Depairing critical currents and self-magnetic field effects in submicron YBa₂Cu₃O₇₋δ microbridges and bicrystal junctions

We report on depairing critical currents in submicron YBa₂Cu₃O₇₋δ microbridges. A small-angle bicrystal grain boundary junction is used as a tool to study the entrance of vortices induced by a transport current and their influence on the I–V curves. The interplay between the depairing and the vortex motion determines a crossover in the temperature dependence of the critical current. The high entrance field of vortices in very narrow superconducting channels creates the possibility of carrying a critical current close to the depairing limit determined by the S–S–S nature of the small-angle grain boundary junction

Commensurate vortex lattices and oscillation effects in superconducting Mo/Si and W/Si multilayers

We report experimental results of the vortex lattice structure investigation in the artificial superconducting Mo/Si and W/Si superlattices. The resistance R and critical current Ic measurements in parallel magnetic fields have been performed as well as measurements in tilted magnetic fields. At temperatures where condition of strong layering is satisfied the dependences Ic(H||) and R(H||) reveal oscillation behavior. It is shown that the appearance of oscillations and of reentrant behavior (vanishing of resistivity in definite ranges of H||) are due to the strong intrinsic pinning and to the effect of commensurability between the vortex lattice period and multilayer wavelength. The locations of Ic(H||) and R(H||) extrema correspond to the stable states of a commensurate vortex lattice. Our experimental data are in good quantitative agreement with Ivlev, Kopnin, and Pokrovsky (IKP) theory. It is shown that the values of the commensurability fields depend exclusively on the superlattice period s and anisotropy coefficient γ and do not depend on the type of materials used for multilayer preparation. The memory effect, i.e., dependence of the oscillation pattern on the magnetic history of the sample, is observed. It is shown experimentally that the state of the vortex matter in the layered superconductors is essentially different from that of type-II superconductors with a random distribution of the pinning centers. Investigation of oscillation and reentrance behavior may be used as a new tool for vortex lattice arrangement study in layered superconductors. The essential advantage of this method is connected with its simplicity and with the possibility of using it in arbitrary large fields. Investigations of the commensurate states may be used for rather precise determination of the anisotropy coefficient γ

Structural investigations of superconducting multilayers consisting of semiconducting materials

There are rather exotic semiconducting superlattices (SL) consisting of monochalcogenides of Pb, Sn and rare-earth metals which exhibit superconductivity at temperature as high as 6 K. Here we report the results of precision x-ray diffractometry and TEM investigations as well as Auger spectroscopy data obtained on some of the epitaxially grown superconducting semiconducting SLs. It is established that essential features of the SL structure determining the appearance of superconductivity are the perfect single-crystallinity of the samples and the presence of continuous dense grids of misfit dislocations on the interfaces between two semiconductors. The segregation of free Pb which was observed in some cases does not correlate, according to experimental data, with the appearance of superconductivity

Matching and surface barrier effects of the flux-line lattice in superconducting films and multilayers.

The flux-line lattice dissipation and the pinning force of Bi2Sr2CaCu2O8 and YBa2Cu3O7 films and a Nb/Cu multilayer are investigated with the vibrating reed technique. In magnetic fields oriented under a small angle with respect to the film surfaces the Bi-2:2:1:2 film shows a series of pronounced dissipation maxima at matching fields BN in the irreversible region of the magnetic phase diagram. The Y-1:2:3 film shows tiny damping maxima, whereas no structure in the dissipation of the Nb/Cu multilayer is detected below the upper critical field. The comparison of the matching fields to an anisotropic London model shows that the dissipation maxima are caused by rearrangements of the flux-line lattice configuration due to interactions with the sample surface. The different behavior of the high-temperature superconductors and the Nb/Cu multilayer is understood by explicitly taking the surface barrier into account. Deviations from the surface induced commensurability of the flux-line lattice due to the intrinsic pinning are discussed. Our results indicate that pancake vortices in the Bi-2:2:1:2 film should be coupled below the irreversibility line and below magnetic fields B??0.5 T perpendicular to the film surface

SCALING BEHAVIOUR OF RESISTIVE SUPERCONDUCTING TRANSITIONS IN THIN FILMS

On a découvert des lois d'échelle pour les transitions résistives de couches supraconductrices, dues à une variation de température, de courant et de champ magnétique extérieur. On peut expliquer les résultats obtenus à l'aide de la théorie générale des fluctuations de transformations de phase de second ordre.Scaling laws for resistive transitions in superconducting films caused by temperature, transport current and applied magnetic field changes have been found. The results obtained may be explained in terms of the general fluctuational theory of second order phase transitions

Quantum size effect and interlayer electron tunneling in metal-semiconductor superlattices

A new type of quantum size effect in metal-semiconductor superlattices is predicted. Giant oscillations of the transverse tunnel conductivity arise if size quantization of the electron spectrum in the metal layers takes place. This effect is due to the fact that the probability of metal electron tunneling through a semiconductor layer depends sharply on the electron incidence angle. The oscillations have been found to exist even in disordered systems, provided the electrons in metal layers undergo low-angle scattering on imperfections

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

Superconducting and normal properties of the set of Mo/Si superlattices with variable Si layer thickness

We report the results of the superconducting and kinetic parameter measurements (transition temperature Tc, parallel and perpendicular critical fields Hc₂, resistivity in the normal state) on a set of Mo/Si superconducting superlattices with a constant metal layer thickness dₘₒ=22 A and variable semiconducting one dₛᵢ(14-44 A ). Our data show a monotonic dependence of all measured parameters on dₛᵢ. It is found that the Josephson interlayer coupling energy depends exponentially on the spacer thickness. The data obtained allowed us to determine the characteristic electron tunneling length for amorphous silicon with high precision. It is equal to 3.9 A. Enhancement of interlayer coupling leads to the Mo/Si multilayer transition temperature increasing, in agreement with Horovitz theory and with the experimental data on high-Tc materials