tunneling, which effectively manifests as an extra
pinning in a FMWC. We also construct a general "effective disorder" model and
effective pinning Hamiltonian for the case of FMWC and AFMWC respectively.
Under this framework, pinning in the presence of IC involves
\textit{interlayer} spatial correlation of disorder in addition to intralayer
correlation, leading to \textit{enhanced} pinning in the FMWC. The pinning mode
frequency (\wpk) of a FMWC is found to decease with the effective layer
separation, whereas for an AFMWC the opposite behavior is expected. An abrupt
drop of \wpk is predicted at a transition from a FMWC to AFMWC. Possible
effects of in-plane magnetic fields and finite temperatures are addressed.
Finally we discuss some other possible ramifications of the FMWC as an
electronic supersolid-like phase.Comment: Slightly revised. The final version is published on PR
A Systematic Identification and Analysis of Scientists on Twitter
Metrics derived from Twitter and other social media---often referred to as
altmetrics---are increasingly used to estimate the broader social impacts of
scholarship. Such efforts, however, may produce highly misleading results, as
the entities that participate in conversations about science on these platforms
are largely unknown. For instance, if altmetric activities are generated mainly
by scientists, does it really capture broader social impacts of science? Here
we present a systematic approach to identifying and analyzing scientists on
Twitter. Our method can identify scientists across many disciplines, without
relying on external bibliographic data, and be easily adapted to identify other
stakeholder groups in science. We investigate the demographics, sharing
behaviors, and interconnectivity of the identified scientists. We find that
Twitter has been employed by scholars across the disciplinary spectrum, with an
over-representation of social and computer and information scientists;
under-representation of mathematical, physical, and life scientists; and a
better representation of women compared to scholarly publishing. Analysis of
the sharing of URLs reveals a distinct imprint of scholarly sites, yet only a
small fraction of shared URLs are science-related. We find an assortative
mixing with respect to disciplines in the networks between scientists,
suggesting the maintenance of disciplinary walls in social media. Our work
contributes to the literature both methodologically and conceptually---we
provide new methods for disambiguating and identifying particular actors on
social media and describing the behaviors of scientists, thus providing
foundational information for the construction and use of indicators on the
basis of social media metrics
Electrical fixing of photorefractive holograms in Sr0.75Ba0.25Nb2O6
Photorefractive holograms stored in Sr0.75Ba0.25Nb2O6 crystals are electrically fixed at room temperature. The fixed holograms can be read out directly or after a positive-voltage pulse is applied that can dramatically enhance the diffraction efficiency. Single gratings as well as images are recorded and fixed
Unstable particle's wave-function renormalization prescription
We strictly define two set Wave-function Renormalization Constants (WRC)
under the LSZ reduction formula for unstable particles at the first time. Then
by introducing antiparticle's WRC and the CPT conservation law we obtain a new
wave-function renormalization condition which can be used to totally determine
the two set WRC. We calculate two physical processes to manifest the
consistence of the present wave-function renormalization prescription with the
gauge theory in standard model. We also prove that the conventional
wave-function renormalization prescription which discards the imaginary part of
unstable particle's WRC leads to physical amplitude gauge dependent.Comment: 10 pages, 3 figure
Probing for Dynamics of Dark-Energy in Mass Varying Neutrinos: Cosmic Microwave Background Radiation and Large Scale Structure
We present cosmological perturbation theory in neutrino probe interacting
dark-energy models, and calculate cosmic microwave background anisotropies and
matter power spectrum. In these models, the evolution of the mass of neutrinos
is determined by the quintessence scalar field, which is responsible for the
cosmic acceleration today. We consider several types of scalar field potentials
and put constraints on the coupling parameter between neutrinos and dark
energy. Assuming the flatness of the universe, the constraint we can derive
from the current observation is at the 95 % confidence
level for the sum over three species of neutrinos.Comment: 12 pages, 8 figures, Present in conferences COSPA-2006, NEPSE-2007
and Yong-Pyung APCTP-200
S-Lemma with Equality and Its Applications
Let and be two quadratic functions
having symmetric matrices and . The S-lemma with equality asks when the
unsolvability of the system implies the existence of a real
number such that . The
problem is much harder than the inequality version which asserts that, under
Slater condition, is unsolvable if and only if for some . In this paper, we
show that the S-lemma with equality does not hold only when the matrix has
exactly one negative eigenvalue and is a non-constant linear function
(). As an application, we can globally solve as well as the two-sided generalized trust region subproblem
without any condition. Moreover, the
convexity of the joint numerical range where is a (possibly non-convex) quadratic
function and are affine functions can be characterized
using the newly developed S-lemma with equality.Comment: 34 page
Phase-locked sustainment of photorefractive holograms using phase conjugation
A method for sustaining multiply exposed photorefractive holograms, in a phase-locked fashion, by using a pair of phase-conjugating mirrors is described. It is shown that a steady state exists where the overall diffraction efficiency is independent of the number of holographic exposures and the final holograms are exactly in phase with the initial ones. Both analytical and experimental results are presented
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