45915 research outputs found
Sort by
Dynamic fragmentation of a spinor Bose-Einstein condensate
No full textWe investigate dynamical behavior of fragmentation of a spinor Bose-Einstein condensate in both the semiclassical and quantum regimes. We show that an unfragmented condensate can become fragmented at later times due to the inherent interactions of elastic atomic collisions, when a certain phase relationship exists among the state amplitudes in different components initially. A close relationship between the fragmentation and population dynamics of atomic distributions within different spin components is found for certain initial states with zero magnetization. The type of fragmentation regime is characterized via atom-number fluctuations. We also address complications of our study when the condensate spatial mode function is different for each spin component
Completely regular quantum stress tensor with w < -1
No full textFor many quantum field theory computations in cosmology it is not possible to use the flat space trick of obtaining full, interacting states by evolving free states over infinite times. State wave functionals must be specified at finite times and, although the free states suffice to obtain the lowest order effects, higher order corrections necessarily involve changes of the initial state. Failing to correctly change the initial state can result in effective field equations which diverge on the initial value surface, or which contain tedious sums of terms that redshift like inverse powers of the scale factor. In this paper we verify a conjecture from 2004 that the lowest order initial state correction can indeed absorb the initial value divergences and all the redshifting terms of the two-loop expectation value (in free, Bunch-Davies vacuum) of the stress tensor of a massless, minimally coupled scalar with a quartic self-interaction on a nondynamical de Sitter background
Optimal-control of laser-generated acoustic-waves in solids
No full textThis paper is concerned with the design of optimal surface-heating patterns that result in focusing acoustic energy inside a subsurface target volume at a specified target time. The surface of the solid is heated by an incident laser beam which gives rise to shear and compressional waves propagating into the solid. The optimal heating design process aims to achieve the desired energy focusing at the target with minimal laser power densities and minimal system disturbance away from the target. Due to the slow motion of the thermal conduction process relative to the propagation of acoustic waves, a general formulation is derived which is the limiting case for a heat-absorbing nonconducting solid. This is consistent with the laser heating of the surface where, for the time duration of interest, thermal effects are confined to a thin layer at the surface. This simplification allows for the solution of the problem through the use of the system Greens function for the purely elastic medium where thermal expansion enters as an external force. The problem is then posed as an optimal control problem, the solution of which is the required heating pattern at the surface. The optimality conditions are secured via the conjugate gradient method and the mechanics of the elastic medium is treated by the finite element method along with using the half-space Greens function matrix. Good quality energy focusing is achieved, with the optimal designs reflecting the high directivity of the photothermally generated shear-wave patterns. © 1994 The American Physical Society
Effect of Co and Ni substitution on the two magnetostructural phase transitions in Fe1.12Te
No full textHere we present the results of high-resolution x-ray diffraction experiments along with specific heat, resistivity, and magnetization measurements of chemically well-characterized Fe1.12-xMx Te (M = Co, Ni) samples. The motivation is to investigate how the two coupled magnetostructural phase transitions in the antiferromagnetic parent compound Fe1.12Te of chalcogenide superconductors can be tuned. While the two-step magnetostructural transition (tetragonal-to-orthorhombic followed by orthorhombic-to-monoclinic) persists in Fe1.10Co0.02Te, only one, tetragonal-to-orthorhombic transition was observed in Fe1.10Ni0.02Te. Upon increasing the Co and Ni substitution, the structural phase transitions and the long-range magnetic order are systematically suppressed without any sign of superconductivity. For high substitution levels (x >= 0.05), a spin-glass-like behavior was observed and the low-temperature structure remains tetragonal. From our results, it can be inferred that the electron doping strongly suppresses the magnetostructural phase transitions
Quantum Monte Carlo study of a nonmagnetic impurity in the two-dimensional Hubbard model
No full textIn order to investigate the effects of nonmagnetic impurities in strongly correlated systems, quantum Monte Carlo (QMC) simulations have been carried out for the doped two-dimensional Hubbard model with one nonmagnetic impurity. Using a bare impurity potential which is onsite and attractive, magnetic and single-particle properties have been calculated. The QMC results show that giant oscillations develop in the Knight shift response around the impurity site due to the short-range antiferromagnetic correlations. These results are useful for interpreting the NMR data on Li- and Zn-substituted layered cuprates
Prediction of optimal folding routes of proteins that satisfy the principle of lowest entropy loss: dynamic contact maps and optimal control
No full textAn optimization model is introduced in which proteins try to evade high energy regions of the folding landscape, and prefer low entropy loss routes during folding. We make use of the framework of optimal control whose convenient solution provides practical and useful insight into the sequence of events during folding. We assume that the native state is available. As the protein folds, it makes different set of contacts at different folding steps. The dynamic contact map is constructed from these contacts. The topology of the dynamic contact map changes during the course of folding and this information is utilized in the dynamic optimization model. The solution is obtained using the optimal control theory. We show that the optimal solution can be cast into the form of a Gaussian Network that governs the optimal folding dynamics. Simulation results on three examples (CI2, Sso7d and Villin) show that folding starts by the formation of local clusters. Non-local clusters generally require the formation of several local clusters. Non-local clusters form cooperatively and not sequentially. We also observe that the optimal controller prefers "zipping" or small loop closure steps during folding. The folding routes predicted by the proposed method bear strong resemblance to the results in the literature
Transport of gold nanoparticles by vascular endothelium from different human tissues
No full textThe selective entry of nanoparticles into target tissues is the key factor which determines their tissue distribution. Entry is primarily controlled by microvascular endothelial cells, which have tissue-specific properties. This study investigated the cellular properties involved in selective transport of gold nanoparticles (<5 nm) coated with PEG-amine/galactose in two different human vascular endothelia. Kidney endothelium (ciGENC) showed higher uptake of these nanoparticles than brain endothelium (hCMEC/D3), reflecting their biodistribution in vivo. Nanoparticle uptake and subcellular localisation was quantified by transmission electron microscopy. The rate of internalisation was approximately 4x higher in kidney endothelium than brain endothelium. Vesicular endocytosis was approximately 4x greater than cytosolic uptake in both cell types, and endocytosis was blocked by metabolic inhibition, whereas cytosolic uptake was energy-independent. The cellular basis for the different rates of internalisation was investigated. Morphologically, both endothelia had similar profiles of vesicles and cell volumes. However, the rate of endocytosis was higher in kidney endothelium. Moreover, the glycocalyces of the endothelia differed, as determined by lectin-binding, and partial removal of the glycocalyx reduced nanoparticle uptake by kidney endothelium, but not brain endothelium. This study identifies tissue-specific properties of vascular endothelium that affects their interaction with nanoparticles and rate of transport
Computation of the canonical lifting via division polynomials
No full textWe study the canonical lifting of ordinary elliptic curves over the ring of Witt vectors. We prove that the canonical lifting is compatible with the base field of the given ordinary elliptic curve which was first proved in Finotti, J. Number Theory 130 (2010), 620-638. We also give some results about division polynomials of elliptic curves de fined over the ring of Witt vectors
Job loss, credit constraints, and consumption growth
No full textWe use direct evidence on credit constraints to study their importance for household consumption growth and for welfare. We distentangle the direct effect on consumption growth of a currently binding credit constraint from the indirect effect of a potentially binding credit constraint that generates consumption risk. Our data are focused on job losers. We find that less than 5% of job losers experience a binding credit constraint, but those who do experience significant welfare losses, and consumption growth is 24% higher than for the rest of the population. However, even among those who are unconstrained and are able to borrow if needed, consumption responds to transitory income