68,678 research outputs found
An Isocurvature Mechanism for Structure Formation
We examine a novel mechanism for structure formation involving initial number
density fluctuations between relativistic species, one of which then undergoes
a temporary downward variation in its equation of state and generates
superhorizon-scale density fluctuations. Isocurvature decaying dark matter
models (iDDM) provide concrete examples. This mechanism solves the
phenomenological problems of traditional isocurvature models, allowing iDDM
models to fit the current CMB and large-scale structure data, while still
providing novel behavior. We characterize the decaying dark matter and its
decay products as a single component of ``generalized dark matter''. This
simplifies calculations in decaying dark matter models and others that utilize
this mechanism for structure formation.Comment: 4 pages, 3 figures, submitted to PRD (rapid communications
Cosmological Perturbations from a Group Theoretical Point of View
We present a new approach to cosmological perturbations based on the theory
of Lie groups and their representations. After re-deriving the standard
covariant formalism from SO(3) considerations, we provide a new expansion of
the perturbed Friedmann-Lemaitre-Robertson-Walker (FLRW) metric in terms of
irreducible representations of the Lorentz group. The resulting decomposition
splits into (scalar, scalar), (scalar, vector) and (vector, vector) terms.
These equations directly correspond to the standard Lifshitz classification of
cosmological perturbations using scalar, vector and tensor modes which arise
from the irreducible SO(3) representation of the spatial part of the metric.
While the Lorentz group basis matches the underlying local symmetries of the
FLRW spacetime better than the SO(3), the new equations do not provide further
simplification compared to the standard cosmological perturbation theory. We
conjecture that this is due to the fact that the so(3,1) ~ su(2) x su(2)
Lorentz algebra has no pair of commuting generators commuting with any of the
translation group generators.Comment: To be published in Classical and Quantum Gravit
Testing flatness of the universe with probes of cosmic distances and growth
When using distance measurements to probe spatial curvature, the geometric
degeneracy between curvature and dark energy in the distance-redshift relation
typically requires either making strong assumptions about the dark energy
evolution or sacrificing precision in a more model-independent approach.
Measurements of the redshift evolution of the linear growth of perturbations
can break the geometric degeneracy, providing curvature constraints that are
both precise and model-independent. Future supernova, CMB, and cluster data
have the potential to measure the curvature with an accuracy of
sigma(Omega_K)=0.002, without specifying a particular dark energy
phenomenology. In combination with distance measurements, the evolution of the
growth function at low redshifts provides the strongest curvature constraint if
the high-redshift universe is well approximated as being purely matter
dominated. However, in the presence of early dark energy or massive neutrinos,
the precision in curvature is reduced due to additional degeneracies, and
precise normalization of the growth function relative to recombination is
important for obtaining accurate constraints. Curvature limits from distances
and growth compare favorably to other approaches to curvature estimation
proposed in the literature, providing either greater accuracy or greater
freedom from dark energy modeling assumptions, and are complementary due to the
use of independent data sets. Model-independent estimates of curvature are
critical for both testing inflation and obtaining unbiased constraints on dark
energy parameters.Comment: 23 pages, 11 figures; submitted to Phys. Rev.
Perturbative analysis of generally nonlocal spatial optical solitons
In analogy to a perturbed harmonic oscillator, we calculate the fundamental
and some other higher order soliton solutions of the nonlocal nonlinear
Schroedinger equation (NNLSE) in the second approximation in the generally
nonlocal case. Comparing with numerical simulations we show that soliton
solutions in the 2nd approximation can describe the generally nonlocal soliton
states of the NNLSE more exactly than that in the zeroth approximation. We show
that for the nonlocal case of an exponential-decay type nonlocal response the
Gaussian-function-like soliton solutions can't describe the nonlocal soliton
states exactly even in the strongly nonlocal case. The properties of such
nonlocal solitons are investigated. In the strongly nonlocal limit, the
soliton's power and phase constant are both in inverse proportion to the 4th
power of its beam width for the nonlocal case of a Gaussian function type
nonlocal response, and are both in inverse proportion to the 3th power of its
beam width for the nonlocal case of an exponential-decay type nonlocal
response.Comment: 13 pages, 16 figures, accepted by Phys. Rev.
Entangling photons using a charged quantum dot in a microcavity
We present two novel schemes to generate photon polarization entanglement via
single electron spins confined in charged quantum dots inside microcavities.
