N. Ya. Fogel

Interfacial superconductivity in bilayer and multilayer IV–VI semiconductor heterostructures

A comprehensive investigation and comparison of the superconducting properties of bilayer and multilayer epitaxial heterostructures of IV–VI semiconductors exhibiting superconductivity at critical temperatures Tc⩽6.5K is carried out. The superconductivity of these systems is due to inversion of the bands in the narrow-gap semiconductors on account of the nonuniform stresses created by the grids of misfit dislocations arising at the interfaces during the epitaxial growth. It is found that Tc and the character of the superconducting transition of bilayer PbTe∕PbS heterostructures depend on the thickness d of the semiconductor layers and are directly related to the quality of the grids of misfit dislocations at the interfaces (the number and type of structural defects in the grids). Substantial differences in the behavior of bilayer sandwiches and superlattices are found. The minimum thickness d at which superconductivity appears is several times larger for bilayer than for multilayer systems. The upper crit...

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-incommensurate phase transition in the system of vortices and their images?

Abstract The authors attempt to explain their own experimental data on peculiar behaviour of critical currents of films placed in a parallel magnetic field in terms of commensurate-incommensurate phase transition in the system of vortices and their images.

Commensurate vortex lattices in thin vanadium films and in V/Si superconducting superlattices

The features of the Shubnikov phase in thin films of type-II superconductors are investigated in the case when the magnetic field is parallel to the surface of the film. Measurements of the nonmonotonic dependence of the critical current Ic on the magnetic field H∥ reveal commensurate vortex lattices with different numbers of vortex chains in the film. It is proved experimentally that in homogeneous films the commensurability effect between the vortex lattice parameter and the thickness of the film can be observed only for the ideal state of the film boundaries, admitting the formation of an infinite lattice of the vortices and their images. Worsening of the film surface smoothness and of the plane-parallel precision of the two surfaces of the film leads to vanishing of the oscillations of Ic and to a sharp decrease of the critical current. The lock-in transition due to the influence of the surface barrier is observed in the films for the first time. It is found that a thin-film layered sample exhibits an...

“Quantum - size effect” in Mo/Si multilayers

Abstract The oscillating dependences of some kinetic and superconducting characteristics (ρn, ρ300/ρn, Tc,dHc⊥/dT|Tc) on the multilayer period D have been observed on superlattices Mo/Si. Extremum positions for all these dependences coincide, the period of oscillations being 35 A. These oscillating dependences resemble closely those observed on single films when the quantum size effect takes place. However the explanation of the results for multilayers in the same terms is rather doubtful due to a strong disorder in Mo layers. Different physical characteristics of the multilayers investigated reveal 2D and 3D behavior on the same samples.

Structural investigations of superconducting multilayers consisting of semiconducting materials

There are rather exotic semiconductor 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 semiconductor SLs. It is established that the 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.

Mo/Si superlattices: phase transitions between the commensurate states in the vortex ensemble

The avalanche-type jumps of the resistivity in the mixed state have been observed on the artificial superconducting multilayers Mo/Si with weak Josephson interlayer coupling when an applied magnetic field was oriented along the layer planes. These jumps are periodic with an inverse magnetic field. The simple model is proposed which explains the jump patterns on different samples as the manifestation of the phase transitions between various commensurate states of the vortex lattice. The values of the jump fields and the distances between these fields are quantitatively consistent with the model considered. It is assumed that the set of the jumps corresponds to the sequence of the upper critical fields for different commensurate lattices in the strongly layered superconductor. These phase transitions belong to the class of the first order transitions in distinction to the corresponding transition in homogeneous type-II superconductors. The resistive curves are hysteretic and reveal considerable supercooling.

Mo/Si and W/Si multilayers: commensurated vortex lattice structures and superconductivity reentrance

Abstract We report on the unusual behavior of Mo/Si and W/Si multilayers (MLs) in parallel and slightly inclined to the ML planes magnetic fields. Field dependencies of resistivity are nonmonotonic and display minima or zero resistance regions (ZRR) in magnetic fields different from zero. We show that such a behavior is connected with strong intrinsic pinning and vortex lattice (VL) commensurability with underlying layered structure. The locations of ZRR correspond to stable VL configurations.

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.

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

We report experimental results of a vortex lattice structure investigation in artificial superconducting Mo/Si and W/Si superlattices. Resistance R and critical current Ic measurements in parallel magnetic fields are performed as well as measurements in tilted magnetic fields. At temperatures where the condition of strong layering is satisfied the dependences Ic(H∥) and R(H∥) exhibit oscillatory behavior. It is shown that the appearance of oscillations and of reentrant behavior (vanishing of the 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 the 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 coe...