Optical Properties of Layered Superconductors near the Josephson Plasma Resonance

We study the optical properties of crystals with spatial dispersion and show that the usual Fresnel approach becomes invalid near frequencies where the group velocity of the wave packets inside the crystal vanishes. Near these special frequencies the reflectivity depends on the atomic structure of the crystal provided that disorder and dissipation are very low. This is demonstrated explicitly by a detailed study of layered superconductors with identical or two different alternating junctions in the frequency range near the Josephson plasma resonance. Accounting for both inductive and charge coupling of the intrinsic junctions, we show that multiple modes are excited inside the crystal by the incident light, determine their relative amplitude by the microscopic calculation of the additional boundary conditions and finally obtain the reflectivity. Spatial dispersion also provides a novel method to stop light pulses, which has possible applications for quantum information processing and the artificial creation of event horizons in a solid.Comment: 25 pages, 20 figures, submitted to Phys. Rev.

Loading rate effect on lateral force measurements on nanostructured Ti and TiN thin films

International audienceTribological properties of titanium and titanium nitride nanostructured thin films as well as of silicon substrate have been studied. We report onthe effect of lateral tip displacement direction and normal force loading mode on lateral force measurements. On hard coatings such as titaniumnitride thin films, we show that the tip's scratch direction has almost no effect on lateral force measurements. A completely different behaviour hasbeen observed on titanium films. In this last case, the measured lateral force is very dependent on the scratch direction.Concerning the effect of normal force loading mode, we will show how the loading rate, rather than the sliding speed, is a very importantparameter in the case of a ramping normal load. For this purpose we have performed two sets of six scratches, for each sample. Scratch length wasfixed to 10 μm in all cases.In the first set, the scratch length and duration were kept constant. In the second one, the scratch duration was tuned in order to achieve aconstant value of the normal loading rate dFzdt. In this last case, the experimental data leads to a unique curve. This means that during nanoscratchmeasurements, normal tip displacement is controlled by the normal force loading rate. Consequently, lateral displacement speed dxdtmust be tunedin order to obtain the required value of dFzdt. This behaviour seems to be universal and not related to material hardness and Young modulus.© 2005 Elsevier B.V. All rights reserved

On the hardness and the inherent ductility of chromium carbide nanostructured coatings prepared by RF sputtering

Homogenous and nanostructured chromium carbide thin coatings are prepared by RF sputtering. Thenanoindentation tests are used to investigate the evolution of plastic deformation and hardness as a functionof the grain size. The Cr3C2 films have been deposited on magnesium oxide substrates from a Cr3C2target in pure argon atmosphere. Based on the effect of the deposition temperature, we have generatednanostructured Cr3C2 coatings with different mechanical properties.We have found that both crystallinetexture and grain size increased with increasing the deposition temperature. The nanostructured chromiumcarbidefilm with grain size in the order of 5 nm, prepared at a deposition temperature of 600 ◦C,showed a higher hardness as well as a good ductility compared to the samples prepared at 850 ◦C or atroom temperature. The cyclic nanoindentation measurements at constant loading rate allowed qualitativecharacterization of the change in the irreversible plastic work for the samples considered in this report

Propriétés tribologiques de films minces nanostructurés

International audienceTribological properties of nanostructured thin films. In order todetermine the tribological properties of nanostructured thin layers of titanium andtitanium nitride with thicknesses of 300 nm, we have studied the influence of Berkovichtip’s lateral displacement orientation and normal loading rate on friction properties.For the hardest films (titanium nitride), we have showed that tip direction scratch doesnot have any effect on tribological properties. A completely different behavior isobserved for titanium. For this last case, the measured lateral force is smaller whenthe scratch test is carried out following forward tip direction. Concerning dynamicseffect on scratch measurements, we have established scaling laws for normaldisplacement versus normal force. We have showed that the relevant parameter forscratch measurements is the loading rate , deduced from these scaling laws. Thus,during scratch tests, the lateral displacement speed must be adapted to the maximumvalue of the applied normal load.Afin de déterminer les propriétés tribologiques de couches mincesnanostructurées de nitrure de titane et de titane de 300 nm d’épaisseur, nous avonsétudié l’influence du sens de déplacement latéral d’une pointe Berkovich et du tauxde chargement normal sur le comportement en frottement. Pour les films les plus durs(nitrure de titane), nous avons montré que le sens de déplacement latéral de la pointen’a aucun effet sur les propriétés tribologiques. Un comportement complètementdifférent est observé pour les films les moins durs (titane). Dans ce dernier cas, laforce latérale mesurée est plus faible lorsque le scratch test est effectué vers l’avantde la pointe. En ce qui concerne l’effet de la dynamique sur les mesures de scratch,nous avons établi des lois d’échelle pour le déplacement normal en fonction de la forcenormale. Nous avons montré que le paramètre pertinent pour les mesures est le tauxde chargement , déduit des lois d’échelle. Ainsi, durant les essais de rayures, lavitesse de déplacement latéral doit être adaptée à la valeur maximale de l’effort normalappliqué

