20,877 research outputs found
Magnetic Excitations in the High Tc Iron Pnictides
We calculate the expected finite frequency neutron scattering intensity based
on the two-sublattice collinear antiferromagnet found by recent neutron
scattering experiments as well as by theoretical analysis on the iron
oxypnictide LaOFeAs. We consider two types of superexchange couplings between
Fe atoms: nearest-neighbor coupling J1 and next-nearest-neighbor coupling J2.
We show how to distinguish experimentally between ferromagnetic and
antiferromagnetic J1. Whereas magnetic excitations in the cuprates display a
so-called resonance peak at (pi,pi) (corresponding to a saddlepoint in the
magnetic spectrum) which is at a wavevector that is at least close to nesting
Fermi-surface-like structures, no such corresponding excitations exist in the
iron pnictides. Rather, we find saddlepoints near (pi,pi/2) and (0,pi/2)(and
symmetry related points). Unlike in the cuprates, none of these vectors are
close to nesting the Fermi surfaces.Comment: 4 pages, 5 figure
Physical Model of the Immune Response of Bacteria Against Bacteriophage Through the Adaptive CRISPR-Cas Immune System
Bacteria and archaea have evolved an adaptive, heritable immune system that
recognizes and protects against viruses or plasmids. This system, known as the
CRISPR-Cas system, allows the host to recognize and incorporate short foreign
DNA or RNA sequences, called `spacers' into its CRISPR system. Spacers in the
CRISPR system provide a record of the history of bacteria and phage
coevolution. We use a physical model to study the dynamics of this coevolution
as it evolves stochastically over time. We focus on the impact of mutation and
recombination on bacteria and phage evolution and evasion. We discuss the
effect of different spacer deletion mechanisms on the coevolutionary dynamics.
We make predictions about bacteria and phage population growth, spacer
diversity within the CRISPR locus, and spacer protection against the phage
population.Comment: 37 pages, 13 figure
Distributed Adaptive Attitude Synchronization of Multiple Spacecraft
This paper addresses the distributed attitude synchronization problem of
multiple spacecraft with unknown inertia matrices. Two distributed adaptive
controllers are proposed for the cases with and without a virtual leader to
which a time-varying reference attitude is assigned. The first controller
achieves attitude synchronization for a group of spacecraft with a leaderless
communication topology having a directed spanning tree. The second controller
guarantees that all spacecraft track the reference attitude if the virtual
leader has a directed path to all other spacecraft. Simulation examples are
presented to illustrate the effectiveness of the results.Comment: 13 pages, 11 figures. To appear in SCIENCE CHINA Technological
Science
Low Mass Dark Matter and Invisible Higgs Width In Darkon Models
The Standard Model (SM) plus a real gauge-singlet scalar field dubbed darkon
(SM+D) is the simplest model possessing a weakly interacting massive particle
(WIMP) dark-matter candidate. In this model, the parameters are constrained
from dark matter relic density and direct searches. The fact that interaction
between darkon and SM particles is only mediated by Higgs boson exchange may
lead to significant modifications to the Higgs boson properties. If the dark
matter mass is smaller than a half of the Higgs boson mass, the Higgs boson can
decay into a pair of darkons resulting in a large invisible branching ratio.
The Higgs boson will be searched for at the LHC and may well be discovered in
the near future. If a Higgs boson with a small invisible decay width will be
found, the SM+D model with small dark matter mass will be in trouble. We find
that by extending the SM+D to a two-Higgs-doublet model plus a darkon (THDM+D)
it is possible to have a Higgs boson with a small invisible branching ratio and
at the same time the dark matter can have a low mass. We also comment on other
implications of this model.Comment: RevTeX, 15 pages, 11 figures. A few typos corrected and some
references adde
The potential role of temporal dynamics in approach biases: delay-dependence of a general approach bias in an alcohol approach-avoidance task
Attractive cues have been shown to evoke automatic approach biases in tasks such as the Automatic Approach Task or Stimulus Response Compatibility task. An important but as yet not studied question is the role of temporal dynamics in such tasks: the impact of automatic processes may depend on the interval between cue and response. The current proof of principle study tested this hypothesized time-dependence of the approach bias. Secondary goals included the exploration of effects of alcohol cues and virtual hand stimuli. 22 participants performed an SRC task in which the delay between the presentation of the cue and the possibility to select the response was manipulated. Results revealed an approach bias that decayed over longer delays. Thus, the approach bias was indeed dependent on processes that are transiently evoked by cues. The results did not show significant effects of alcohol cues or a virtual hand. Temporal dynamics may be an essential feature of approach biases
Electronic Structure of ZnCNi3
According to a recent report by Park et al, ZnCNi3 is isostructural and
isovalent to the superconducting (Tc = 8 K) anti-perovskite, MgCNi3, but shows
no indication of a superconducting transition down to 2K. A comparison of
calculated electronic structures shows that the main features of MgCNi3,
particularly the van Hove singularity near the Fermi energy, are preserved in
ZnCNi3. Thus the reported lack of superconductivity in ZnCNi3 is not
explainable in terms of Tc being driven to a very low value by a small Fermi
level density of states. We propose that the lack of superconductivity, the
small value of the linear specific heat coefficient, gamma, and the discrepancy
between theoretical and experimental lattice constants can all be explained if
the material is assumed to be a C-deficient alpha-ZnCNi3 similar to the
analogous non-superconducting phase of MgCNi3
Recurrence interval analysis of high-frequency financial returns and its application to risk estimation
We investigate the probability distributions of the recurrence intervals
between consecutive 1-min returns above a positive threshold or
below a negative threshold of two indices and 20 individual stocks in
China's stock market. The distributions of recurrence intervals for positive
and negative thresholds are symmetric, and display power-law tails tested by
three goodness-of-fit measures including the Kolmogorov-Smirnov (KS) statistic,
the weighted KS statistic and the Cram\'er-von Mises criterion. Both long-term
and shot-term memory effects are observed in the recurrence intervals for
positive and negative thresholds . We further apply the recurrence interval
analysis to the risk estimation for the Chinese stock markets based on the
probability , Value-at-Risk (VaR) analysis and VaR analysis
conditioned on preceding recurrence intervals.Comment: 17 pages, 10 figures, 1 tabl
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