30,724 research outputs found
Diffusion coefficients for multi-step persistent random walks on lattices
We calculate the diffusion coefficients of persistent random walks on
lattices, where the direction of a walker at a given step depends on the memory
of a certain number of previous steps. In particular, we describe a simple
method which enables us to obtain explicit expressions for the diffusion
coefficients of walks with two-step memory on different classes of one-, two-
and higher-dimensional lattices.Comment: 27 pages, 2 figure
Persistence of coherent quantum dynamics at strong dissipation
The quantum dynamics of a two state system coupled to a bosonic reservoir
with sub-Ohmic spectral density is investigated for strong friction.
Numerically exact path integral Monte Carlo methods reveal that in contrast to
conventional expectations, coherent dynamics never turns into incoherent decay
for a broad class of spectral distributions. Coherences associated with
substantial system-reservoir entanglement exist in non-equilibrium even when
strong dissipation makes the thermodynamic state of the system to behave
essentially classical. This may be of relevance for current experiments with
nanoscale devices and bio-molecular aggregates.Comment: 5 pages, 5 figure
A test of "fluctuation theorem" in non-Markovian open quantum systems
We study fluctuation theorems for open quantum systems with a non-Markovian
heat bath using the approach of quantum master equations and examine the
physical quantities that appear in those fluctuation theorems. The approach of
Markovian quantum master equations to the fluctuation theorems was developed by
Esposito and Mukamel [Phys. Rev. E {\bf73}, 046129 (2006)]. We show that their
discussion can be formally generalized to the case of a non-Markovian heat bath
when the local system is linearly connected to a Gaussian heat bath with the
spectrum distribution of the Drude form. We found by numerically simulating the
spin-boson model in non-Markovian regime that the "detailed balance" condition
is well satisfied except in a strongly non-equilibrium transient situation, and
hence our generalization of the definition of the "entropy production" is
almost always legitimate. Therefore, our generalization of the fluctuation
theorem seems meaningful in wide regions.Comment: 21 pages, 5 figure
Impaired Intracellular Transport and Cell Surface Expression of Nonpolymorphic HLA-E
The assembly of the classical, polymorphic major histocompatibility complex class I molecules
in the endoplasmic reticulum requires the presence of peptide ligands and ~2-microglobulin
(~2m). Formation of this trimolecular complex is a prerequisite for e~cient transport to the
cell surface, where presented peptides are scanned by T lymphocytes. The function of the other
class I molecules is in dispute. The human, nonclassical class I gene, HLA-E, was found to be
ubiquitously transcribed, whereas cell surface expression was dif~cult to detect upon transfection.
Pulse chase experiments revealed that the HLA-E heavy chain in transfectants, obtained with
the murine myeloma cell line P3X63-Ag8.653 (X63), displays a significant reduction in
oligosaccharide maturation and intracellular transport compared with HLA-B27 in corresponding
transfectants. The accordingly low HLA-E cell surface expression could be significantly enhanced
by either reducing the culture temperature or by supplementing the medium with human ~2m,
suggesting inefficient binding of endogenous peptides to HLA-E. To analyze whether HLA-E
binds peptides and to identify the corresponding ligands, fractions of acid-extracted material from
HLA-E/X63 transfectants were separated by reverse phase HPLC and were tested for their ability
to enhance HLA-E cell surface expression. Two fractions specifically increased the HLA class
I expression on the HLA-E transfectant clone
Fighting Decoherence by Feedback-controlled Dissipation
Repeated closed-loop control operations acting as piecewise-constant
Liouville superoperators conditioned on the outcomes of regularly performed
measurements may effectively be described by a fixed-point iteration for the
density matrix. Even when all Liouville superoperators point to the completely
mixed state, feedback of the measurement result may lead to a pure state, which
can be interpreted as selective dampening of undesired states. Using a
microscopic model, we exemplify this for a single qubit, which can be purified
in an arbitrary single-qubit state by tuning the measurement direction and two
qubits that may be purified towards a Bell state by applying a special
continuous two-local measurement. The method does not require precise knowledge
of decoherence channels and works for large reservoir temperatures provided
measurement, processing, and control can be implemented in a continuous
fashion.Comment: to appear in PR
Coherent and incoherent dynamics in excitonic energy transfer: correlated fluctuations and off-resonance effects
We study the nature of the energy transfer process within a pair of coupled
two-level systems (donor and acceptor) subject to interactions with the
surrounding environment. Going beyond a standard weak-coupling approach, we
derive a master equation within the polaron representation that allows for
investigation of both weak and strong system-bath couplings, as well as
reliable interpolation between these two limits. With this theory, we are then
able to explore both coherent and incoherent regimes of energy transfer within
the donor-acceptor pair. We elucidate how the degree of correlation in the
donor and acceptor fluctuations, the donor-acceptor energy mismatch, and the
range of the environment frequency distribution impact upon the energy transfer
dynamics. In the resonant case (no energy mismatch) we describe in detail how a
crossover from coherent to incoherent transfer dynamics occurs with increasing
temperature [A. Nazir, Phys. Rev. Lett. 103, 146404 (2009)], and we also
explore how fluctuation correlations are able to protect coherence in the
energy transfer process. We show that a strict crossover criterion is harder to
define when off-resonance, though we find qualitatively similar population
dynamics to the resonant case with increasing temperature, while the amplitude
of coherent population oscillations also becomes suppressed with growing site
energy mismatch.Comment: 14 pages, 7 figures, builds upon PRL 103, 146404 (2009)
(arXiv:0906.0592). Comments welcome. V2 - Section IV shortened to improve
presentation, references updated, new Imperial College affiliation added for
A. Nazir. Published versio
Allelic variation in HLA-B and HLA-C sequences and the evolution of the HLA-B alleles
Several new HLA-B (B8, B51, Bw62)- and
HLA-C (Cw6, Cw7)-specific genes were isolated either as
genomic cosmid or cDNA clones to study the diversity
of HLA antigens. The allele specificities were identified
by sequence analysis in comparison with published HLAB
and -C sequences, by transfection experiments, and
Southern and northern blot analysis using oligonucleotide
probes. Comparison of the classical HLA-A, -B, and -C
sequences reveals that allele-specific substitutions seem
to be rare events. HLA-B51 codes only for one allelespecific
residue: arginine at position 81 located on the cd
helix, pointing toward the antigen binding site. HLA-B8
contains an acidic substitution in amino acid position 9
on the first central/3 sheet which might affect antigen binding
capacity, perhaps in combination with the rare
replacement at position 67 (F) on the Alpha-l helix. HLA-B8
shows greatest homology to HLA-Bw42, -Bw41, -B7, and
-Bw60 antigens, all of which lack the conserved restriction
sites Pst I at position 180 and Sac I at position 131.
Both sites associated with amino acid replacements seem
to be genetic markers of an evolutionary split of the HLA-B
alleles, which is also observed in the leader sequences.
HLA-Cw7 shows 98% sequence identity to the JY328
gene. In general, the HLA-C alleles display lower levels
of variability in the highly polymorphic regions of the Alpha 1
and Alpha 2 domains, and have more distinct patterns of locusspecific
residues in the transmembrane and cytoplasmic
domains. Thus we propose a more recent origin for the
HLA-C locus
Reaction of the chick to one atmosphere of oxygen
Experiment to determine chicken reaction to 100 percent oxygen at atmospheric pressur
Embryo development and chick growth in a helium - oxygen atmosphere
Embryo development and chick growth in helium- oxygen atmospher
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