16,613 research outputs found
Statistical properties of dark matter mini-haloes at z >= 15
Understanding the formation of the first objects in the universe critically
depends on knowing whether the properties of small dark matter structures at
high-redshift (z > 15) are different from their more massive lower-redshift
counterparts. To clarify this point, we performed a high-resolution N-body
simulation of a cosmological volume 1 Mpc/h comoving on a side, reaching the
highest mass resolution to date in this regime. We make precision measurements
of various physical properties that characterize dark matter haloes (such as
the virial ratio, spin parameter, shape, and formation times, etc.) for the
high-redshift (z > 15) dark matter mini-haloes we find in our simulation, and
compare them to literature results and a moderate-resolution comparison run
within a cube of side-length 100 Mpc/h. We find that dark matter haloes at
high-redshift have a log-normal distribution of the dimensionless spin
parameter centered around {\lambda} 0.03, similar to their more massive
counterparts. They tend to have a small ratio of the length of the shortest
axis to the longest axis (sphericity), and are highly prolate. In fact, haloes
of given mass that formed recently are the least spherical, have the highest
virial ratios, and have the highest spins. Interestingly, the formation times
of our mini-halos depend only very weakly on mass, in contrast to more massive
objects. This is expected from the slope of the linear power spectrum of
density perturbations at this scale, but despite this difference, dark matter
structures at high-redshift share many properties with their much more massive
counterparts observed at later times.Comment: 17 pages. Accepted for publication in MNRA
A cluster expansion approach to exponential random graph models
The exponential family of random graphs is among the most widely-studied
network models. We show that any exponential random graph model may
alternatively be viewed as a lattice gas model with a finite Banach space norm.
The system may then be treated by cluster expansion methods from statistical
mechanics. In particular, we derive a convergent power series expansion for the
limiting free energy in the case of small parameters. Since the free energy is
the generating function for the expectations of other random variables, this
characterizes the structure and behavior of the limiting network in this
parameter region.Comment: 15 pages, 1 figur
Hybrid exciton-polaritons in a bad microcavity containing the organic and inorganic quantum wells
We study the hybrid exciton-polaritons in a bad microcavity containing the
organic and inorganic quantum wells. The corresponding polariton states are
given. The analytical solution and the numerical result of the stationary
spectrum for the cavity field are finishedComment: 3 pages, 1 figure. appear in Communications in Theoretical Physic
Do Linear Dispersions of Classical Waves Mean Dirac Cones?
By using the \vec{k}\cdot\vec{p} method, we propose a first-principles theory
to study the linear dispersions in phononic and photonic crystals. The theory
reveals that only those linear dispersions created by doubly-degenerate states
can be described by a reduced Hamiltonian that can be mapped into the Dirac
Hamiltonian and possess a Berry phase of -\pi. Triply-degenerate states can
also generate Dirac-like cone dispersions, but the wavefunctions transform like
a spin-1 particle and the Berry phase is zero. Our theory is capable of
predicting accurately the linear slopes of Dirac/Dirac-like cones at various
symmetry points in a Brilliouin zone, independent of frequency and lattice
structure
Broadband spin-controlled focusing via logarithmic-spiral nanoslits of varying width
This work presents analytical, numerical and experimental demonstrations of light diffracted through a logarithmic spiral (LS) nanoslit, which forms a type of switchable and focus-tunable structure. Owing to a strong dependence on the incident photon spin, the proposed LS-nanoslit converges incoming light of opposite handedness (to that of the LS-nanoslit) into a confined subwavelength spot, while it shapes light with similar chirality into a donut-like intensity profile. Benefitting from the varying width of the LS-nanoslit, different incident wavelengths interfere constructively at different positions, i.e., the focal length shifts from 7.5 μm (at λ = 632.8 nm) to 10 μm (at λ = 488 nm), which opens up new opportunities for tuning and spatially separating broadband light at the micrometer scale
Cosmogenic activation of materials used in rare event search experiments
We evaluate the cosmogenic production rates in some materials that are commonly used as targets and shielding/supporting components for detecting rare events. The results from Geant4 simulations and the calculations of ACTIVIA are compared with the available experimental data. We demonstrate that the production rates from the Geant4-based simulations agree with the available data reasonably well. As a result, we report that the cosmogenic production of several isotopes in various materials can generate potential backgrounds for direct detection of dark matter and neutrinoless double-beta decay
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