2014 Des transitions de l'orientation moléculaire induites par un champ électrique ou magnétique ont été mises en évidence dans différents composes smectiques A. Deux régimes de distorsion ont été observés : 2014 près de la transition smectique A ~ nématique, une distorsion périodique, de vecteur d'onde perpendiculaire au plan (n, n0) ; 2014 à plus basse température, deux réseaux périodiques de défauts, perpendiculaires au plan (n, n0) permettent le pliage des couches. Les dépendances du champ seuil en fonction de l'épaisseur de l'échantillon et de la température, et la variation de l'angle de basculement des molécules en fonction du champ appliqué sont discutées pour le second régime de distorsion. Abstract. 2014 Observations of electric and magnetic field induced transitions of the molecular orientation in smectics A are presented. Two regimes of distortion can be distinguished : 2014 close to the nematic/smectic A transition, a periodic distortion appears with a wave vector normal to the plane (n, n0). 2014 for lower temperatures, two periodic arrays of defects, normal to the plane (n, n0), allow for the folding of the smectic layers. The dependence of threshold field versus sample thickness and temperature, and the variation of the tilt angle of the molecules versus applied field are discussed for the second regime of distortion.
Quantitative Mn, Si, Cr, and Fe profiles have been obtained by secondary ion mass spectrometry on Cr-doped semi-insulating GaAs subjected to unencapsulated anneals at 750 °C in a hydrogen ambient. The chemical concentration of Mn induced by annealing is greater than can be realized by diffusion from the bulk (Mn = 3×1013 cm−3), and relates directly to both the free-hole concentration measured in the conducting surface layer and the 5 K photoluminescence (PL) intensity of the zero-phonon emission at 1.409 eV. Post anneal profiles of the elements Si and Cr, which are present in the bulk prior to annealing, indicate that Si remains uniformly distributed while Cr suffers out-diffusion in conjunction with surface accumulation. The distribution of Fe which peaks at 5×1015 atoms cm−3 may, like Mn, arise from incorporation from the annealing ambient.
Electrical conductivities parallel and perpendicular to the long axis of the molecules (respectively σ|| and σ⊥) are presented for the nematic and smectic (A and B) phases of various compounds having a low ratent heat of transition smectic A ↔ nematic. All the materials show pretransitional effects in their nematic phase. The study of conductivity in smectics A has led us to distinguish two groups of materials. In the first one σ|| becomes equal to σ⊥ a few degrees above the smectic A-nematic transition, while σ|| is at least five times smaller than σ⊥, in the smectic A phase. Furthermore σ⊥ increases anomalously with decreasing temperature, which is observed also for the dye diffusion coefficient normal to the molecular axis. In the second group σ|| differs only slightly from σ⊥. The different behaviour of the conductivity in this group is probably related to the fact that the smectic A layers comprise two molecules
Low-temperature photoluminescence on metalorganic vapor phase epitaxial AlxGa1−xAs layers (0⩽x⩽0.37) exhibiting relatively sharp bound exciton and band acceptor lines, combined with effective mass-type calculations of the expected transition energies, has allowed the identification of carbon, zinc, and possibly silicon as residual acceptors. Carbon seems to be the predominant residual acceptor impurity species in layers grown at high temperature. Large spectral shifts in direct-band-gap AlxGa1−xAs as a function of excitation density or in time resolved spectroscopy reported in literature are attributed to the presence of several acceptor species rather than to D-A pair recombination.
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