105 research outputs found
Simple Model for the Variation of Superfluid Density with Zn Concentration in YBCO
We describe a simple model for calculating the zero-temperature superfluid
density of Zn-doped YBa_2Cu_3O_{7-\delta} as a function of the fraction x of
in-plane Cu atoms which are replaced by Zn. The basis of the calculation is a
``Swiss cheese'' picture of a single CuO_2 layer, in which a substitutional Zn
impurity creates a normal region of area around it as
originally suggested by Nachumi et al. Here is the zero-temperature
in-plane coherence length at x = 0. We use this picture to calculate the
variation of the in-plane superfluid density with x at temperature T = 0, using
both a numerical approach and an analytical approximation. For ,
if we use the value = 18.3 angstrom, we find that the in-plane
superfluid decreases with increasing x and vanishes near in the
analytical approximation, and near in the numerical approach.
is quite sensitive to , whose value is not widely agreed upon.
The model also predicts a peak in the real part of the conductivity,
Re, at concentrations , and low frequencies,
and a variation of critical current density with x of the form near percolation, where is the in-plane
superfluid density.Comment: 19 pages including 6 figures, submitted to Physica
Quiet SDS Josephson Junctions for Quantum Computing
Unconventional superconductors exhibit an order parameter symmetry lower than
the symmetry of the underlying crystal lattice. Recent phase sensitive
experiments on YBCO single crystals have established the d-wave nature of the
cuprate materials, thus identifying unambiguously the first unconventional
superconductor. The sign change in the order parameter can be exploited to
construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting
a degenerate ground state and a double-periodic current-phase characteristic.
Here we discuss how to make use of these special junction characteristics in
the construction of a quantum computer. Combining such junctions together with
a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit
--- a solid state implementation of a quantum bit which remains optimally
isolated from its environment.Comment: 4 pages, 2 ps-figure
The pseudogap: friend or foe of high Tc?
Although nineteen years have passed since the discovery of high temperature
superconductivity, there is still no consensus on its physical origin. This is
in large part because of a lack of understanding of the state of matter out of
which the superconductivity arises. In optimally and underdoped materials, this
state exhibits a pseudogap at temperatures large compared to the
superconducting transition temperature. Although discovered only three years
after the pioneering work of Bednorz and Muller, the physical origin of this
pseudogap behavior and whether it constitutes a distinct phase of matter is
still shrouded in mystery. In the summer of 2004, a band of physicists gathered
for five weeks at the Aspen Center for Physics to discuss the pseudogap. In
this perspective, we would like to summarize some of the results presented
there and discuss its importance in the context of strongly correlated electron
systems.Comment: expanded version, 20 pages, 11 figures, to be published, Advances in
Physic
Excitation energy transfer in native and unstacked thylakoid membranes studied by low temperature and ultrafast fluorescence spectroscopy
In this work, the transfer of excitation energy was studied in native and cation-depletion induced, unstacked thylakoid membranes of spinach by steady-state and time-resolved fluorescence spectroscopy. Fluorescence emission spectra at 5 K show an increase in photosystem I (PSI) emission upon unstacking, which suggests an increase of its antenna size. Fluorescence excitation measurements at 77 K indicate that the increase of PSI emission upon unstacking is caused both by a direct spillover from the photosystem II (PSII) core antenna and by a functional association of light-harvesting complex II (LHCII) to PSI, which is most likely caused by the formation of LHCII-LHCI-PSI supercomplexes. Time-resolved fluorescence measurements, both at room temperature and at 77 K, reveal differences in the fluorescence decay kinetics of stacked and unstacked membranes. Energy transfer between LHCII and PSI is observed to take place within 25 ps at room temperature and within 38 ps at 77 K, consistent with the formation of LHCII-LHCI-PSI supercomplexes. At the 150-160 ps timescale, both energy transfer from LHCII to PSI as well as spillover from the core antenna of PSII to PSI is shown to occur at 77 K. At room temperature the spillover and energy transfer to PSI is less clear at the 150 ps timescale, because these processes compete with charge separation in the PSII reaction center, which also takes place at a timescale of about 150 ps. © 2007 Springer Science+Business Media B.V
Kinetochore fiber formation in animal somatic cells : dueling mechanisms come to a draw
Author Posting. © The Author, 2005. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Chromosoma 114 (2005): 310-318, doi:10.1007/s00412-005-0028-2.The attachment to and movement of a chromosome on the mitotic spindle is
mediated by the formation of a bundle of microtubules (MTs) that tethers the
kinetochore on the chromosome to a spindle pole. The origin of these âkinetochore
fibersâ (K-fibers) has been investigated for over 125 years. As noted in 1944 by
Schrader, there are only three possible ways to form a K-fiber: either it a) grows from
the pole until it contacts the kinetochore; b) grows directly from the kinetochore; or c)
it forms as a result of an interaction between the pole and the chromosome. Since
Schraderâs time it has been firmly established that K-fibers in centrosome-containing
animal somatic cells form as kinetochores capture MTs growing from the spindle pole
(route a). It is now similarly clear that in cells lacking centrosomes, including plants
and many animal oocytes, K-fibers âself-assembleâ from MTs generated by the
chromosomes (route b). Can animal somatic cells form K-fibers in the absence of
centrosomes by the âself-assemblyâ pathway? In 2000 the answer to this question
was shown to be a resounding âyesâ. With this result, the next question became
whether the presence of a centrosome normally suppresses K-fiber self-assembly, or
if this route works concurrently with centrosome-mediated K-fiber formation. This
question, too, has recently been answered: observations on untreated live animal cells
expressing GFP-tagged tubulin clearly show that kinetochores can nucleate the
formation of their associated MTs in the presence of functional centrosomes. The
concurrent operation of these two âduelingâ routes for forming K-fibers in animals
helps explain why the attachment of kinetochores and the maturation of K-fibers
occur as quickly as it does on all chromosomes within a cell.The work is sponsored by
NIH grant GMS 40198
Barcoding T Cell Calcium Response Diversity with Methods for Automated and Accurate Analysis of Cell Signals (MAAACS)
International audienceWe introduce a series of experimental procedures enabling sensitive calcium monitoring in T cell populations by confocal video-microscopy. Tracking and post-acquisition analysis was performed using Methods for Automated and Accurate Analysis of Cell Signals (MAAACS), a fully customized program that associates a high throughput tracking algorithm, an intuitive reconnection routine and a statistical platform to provide, at a glance, the calcium barcode of a population of individual T-cells. Combined with a sensitive calcium probe, this method allowed us to unravel the heterogeneity in shape and intensity of the calcium response in T cell populations and especially in naive T cells, which display intracellular calcium oscillations upon stimulation by antigen presenting cells
Cell-signalling dynamics in time and space
The specificity of cellular responses to receptor stimulation is encoded by the spatial and temporal dynamics of downstream signalling networks. Computational models provide insights into the intricate relationships between stimuli and responses and reveal mechanisms that enable networks to amplify signals, reduce noise and generate discontinuous bistable dynamics or oscillations. These temporal dynamics are coupled to precipitous spatial gradients of signalling activities, which guide pivotal intracellular processes, but also necessitate mechanisms to facilitate signal propagation across a cell
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