544 research outputs found
A sequence motif responsible for ER export and surface expression of Kir2.0 inward rectifier K+ channels
AbstractIntegral membrane proteins are sorted via the secretory pathway. It was proposed that this pathway is non-selective provided that the cargo protein is properly assembled and lacks an endoplasmic reticulum (ER) retention signal. However, recent experimental evidence suggests that efficient export of proteins from the ER to the Golgi complex is not simply a default pathway. Here we demonstrate a novel sequence motif (FxYENEV) in the cytoplasmic C-terminus of mammalian inward rectifier potassium (Kir) channels which determines ER export. This motif is found to be both necessary and sufficient for efficient export from the ER that eventually leads to efficient surface expression of Kir2.1 channels
Molecular states in a one-electron double quantum dot
The transport spectrum of a strongly tunnel-coupled one-electron double
quantum dot electrostatically defined in a GaAs/AlGaAs heterostructure is
studied. At finite source-drain-voltage we demonstrate the unambiguous
identification of the symmetric ground state and the antisymmetric excited
state of the double well potential by means of differential conductance
measurements. A sizable magnetic field, perpendicular to the two-dimensional
electron gas, reduces the extent of the electronic wave-function and thereby
decreases the tunnel coupling. A perpendicular magnetic field also modulates
the orbital excitation energies in each individual dot. By additionally tuning
the asymmetry of the double well potential we can align the chemical potentials
of an excited state of one of the quantum dots and the ground state of the
other quantum dot. This results in a second anticrossing with a much larger
tunnel splitting than the anticrossing involving the two electronic ground
states.Comment: 4 pages, 4 figures; EP2DS-16 conference contributio
Full Relativistic Electronic Structure and Fermi Surface Sheets of the First Honeycomb-Lattice Pnictide Superconductor SrPtAs
We report full-potential density functional theory (DFT)-based {\it ab
initio} band structure calculations to investigate electronic structure
properties of the first pnictide superconductor with a honeycomb-lattice
structure: SrPtAs. As a result, electronic bands, density of states, Fermi
velocities and the topology of the Fermi surface for SrPtAs are obtained. These
quantities are discussed in comparison to the first available experimental
data. Predictions for future measurements are provided
Nonequilibrium phenomena in adjacent electrically isolated nanostructures
We report on nonequilibrium interaction phenomena between adjacent but
electrostatically separated nanostructures in GaAs. A current flowing in one
externally biased nanostructure causes an excitation of electrons in a circuit
of a second nanostructure. As a result we observe a dc current generated in the
unbiased second nanostructure. The results can be qualitatively explained in
terms of acoustic phonon based energy transfer between the two mutually
isolated circuits.Comment: EP2DS-2007 proceedings; as publishe
Response, relaxation and transport in unconventional superconductors
We investigate the collision-limited electronic Raman response and the
attenuation of ultrasound in spin-singlet d-wave superconductors at low
temperatures. The dominating elastic collisions are treated within a t-matrix
approximation, which combines the description of weak (Born) and strong
(unitary) impurity scattering. In the long wavelength limit a two-fluid
description of both response and transport emerges. Collisions are here seen to
exclusively dominate the relaxational dynamics of the (Bogoliubov)
quasiparticle system and the analysis allows for a clear connection of response
and transport phenomena. When applied to quasi-2-d superconductors like the
cuprates, it turns out that the transport parameter associated with the Raman
scattering intensity for B1g and B2g photon polarization is closely related to
the corresponding components of the shear viscosity tensor, which dominates the
attenuation of ultrasound. At low temperatures we present analytic solutions of
the transport equations, resulting in a non-power-law behavior of the transport
parameters on temperature.Comment: 22 pages, 3 figure
Freezing transitions and the density of states of 2D random Dirac Hamiltonians
Using an exact mapping to disordered Coulomb gases, we introduce a novel
method to study two dimensional Dirac fermions with quenched disorder in two
dimensions which allows to treat non perturbative freezing phenomena. For
purely random gauge disorder it is known that the exact zero energy eigenstate
exhibits a freezing-like transition at a threshold value of disorder
. Here we compute the dynamical exponent which
characterizes the critical behaviour of the density of states around zero
energy, and find that it also exhibits a phase transition. Specifically, we
find that (and ) with for and
for . For a finite system size we find large
sample to sample fluctuations with a typical .
