261 research outputs found
On general features of warm dark matter with reduced relativistic gas
Reduced Relativistic Gas (RRG) is a useful approach to describe the warm dark
matter (WDM) or the warmness of baryonic matter in the approximation when the
interaction between the particles is irrelevant. The use of Maxwell
distribution leads to the complicated equation of state of the J\"{u}ttner
model of relativistic ideal gas. The RRG enables one to reproduce the same
physical situation but in a much simpler form. For this reason RRG can be a
useful tool for the theories with some sort of a "new Physics". On the other
hand, even without the qualitatively new physical implementations, the RRG can
be useful to describe the general features of WDM in a model-independent way.
In this sense one can see, in particular, to which extent the cosmological
manifestations of WDM may be dependent on its Particle Physics background. In
the present work RRG is used as a complementary approach to derive the main
observational exponents for the WDM in a model-independent way. The only
assumption concerns a non-negligible velocity for dark matter particles
which is parameterized by the warmness parameter . The relatively high
values of ( ) erase the radiation (photons and
neutrinos) dominated epoch and cause an early warm matter domination after
inflation. Furthermore, RRG approach enables one to quantify the lack of power
in linear matter spectrum at small scales and in particular, reproduces the
relative transfer function commonly used in context of WDM with accuracy of
. A warmness with (equivalent to ) does not alter significantly the CMB power spectrum and is in
agreement with the background observational tests.Comment: 15 pages, 8 figures. Essential improvements in style and presentatio
Enhanced Optical Dichroism of Graphene Nanoribbons
The optical conductivity of graphene nanoribbons is analytical and exactly
derived. It is shown that the absence of translation invariance along the
transverse direction allows considerable intra-band absorption in a narrow
frequency window that varies with the ribbon width, and lies in the THz range
domain for ribbons 10-100nm wide. In this spectral region the absorption
anisotropy can be as high as two orders of magnitude, which renders the medium
strongly dichroic, and allows for a very high degree of polarization (up to
~85) with just a single layer of graphene. The effect is resilient to level
broadening of the ribbon spectrum potentially induced by disorder. Using a
cavity for impedance enhancement, or a stack of few layer nanoribbons, these
values can reach almost 100%. This opens a potential prospect of employing
graphene ribbon structures as efficient polarizers in the far IR and THz
frequencies.Comment: Revised version. 10 pages, 7 figure
Embodied Skillful Performance: Where the Action Is
When someone masters a skill, their performance looks to us like second nature: it looks as if their actions are performed smoothly without explicit, knowledge-driven, online monitoring of their performance. Contemporary computational models in motor control theory, however, are instructionist. That is, they cast skilful performance as a knowledge-driven process, one that is driven by explicit motor representations of the action to be performed skillfully, which harness instructions for performance. Optimal control theory, a popular representative of such approaches, casts skillful performance as the execution of motor commands, the deliverances of a motor control system implemented by separable forward and inverse models that work in tandem with a state estimator to control the motor plant. These models rest on the principle that motor control is realized by the concerted action of separate modular subsystems, which transform an explicit motor representation into a sequence of physical movements. This paper aims to show the limitations of such instructionist approaches to skillful performance. Specifically, we address whether the assumption of modular knowledge-driven motor control in optimal control theory (based on motor commands computed by separable state estimators, forward models, and inverse models) is warranted. The first section of this paper examines the instructionist assumption, according to which skillful performance consists in the execution of instructions invested in motor representations. The second and third sections characterize the implementation of motor representations as motor commands, with a special focus on formulations from optimal control theory. The final sections of this paper examine predictive coding and active inference – behavioral modeling frameworks that descend, but are distinct, from optimal control theory – and argue that the instructionist assumption is ill-motivated in light of new developments in motor control theory, which cast motor control and motor planning as a form of (active) inference
A geometric perspective on the -cluster morphism category
We show how the -cluster morphism category may be defined in terms of
the wall-and-chamber structure of an algebra. This geometric perspective leads
to a simplified proof that the category is well-defined.Comment: 20 pages, 5 figures. Comments welcome! v2: added a little more
discussio
Embodied Skillful Performance: Where the Action Is
