3,172 research outputs found
Biophysical modelling of a drosophila photoreceptor
It remains unclear how visual information is co-processed
by different layers of neurons in the retina. In particular, relatively little is known how retina translates vast environmental light changes into
neural responses of limited range. We began examining this question in a bottom-up way in a relatively simple °y eye. To gain understanding of how complex bio-molecular interactions govern the conversion of light input into voltage output (phototransduction), we are building a
biophysical model of the Drosophila R1-R6 photoreceptor. Our model, which relates molecular dynamics of the underlying biochemical reactions to external light input, attempts to capture the molecular dynamics of
phototransduction gain control in a quantitative way
The Past Has Ears (PHE): XR Explorations of Acoustic Spaces as Cultural Heritage
Hearing is one of our most pervasive senses. There is no
equivalent to closing our eyes, or averting our gaze, for the ears. When
we think about great architectural achievements in European history,
such as ancient amphitheatres or Gothic cathedrals, their importance
is strongly tied to their acoustic environment. The acoustics of a heritage
site is an intangible consequence of the space's tangible construction
and furnishings. Inspired by the project's namesake (Phe, for the
constellation Phoenix), and the relatively recent res at Cathedrale de
Notre Dame de Paris and Teatro La Fenice opera hall, the PHE project
focuses on virtual reconstruction of heritage sites, bringing them back
from the ashes. In addressing the intangible acoustic heritage of architectural
sites, three main objectives have been identied for this research
project: Documentation, Modelling, and Presentation. In parallel, three
heritage sites are participating as case studies: Tindari Theatre (IT),
Notre-Dame de Paris Cathedral (FR), and The Houses of Parliament
(UK). The acoustics of a space is immersive, spatial, and due to the nature
of auditory perception egocentric, in contrast to visual perception
of an object, which can be observed from outside". Consequently, presentation
methods for communicating acoustic heritage must represent
the spatially immersive and listener-centric nature of acoustics. PHE will
lead development of a museum grade hardware/software prototype for
the presentation of immersive audio experiences adaptable to multiple
platforms, from on-site immersive speaker installations, to mobile XR
via smartphone applications
Hooge's Constant of Carbon Nanotube Field Effect Transistors
The 1/f noise in individual semiconducting carbon nanotubes (s-CNT) in a
field effect transistor configuration has been measured in ultra-high vacuum
and following exposure to air. The amplitude of the normalized current spectral
noise density is independent of source-drain current, indicating the noise is
due to mobility rather than number fluctuations. Hooge's constant for s-CNT is
found to be 9.3 plus minus 0.4x10^-3. The magnitude of the 1/f noise is
substantially degreased by exposing the devices to air
Curvaton Potential Terms, Scale-Dependent Perturbation Spectra and Chaotic Initial Conditions
The curvaton scenario predicts an almost scale-invariant spectrum of
perturbations in most inflation models. We consider the possibility that
renormalisable phi^4 or Planck scale-suppressed non-renormalisable curvaton
potential terms may result in an observable deviation from scale-invariance. We
show that if the curvaton initially has a large amplitude and if the total
number of e-foldings of inflation is less than about 300 then a running blue
perturbation spectrum with an observable deviation from scale-invariance is
likely. D-term inflation is considered as an example with a potentially low
total number of e-foldings of inflation. A secondary role for the curvaton, in
which it drives a period of chaotic inflation leading to D-term or other flat
potential inflation from an initially chaotic state, is suggested.Comment: 12 pages LaTeX, minor corrections, to be published in JCA
Resonant decay of flat directions
We study preheating, i.e., non-perturbative resonant decay, of flat direction
fields, concentrating on MSSM flat directions and the right handed sneutrino.
The difference between inflaton preheating and flaton preheating, is that the
potential is more constraint in the latter case. The effects of a complex
driving field, quartic couplings in the potential, and the presence of a
thermal bath are important and cannot be neglected.
Preheating of MSSM flat directions is typically delayed due to out-of-phase
oscillations of the real and imaginary components and may be preceded by
perturbative decay or -ball formation. Particle production due to the
violation of adiabaticity is expected to be inefficient due to back reaction
effects. For a small initial sneutrino VEV, with
the mass of the right handed sneutrino and a yakawa coupling, there are
tachyonic instabilities. The -term quartic couplings do not generate an
effective mass for the tachyonic modes, making it an efficient decay channel.