Adding a scalar random potential of small variance , as in the
corresponding quantum Hall system, yields a finite noncritical whose scaling exponent exhibits two transitions, one
at and the other at . These transitions are shown
to be related to the one of a directed polymer on a Cayley tree with random
signs (or complex) Boltzmann weights. Some observations are made for the strong
disorder regime relevant to describe transport in the quantum Hall system
Resonant dipole-dipole interaction in the presence of dispersing and absorbing surroundings
Within the framework of quantization of the macroscopic electromagnetic
field, equations of motion and an effective Hamiltonian for treating both the
resonant dipole-dipole interaction between two-level atoms and the resonant
atom-field interaction are derived, which can suitably be used for studying the
influence of arbitrary dispersing and absorbing material surroundings on these
interactions. The theory is applied to the study of the transient behavior of
two atoms that initially share a single excitation, with special emphasis on
the role of the two competing processes of virtual and real photon exchange in
the energy transfer between the atoms. In particular, it is shown that for weak
atom-field interaction there is a time window, where the energy transfer
follows a rate regime of the type obtained by ordinary second-order
perturbation theory. Finally, the resonant dipole-dipole interaction is shown
to give rise to a doublet spectrum of the emitted light for weak atom-field
interaction and a triplet spectrum for strong atom-field interaction.Comment: 15 pages, 1 figure, RevTE
Dimensional Crossover of Localisation and Delocalisation in a Quantum Hall Bar
The 2-- to 1--dimensional crossover of the localisation length of electrons
confined to a disordered quantum wire of finite width is studied in a
model of electrons moving in the potential of uncorrelated impurities. An
analytical formula for the localisation length is derived, describing the
dimensional crossover as function of width , conductance and
perpendicular magnetic field . On the basis of these results, the scaling
analysis of the quantum Hall effect in high Landau levels, and the
delocalisation transition in a quantum Hall wire are reconsidered.Comment: 12 pages, 7 figure
Can forest management based on natural disturbances maintain ecological resilience?
Given the increasingly global stresses on forests, many ecologists argue that managers must maintain ecological resilience: the capacity of ecosystems to absorb disturbances without undergoing fundamental change. In this review we ask: Can the emerging paradigm of natural-disturbance-based management (NDBM) maintain ecological resilience in managed forests? Applying resilience theory requires careful articulation of the ecosystem state under consideration, the disturbances and stresses that affect the persistence of possible alternative states, and the spatial and temporal scales of management relevance. Implementing NDBM while maintaining resilience means recognizing that (i) biodiversity is important for long-term ecosystem persistence, (ii) natural disturbances play a critical role as a generator of structural and compositional heterogeneity at multiple scales, and (iii) traditional management tends to produce forests more homogeneous than those disturbed naturally and increases the likelihood of unexpected catastrophic change by constraining variation of key environmental processes. NDBM may maintain resilience if silvicultural strategies retain the structures and processes that perpetuate desired states while reducing those that enhance resilience of undesirable states. Such strategies require an understanding of harvesting impacts on slow ecosystem processes, such as seed-bank or nutrient dynamics, which in the long term can lead to ecological surprises by altering the forest's capacity to reorganize after disturbance
Nutrient flux and budget in the Ebro estuary
The Ebro river flows to the Mediterranean coast of Spain. During its final stretch, the Ebro behaves in a similar way to a highly stratified estuary. This paper describes the transport of nutrients to the Ebro estuary, evaluates the general movement of nutrients in the estuarine region, using a mass balance approach, and estimates the amounts of nutrients discharged to the coastal environment. Given the strong saline stratification, this study only includes the surface layer that contains the continental freshwater. The annual nutrient budget for the Ebro estuary shows a net excess for nitrogen and phosphorus, while silicate almost attains equilibrium between addition and removal. There are several reasons for gains in nitrogen and phosphorous: a contribution of dissolved and particulate compounds in the freshwater (some of which are mineralized); a lower uptake of phytoplankton indicated by chlorophyll reduction in the estuary; an entrainment of the nutrient-rich upper part of the salt wedge; and, to a lesser extent, the impact of wastewater and agricultural water use. The biggest load discharged into the Mediterranean Sea by the Ebro is nitrogen, followed by silicate with over 10 000 tons of each deposited annually. Phosphorus is discharged at relatively low concentrations and with an annual load of about 200 t yr¿1.This project was funded by the European Union in the framework of the MAST-III research project: "Preparation and Integration of Analysis Tools towards Operational Forecast of Nutrients in Estuaries of European Rivers (PIONEER)", Reference No. MAS3-CT98-0170.Falco Giaccaglia, SL.; Niencheski, L.; Rodilla Alamá, M.; Romero Gil, I.; González Del Rio Rams, J.; Sierra, J.; Mösso, C. (2010). Nutrient flux and budget in the Ebro estuary. Estuarine, Coastal and Shelf Science. 87(1):92-102. doi:10.1016/j.ecss.2009.12.020S9210287
- …