Convection in the presence of a first-order phase change

We report experimental and theoretical results for two-phase convection in a thin horizontal layer of a fluid with a first-order phase change and heated from below. A top layer of the nematic phase of a liquid crystal is located above the bottom layer of the isotropic phase of the same substance. A horizontal field of 1000 G is applied in order to align the director of the nematic phase. Over some ranges of the thickness of the isotropic phase, and in sufficiently large thermal gradients, the more dense nematic phase can be stably stratified above the less dense isotropic one, with a stable interface between them. Based on the equations of motion derived for this problem by Busse and Schubert [J. Fluid Mech. 46, 801 (1971)], we evaluate the bifurcation lines between the quiescent and convecting states and the corresponding critical wave vectors as a function of the interface position. We report experimental measurements based on Nusselt-number determinations for the locations of the bifurcation lines. They are in good agreement with the theoretical results. We also report approximate determinations of the critical wave numbers which are semiquantitatively consistent with the theory. A great diversity of patterns is observed in the convecting states, including normal and parallel rolls, rolls with defects and disorder, target patterns and spirals, and cellular flow with upflow or downflow at the cell center. These patterns are discussed in terms of the breaking of the mirror symmetry at the horizontal midplane by the interface, and in terms of the orienting effects of the magnetic field.     

Observation of the dynamical change in a water monolayer adsorbed on a ZnO surface

A combined scanning tunneling microscopy and density-functional theory (DFT) study shows a rich structure of water monolayers adsorbed on ZnO(1010) at room temperature. Most of the water is in a lowest-energy configuration where every second molecule is dissociated. It coexists with an energetically almost degenerate configuration consisting of a fully molecular water monolayer. Parts of the layer continuously switch back and forth between these two states. DFT calculations reveal that water molecules repeatedly associate and dissociate in this sustained dynamical process.     

Classical limit of the probability density for a perturbed change in the Hamiltonian

The stationary perturbation series is derived from the time-dependent perturbation series up to any order by using a formula derived from the theory of divergent series. The probability density of the perturbed state turns out as the evolution of the density operator of the pure unperturbed initial state as 0. This result, for 0, indicates that the particle moves in the classical trajectory with the HamiltonianH=H ₀+V immediately afterV is turned on, if it is in the trajectory ofH ₀ initially. This confirms the classical dynamics. Further, the adiabatic theorem is introduced in order to get the same conclusion for arbitrary finite potentialV.This paper was developed from part of a report (Su, 1968) which was supported by National Science Council, Republic of China.     

On the change in electromagnetic wave polarization in a smooth one-dimensionally inhomogeneous medium

Propagation of an electromagnetic wave in a smooth one-dimensionally inhomogeneous isotropic medium is considered in the second approximation of geometrical optics. The polarization evolution is studied extensively. It is known that in the first (Rytov) approximation of geometrical optics, there is only the rotation of the plane of polarization (with no change in the polarization shape and sign) for rays with torsion. In the case considered, both the shape of polarization ellipse and the sign of polarization change proportionally to the integral of the squared ray curvature even for plane rays. The effect is of nonlocal geometrical nature and can be described in terms of the generalized geometrical phase incursion between two linear polarizations.     

Intensity-induced mode shift in a femtosecond laser by a change in the nonlinear index of refraction

The modes of the comb spectrum of a Kerr-lens mode-locked laser are frequency shifted versus the corresponding cw modes of the cavity by an intensity-induced change in the index of refraction in the Kerr medium. We demonstrate this effect and discuss novel schemes for fast frequency control of the comb spectrum.     

On the cause of a contrast change in the SR images of micropipes in SiC

We propose a theoretical explanation for the experimentally observed feature in phase contrast images of micropipes in silicon carbide in a white synchrotron radiation (SR) beam, which consists in the fact that the contrast of various micropipes or various regions of the same micropipe is of different colors. The contrast is most often white (the intensity is higher than the background) at the center and black at the edges; however, the sign sometimes changes, and the contrast becomes black at the center and white at the edges. We discuss the results of experiments performed at the SR source in Pohang, Republic of Korea. The cause of the contrast change can be a change in the angle between the micropipe axis and the SR beam direction. At not overly small angles, the phase progression in a section of the micropipe is small and the contrast is standard. If the angle becomes very small, the size of the longitudinal section of the micropipe by the beam increases, which leads to wave field oscillations in the region of the section. At large distances in the white beam, these oscillations are averaged, and averaging results in black contrast.     

