6,027 research outputs found
Eternally accelerating spacelike braneworld cosmologies
We construct an eternally inflating spacelike brane world model. If the space
dimension of the brane is three (SM2) or six (SM5) for M theory or four (SD3)
for superstring theory, a time-dependent -form field would supply a constant
energy density and cause exponentially expansion of the spacelike brane. In
these cases, the hyperbolic space perpendicular to the brane would not vary in
size. In the other cases, however, the extra space would vary in size.Comment: 8 pages, Mod. Phys. Lett. A Vol.21, No.40(2006) 2989-299
Eternally inflating cosmologies from intersecting spacelike branes
Intersecting spacelike braneworld cosmologies are investigated. The time axis
is set on the scale parameter of extra space, which may include more than one
timelike metric. Obtained are eternally inflating (i.e. undergoing late-time
inflation) Robertson-Walker spacetime and extra space with a constant scale
factor. In the case of multibrane solutions, some dimensions are static or
shrink. The fact that the largest supersymmetry algebra contains 32
supercharges in 4 dimensions imposes a restriction on the geometry of extra
space.Comment: 19 page
Flavor Mixing in the Gauge-Higgs Unification
Gauge-Higgs unification is the fascinating scenario solving the hierarchy
problem without supersymmetry. In this scenario, the Standard Model (SM) Higgs
doublet is identified with extra component of the gauge field in higher
dimensions and its mass becomes finite and stable under quantum corrections due
to the higher dimensional gauge symmetry. On the other hand, Yukawa coupling is
provided by the gauge coupling, which seems to mean that the flavor mixing and
CP violation do not arise at it stands. In this talk, we discuss that the
flavor mixing is originated from simultaneously non-diagonalizable bulk and
brane mass matrices. Then, this mechanism is applied to various flavor changing
neutral current (FCNC) processes via Kaluza-Klein (KK) gauge boson exchange at
tree level and constraints for compactification scale are obtained.Comment: 5 pages, prepared for the proceedings of the International Workshop
on Grand Unified Theories (GUT2012) held at Yukawa Institute for Theoretical
Physics, March 15-17 2012, Kyoto, Japa
An augmented moment method for stochastic ensembles with delayed couplings: II. FitzHugh-Nagumo model
Dynamics of FitzHugh-Nagumo (FN) neuron ensembles with time-delayed couplings
subject to white noises, has been studied by using both direct simulations and
a semi-analytical augmented moment method (AMM) which has been proposed in a
recent paper [H. Hasegawa, E-print: cond-mat/0311021]. For -unit FN neuron
ensembles, AMM transforms original -dimensional {\it stochastic} delay
differential equations (SDDEs) to infinite-dimensional {\it deterministic} DEs
for means and correlation functions of local and global variables.
Infinite-order recursive DEs are terminated at the finite level in the
level- AMM (AMM), yielding -dimensional deterministic DEs. When a
single spike is applied, the oscillation may be induced if parameters of
coupling strength, delay, noise intensity and/or ensemble size are appropriate.
Effects of these parameters on the emergence of the oscillation and on the
synchronization in FN neuron ensembles have been studied. The synchronization
shows the {\it fluctuation-induced} enhancement at the transition between
non-oscillating and oscillating states. Results calculated by AMM5 are in
fairly good agreement with those obtained by direct simulations.Comment: 15 pages, 3 figures; changed the title with correcting typos,
accepted in Phys. Rev. E with some change
A Toy Model of Flying Snake's Glide
We have developed a toy model of flying snake's glide [J.J. Socha, Nature
vol. 418 (2002) 603.] by modifying a model for a falling paper. We have found
that asymmetric oscillation is a key about why snake can glide. Further
investigation for snake's glide will provide us details about how it can glide
without a wing.Comment: 6 pages, to be submitted to J. Phys. Soc. Jpn. Revised Version
submitted to the abov
Interaction of numerosity and time in prefrontal and parietal cortex
It has been proposed that numerical and temporal information are processed by partially overlapping magnitude systems. Interactions across different magnitude domains could occur both at the level of perception and decision-making. However, their neural correlates have been elusive. Here, using functional magnetic resonance imaging in humans, we show that the right intraparietal cortex (IPC) and inferior frontal gyrus (IFG) are jointly activated by duration and numerosity discrimination tasks, with a congruency effect in the right IFG. To determine whether the IPC and the IFG are involved in response conflict (or facilitation) or modulation of subjective passage of time by numerical information, we examined their functional roles using transcranial magnetic stimulation (TMS) and two different numerosity-time interaction tasks: duration discrimination and time reproduction tasks. Our results show that TMS of the right IFG impairs categorical duration discrimination, whereas that of the right IPC modulates the degree of influence of numerosity on time perception and impairs precise time estimation. These results indicate that the right IFG is specifically involved at the categorical decision stage, whereas bleeding of numerosity information on perception of time occurs within the IPC. Together, our findings suggest a two-stage model of numerosity-time interactions whereby the interaction at the perceptual level occurs within the parietal region and the interaction at categorical decisions takes place in the prefrontal cortex
Optical excitation and external photoluminescence quantum efficiency of Eu3+ in GaN
We investigate photoluminescence of Eu-related emission in a GaN host consisting of thin layers grown by organometallic vapor-phase epitaxy. By comparing it with a reference sample of Eu-doped Y2O3, we find that the fraction of Eu3+ ions that can emit light upon optical excitation is of the order of 1%. We also measure the quantum yield of the Eu-related photoluminescence and find this to reach (similar to 10%) and (similar to 3%) under continuous wave and pulsed excitation, respectively.Stichting voor de Technologische Wetenschappen (STW); Japan Society for the Promotion of Science [19GS1209, 24226009]; Ministry of Education, Culture, Sports, Science and Technology of Japaninfo:eu-repo/semantics/publishedVersio
Exponentially growing solutions in homogeneous Rayleigh-Benard convection
It is shown that homogeneous Rayleigh-Benard flow, i.e., Rayleigh-Benard
turbulence with periodic boundary conditions in all directions and a volume
forcing of the temperature field by a mean gradient, has a family of exact,
exponentially growing, separable solutions of the full non-linear system of
equations. These solutions are clearly manifest in numerical simulations above
a computable critical value of the Rayleigh number. In our numerical
simulations they are subject to secondary numerical noise and resolution
dependent instabilities that limit their growth to produce statistically steady
turbulent transport.Comment: 4 pages, 3 figures, to be published in Phys. Rev. E - rapid
communication
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