Stability limits, structure, and relaxation of a mixed state in superconducting films with an edge barrier

The interval of external magnetic fields where a mixed state can exist in a superconducting film with a fixed value of the flux is determined. The equilibrium and reversible magnetization curves of the sample are calculated, and the dynamics of the magnetic relaxation in the film is described. Nonuniform deformation of the Abrikosov lattice is predicted. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 5, 405–410 (10 March 1997)     

Propagation of magnetostatic waves in a ferrite plus high-T c superconductor structure in the presence of transport current in the superconductor

A thin-film structure consisting of a ferrite and a high-T _c superconductor was used to investigate the effct of the transport current in the superconductor on the amplitude-frequency characteristic and dispersion of surface magnetostatic waves (MSWs) in a ferrite film. It was found that the nature of energy transfer between the MSWs and the superconducting film undergoes a significant change as the transport current is varied. In particular, in one of the current ranges, energy can be transferred both from the MSWs to the superconductor and back again, whereas in another range it can only be efficently transferred to the superconductor. Fiz. Tverd. Tela (St. Petersburg) 39, 2195–2202 (December 1997)     

The statistical distribution of magnetic critical currents determined by HTSC film morphology

An analysis is made of the geometric and morphological properties of YBCO superconducting films and of their effect on the magnetic and transport phenomena. A study is presented of the statistical characteristics of critical currents derived from the variation of trapped magnetic flux induced by passing a pulsed transport current. It is shown that the critical currents in the materials under study have a specific statistical distribution, whose main properties are due to the morphology of their structure and can be determined by geometric-probability analysis. The superconducting film is considered as a percolation system. An empirical normal-phase cluster-area distribution function was used to derive the distribution function of magnetic critical currents, which describes adequately the experimental data on how transport current affects trapped magnetic flux. The critical current for transition of a film to the resistive state has been calculated. Fiz. Tverd. Tela (St. Petersburg) 41, 1739–1742 (October 1999)     

Ac magnetic field screening by high-Tc superconductor films and single-crystals

A method is proposed to measure the screening efficiency of an ac magnetic field by superconducting films with the use of a strip line as an ac magnetic field source capable of operating within a broad frequency range. The dependence of the efficiency of ac magnetic field screening by YBCO films and single-crystal BiSrCaCuO on temperature and the frequency of magnetic field oriented perpendicular to the sample plane has been studied in the 10⁴–10⁹-Hz range. This relation is shown to depend substantially on the technique used in the film fabrication. Fiz. Tverd. Tela (St. Petersburg) 39, 228–230 (February 1997)     

Nonlinear acoustoelectronic interaction in thin superconducting lead films

The effect of high-power surface acoustic waves (SAW) on the superconducting state of Pb films has been studied in different transverse magnetic fields. It was established that a high-intensity sound wave affects the T _c of the film and the character of the superconducting transition, and that the observed changes are the larger, the higher is the SAW intensity. It was found that high-power SAW are capable of inducing vortex depinning in the film and reducing the critical current. Various mechanisms which are responsible for the nature of these effects are discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1753–1760 (October 1997)     

Structure of the mixed state induced in thin YBaCuO superconducting films by the field of a small ferromagnetic particle

The temperature dependence of the local energy barrier to formation of the mixed state in YBaCuO thin-film superconducting samples has been determined. The measurement technique is based on use of a small ferromagnetic particle as the magnetic field source. It is found that the energy barrier to creation of vortices (for the field oriented parallel to the CuO planes) is anomalously small while the dependence of the corresponding threshold current j _c(T) differs substantially from the temperature dependence of the pair-breaking current. The experimental results are interpreted in terms of the model of a Josephson medium. The observed temperature dependence of j _c points to a strong suppression of the superconducting order parameter at the intergrain boundaries, which for the most probable type of boundaries: superconductor-insulator-superconductor, is evidence of anisotropic pairing. Zh. éksp. Teor. Fiz. 116, 1735–1749 (November 1999)     

Anomalous millimeter-wave absorption in the superconducting phase of La2−x SrxCuO4

