Linear Temperature Variation of the Penetration Depth in YBCO Thin Films

We have measured the penetration depth $\lambda(T)$ on $\rm YBa_{2}Cu_{3}O_{7}$ thin films from transmission at 120, 330 and 510~GHz, between 5 and 50~K. Our data yield simultaneously the absolute value and the temperature dependence of $\lambda(T)$. In high quality films $\lambda(T)$ exhibits the same linear temperature dependence as single crystals, showing its intrinsic nature, and $\lambda(0)=1750\,{\rm \AA}$. In a lower quality one, the more usual $T^2$ dependence is found, and $\lambda(0)=3600\,{\rm \AA}$. This suggests that the $T^2$ variation is of extrinsic origin. Our results put the $d$-wave like interpretation in a much better position.Comment: 12 pages, revtex, 4 uuencoded figure

Détection du contour exterieur d'objets en mouvement sur fond non uniforme

Nous proposons un nouvel opérateur basé sur la différence d'images, permettant la recherche des contours en mouvement dans une scène où se déplacent des objets quelconques en lumière naturelle et sur fond non uniforme. Les contours en mouvement sont d'abord détectés en combinant des opérations de différence et de détection de contours sur les images de la séquence. Nous montrons comment, grâce à cet opérateur, nous pouvons ensuite modéliser de manière satisfaisante la scène à l'aide de modèle octogonaux. Des résultats sur scènes réelles de trafic routier urbain montrent la robustesse de l'opérateur et permettent d'envisager le contrôle de trafic

Low temperature penetration depth and the effect of quasi-particle scattering measured by millimeter wave transmission in YBa

Measurement of the penetration depth $\lambda(T)$ as a function of temperature using millimeter wave transmission in the range 130-500 GHz are reported for three $\rm YBa_2Cu_3O_{7-\delta}$ (YBCO) laser ablated thin films. Two films, deposited on a $\rm LaAlO_3$ substrate ($T_c = 90.2 \rm K$), exhibit a narrow resistive transition ($0.3 \rm K$). One has been subsequently irradiated with $\rm He^+$ ions in order to increase the scattering rate of the quasi-particles ($T_c = 87.8 \rm K$). The third film, grown on MgO ($T_c = 88.5 \rm K$), exhibits also a fairly narrow transition ($0.8 \rm K$) and a high crystalline quality. The experiment provides the absolute value $\lambda(T \leq 30 \rm K)$ for the penetration depth at low temperature: the derivation from the transmission data and the experimental uncertainty are discussed. We find a zero temperature penetration depth $\tilde{\lambda_0} = 1990 \pm 200$ Å, $2180 \pm 200$ Å and $2180 \pm 200$ Å, for YBCO-500 Å/$\rm LaAlO_3$ (pristine), YBCO-1300 Å /MgO and YBCO-500 Å /$\rm LaAlO_3$ (irradiated) respectively. $\lambda(T \leq 30 \rm K)$ exhibits a different behavior for the three films. In the pristine sample, $\lambda(T \leq 30 \rm K)$ shows a clear temperature and frequency dependence, namely the temperature dependence is consistent with a linear variation, whose slope decreases with frequency: this is considered as an evidence for the scattering rate being of the order of the measuring frequency. A two fluids analysis yields $1/\tau(T \leq 30 \rm K) \sim 1.7 \times 10^{12} s^{-1}$. In the two other samples, $\lambda(T \leq 30 \rm K)$ does not display any frequency dependence, suggesting a significantly larger scattering rate. The temperature dependence is different in these latter samples. It is consistent with a linear variation for the YBCO/MgO sample, not for the YBCO/$\rm LaAlO_3$ irradiated one, which exhibits a T2 dependence up to $40 \rm K$. We have compared our data to the predictions of the d-wave model incorporating resonant scattering and we do not find a satisfactory agreement. However, the large value of $\tilde{\lambda_0}$ in the pristine sample is a puzzle and sheds some doubt on a straightforward comparison with the theory of data from thin films, if considered as dirty d-wave superconductors

Influence of the depletion length on the commensurability effects in tunable antidots

Commensurability effects between the period of an artificial array of scatterers (antidots) in a two-dimensional electron gas and the classical cyclotron radius are found to depend strongly on the size of the antidots in comparison with the period. Magnetotransport experiments on tunable antidot arrays are reported and analyzed. In the region of large depletion zones we find strong deviations of the Hall effect from the Drude slope in magnetic fields up to several teslas. From this, we deduce undepleted regions of less than 60 nm in typical 500-nm period structures. The suppression of the commensurability motion around several antidots observed in the longitudinal magnetoresistance in the case of strong depletion has the same origin and allows us to determine independently and consistently the depletion radius of the antidots

Leukocyte adhesion:a fundamental process in leukocyte physiology

Leukocyte adhesion is of pivotal functional importance. The adhesion involves several different adhesion molecules, the most important of which are the leukocyte ß2-integrins (CD11/CD18), the intercellular adhesion molecules, and the selectins. We and others have extensively studied the specificity and binding sites in the integrins and the intercellular adhesion molecules for their receptors and ligands. The integrins have to become activated to exert their functions but the possible mechanisms of activation remain poorly understood. Importantly, a few novel intercellular adhesion molecules have been recently described, which seem to function only in specific tissues. Furthermore, it is becoming increasingly apparent that changes in integrins and intercellular adhesion molecules are associated with a number of acute and chronic diseases