Photoresponse of epitaxial YBa2Cu3O7-x ultrathin films

The voltage photoresponse of YBa2CH307_ x/SrTiO3 10 nm-tlfick films\ud on laser irradiation is studied using the low temperature scanning laser\ud microscopy (LTSLM) teclmique. The irradiation wavelength is 633 run.\ud The spatial response dependencies via temperature (4-100 K), beam\ud intensity modulation frequency (0-150 kHz) and bias current are\ud examined. The bolometric and non-bolometric components are\ud separated. The amplitude of the non-bolometric component at 4.2 K is\ud about 10 times higher than that of bolometric one measured near the\ud midpoint of the superconducting transition. The non-bolometric\ud component is presumably associated with weak links in the film due to\ud interdiffusion of SrTiO3 into YBCO layer. The spatial irregularity in\ud superconducting parameters is not resolved by the LTSLM method\ud since the spacing between weak links is appeared to be less than 1 um

Laser scanning imaging and local characterization of superconducting properties in high-Tc thin film multiturn coil

Low-temperature scanning laser microscopy has been used to investigate the spatial variation of the critical temperature Tc and critical current Ic in thin-film high-Tc multilayer structures that include dielectric layers. The method is described and measurements are presented on an YBa2Cu3O7-x-based multiturn coil with SrTiO3 insulating layer. We found that the critical temperature Tc of the YBa2Cu3O7-x top layer, from which the return strip of the coil is formed, is higher than that of the YBa2Cu3O7-x base layer. The critical current of the coil is limited by the quality of the YBa2Cu3O7-x base layer and not by the edges of the crossovers

Superconducting Quantum Interference in Fractal Percolation Films. Problem of 1/f Noise

An oscillatory magnetic field dependence of the DC voltage is observed when a low-frequency current flows through superconducting Sn-Ge thin-film composites near the percolation threshold. The paper also studies the experimental realisations of temporal voltage fluctuations in these films. Both the structure of the voltage oscillations against the magnetic field and the time series of the electric "noise" possess a fractal pattern. With the help of the fractal analysis procedure, the fluctuations observed have been shown to be neither a noise with a large number of degrees of freedom, nor the realisations of a well defined dynamic system. On the contrary the model of voltage oscillations induced by the weak fluctuations of a magnetic field of arbitrary nature gives the most appropriate description of the phenomenon observed. The imaging function of such a transformation possesses a fractal nature, thus leading to power-law spectra of voltage fluctuations even for the simplest types of magnetic fluctuations including the monochromatic ones. Thus, the paper suggests a new universal mechanism of a "1/f noise" origin. It consists in a passive transformation of any natural fluctuations with a fractal-type transformation function.Comment: 17 pages, 13 eps-figures, Latex; title page and figures include

Current-induced highly dissipative domains in high Tc thin films

We have investigated the resistive response of high Tc thin films submitted to a high density of current. For this purpose, current pulses were applied into bridges made of Nd(1.15)Ba(1.85)Cu3O7 and Bi2Sr2CaCu2O8. By recording the time dependent voltage, we observe that at a certain critical current j*, a highly dissipative domain develops somewhere along the bridge. The successive formation of these domains produces stepped I-V characteristics. We present evidences that these domains are not regions with a temperature above Tc, as for hot spots. In fact this phenomenon appears to be analog to the nucleation of phase-slip centers observed in conventional superconductors near Tc, but here in contrast they appear in a wide temperature range. Under some conditions, these domains will propagate and destroy the superconductivity within the whole sample. We have measured the temperature dependence of j* and found a similar behavior in the two investigated compounds. This temperature dependence is just the one expected for the depairing current, but the amplitude is about 100 times smaller.Comment: 9 pages, 9 figures, Revtex, to appear in Phys. Rev.

CRITICAL CURRENT HYSTERESIS DUE TO ELECTRON OVERHEAT IN NARROW SUPERCONDUCTING FILMS

On a étudié l'hystérésis des caractéristiques courant-tension des couches minces supraconductrices d'étain et d'indium dans un large intervalle de température. On a montré, que si ce transfert de chaleur est parfait, la valeur de l'hystérésis et sa variation avec latemperature s'accordent avec les résultats de la théorie de Shklovski, expliquant l'apparition de l'hystérésis par la surchauffe des électrons par rapport au réseau. Sur ces mêmes couches, on a découvert l'influence de la surchauffe des électrons sur la valeur de la résistance thermique de l'interface métal-isolant.The hysteresis of IVC has been studied on thin superconducting films of tin and indium in a wide temperature range. It has been shown that under perfect heat transfer, the value of hysteresis and its temperature dependence are in agreement with the results of Shklovsky's theory according to which the hysteresis is due to electron overheat with respect to the lattice. The effect of electron overheat on thermal resistivity of a metal-insulator interface has been found

CROSS-LIKE FILM STRUCTURES, MICROWAVE IRRADIATION STIMULATED SUPERCONDUCTIVITY AND JOSEPHSON EFFECT

On a étudié les structures en couches cruciformes de l'étain. Les observations de la supraconductivité stimulée par irradiation d'hyperfréquence sur les couches homogènes dans un large intervalle de libre parcours l sont en accord quantitatif avec la théorie d'Eliashberg. L'effet relatif augmente quand une zone faible est créée par le passage du courant transversal i. Aux forts i et au voisinage de Tc, on observe des marches Josephson induites sur les caractéristiques I-V par l'irradiation.Cross-like tin film structures are studied. Superconductivity stimulated by microwave irradiation was observed on homogeneous films in a wide range of mean free paths l and agree quantitatively with Eliashberg's theory. In a weak portion created by passing transverse current i through it, the relative effect increases. At high i near Tc irradiation-induced Josephson steps appear in V-I curves

Microwave irradiation stimulated superconductivity and Josephson effect in cross-like film structures

The cross-like film structure is studied experimentally. The occurrence and magnitude of the induced superconductivity effect in the microwave field are shown to depend on the mean free path (l = 350-2 000 Å). The homogeneity of a long strip was found through a comparison with Galaiko's theory [1] which gives quantitative agreement. As the control current i passes through a transverse strip, the relative effect of induced superconductivity increases. The i growth causes the coupling energy of the controlled cross-like element to decrease leading to microwave induced Josephson steps in the I-V characteristics.On a réalisé l'étude expérimentale d'un film cruciforme. On a montré que l'apparition et la grandeur de la supraconductivité induite par le champ haute fréquence dépendent de la longueur du libre parcours moyen (l = 350-2 000 Å). L'homogénéité le long du film a été déterminée par comparaison avec la théorie de Galaiko [1], on constate un bon accord quantitatif. En présence du courant de contrôle dans la bande transversale, nous observons une augmentation relative de la supraconductivité induite. Dans ces conditions, le couplage de l'élément de Josephson cruciforme diminue, et des marches de Josephson apparaissent sur la caractéristique V-I

Commensurability effect and lock-in transition in Mo/Si superconducting superlattices

We report the first observation of the lock-in transition in artificial superconducting superlattices, which takes place in tilted magnetic fields. The measurements were carried out on the Mo/Si layered system. The temperature dependence of the critical angle for the trapping of the vortices in the orientation parallel to the layer planes is determined by the previously known resistive method and by a new method based on the effect of commensurability between the intervortex distance and the superlattice wavelength. The temperature dependences of the critical angle obtained by the two methods practically coincide. The experimental results are consistent with the theoretical predictions of Feinberg and Villard