Direct measurements of the complex conductivity spectra of thin-film La_2−2x Sr_xCuO₄ are made at frequencies of 5–40 cm⁻¹. Narrow, intense Drude-type excitation is observed in the superconducting phase. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 406–409 (10 September 1998)     

Calculation of the force characteristics of a multi-coil suspension of a superconducting sphere

The theory presented in the accompanying paper [Ref. 1: Zh. Tekh. Fiz. 67(1), 3 (1997); Tech. Phys. 42, 1 (1997)] for the calculation of the magnetic field and the force characteristics of an electromagnetic suspension for a superconducting body is used to carry out specific calculations of the force characteristics of a multi-coil suspension of a superconducting sphere. Formulas are obtained for the dependence of the force and the stiffness, from which it is possible to approach the problem of the stable suspension of a superconducting sphere in the field of a system of circular currents. The problem of determining the magnetic field at the sphere is analyzed and the conditions are found for which it is less than the critical field. Zh. Tekh. Fiz. 67, 10–16 (January 1997)     

Dielectric relaxation in thin-film metal-PZT-ferroelectric-metal structures

The parameters of the Colle-Colle-type distribution function for the time constant of Pb(Zr_0.52Ti_0.48)O₃ thin-film layers and the pattern of the bulk resistivity distribution through the film thickness have been derived from measured frequency dependences of the complex capacitance of metal-PZT-ferroelectric-metal thin-film structures within the 100 Hz to 100 MHz range. Fiz. Tverd. Tela (St. Petersburg) 40, 1915–1918 (October 1998)     

Characteristics of film type inductors using a FeZrBAg magnetic material

Two types of double rectangular spiral thin-film inductors with a multi-layer structure of magnetic film/conduction film/magnetic film were fabricated using a Fe_86.7Zr_3.3B₄Ag₆ thin film with high permeability and resistance, in which easy axis of magnetization of the thin-film was perpendicular or parallel to the current direction. The dependence of inductance and resonance frequency on the inductor type indicated that the perpendicular geometry inductor has higher inductance than the parallel geometry one, because spins of magnetic film align more easily along the field direction due to higher field intensity in the perpendicular geometry. However, the increase of the inductance, resulted in the decrease of resonance frequency. Then the effects of permeability of the magnetic film on inductance and resistance of the perpendicular geometry inductor were investigated. The permeability was controlled by annealing the film at different temperatures. With increasing the permeability, the inductance increased, but the resistance also increased due to the increase in magnetic core loss. As the resonance frequency was higher in air-core inductor than in magnetic thin-film core inductor, it is suggested to increase the resonance frequency which is characteristic of the air-core inductor rather than magnetic properties of the magnetic thin-film should be enhanced.     

Angular dependence of magnetoresistivity in c-oriented MgB thin film

The anisotropy of MgB₂ is still under debate: its value, strongly dependent on the kind of sample and on the measuring method, ranges between 1.2 and 13. In this work we present our results on MgB₂ c-oriented superconducting thin film. To evaluate the anisotropy, we followed two different approaches. Firstly, magnetoresistivity was measured as a function of temperature at selected magnetic fields applied both parallel and perpendicular to the c-axis; secondly, we measured magnetoresistivity at selected temperatures and magnetic fields, varying the angle θ between the magnetic field and the c-axis. The anisotropy estimated from the ratio between the upper critical fields parallel and perpendicular to the c-axis and the one obtained in the framework of the scaling approach within the anisotropic Ginzburg-Landau theory are different but show a similar trend in the temperature dependence. Some differences in the upper critical field and in its anisotropy of our film with respect to single crystals are emphasized: some of these aspects can be accounted for by an analysis of upper critical fields within a two-band model in presence of disorder and/or crystallographic strain. Received 12 July 2002 / Received in final form 17 September 2002 Published online 29 November 2002     

