Magnetotransport measurements have been carried out for pristine and irradiated Y Ba2Cu3O7−x films. The measurements reveal a crossover from the clean superconducting limit to the dirty one with an increase in the impurity scattering rate Γ produced by a certain dose of irradiation. For the clean limit, the critical temperature Tc0 decreases with Γ, while the initial slope K=|dHc2/dT|Tc0 increases. We find a nice agreement between these experimental data and a model of d-wave superconductors with anisotropic impurity scattering. Such dose-dependencies of Tc0 and K reveal the superconducting coherence length ξ0 to be quasi-invariant in the clean limit. For the dirty limit, ξ0 increases with Γ due to a decrease in both the quantities Tc0 and K. Though, this decrease appears slower than predicted by the textbook picture of d-wave superconductors. Such discrepancy can be produced by an irradiation-induced modulated disorder responsible for the formation of (d+s)-wave pairing.

Magnetotransport measurements were carried out for YBa 2 Cu 3 O 7-x (YBCO) thin films in external perpendicular magnetic fields of H. The studies were performed both for the virgin samples and for the irradiated ones. Xenon ions were used as an external irradiation. Thus we studied features of the broadening of superconducting transition in YBCO films (virgin and irradiated). The broadening of superconducting drop was analyzed depending on an external magnetic field H, as well as on an irradiation dose n D . When processing the experimental data R(H,T), we studied a criterion for determination of temperature dependence of the upper critical field H c2 (T). The criterion was analyzed depending on the defect concentration in the film corresponding to a certain value of n D . It was found out that for a virgin sample, H c2 should be determined by the resistance level R=0.4R N inside the superconducting transition, where R N =R(T=100 K). With a gradual increase in n D , this resistance level decreases. At sufficiently high radiation doses n D >7·10 12 cm -2 , the H c2 (T) phase transition line should be determined by the level R~0. Keywords: thin films, HTSC, xenon ion irradiation, magnetotransport studies, broadening of the superconducting transition, upper critical field, phase transition line, Tinkham's formula, Abrikosov vortices.

Magnetotransport measurements were carried out for YBa_2Cu_3O_{7-x} (YBCO) thin films in external perpendicular magnetic fields of H. The studies were performed both for the virgin samples and for the irradiated ones. Xenon ions were used as an external irradiation. Thus we studied features of the broadening of superconducting transition in YBCO films (virgin and irradiated). The broadening of superconducting drop was analyzed depending on an external magnetic field H, as well as on an irradiation dose n_D. When processing the experimental data R(H, T), we studied a criterion for determination of temperature dependence of the upper critical field H_{c2}(T). The criterion was analyzed depending on the defect concentration in the film corresponding to a certain value of n_D. It was found out that for a virgin sample, H_{c2} should be determined by the resistance level R = 0.4R_N inside the superconducting transition, where R_N = R(T = 100 K). With a gradual increase in n_D, this resistance level decreases. At sufficiently high radiation doses n_D > 7·10^{12} cm^{-2}, the H_{c2}(T) phase transition line should be determined by the level R ≈ 0.

The disorder effect on superconducting properties of thin-film YBCO nanostructures in external magnetic fields is experimentally studied. The disorder was produced by irradiation with xenon ions. The research included transport measurements of narrow bridges based on HTSC YBCO films (thickness 50 nm) in strong magnetic fields (up to 12 T). Thus, for samples with different degrees of disorder, critical dependencies have been studied, i.e. the Hc2(T) phase transition line, the Hirr(T) irreversibility line, etc. The dependences of the mean-free path and critical temperature on the concentration of defects created by ion irradiation have been experimentally studied. The experimental data are described using formulas obtained within the framework of well-known models, such as the Ginzburg-Landau theory, the Drude theory and the Gorkov equations.

The disorder effect on superconducting properties of thin-film YBCO nanostructures in external magnetic fields is experimentally studied. The disorder was produced by irradiation with xenon ions. The research included transport measurements of narrow bridges based on HTSC YBCO films (thickness 50 nm) in strong magnetic fields (up to 12 T). Thus, for samples with different degrees of disorder, critical dependences have been studied, i.e. the H c2 (T) phase-transition line, the H irr (T) irreversibility line, etc. The dependences of the mean-free path and critical temperature on the concentration of defects created by ion irradiation have been experimentally studied. The experimental data are described using formulas obtained within the framework of well-known models, such as the Ginzburg-Landau theory, the Drude theory, and the Gorkov equations. Keywords: B HTSP, thin films, ion irradiation, defects, resistive measurements, upper critical field, line of irreversibility, vortices.

