Descripción: Reducible gauge theories with constraints linear in the momenta are quantized. The equivalence of the reduced phase space quantization, Dirac quantization, and BRST quantization is established, provided one transforms appropriately the Dirac wave functions under changes of representation of the constraint surface and of the reducibility functions. The ghosts of ghosts are found to play a crucial role in the equivalence proof. © 1993 American Institute of Physics.

Descripción: We numerically and theoretically investigate the evolution of the ridges and rifts produced by the convergent and divergent motions of two substrates over which an initially uniform layer of a Newtonian liquid rests. We put particular emphasis on the various asymptotic self-similar and quasi-self-similar regimes that occur in these processes. During the growth of a ridge, two self-similar stages occur; the first takes place in the initial linear phase, and the second is obtained for a large time. Initially, the width and the height of the ridge increase as t 1/2. For a very large time, the width grows as t 3/4, while the height increases as t 1/4. On the other hand, in the process of formation of a rift, there are three self-similar asymptotics. The initial linear phase is similar to that for ridges. The second stage corresponds to the separation of the current in two parts, leaving a dry region in between. Last, for a very large t, each of the two parts in which the current has separated approaches the self-similar viscous dam break solution. © 2008 American Institute of Physics.

Descripción: Imaging of samples by different microscopy techniques has produced a relevant impact in the development of new diagnosis techniques in biology, medicine and material science. In many biological, applications, where the sample changes or moves during the observation, a moving spot to track an identified sample is required. We introduce here an optical system that can perform, this tracking without mechanical components. The system, is based on the use of a high resolution liquid crystal on silicon (LCoS) device working as a mostly phase wave front modulator. The additional advantage of this system is performing the motion of the spot at video rate. In general, these devices produce coupled phase and amplitude modulation responses as a function of the applied voltage. This coupling effect deteriorates the response of those ideal optical elements designed as phase only or amplitude only functions. By means of an elliptical polarization light we can reduce the amplitude modulation and improve the phase modulation. We have experimentally found a configuration where the amplitude is almost constant while the phase reaches a high modulation. For this configuration we show how the spot can be moved through, focus plane by means of linear phases, or displaced out of this plane by using a quadratic phase. © 2008 American Institute of Physics.

Descripción: The linear response within the elimination of the small component formalism is aimed at obtaining the leading order relativistic corrections to magnetic molecular properties in the context of the elimination of the small component approximation. In the present work we extend the method in order to include two-body effects in the form of a mean field one-body operator. To this end we consider the four-component Dirac-Hartree-Fock operator as the starting point in the evaluation of the second order relativistic expression of magnetic properties. The approach thus obtained is the fully consistent leading order approximation of the random phase approximation four-component formalism. The mean field effect on the relativistic corrections to both the diamagnetic and paramagnetic terms of magnetic properties taking into account both the Coulomb and Breit two-body interactions is considered. © 2006 American Institute of Physics.

Descripción: In the present work, numerical results of the nuclear spin-rotation (SR) tensor in the series of compounds HX (X=H,F,Cl,Br,I) within relativistic 4-component expressions obtained by Aucar [J. Chem. Phys. 136, 204119 (2012)10.1063/1.4721627] are presented. The SR tensors of both the H and X nuclei are discussed. Calculations were carried out within the relativistic Linear Response formalism at the Random Phase Approximation with the DIRAC program. For the halogen nucleus X, correlation effects on the non-relativistic values are shown to be of similar magnitude and opposite sign to relativistic effects. For the light H nucleus, by means of the linear response within the elimination of the small component approach it is shown that the whole relativistic effect is given by the spin-orbit operator combined with the Fermi contact operator. Comparison of "best estimate" calculated values with experimental results yield differences smaller than 2-3 in all cases. The validity of "Flygares relation" linking the SR tensor and the NMR nuclear magnetic shielding tensor in the present series of compounds is analyzed. © 2013 American Institute of Physics.

Descripción: The noncovalent complex formed by the association of two fragments of chymotrypsin inhibitor-2 is reversibly denatured by pressure in the absence of chemical denaturants. On pressure release, the complex returned to its original conformation through a biphasic reaction, with first-order rate constants of 0.012 and 0.002 s-1, respectively. The slowest phase arises from an interconversion of the pressure-denatured state, as revealed by double pressure-jump experiments. Below 5 μM, the process was concentration dependent with a second-order rate constant of 1,700 s-1 M-1. Fragment association at atmospheric pressure showed a similar break in the order of the reaction above 5 μM, but both first- and second-order folding/association rates are 2.5 times faster than those for the refolding of the pressure-denatured state. Although the folding rates of the intact protein and the association of the fragments displayed nonlinear Eyring behavior for the temperature dependence, refolding of the pressure-denatured complex showed a linear response. The negligible heat capacity of activation reflects a balance of minimal change in the burial of residues from the pressure-denatured state to the transition state. If we add the higher energy barrier in the refolding of the pressure-denatured state, the rate differences must lie in the structure of this state, which has to undergo a structural rearrangement. This clearly differs from the conformational flexibility of the isolated fragments or the largely unfolded denatured state of the intact protein in acid and provides insight into denatured states of proteins under folding conditions.