2,580 research outputs found
Universality of thermally assisted magnetic domain wall motion under spin torque
Thermally assisted motion of magnetic domain wall under spin torque is
studied theoretically. It is shown that the wall velocity depends
exponentially on the spin current,
\Is, below the threshold value, in the same way as in a thermally activated
motion driven by a force. A novel property of the spin torque driven case at
low temperature is that the linear term in spin current is universal, i.e.,
\ln v \sim \frac{\pi\hbar}{2e}(\Is/\kB T). This behavior, which is
independent of pinning and material constants, could be used to confirm
experimentally the spin torque as the driving mechanism
Critical Behavior in the Rotating D-branes
The low energy excitation of the rotating D3-branes is thermodynamically
stable up to a critical angular momentum density. This indicates that there is
a corresponding phase transition of the =4 large super Yang-Mills
theory at finite temperature. On the side of supergravity, we investigate the
phase transition in the grand canonical ensemble and canonical ensemble. Some
critical exponents of thermodynamic quantities are calculated. They obey the
static scaling laws. Using the scaling laws related to the correlation length,
we get the critical exponents of the correlation function of gauge field. The
thermodynamic stability of low energy excitations of the rotating M5-branes and
rotating M2-branes is also studied and similar critical behavior is observed.
We find that the critical point is shifted in the different ensembles and there
is no critical point in the canonical ensemble for the rotating M2-branes. We
also discuss the Hawking-Page transition for these rotating branes. In the
grand canonical ensemble, the Hawking-Page transition does not occur. In the
canonical ensemble, however, the Hawking-Page transition may appear for the
rotating D3- and M5-branes, but not for the rotating M2-branes.Comment: Revtex, 17 pages, minor changes, the discussion on the Hawking-Page
transition and references adde
Direction-Projection-Permutation for High Dimensional Hypothesis Tests
Motivated by the prevalence of high dimensional low sample size datasets in
modern statistical applications, we propose a general nonparametric framework,
Direction-Projection-Permutation (DiProPerm), for testing high dimensional
hypotheses. The method is aimed at rigorous testing of whether lower
dimensional visual differences are statistically significant. Theoretical
analysis under the non-classical asymptotic regime of dimension going to
infinity for fixed sample size reveals that certain natural variations of
DiProPerm can have very different behaviors. An empirical power study both
confirms the theoretical results and suggests DiProPerm is a powerful test in
many settings. Finally DiProPerm is applied to a high dimensional gene
expression dataset
Quantum Larmor radiation in conformally flat universe
We investigate the quantum effect on the Larmor radiation from a moving
charge in an expanding universe based on the framework of the scalar quantum
electrodynamics (SQED). A theoretical formula for the radiation energy is
derived at the lowest order of the perturbation theory with respect to the
coupling constant of the SQED. We evaluate the radiation energy on the
background universe so that the Minkowski spacetime transits to the Milne
universe, in which the equation of motion for the mode function of the free
complex scalar field can be exactly solved in an analytic way. Then, the result
is compared with the WKB approach, in which the equation of motion of the mode
function is constructed with the WKB approximation which is valid as long as
the Compton wavelength is shorter than the Hubble horizon length. This
demonstrates that the quantum effect on the Larmor radiation of the order
e^2\hbar is determined by a non-local integration in time depending on the
background expansion. We also compare our result with a recent work by Higuchi
and Walker [Phys. Rev. D80 105019 (2009)], which investigated the quantum
correction to the Larmor radiation from a charged particle in a
non-relativistic motion in a homogeneous electric field.Comment: 12 pages, 4 figure, accepted for publication in Physical Review
Relation between the Dynamics of the Reduced Purity and Correlations
A general property of the relation between the dynamics of the reduced purity
and correlations is investigated in quantum mechanical systems. We show that a
non-zero time-derivative of the reduced purity of a system implies the
existence of non-zero correlations with its environment under any unbounded
Hamiltonians with finite variance. This shows the role of local dynamical
information on the correlations, as well as the role of correlations in the
mechanism of purity change.Comment: 7 page
Spectrum from the warped compactifications with the de Sitter universe