The original research results for thin disordered HTSC films based on YBCO are presented in this article. Several experiments have been carried out to confirm a theoretical prediction that the s-phase of superconducting pairing in disordered d-type superconductors can occur with a gradual decrease in the mean free path. The YBa_2Cu_3O_{7-x} films were used as samples which possess a d-type of superconducting state that was experimentally confirmed. A gradual decrease in the mean free path was achieved by both thermal annealing and ion irradiation. The experiments included measurements of temperature dependence of the London penetration depth λ and resistive studies.

We report the effect of disorder on superconducting phase transition of YBa2Cu3O7-x epitaxial thin films in external magnetic fields. The disorder was produced by several successive acts of oxygen ion implantation. Controlling a total accumulated dose of implanted ions n(D), we carried out transport measurements in ab-plane for temperatures T below 91 K and external magnetic fields H up to 11 T. Temperature-field dependencies of in-plane resistivity allow us to analyze H - T phase diagrams for primary compound as well as for disordered structure. Considering the upper critical field H-c2 as the magnetic field which corresponds to a local resistivity drop at the onset of superconducting transition, we found out the following results. By gradual increasing of n(D), the phase-transition line H-c2(T) suffers an unconventional critical-field slope reduction, while larger defect concentrations usually enhance the upper critical field in the vicinity of T-c0. Besides, for rather large n(D), the curvature of H-c2(T) becomes upward for temperatures close to T-c0. Theoretical interpretation of the experimental data is developed in the framework of linearized Ginzburg-Landau theory with an inhomogeneous superconducting coherence length. A simple expression for the critical temperature T-c is obtained: T-c = T-c0 (1 - h + alpha h(3/2)) ,where h is the dimensionless magnetic field and alpha is a constant which describes the defects in a specimen. The formula nicely fits our experimental results.

An unusual decrease in the slope of the upper critical field near Tc0 at a gradual increase in the ion implantation dose has been experimentally observed in narrow bridges formed on the base of thin HTSC YBa2Cu3O7 – x films, while an increase in the defect concentration usually leads to an increase in the local slope of the phase transition line Hc2(T). In addition, it has been found that the temperature dependence of the upper critical field has a positive curvature near Tc0. A possible theoretical interpretation of the results is proposed. It is based on the modified Ginzburg–Landau theory with a nonuniform length of superconducting correlations.

AbstractAn unusual decrease in the slope of the upper critical field near T _ c 0 at a gradual increase in the ion implantation dose has been experimentally observed in narrow bridges formed on the base of thin HTSC YBa_2Cu_3O_7 – _ x films, while an increase in the defect concentration usually leads to an increase in the local slope of the phase transition line H _ c 2( T ). In addition, it has been found that the temperature dependence of the upper critical field has a positive curvature near T _ c 0. A possible theoretical interpretation of the results is proposed. It is based on the modified Ginzburg–Landau theory with a nonuniform length of superconducting correlations.

Within the framework of the Ginzburg–Landau theory, we study the features of the localized nucleation of the order parameter in superconducting systems with inhomogeneous effective mass m of the Cooper pairs, which is due to the spatial modulation of the diffusion coefficient and/or fluctuations in the local anisotropy axis in the sample. In the asymptotics of the weak magnetic fields H, for which the magnetic length [Φ0/(2πH)]1/2, where Φ0 is the magnetic-flux quantum, is much shorter than the inhomogeneity scale, the spatial scale of the order parameter is determined by the sample-average coherence length and the regular lattice of the Abrikosov vortices is formed in the superconductor. In sufficiently strong magnetic fields H, the order parameter is localized near the minima of the coherence length ξ ∝ m −1/2, which results in an increase in the critical temperature and destruction of the regular lattice of the Abrikosov vortices. Therefore, competition between the two superconductivity-nucleation types is observed during a gradual increase in the magnetic field, which leads to the positive curvature of the phase-transition line. We have also studied the features of the temperature dependences of the upper critical magnetic field for some model spatial mass profiles of the Cooper pairs. The obtained results are in good agreement with direct numerical calculations.