When someone masters a skill, their performance looks to us like second nature: it looks as if their actions are performed smoothly without explicit, knowledge-driven, online monitoring of their performance. Contemporary computational models in motor control theory, however, are instructionist. That is, they cast skilful performance as a knowledge-driven process, one that is driven by explicit motor representations of the action to be performed skillfully, which harness instructions for performance. Optimal control theory, a popular representative of such approaches, casts skillful performance as the execution of motor commands, the deliverances of a motor control system implemented by separable forward and inverse models that work in tandem with a state estimator to control the motor plant. These models rest on the principle that motor control is realized by the concerted action of separate modular subsystems, which transform an explicit motor representation into a sequence of physical movements. This paper aims to show the limitations of such instructionist approaches to skillful performance. Specifically, we address whether the assumption of modular knowledge-driven motor control in optimal control theory (based on motor commands computed by separable state estimators, forward models, and inverse models) is warranted. The first section of this paper examines the instructionist assumption, according to which skillful performance consists in the execution of instructions invested in motor representations. The second and third sections characterize the implementation of motor representations as motor commands, with a special focus on formulations from optimal control theory. The final sections of this paper examine predictive coding and active inference – behavioral modeling frameworks that descend, but are distinct, from optimal control theory – and argue that the instructionist assumption is ill-motivated in light of new developments in motor control theory, which cast motor control and motor planning as a form of (active) inference
Embodied Skillful Performance: Where the Action Is
When someone masters a skill, their performance looks to us like second nature: it looks as if their actions are performed smoothly without explicit, knowledge-driven, online monitoring of their performance. Contemporary computational models in motor control theory, however, are instructionist. That is, they cast skilful performance as a knowledge-driven process, one that is driven by explicit motor representations of the action to be performed skillfully, which harness instructions for performance. Optimal control theory, a popular representative of such approaches, casts skillful performance as the execution of motor commands, the deliverances of a motor control system implemented by separable forward and inverse models that work in tandem with a state estimator to control the motor plant. These models rest on the principle that motor control is realized by the concerted action of separate modular subsystems, which transform an explicit motor representation into a sequence of physical movements. This paper aims to show the limitations of such instructionist approaches to skillful performance. Specifically, we address whether the assumption of modular knowledge-driven motor control in optimal control theory (based on motor commands computed by separable state estimators, forward models, and inverse models) is warranted. The first section of this paper examines the instructionist assumption, according to which skillful performance consists in the execution of instructions invested in motor representations. The second and third sections characterize the implementation of motor representations as motor commands, with a special focus on formulations from optimal control theory. The final sections of this paper examine predictive coding and active inference – behavioral modeling frameworks that descend, but are distinct, from optimal control theory – and argue that the instructionist assumption is ill-motivated in light of new developments in motor control theory, which cast motor control and motor planning as a form of (active) inference
Optical Self Energy in Graphene due to Correlations
In highly correlated systems one can define an optical self energy in analogy
to its quasiparticle (QP) self energy counterpart. This quantity provides
useful information on the nature of the excitations involved in inelastic
scattering processes. Here we calculate the self energy of the intraband
optical transitions in graphene originating in the electron-electron
interaction (EEI) as well as electron-phonon interaction (EPI). Although optics
involves an average over all momenta () of the charge carriers, the
structure in the optical self energy is nevertheless found to mirror mainly
that of the corresponding quasiparticles for equal to or near the Fermi
momentum . Consequently plasmaronic structures which are associated with
momenta near the Dirac point at are not important in the intraband
optical response. While the structure of the electron-phonon interaction (EPI)
reflects the sharp peaks of the phonon density of states, the excitation
spectrum associated with the electron-electron interaction is in comparison
structureless and flat and extends over an energy range which scales linearly
with the value of the chemical potential. Modulations seen on the edge of the
interband optical conductivity as it rises towards its universal background
value are traced to structure in the quasiparticle self energies around
of the lower Dirac cone associated with the occupied states.Comment: 30 pages, 10 figure
Radio-Optically- and Thermally Stimulated Luminescence of Zn(BO2)2:Tb3+ exposed to Ionizing Radiation
The optical absorption of zinc tetraborate at different concentrations of the terbium impurity (0, 0.5, 1, 2, 4, 8 mol%) was analyzed. The radioluminescence (RL) emission spectra was obtained after beta irradiation of a 90Sr/90Y source. The RL spectrum showed the characteristics bands of Tb3+ with two main emissions at 489 nm and 546 nm which corresponding to the5D4→7F6 and 5D4→7F5 transitions respectively in this ion. The OSL and TL characteristics have been analyzed. The stimulation blue light (497 nm) of a diode laser at 500 mA was used to bleach the thermoluminescent (TL) signals obtained with 5Gy of 60Co source. The two main glow peaks (79 and 161 °C) are sensitives under 497 nm stimulation, and they were shifted to higher temperature values and faded their TL intensities. Similar behavior of TL glow curves before and after OSL stimulation with blue light was observed when the samples were exposed to 30 Gy gamma dose of 137Cs irradiator. The OSL signal response was linear with the dose range of 1-10 Gy and increased their response up to 200 Gy gamma dose. The OSL shows a bleaching sensitive shallow traps and diminishing the intensity of the TL glow curves remaining a complex traps distribution. The RL, TL and OSL properties were investigated in Zn(BO2)2:Tb3+ phosphor
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