We discuss the spectrum of the tensor metric perturbations and the stability
of warped compactifications with the de Sitter spacetime in the
higher-dimensional gravity. The spacetime structure is given in terms of the
warped product of the non-compact direction, the spherical internal dimensions
and the four-dimensional de Sitter spacetime. To realize a finite bulk volume,
we construct the brane world model, using the cut-copy-paste method. Then, we
compactify the spherical directions on the brane. In any case, we show the
existence of the massless zero mode and the mass gap of it with massive
Kaluza-Klein modes. Although the brane involves the spherical dimensions, no
light massive mode is excited. We also investigate the scalar perturbations,
and show that the model is unstable due to the existence of a tachyonic bound
state, which seems to have the universal negative mass square, irrespective of
the number of spacetime dimensions.Comment: Journal version (JHEP
New interpretation of matter-antimatter asymmetry based on branes and possible observational consequences
Motivated by the AMS project, we assume that after the Big Bang or inflation
epoch, antimatter was repelled onto one brane which is separated from our brane
where all the observational matter resides. It is suggested that CP may be
spontaneously broken, the two branes would correspond to ground states for
matter and antimatter respectively. Generally a complex scalar field which is
responsible for the spontaneous CP violation, exists in the space between the
branes and causes a repulsive force against the gravitation. A possible
potential barrier prevents the mater(antimatter) particles to enter the space
between two branes. However, by the quantum tunnelling, a sizable anti-matter
flux may come to our brane. In this work by considering two possible models,
i.e. the naive flat space-time and Randall-Sundrum models and using the
observational data on the visible matter in our universe as inputs, we derive
the antimatter flux which would be observed by the AMS detector.Comment: 10 pages, 4 figures and 2 tables. Replaced by new versio
Opportunities of artificial neural network generated VGA: Training a Multilayer Perceptron to recognize the underlying structures of space
This paper presents the research conducted with the aim of understanding if new advances in computer science, more specifically a type of supervised, feedforward Artificial Neural Network, a Multilayer Perceptron (MLP) is able to estimate the values of Visibility Graph Analysis (VGA) without the need for expensive calculation. The overarching hypothesis is that an MLP can be setup in a way that it can be trained to learn the relationship between spatial configuration and the VGA (neighbourhood size and clustering coefficient) derived from it. Two hypotheses are stated: firstly, if such an MLP can be created than it will be able to generate spatial configurations for specific VGA values as inputs (mode A); secondly, the network would be able to generate VGA when presented with spatial configuration faster, compared to current method and with negligible error (mode B). The hypotheses were tested by creating unique setups of an MLP for each mode, all of which had a different configuration. As each combination of possible setups were tested, the performance of the networks could be compared to each other and to the traditional method of VGA calculation. Both mode A and mode B was able to achieve satisfying results that prove that an MLP is able to generate –with limitations- configurations based on VGA input and it is able to calculate the neighbourhood size and the clustering coefficient of a 2D layout substantially faster and with negligible error. All MLPs were created at a generic space, therefore the MLP taught once can be adopted universally to most spaces. The implications of the two systems is that spatial analysis can be integrated into the design process, enabling interactive, instant analysis and the possible deployment of optimisation procedures, for instance a Genetic Algorithm
Notes on Ghost Dark Energy
We study a phenomenological dark energy model which is rooted in the
Veneziano ghost of QCD. In this dark energy model, the energy density of dark
energy is proportional to Hubble parameter and the proportional coefficient is
of the order , where is the mass scale of QCD.
The universe has a de Sitter phase at late time and begins to accelerate at
redshift around . We also fit this model and give the
constraints on model parameters, with current observational data including
SnIa, BAO, CMB, BBN and Hubble parameter data. We find that the squared sound
speed of the dark energy is negative, which may cause an instability. We also
study the cosmological evolution of the dark energy with interaction with cold
dark matter.Comment: 20 pages,10 figures,Correct some typos and add new reference
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