743 research outputs found
Non-trivial \theta-Vacuum Effects in the 2-d O(3) Model
We study \theta-vacua in the 2-d lattice O(3) model using the standard action
and an optimized constraint action with very small cut-off effects, combined
with the geometric topological charge. Remarkably, dislocation lattice
artifacts do not spoil the non-trivial continuum limit at \theta\ non-zero, and
there are different continuum theories for each value of \theta. A very precise
Monte Carlo study of the step scaling function indirectly confirms the exact
S-matrix of the 2-d O(3) model at \theta = \pi.Comment: 4 pages, 3 figure
The Freezing of Random RNA
We study secondary structures of random RNA molecules by means of a
renormalized field theory based on an expansion in the sequence disorder. We
show that there is a continuous phase transition from a molten phase at higher
temperatures to a low-temperature glass phase. The primary freezing occurs
above the critical temperature, with local islands of stable folds forming
within the molten phase. The size of these islands defines the correlation
length of the transition. Our results include critical exponents at the
transition and in the glass phase.Comment: 4 pages, 8 figures. v2: presentation improve
Study of theta-Vacua in the 2-d O(3) Model
We investigate the continuum limit of the step scaling function in the 2-d
O(3) model with different theta-vacua. Since we find a different continuum
value of the step scaling function for each value of theta, we can conclude
that theta indeed is a relevant parameter of the theory and does not get
renormalized non-perturbatively. Furthermore, we confirm the result of the
conjectured exact S-matrix theory, which predicts the continuum value at theta
= pi. To obtain high precision data, we use a modified Hasenbusch improved
estimator and an action with an optimized constraint, which has very small
cut-off effects. The optimized constraint action combines the standard action
of the 2-d O(3) model with a topological action. The topological action
constrains the angle between neighboring spins and is therefore invariant
against small deformations of the field.Comment: 7 pages, 4 figures, The 30 International Symposium on Lattice Field
Theory - Lattice 2012, June 24-29, 2012, Cairns, Australi
Congregational Enrollment Support for the Denominational Institution of Higher Education: A Local-Cosmopolitan Cultural Analysis
Quantification of leaflet flutter in bioprosthetic heart valves using fluid-structure interaction analysis
Many studies have indicated that leaflet fluttering and associated bending in biopros-thetic heart valves (BHVs) is an important criterion in determining the durability of BHVimplants. In this thesis work, a computational methodology for the flutter quantificationof BHV leaflets is presented using an immersogeometric fluid–structure interaction (FSI)framework. The proposed approach is based upon displacement tracking of the BHV leafletfree edges. Integrating over the discrete Fourier transform of free edge displacement data,the energy spectral density is computed for a measure of leaflet flutter. This methodologyseeks to improve approaches used in experimental flutter quantification through utiliza-tion of highly accurate simulation solutions and visualizations to capture a measurement ofleaflet flutter. A set of sampling cases with varying valve material thickness are generatedand FSI-based flutter quantification is performed to investigate the effect of leaflet materialthickness on the presence of flutter and bending in BHVs
Description of an aeronautical geometry conversion package: Wave-drag to Langley Wireframe Geometry Standard (LaWGS) to Supersonic Implicit Marching Potential (SIMP)
Documented is an aeronautical geometry conversion package which translates wave-drag geometry into the Langley Wireframe Geometry Standard (LaWGS) format and then into a format which is used by the Supersonic Implicit Marching Potential (SIMP) program. The programs described were developed by Computer Sciences Corporation for the Advanced Vehicles Division/Advanced Concepts Branch at NASA Langley Research Center. Included also are the input and output from a benchmark test case
Astrocytes as a Source for Extracellular Matrix Molecules and Cytokines
Research of the past 25 years has shown that astrocytes do more than participating and building up the blood-brain barrier and detoxify the active synapse by reuptake of neurotransmitters and ions. Indeed, astrocytes express neurotransmitter receptors and, as a consequence, respond to stimuli. Within the tripartite synapse, the astrocytes owe more and more importance. Besides the functional aspects the differentiation of astrocytes has gained a more intensive focus. Deeper knowledge of the differentiation processes during development of the central nervous system might help explaining and even help treating neurological diseases like Alzheimer’s disease, Amyotrophic lateral sclerosis, Parkinsons disease, and psychiatric disorders in which astrocytes have been shown to play a role. Specific differentiation of neural stem cells toward the astroglial lineage is performed as a multi-step process. Astrocytes and oligodendrocytes develop from a multipotent stem cell that prior to this has produced primarily neuronal precursor cells. This switch toward the more astroglial differentiation is regulated by a change in receptor composition on the cell surface and responsiveness to Fibroblast growth factor and Epidermal growth factor (EGF). The glial precursor cell is driven into the astroglial direction by signaling molecules like Ciliary neurotrophic factor, Bone Morphogenetic Proteins, and EGF. However, the early astrocytes influence their environment not only by releasing and responding to diverse soluble factors but also express a wide range of extracellular matrix (ECM) molecules, in particular proteoglycans of the lectican family and tenascins. Lately these ECM molecules have been shown to participate in glial development. In this regard, especially the matrix protein Tenascin C (Tnc) proved to be an important regulator of astrocyte precursor cell proliferation and migration during spinal cord development. Nevertheless, ECM molecules expressed by reactive astrocytes are also known to act mostly in an inhibitory fashion under pathophysiological conditions. Thus, we further summarize resent data concerning the role of chondroitin sulfate proteoglycans and Tnc under pathological conditions
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