3,984 research outputs found
Coherent Destruction of Coulomb Blockade Peaks in Molecular Junctions
Coherent electronic transport in single-molecule junctions is investigated in
the Coulomb blockade regime. Both the transmission phase and probability are
calculated for junctions with various contact symmetries. A dramatic
suppression of the Coulomb blockade peaks is predicted for junctions where
multiple atomic orbitals of the molecule couple to a single electrode although
the charging steps are unaffected.Comment: 6 pages, 4 figure
Stability of Metal Nanowires at Ultrahigh Current Densities
We develop a generalized grand canonical potential for the ballistic
nonequilibrium electron distribution in a metal nanowire with a finite applied
bias voltage. Coulomb interactions are treated in the self-consistent Hartree
approximation, in order to ensure gauge invariance. Using this formalism, we
investigate the stability and cohesive properties of metallic nanocylinders at
ultrahigh current densities. A linear stability analysis shows that metal
nanowires with certain {\em magic conductance values} can support current
densities up to 10^11 A/cm^2, which would vaporize a macroscopic piece of
metal. This finding is consistent with experimental studies of gold nanowires.
Interestingly, our analysis also reveals the existence of reentrant stability
zones--geometries that are stable only under an applied bias.Comment: 12 pages, 6 figures, version published in PR
Circadian and Ultradian Rhythms of Free Glucocorticoid Hormone Are Highly Synchronized between the Blood, the Subcutaneous Tissue, and the Brain
Total glucocorticoid hormone levels in plasma of various species, including humans, follow a circadian rhythm that is made up from an underlying series of hormone pulses. In blood most of the glucocorticoid is bound to corticosteroid-binding globulin and albumin, resulting in low levels of free hormone. Although only the free fraction is biologically active, surprisingly little is known about the rhythms of free glucocorticoid hormones. We used single-probe microdialysis to measure directly the free corticosterone levels in the blood of freely behaving rats. Free corticosterone in the blood shows a distinct circadian and ultradian rhythm with a pulse frequency of approximately one pulse per hour together with an increase in hormone levels and pulse height toward the active phase of the light/dark cycle. Similar rhythms were also evident in the subcutaneous tissue, demonstrating that free corticosterone rhythms are transferred from the blood into peripheral target tissues. Furthermore, in a dual-probe microdialysis study, we demonstrated that the circadian and ultradian rhythms of free corticosterone in the blood and the subcutaneous tissue were highly synchronized. Moreover, free corticosterone rhythms were also synchronous between the blood and the hippocampus. These data demonstrate for the first time an ultradian rhythm of free corticosterone in the blood that translates into synchronized rhythms of free glucocorticoid hormone in peripheral and central tissues. The maintenance of ultradian rhythms across tissue barriers in both the periphery and the brain has important implications for research into aberrant biological rhythms in disease and for the development of improved protocols for glucocorticoid therapy
Simple model for decay of superdeformed nuclei
Recent theoretical investigations of the decay mechanism out of a
superdeformed nuclear band have yielded qualitatively different results,
depending on the relative values of the relevant decay widths. We present a
simple two-level model for the dynamics of the tunneling between the
superdeformed and normal-deformed bands, which treats decay and tunneling
processes on an equal footing. The previous theoretical results are shown to
correspond to coherent and incoherent limits of the full tunneling dynamics.