Temporal predictions based on a gradual change in tempo

Previous studies investigating sensitivity to step changes in tempo and prediction of tone onset time have generally utilized isochronous sequences. This study investigates subjects' ability to detect deviations from a gradual change in the tempo of a tone sequence (experiment 1) and their judgment of the perceptually optimal timing of this tone (experiment 2). In experiment 1, inter-onset-intervals within pairs of eight-tone sequences followed a geometric progression to create a gradual tempo change. In one sequence, the final tone was presented either earlier or later than specified by the progression. Subjects performed well at detecting deviations that exaggerated the tempo progression but poorly when it was counteracted. Experiment 2 used similar pairs except that the final tone was always presented earlier in one sequence than the other. Final interval length was adaptively adjusted to subjects' judgments; it was adjudged in best agreement with the progression when its length was roughly half way between the mathematically correct value and the length of the penultimate interval. The data support "multiple-look" and entrainment models of tempo sensitivity and suggest that temporal prediction is based less on the tempo contour of a whole sequence than on the duration of the preceding interval.     

Effect of a change in the optical density of a magnetic emulsion in electric and magnetic fields

The change in the optical density of an emulsion of magnetic fluid drops suspended in a mineral oil under the action of electric and magnetic fields has been investigated. It is found that the sign of the change in transparency in an ac electric field depends on the field frequency. It is shown that, joint action of codirectional low-frequency electric and dc magnetic fields can compensate for the change in the optical density. Calculation of the change in the emulsion optical density within the anomalous diffraction approximation showed that this effect can be explained by small field-induced deformation of microdrops.     

Structural change in the photoexcited cis-polyacetylene

The structural change in the cis-polyacetylene photoexcited with the energy corresponding to its band gap was examined. The growth of four new infrared absorption peaks at 802, 1062, 1112 and 1259 cm^?1 were found after the irradiation. The appearance of these peaks strongly suggests the formation of the trans-cisoid segment in the cis-transoid chain of polyacetylene.     

On the spontaneous-coherence-transfer-induced sign change of a sub-natural-width nonlinear resonance

A simple theoretical model is proposed which describes the positive sign of the nonlinear absorption resonances in the experiments of Akul’shin et al. [Phys. Rev. A 57, 2996 (1998)]. An analytical expression for the linear response to a weak probe field is obtained in the weak-saturation limit. It is shown that the positive sign of sub-natural-width resonances is due to the spontaneous transfer of low-frequency coherence from an excited level to the ground state. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 11, 776–781 (10 June 1999)     

Sign change of the surface energy of a two-component superconductor

It is shown that the sign of the surface energy of a two-component superconductor is determined not only by the Ginzburg-Landau parameters of two superconducting components, but also by a temperature-independent parameter κξ, which is defined as the ratio of the coherence lengths of two components.     

Localization of a small change in a multiple scattering environment without modeling of the actual medium

A method to actively localize a small perturbation in a multiple scattering medium using a collection of remote acoustic sensors is presented. The approach requires only minimal modeling and no knowledge of the scatterer distribution and properties of the scattering medium and the perturbation. The medium is ensonified before and after a perturbation is introduced. The coherent difference between the measured signals then reveals all field components that have interacted with the perturbation. A simple single scatter filter (that ignores the presence of the medium scatterers) is matched to the earliest change of the coherent difference to localize the perturbation. Using a multi-source/receiver laboratory setup in air, the technique has been successfully tested with experimental data at frequencies varying from 30 to 60 kHz (wavelength ranging from 0.5 to 1 cm) for cm-scale scatterers in a scattering medium with a size two to five times bigger than its transport mean free path.     

First hint on a change of the 210Po alpha-decay half-life in the metal Cu

In the metal Cu cooled to T = 12K the α-decay half-life of ²¹⁰Po nuclides, located inside the metal, was measured to be shorter by 6.3±1.4% compared to that at room temperature.     

Change in the spectral parameters of a laser plasma on a change to two-pulse irradiation of a dielectric in air

Translated from Zhurnal Prlkladnoi Spektroskopii, Vol. 51, No. 4, pp. 564–571, October, 1989.