It is unclear how thermal scattering affects the resonance.Comment: 20 pages, 4 figure
Quantized Adiabatic Charge Transport in a Carbon Nanotube
The coupling of a metallic Carbon nanotube to a surface acoustic wave (SAW)
is proposed as a vehicle to realize quantized adiabatic charge transport in a
Luttinger liquid system. We demonstrate that electron backscattering by a
periodic SAW potential, which results in miniband formation, can be achieved at
energies near the Fermi level. Electron interaction, treated in a Luttinger
liquid framework, is shown to enhance minigaps and thereby improve current
quantization. Quantized SAW induced current, as a function of electron density,
changes sign at half-filling.Comment: 5 pages, 2 figure
Technical Note: Mesocosm approach to quantify dissolved inorganic carbon percolation fluxes
Dissolved inorganic carbon (DIC) fluxes across the vadose zone are
influenced by a complex interplay of biological, chemical and physical
factors. A novel soil mesocosm system was evaluated as a tool for providing
information on the mechanisms behind DIC percolation to the groundwater from
unplanted soil. Carbon dioxide partial pressure (<i>p</i>CO<sub>2</sub>), alkalinity, soil
moisture and temperature were measured with depth and time, and DIC in the
percolate was quantified using a sodium hydroxide trap. Results showed good
reproducibility between two replicate mesocosms. The <i>p</i>CO<sub>2</sub> varied between
0.2 and 1.1%, and the alkalinity was 0.1–0.6 meq L<sup>−1</sup>. The measured cumulative
effluent DIC flux over the 78-day experimental period was 185–196 mg L<sup>−1</sup> m<sup>−2</sup> and in the same range as estimates derived from
<i>p</i>CO<sub>2</sub> and alkalinity in samples extracted from the side of the mesocosm
column and the drainage flux. Our results indicate that the mesocosm system
is a promising tool for studying DIC percolation fluxes and other
biogeochemical transport processes in unsaturated environments
Aspects of warm-flat directions
Considering the mechanism of dissipative slow-roll that has been used in warm
inflation scenario, we show that dissipation may alter usual cosmological
scenarios associated with SUSY-flat directions. We mainly consider SUSY-flat
directions that have strong interactions with non-flat directions and may cause
strong dissipation both in thermal and non-thermal backgrounds. An example is
the Affleck-Dine mechanism in which dissipation may create significant (both
qualitative and quantitative) discrepancies between the conventional scenario
and the dissipative one. We also discuss several mechanisms of generating
curvature perturbations in which the dissipative field, which is distinguished
from the inflaton field, can be used as the source of cosmological
perturbations. Considering the Morikawa-Sasaki dissipative coefficient, the
damping caused by the dissipation may be significant for many MSSM flat
directions even if the dissipation is far from thermal equilibrium.Comment: 22 pages, accepted for publication in International Journal of Modern
Physics
Identifying the curvaton within MSSM
We consider inflaton couplings to MSSM flat directions and the thermalization
of the inflaton decay products, taking into account gauge symmetry breaking due
to flat direction condensates. We then search for a suitable curvaton candidate
among the flat directions, requiring an early thermally induced start for the
flat direction oscillations to facilitate the necessary curvaton energy density
dominance. We demonstrate that the supersymmetry breaking -term is crucial
for achieving a successful curvaton scenario. Among the many possible
candidates, we identify the flat direction as a viable MSSM
curvaton.Comment: 9 pages. Discussion on the evaporation of condensate added, final
version published in JCA
Interference effects in electronic transport through metallic single-wall carbon nanotubes
In a recent paper Liang {\it et al.} [Nature {\bf 411}, 665 (2001)] showed
experimentally, that metallic nanotubes, strongly coupled to external
electrodes, may act as coherent molecular waveguides for electronic transport.
The experimental results were supported by theoretical analysis based on the
scattering matrix approach. In this paper we analyze theoretically this problem
using a real-space approach, which makes it possible to control quality of
interface contacts. Electronic structure of the nanotube is taken into account
within the tight-binding model. External electrodes and the central part
(sample) are assumed to be made of carbon nanotubes, while the contacts between
electrodes and the sample are modeled by appropriate on-site (diagonal) and
hopping (off-diagonal) parameters. Conductance is calculated by the Green
function technique combined with the Landauer formalism. In the plots
displaying conductance {\it vs.} bias and gate voltages, we have found typical
diamond structure patterns, similar to those observed experimentally. In
certain cases, however, we have found new features in the patterns, like a
double-diamond sub-structure.Comment: 15 pages, 4 figures. To apear in Phys. Rev.
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