E. E. Pestov

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...

Phase transitions in hybrid SFS structures with thin superconducting layers

Features of a phase transition between 0 and π states in superconductor/ferromagnet/superconductor (SFS) Josephson structures with thin superconducting layers and a ferromagnetic barrier are studied experimentally and theoretically. The dependence of the critical temperature Tc of a transition of the hybrid structure to a superconducting state on the thickness of superconducting layers ds is analyzed by a local method involving measurements of the nonlinear microwave response of the system by a near-field probe. An anomalous increase in the measured temperature Tc at the reduction of the thickness ds is detected and is attributed to the 0-π transition.

Effect of the microstructure of epitaxial YBa2Cu3O7 − x films on their electrophysical and nonlinear microwave properties

The local nonlinear microwave response of YBa2Cu3O7 − x films was measured by near-zone field microscopy with a spatial resolution of 50 μm, and YBa2Cu3O7 − x film microbridges were locally studied by low-temperature scanning microscopy with a spatial resolution of 4 μm. The microstructure of epitaxial YBa2Cu3O7 − x films was examined using x-ray diffraction and electron microscopy. A correlation is detected between the average crystallite size and the half-widths of the temperature dependences of the third-harmonic power (WTH) and the electron-beam-induced voltage (WEBIV). The experimental results are described in terms of a model of a two-phase medium taking into account the nonlinear I-V characteristic of the superconductor. For large crystallites, the nonlinear microwave response is shown to be caused by intracrystallite vortex pinning. As the average crystallite size decreases, an additional contribution to the nonlinear response appears due to the pinning of a magnetic flux by the Josephson network of crystallite boundaries. Calculations show that a three-fold increase in the crystallite size decreases the nonlinearity coefficient of YBa2Cu3O7 − x films by two orders of magnitude.

Accuracy of the new voltage standard using Josephson junctions cooled to 77 K

A practical dc voltage standard with output voltages Ł/hts from 0.1 V to 10 V using high temperature superconductor (HTS) arrays of Josephson junctions cooled to 77 K was characterized. HTS arrays with increased current margins and the voltage divider with a high stability of the dividers coefficients enable stable and reproducible automated calibration of the new HTS standard. The direct comparison of UHTS against the voltage generated on an array of niobium Josephson junctions cooled to 4.2 K reveals the agreement of voltages at 1 V level with a Type A uncertainty equal to a few parts in 108.

High-temperature superconductor Josephson junctions for voltage standards

This paper presents the results of investigations of the high-frequency properties of multi-junction Josephson circuits based on high-temperature superconductors and the specific features of their use in voltage standards.

Study of the Nonlinear Response of Superconductors in the Microwave Band Using a Local Technique

Using a local technique, we study the microwave radiation power P3ω(T,Pω,Hdc) at the triple frequency of the main signal as a function of the temperature T, the input power Pω, and the external magnetic field Hdc perpendicular to the superconductor surface for YBa2Cu3O7 (YBCO) films, monocrystals, and polycrystals and for Nb films. The most distinctive feature of the temperature dependence P3ω(T) of nonlinear response of superconductors is a maximum of the nonlinearity near the critical temperature Tc. Spatial distributions of the third-harmonic power are obtained for YBCO films at various temperatures. These distributions are indicative of a nonuniform distribution of Tc over the superconductor surface. Additional nonlinearity maxima are discovered in the dependence P3ω(T) for YBCO films and monocrystals at temperatures about 2Tc/3. These maxima are probably related to the existence of several superconducting phases with different critical temperatures. For Nb films, the second nonlinearity maximum in the dependence P3ω(T) appears only in the presence of an external magnetic field. The experimental data are interpreted within the framework of a two-fluid model of a superconductor, which takes into account the phenomenological nonlinear relationship between the vector potential A and the supercurrent js(A). The origin of nonlinearity in the studied superconductors is discussed.

Peculiarities of the Resistive State in Mo/Si Superlattices in a Magnetic Field

The experimental investigations of the transport properties of the superconducting Mo/Si multilayered structures are presented. The observed anisotropy of the electrical resistivity of Mo/Si bridges in a weak magnetic fields H ≲ 5 × 102Oe is in agreement with the data reported earlier. However, for the Mo/Si microbridges with the width 2 μm in a transverse magnetic field H ≳ 5 × 102Oe we have found out that the temperature dependence of the in-plane resistivity R(T) demonstrates a sharp change of the resistivity at a certain critical temperature T* < Tc, where Tc is the superconducting transition temperature in a zero magnetic field. For a parallel magnetic field this step-like behavior has not been revealed; we neither observed this effect in wider bridges. The comparison of the transport properties and the nonlinear microwave characteristics is presented. A possible origin of this unusual temperature dependence of R(T) is proposed.

Microwave properties of high-temperature Josephson contacts on a sapphire bicrystal substrate

HTS bicrystal junctions up to 50 μm wide on sapphire substrates have been studied. The dependences of the critical current on the temperature and external magnetic field of these contacts have been measured. The irradiation of the Josephson junction on a sapphire substrate at the frequency of 73 GHz at the temperature of 77 K resulted in the appearance of Shapiro steps in the current–voltage characteristic (IVC) at the voltage of 150 μV. The possibility of using such contacts in voltage standards at the temperature of 77 K in the ranges of microwave and terahertz frequencies has been analyzed.

Nonlinear microwave properties of YBa2Cu3O7−x block epitaxial films near the superconducting transition temperature

The structure and local properties of a nonlinear microwave response of YBa2Cu3O7−x block films, as well as microbridges of these films, are studied using low-temperature scanning microscopy. Correlation between the halfwidth of the temperature dependence of the third harmonic power WTH and the curve of the electron-beam induced voltage WEBIV is observed for the medium size of the block. A theoretical model is proposed, which demonstrates that the nonlinear microwave response for large blocks is determined by intrablock pinning of Abrikosov vortices; for blocks of smaller size, the response acquires an additional contribution associated with pinning of hypervortices on the Josephson network of the boundaries between the blocks.

Effect of cation composition on the superconducting properties and on the microstructure of YBaCuO thin films

The surface morphology and superconducting properties of YBaCuO epitaxial films prepared through magnetron sputtering from targets of different cation composition were systematically studied. It was shown that small changes in the growth conditions and relatively small variations in the cation composition of the condensate noticeably affect the surface morphology of the films and their structural and superconducting properties, thus offering an efficient way of controlling the YBCO film parameters. It was found that the 90° off-axis configuration of the magnetron sputtering system permits realization of growth conditions in which the grown films do not contain CuO precipitates and exhibit good superconducting properties (Tc≥88 K, jc(77 K)≥4×106 A/cm2).