We suggest a simple model describing the temperature-driven crossover between Abrikosov vortex lattice and superconducting droplet state in dirty superconductors with fluctuations either in the impurity concentration or in the crystal axes orientation. Our analysis is based on the Usadel-type theory with a spatially modulated diffusion coefficient. This modulation appears to break a regular vortex lattice into a random set of weakly coupled superconducting droplets emerging below the fluctuating upper critical field H-c2(T). These droplets cause the resistivity drop at the onset of superconducting transition, being responsible for the increasing broadening of the resistive transition in the increasing magnetic field. The above crossover reveals itself in a positive curvature of the H-c2(T) curves, allowing us, thus, to explain the phase diagrams observed in a wide class of disordered superconducting materials.

The distinctive features of current-voltage characteristics are studied for mesoscopic multiterminal structures effected by external irradiation. Considering a simple model of applied dc+ac voltage, we calculate Shapiro-like steps in Josephson systems with several weakly coupled superconducting electrodes. Owing to the action of an external alternating signal, the dc current is found to be rather increased than it appears in the same multiterminal Josephson node without the radiation. The possible applications of our results for the experimental observation of the Shapiro-like steps in such Josephson structures are discussed. (C) 2016 Elsevier B.V. All rights reserved.

•The scattering-matrix approach was applied for multiterminal Josephson nodes.•The current-phase relations were calculated for a three-terminal system.•The applications of our results and typical experimental examples were discussed.

Within the Ginzburg-Landau model we study the critical field and temperature enhancement for crossing superconducting channels formed either along the sample edges or domain walls in thin-film magnetically coupled superconducting - ferromagnetic bilayers. The corresponding Cooper pair wave function can be viewed as a hybridization of two order parameter (OP) modes propagating along the boundaries and/or domain walls. Different momenta of hybridized OP modes result in the formation of vortex chains outgoing from the crossing point of these channels. Near this crossing point the wave functions of the modes merge giving rise to the increase in the critical temperature for a localized superconducting state. The origin of this critical temperature enhancement caused by the wave function squeezing is illustrated for a limiting case of approaching parallel boundaries and/or domain walls. Using both the variational method and numerical simulations we have studied the critical temperature dependence and OP structure vs the applied magnetic field and the angle between the crossing channels.

We have studied distinctive features of magnetic field distributions induced by vortices in thin films of anisotropic superconductors for an arbitrary orientation of external tilted magnetic fields. The magnetic field profiles are found to be strongly modified for rather small film thickness and large field tilting angle. The single-vortex field profile reveals two maxima instead of one peak observed in relatively thick films or in the films placed in magnetic fields perpendicular to the film plane. Our calculations are based on the study of the energetically favorable shape of an isolated vortex line in the presence of the inhomogeneous supercurrents which screen the field component parallel to the film plane. Starting from the London theory with anisotropic mass tensor we justify the elastic string approximation for the vortex lineshape affected by the screening currents. The exotic magnetic field profiles typical for curved vortices can be observed by modern vortex imaging techniques.

The distinctive features of equilibrium vortex structures in thin films of anisotropic superconductors in tilted magnetic fields are studied for the limits of moderate and strong anisotropy. The energetically favorable shape of isolated vortex lines is found in the framework of two particular models describing these limiting cases: London theory with an anisotropic mass tensor and London-type model for a stack of Josephson--decoupled superconducting layers. The increase of the field tilting is shown to result in qualitative changes in the vortex--vortex interaction potential: the balance between long--range attractive and repulsive forces occurs to be responsible for a formation of a minimum of the interaction potential vs the intervortex distance. This minimum appears to exist only for a certain restricted range of the vortex tilting angles which shrinks with the decrease of the system anisotropy parameter. Tilted vortices with such unusual interaction potential form clusters with the size depending on the field tilting angle and film thickness or/and can arrange into multiquanta flux lattice. The magnetic flux through the unit cells of the corresponding flux line lattices equals to an integer number $M$ of flux quanta. Thus, the increase in the field tilting should be accompanied by the series of the phase transitions between the vortex lattices with different $M$.