One scheme is via entangled remote electron spins followed by
negatively-charged exciton emissions, and another scheme is via a single
electron spin followed by the spin state measurement. Both schemes are based on
giant circular birefringence and giant Faraday rotation induced by a single
electron spin in a microcavity. Our schemes are deterministic and can generate
an arbitrary amount of multi-photon entanglement. Following similar procedures,
a scheme for a photon-spin quantum interface is proposed.Comment: 4 pages, 4 figure
M-atom conductance oscillations of a metallic quantum wire
The electron transport through a monoatomic metallic wire connected to leads
is investigated using the tight-binding Hamiltonian and Green's function
technique. Analytical formulas for the transmittance are derived and M-atom
oscillations of the conductance versus the length of the wire are found. Maxima
of the transmittance function versus the energy, for the wire consisted of N
atoms, determine the (N+1) period of the conductance. The periods of
conductance oscillations are discussed and the local and average quantum wire
charges are presented. The average charge of the wire is linked with the period
of the conductance oscillations and it tends to the constant value as the
length of the wire increases. For M-atom periodicity there are possible (M-1)
average occupations of the wire states.Comment: 8 pages, 5 figures. J.Phys.: Condens. matter (2005) accepte
Multiple phase transitions in single-crystalline NaFeAs
Specific heat, resistivity, susceptibility and Hall coefficient measurements
were performed on high-quality single crystalline NaFeAs. This
compound is found to undergo three successive phase transitions at around 52,
41, and 23 K, which correspond to structural, magnetic and superconducting
transitions, respectively. The Hall effect result indicates the development of
energy gap at low temperature due to the occurrence of spin-density-wave
instability. Our results provide direct experimental evidence of the magnetic
ordering in the nearly stoichiometric NaFeAs.Comment: 4 pages, 4 figure
A Survey on Multisensor Fusion and Consensus Filtering for Sensor Networks
Multisensor fusion and consensus filtering are two fascinating subjects in the research of sensor networks. In this survey, we will cover both classic results and recent advances developed in these two topics. First, we recall some important results in the development ofmultisensor fusion technology. Particularly, we pay great attention to the fusion with unknown correlations, which ubiquitously exist in most of distributed filtering problems. Next, we give a systematic review on several widely used consensus filtering approaches. Furthermore, some latest progress on multisensor fusion and consensus filtering is also presented. Finally,
conclusions are drawn and several potential future research directions are outlined.the Royal Society of the UK, the National Natural Science Foundation of China under Grants 61329301, 61374039, 61304010, 11301118, and 61573246, the Hujiang Foundation of China under Grants C14002
and D15009, the Alexander von Humboldt Foundation of Germany, and the Innovation Fund Project for Graduate Student of Shanghai under Grant JWCXSL140
The entanglement beam splitter: a quantum-dot spin in a double-sided optical microcavity
We propose an entanglement beam splitter (EBS) using a quantum-dot spin in a
double-sided optical microcavity. In contrast to the conventional optical beam
splitter, the EBS can directly split a photon-spin product state into two
constituent entangled states via transmission and reflection with high fidelity
and high efficiency (up to 100 percent). This device is based on giant optical
circular birefringence induced by a single spin as a result of cavity quantum
electrodynamics and the spin selection rule of trion transition (Pauli
blocking). The EBS is robust and it is immune to the fine structure splitting
in a realistic quantum dot. This quantum device can be used for
deterministically creating photon-spin, photon-photon and spin-spin
entanglement as well as a single-shot quantum non-demolition measurement of a
single spin. Therefore, the EBS can find wide applications in quantum
information science and technology.Comment: 7 pages, 5 figure
The Structure of Structure Formation Theories
We study the general structure of models for structure formation, with
applications to the reverse engineering of the model from observations. Through
a careful accounting of the degrees of freedom in covariant gravitational
instability theory, we show that the evolution of structure is completely
specified by the stress history of the dark sector. The study of smooth,
entropic, sonic, scalar anisotropic, vector anisotropic, and tensor anisotropic
stresses reveals the origin, robustness, and uniqueness of specific model
phenomenology. We construct useful and illustrative analytic solutions that
cover cases with multiple species of differing equations of state relevant to
the current generation of models, especially those with effectively smooth
components. We present a simple case study of models with phenomenologies
similar to that of a LambdaCDM model to highlight reverse-engineering issues. A
critical-density universe dominated by a single type of dark matter with the
appropriate stress history can mimic a LambdaCDM model exactly.Comment: 31 pages, 18 figures, RevTeX, submitted to Phys. Rev.
- …