Relationships between x-ray photoelectron spectra and superconductivity in Bi-O thin films

Detailed X-ray photoelectron spectra (XPS) have been obtained from BiSrCaCuO thin fums without lead doping (Tc = 86°K, 2212 phase) and from thin films with lead doping (Tc = 107°K, mainly 2223 phase). Films were prepared by single target sputtering and post-annealed tosynthesize superconducting films 1t 2.The spectral features of the Cu 2p, 0 1s, Sr 3p, Ca 2p and Bi 4f are considered in some detail. It is found that in general the XPS signatures of the Cu 2p peaks are consistent with the presence of a mixed oxide environment with a main contribution from a binding energy similar to that for CuO but also with a significant contribution from a component with a binding energy shifted up to 1.7 eV for the low Tc phase and to 2 eV for the high Tc phase consistent with some trivalent state of copper for lead doped films. The Cu 2p structure exhibits the main line and satellite peaks familiarfrom screening of the 2p hole by the two valence-band configurations 2p 3d10 and 2p 3d9 in the intensity ratio -2: 1. Line-shape decomposition of the 0 1s emission suggests that the main line corresponds to emission from oxygen bonded to copper in the Cu-O planes while the satellite peak with the higher binding energy corresponds to oxygen bonded to bismuth in Bi-O planes. No oxygen species bonded to Ca and Sr is clearly identified by line-shape analysis. Such explanations agree with angular-resolved photoemission spectra on our c axis oriented thin fums. The Ca 2p and Sr 3d core-level spectra do not exhibit two sets of doublets as reported in other papers but their corresponding binding energies are shifted in the opposite trend of that observed for oxides. The bismuth ionization state in unleaded films seems inert under different annealing conditions and is very similar to Bi20:3.It is equally shown that the valence band structures are quite sensitive to small changes of the surface conditions and lead doping. In essence, this strongly suggests that valence band spectra may be able to be used as a finger print for the surface of the superconducting films

Anisotropy of superconducting properties of Bi-Sr-Ca-Cu-O thin films

International audienceAngular dependence of the magnetoresistance and of the critical current up to 20 tesla in the bismuth 2212 phase shows remarkable anisotropic behavior. Samples are highly c-axis oriented thin films. The field dependence of the critical current is shown to be essentially dominated by the component of the magneticfield perpendicular to the CU02 layers

Nanostructure Study of Ti/TiN Multilayers: Effect of the Deposition Temperature

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Scaling of the mixed state magnetoresistance of high-Tc superconductors in terms of two-dimensional superconducting fluctuations

International audienceWe show scaling properties of the magnetoresistance of Bi2Sr2CaCu2Os thin films below T¢=89 K in fields up to 20 T. Thefunctional dependence is that of two-dimensional superconducting fluctuations above a critical temperature of 30 K. We proposethat the double CuOz sheets in each unit cell behave as 2D superconducting units with giant fluctuations

Angular scaling of the magnetoresistance of BiSrCaCuO thin films below Tc

International audienceWe have studied the magnetoresistance (MR) of c-axis oriented superconducting BiSrCaCuO thin films at, various te~perat~re (T~Tc) and various orientations of the magnetic field (H~20T) with respect to the cnstallographlc c-axls. A scaling law of the MR for different tilt angle of the magnetic field is presented. It results from the predominant role of the component of the magnetic field along the c-axis

Elaboration and characterization of Ti and TiN thin films and multilayers for hard coating applications

In this paper, we report on the growth of titanium and titanium nitride thin films and of Ti/TiN nanometric multilayers. The elaboration ofthese films has been carried out by high-vacuum diode r.f. sputtering. The growth was monitored in-situ by kinetic ellipsometry. The filmthickness ranged from 50 to 200 nm for the Ti and TiN single layers. For the multilayers, the thickness of each component was varied from1 nm to 10 nm and alternately repeated in order to obtain a total thickness of 200 nm. After deposition, the films were characterised by meansof X-ray diffraction, grazing incidence X-ray reflectometry, atomic force microscopy and transmission electron microscopy for structuraldetermination. A comparison is made between the microstructural and the mechanical properties of Ti and TiN films on the one hand and themultilayers on the other hand (porosity, density and so on). We show that the wear properties are increased by multilayering