E. Guyon

Domain Structure in the Nematic Freedericksz Transition

Abstract In the course of experiments using the Freedericksz transition on a planar nematic film in a magnetic field perpendicular to the limiting plates, we have observed the transient existence of a periodic roll structure very similar to published observations by Carr, and apparently similar to Williams domains. The present paper develops a model based on the effect of the decrease of the effective viscosity in the presence of the back flow, induced by the rotation of n and taking place as a convective roll pattern. Direct observations and order of magnitude estimates of the wavelength of the roll and of the time constant for the development of the rolls agree with the model. We also addressed ourselves with the problem of distortion in an initially well aligned bulk material placed in a field perpendicular to n and find that inertia effects can also lead to a roll structure in the problem.

Shear-flow-induced transition in nematics

Abstract The distortion, induced by a shear flow in a nematic (MBBA) layer having its director initially normal to the velocity and to the velocity gradient, has been observed by a conoscopic technique. At low velocity, the sample retains an ideal planar texture. Above a certain velocity threshold vc, a distortion is observed. The threshold is inversely proportional to the sample thickness, and can also be modified by a magnetic field. These results are in agreement with a simple calculation based on the Leslie equations.

Heat Convection in a Nematic Liquid Crystal

Abstract We present a theoretical and experimental study of the thermal properties of a well aligned nematic Liquid Crystal in a vertical temperature gradient. Due to the long time constant of the relaxation of the director and to the anisotropy of the heat conductivity, the value of the temperature gradient threshold for convective instabilities is strongly modified as compared to the isotropic case. Experiments on planar MBBA films show a drastic reduction of the threshold. A detailed description of the shape and temperature distribution of the convective rolls is given.

Optical Study of a Chiral Smectic C Under Shear

Abstract We study the distortion of a chiral smectic C, under a shear applied parallel to the plane of the layers, using an original technique based on the modification of the conoscopic image. Both the amplitude of the distortion and the phase lag between the alternating applied shear and the deformation are found to vary like the flow induced transverse polarization (described in Ref. 7). The behaviour is understood in terms of a viscoelastic-nematic-like model and of a simplified optical description which gives account of the displacement without deformation of the conoscopic image.

Deformation of nematic layers in crossed electric and magnetic fields

Abstract A parallel nematic layer will show a Freedericksz-transition in an electric field when the voltage exceeds a threshold value U c . If a magnetic field is applied parallel to the layer but perpendicular to the molecular orientation the threshold voltage U c increases with increasing magnetic field field H above a threshold field H c2 . From a plot of U c vs H all three elastic constants can be obtained. Theoretical results are compared with data of HBAB.

STUDY OF DILUTE MAGNETIC ALLOYS USING SUPERCONDUCTING PROXIMITY.

Abstract We have investigated the magnetic properties of the Cu series of dilute alloy films (X = Mn, Cr, Fe), from the critical temperature of Pb/uX superconducting sandwiches, as compared with Pb/Cu ones. A quantitative analysis of the thickness dependence of Tc, based on the tunneling model of the proximity effect, leads to an estimation of the magnetic depairing energy for different concentrations. A tentative analysis of the data is given in terms of the Kondo effect in these dilute alloys.

Dependence of critical current and fields of periodically modulated superconducting alloys on modulation amplitude

The critical current Jc(H//) of superconducting alloy films with a modulation of composition across thickness is shown to be a sensitive function of the periodic heterogeneity when the period is of the order of magnitude of the characteristic length scales ξ(T), λ(T) of the vortex (A magnetic field H// is applied along the layers so that the Lorentz force is exerted along the direction of modulation). We stress in particular that it is a more sensitive function than critical fields which hardly depend on the amplitude of modulation.

Comparative study of size effects in solid and liquid films

Abstract We consider the behaviour, near a second order phase transition, of superconducting, superfluid and nematic liquid-crystal films, subject to various boundary conditions. A comparative study of size effects in these several types of films leads to considerable physical insight, and we find their general properties at least qualitatively described by a mean-field model—the Landau theory of second order transitions. We mention a few applications of the observed phenomena, and touch upon further areas where a similar approach may prove fruitful.

Tomasch effect and superconducting proximity

Tunneling conductance measurements (dI/dV)(V) on thick and clean super-conducting films (S1) backed by normal metals (M2) show geometrical resonance effects, which have been associated with the variation δΔ(x) of the pair potential near the interface. We analyze both theoretically and experimentally the three factors involved in the resonance effect on S1/M1 sandwiches where one of the films is superconducting. The first factor, period, has already received considerable attention and is connected with the ratioVF*/d1 of the renormalized Fermi velocity and the thicknessd1 of the film S1. We show that generally subharmonic effects, related to multiple interference effects, are expected. This result is demonstrated experimentally on Pb/M2 sandwiches. The phase depends on the sign of δΔ(x), the detailed shape of its variation, and on the nature of the tunneling process. Unfortunately the interplay among these parameters is difficult to analyze, although directive tunneling is found to fit better with experiments than the generally assumed diffuse tunneling. The amplitude of the resonance should be proportional to the change of δΔ(x). The ambiguity in the experimental results is pointed out and an experiment that shows clearly the role of the superconducting proximity mechanisms is described where the proximity on several simultaneously prepared Pb/Al films is controlled by deposition of various third upper layers. The theoretical discussion is an extension of the Ishii matrix formulation, itself based on an application of the Andreev scattering process.