241 research outputs found
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
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
Cosmological framework for renormalization group extended gravity at the action level
General relativity (GR) extensions based on renormalization group (RG) flows
may lead to scale-dependent couplings with nontrivial effects at large distance
scales. Here we develop further the approach in which RG effects at large
distance scales are fully encoded in an effective action and we apply it to
cosmology. In order to evaluate the cosmological consequences, our main
assumption is the use of a RG scale such that the (infrared) RG effects only
appear at perturbative order (not at the background level). The emphasis here
is on analytical results and qualitative understanding of the implied
cosmology. We employ commonly used parametrizations for describing modified
gravity in cosmology (as the slip parameter). From them, we describe the
dynamics of the first order perturbations and estimate bounds on the single
dimensionless parameter () introduced by this framework. Possible impacts
on dark matter and dark energy are discussed. It is also shown here that the
parameter effects to are stronger at low redshifts (),
while different values for do not appreciably change at
higher redshifts, thus opening a window to alleviate an issue that is currently
faced by CDM.Comment: v3: 13 pages, 1 figure. Added analysis and text
improvements. Version to appear in EPJ
Bulk viscous cosmology with causal transport theory
We consider cosmological scenarios originating from a single imperfect fluid
with bulk viscosity and apply Eckart's and both the full and the truncated
M\"uller-Israel-Stewart's theories as descriptions of the non-equilibrium
processes. Our principal objective is to investigate if the dynamical
properties of Dark Matter and Dark Energy can be described by a single viscous
fluid and how such description changes when a causal theory
(M\"uller-Israel-Stewart's, both in its full and truncated forms) is taken into
account instead of Eckart's non-causal theory. To this purpose, we find
numerical solutions for the gravitational potential and compare its behaviour
with the corresponding LambdaCDM case. Eckart's and the full causal theory seem
to be disfavoured, whereas the truncated theory leads to results similar to
those of the LambdaCDM model for a bulk viscous speed in the interval 10^{-11}
<< c_b^2 < 10^{-8}. Tentatively relating such value to a square propagation
velocity of the order of T/m of perturbations in a non-relativistic gas of
particles with mass m at the epoch of matter-radiation equality, this may be
compatible with a mass range 0.1 GeV < m << 100 GeV.Comment: 23 pages, 7 figure
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
Increasing the length of EM-9 interstock enhances production efficiency in Imperial Gala apples.
The objective of this study was to evaluate the influence of different lengths of EM-9 interstock, on the production and fruit characteristics of Imperial Gala apples more than eight years old. This experiment was conducted in a commercial orchard located in Vacaria-RS, Brazil, situated at an altitude of 955 m , during seasons 2005/06, 2007/08, 2010/11, 2011/12 and 2012/13. The treatments consisted of five lengths of EM-9 interstock (10, 15, 20, 25 and 30 cm) connecting the Marubakaido rootstock to the Imperial Gala scion. For production efficiency a positive correlation between increased efficiency and the length of interstock was observed. Likewise a positive correlation was also noted for quality parameters of the fruit; firmness and classification (Category 1). From this study we conclude that a EM-9 interstock of 30 cm on Marubakaido rootstock is the most suitable for the vigor control of Imperial Gala apples, it ensures greater production efficiency and firmer fruit
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Pattern breaking: a complex systems approach to psychedelic medicine
Recent research has demonstrated the potential of psychedelic therapy for mental health care. However, the psychological experience underlying its therapeutic effects remains poorly understood. This paper proposes a framework that suggests psychedelics act as destabilizers, both psychologically and neurophysiologically. Drawing on the ‘entropic brain’ hypothesis and the ‘RElaxed Beliefs Under pSychedelics’ model, this paper focuses on the richness of psychological experience. Through a complex systems theory perspective, we suggest that psychedelics destabilize fixed points or attractors, breaking reinforced patterns of thinking and behaving. Our approach explains how psychedelic-induced increases in brain entropy destabilize neurophysiological set points and lead to new conceptualizations of psychedelic psychotherapy. These insights have important implications for risk mitigation and treatment optimization in psychedelic medicine, both during the peak psychedelic experience and during the subacute period of potential recovery
Effects of differential mobility on biased diffusion of two species
Using simulations and a simple mean-field theory, we investigate jamming
transitions in a two-species lattice gas under non-equilibrium steady-state
conditions. The two types of particles diffuse with different mobilities on a
square lattice, subject to an excluded volume constraint and biased in opposite
directions. Varying filling fraction, differential mobility, and drive, we map
out the phase diagram, identifying first order and continuous transitions
between a free-flowing disordered and a spatially inhomogeneous jammed phase.
Ordered structures are observed to drift, with a characteristic velocity, in
the direction of the more mobile species.Comment: 15 pages, 4 figure
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