Our model accounts for experimental data in both the A~150 mass region, where
the tunneling dynamics is coherent, and in the A~190 mass region, where the
tunneling dynamics is incoherent.Comment: 4 page
Spaceflight modulates gene expression in the whole blood of astronauts
Astronauts are exposed to a unique combination of stressors during spaceflight, which leads to alterations in their physiology and potentially increases their susceptibility to disease, including infectious diseases. To evaluate the potential impact of the spaceflight environment on the regulation of molecular pathways mediating cellular stress responses, we performed a first-of-its-kind pilot study to assess spaceflight-related gene-expression changes in the whole blood of astronauts. Using an array comprised of 234 well-characterized stress-response genes, we profiled transcriptomic changes in six astronauts (four men and two women) from blood preserved before and immediately following the spaceflight. Differentially regulated transcripts included those important for DNA repair, oxidative stress, and protein folding/degradation, including HSP90AB1, HSP27, GPX1, XRCC1, BAG-1, HHR23A, FAP48, and C-FOS. No gender-specific differences or relationship to number of missions flown was observed. This study provides a first assessment of transcriptomic changes occurring in the whole blood of astronauts in response to spaceflight
A non-Hermitian critical point and the correlation length of strongly correlated quantum systems
We study a non-Hermitian generalization of quantum systems in which an
imaginary vector potential is added to the momentum operator. In the
tight-binding approximation, we make the hopping energy asymmetric in the
Hermitian Hamiltonian. In a previous article, we conjectured that the
non-Hermitian critical point where the energy gap vanishes is equal to the
inverse correlation length of the Hermitian system and we confirmed the
conjecture for two exactly solvable systems. In this article, we present more
evidence for the conjecture. We also argue the basis of our conjecture by
noting the dispersion relation of the elementary excitation.Comment: 25 pages, 18 figure
Coherent Resonant Tunneling Through an Artificial Molecule
Coherent resonant tunneling through an artificial molecule of quantum dots in
an inhomogeneous magnetic field is investigated using an extended Hubbard
model. Both the multiterminal conductance of an array of quantum dots and the
persistent current of a quantum dot molecule embedded in an Aharanov-Bohm ring
are calculated. The conductance and persistent current are calculated
analytically for the case of a double quantum dot and numerically for larger
arrays using a multi-terminal Breit-Wigner type formula, which allows for the
explicit inclusion of inelastic processes. Cotunneling corrections to the
persistent current are also investigated, and it is shown that the sign of the
persistent current on resonance may be used to determine the spin quantum
numbers of the ground state and low-lying excited states of an artificial
molecule. An inhomogeneous magnetic field is found to strongly suppress
transport due to pinning of the spin-density-wave ground state of the system,
and giant magnetoresistance is predicted to result from the ferromagnetic
transition induced by a uniform external magnetic field.Comment: 23 pages, 12 figure
Continuous-culture fermentation as a tool for forage evaluation
Ruminal degradation of organic matter and protein in alfalfa and prairie hay were evaluated in vivo, using cannulated cows, and in vitro, using a continuous-culture fermenter to simulate ruminal fermentation. Estimates of organic matter degradability, microbial N flow per unit feed N input, and efficiency of microbial growth were not different (P\u3e.10) between the in vivo and in vitro systems. However, for both forages, estimates of nitrogen degradability were greater with the in vitro system. Despite the differences between in vivo and in vitro techniques for some variables, continuous-culture fermentation will allow us to compare the effects of dietary treatments on forage digestion and will aid in the formulation of supplements to meet specific nutrient requirements for cattle consuming forage-based diets
The Order of Phase Transitions in Barrier Crossing
A spatially extended classical system with metastable states subject to weak
spatiotemporal noise can exhibit a transition in its activation behavior when
one or more external parameters are varied. Depending on the potential, the
transition can be first or second-order, but there exists no systematic theory
of the relation between the order of the transition and the shape of the
potential barrier. In this paper, we address that question in detail for a
general class of systems whose order parameter is describable by a classical
field that can vary both in space and time, and whose zero-noise dynamics are
governed by a smooth polynomial potential. We show that a quartic potential
barrier can only have second-order transitions, confirming an earlier
conjecture [1]. We then derive, through a combination of analytical and
numerical arguments, both necessary conditions and sufficient conditions to
have a first-order vs. a second-order transition in noise-induced activation
behavior, for a large class of systems with smooth polynomial potentials of
arbitrary order. We find in particular that the order of the transition is
especially sensitive to the potential behavior near the top of the barrier.Comment: 8 pages, 6 figures with extended introduction and discussion; version
accepted for publication by Phys. Rev.
Quantitative and Dynamic Catalogs of Proteins Released during Apoptotic and Necroptotic Cell Death
The inflammatory functions of the cytokine tumor necrosis factor (TNF) rely on its ability to induce cytokine production and to induce cell death. Caspase-dependent and caspase-independent pathways-apoptosis and necroptosis, respectively-regulate immunogenicity by the release of distinct sets of cellular proteins. To obtain an unbiased, systems-level understanding of this important process, we here applied mass spectrometry-based proteomics to dissect protein release during apoptosis and necroptosis. We report hundreds of proteins released from human myeloid cells in time course experiments. Both cell death types induce receptor shedding, but only apoptotic cells released nucleosome components. Conversely, necroptotic cells release lysosomal components by activating lysosomal exocytosis at early stages of necroptosis-induced membrane permeabilization and show reduced release of conventionally secreted cytokines
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