Incommensurateness effects in a lattice Ginzburg-Landau model

This paper discusses the effect of magnetic translational symmetry on the vortex structure in superconducting crystals with a large basis in artificial Josephson media (regular lattices of superconducting clusters) prepared with opal as the base material. For external magnetic fields lower than the upper critical field, the lattice Ginzburg-Landau model reduces to the two-dimensional Frenkel’-Kontorova model which in some cases is exactly solvable, in which the crystal lattice plays the role of an “hard sublattice” while the deformable vortex lattice plays the role of a “soft sublattice.” It is shown that static shear waves in the vortex lattice are solutions to the two-dimensional sine-Gordon equation with an additional condition of incompressibility implied by flux quantization. The pinning energy is found as a function of the magnetic field, nearness to the transition line, and the crystal lattice constant. Fiz. Tverd. Tela (St. Petersburg) 39, 1158–1162 (July 1997)     

The phase diagram of the superconducting state of superconductor-band-antiferromagnet superlattices

The formation of the superconducting phase in short-period proximity-effect layered superlattices of the superconductor-band-antiferromagnetic-metal (SC/AF) type is studied. The exact solution of the Usadel equations is used to discuss the possibility of formation in such structures of a ground state in which the order parameters of the adjacent superconducting layers have opposite signs (the “π-phase”). The dependence of the superconducting transition temperature and the upper critical field normal to the layers on the lattice period, the intensity of magnetic interaction in the antiferromagnetic layer, and the state of the interface between the layers is examined. It is found that there exists a nonlinear dependence of the conditions for the appearance of the superconducting state in a layered SC/AF system on the system’s parameters. Finally, the conditions for the appearance of the superconducting phase in proximity-effect superlattices consisting of a superconductor with nonmagnetic, ferromagnetic, and antiferromagnetic metals are compared. Zh. éksp. Teor. Fiz. 111, 547–561 (February 1997)     

Composite spin-triplet superconductivity in an symmetric lattice model

The two-channel Anderson lattice model which has SU (2) ⊗ SU (2) symmetry is of relevance to understanding of the magnetic, quadrupolar and superconducting phases in U_1-xTh_xBe₁₃ or Pr based skutterudite compounds such as PrFe₄P₁₂ or PrOs₄Sb₁₂. Possible unconventional superconducting phases of the model are explored. They are characterized by a composite order parameter comprising of a local magnetic or quadrupolar moment and a triplet conduction electron Cooper-pair. This binding of local degrees of freedom removes the entropy of the non Fermi-liquid normal state. We find superconducting transitions in the intermediate valence regime which are suppressed in the stable moment regime. The gap function is non analytic and odd in frequency: a pseudo-gap develops in the conduction electron density of states which vanishes as |ω| close to ω = 0. In the strong intermediate valent regime, the gap function acquires an additional -dependence. Received 28 February 2002 / Received in final form 18 April 2002 Published online 9 July 2002     

Superconducting resonator in strong microwave fields: Thermal domain, nonlinear thermal effects

The thermal nonlinearity of superconducting films in an electromagnetic field is investigated theoretically. It is shown that the transition of the superconductor from the superconducting to the normal state as a result of Joule heating exhibits hysteresis, corresponding to the onset of a bistable state in the film. It is shown that as a result of the nonuniformity of the amplitude of the electromagnetic field along the surface of the superconducting film, above some threshold a normal-phase domain forms in the superconductor. It is ascertained that the temporal evolution of the domain walls exhibits the character of thermal autowaves of heating or cooling. The nonlinear radio-frequency characteristics (reflection and transmission coefficients and frequency responses) of a stripline resonator under conditions of thermal nonlinearity are calculated. Zh. Tekh. Fiz. 67, 81–89 (October 1997)     

Dynamic magnetic susceptibility of a superconductor with a relaxing magnetic moment

Within the framework of the diffusion approximation for the dynamic magnetic flux, a model is constructed for the dynamic magnetic susceptibility of a superconductor with a nonuniform and time-dependent magnetic field distribution in the sample. The possibility of appearance of peaks in the temperature (or frequency) dependence of χ″ in a structurally homogeneous superconducting sample in the presence of a magnetic-field-induced nonuniformity of the diffusion parameter is demonstrated. The character of the temporal evolution of the magnetic susceptibility during relaxation of the magnetic moment of the sample is predicted and its properties are investigated. Fiz. Tverd. Tela (St. Petersburg) 39, 811–815 (May 1997)     

de Haas-van Alphen effect in superconductors

A theory of the de Haas-van Alphen effect in type-II superconductors is proposed. The effect of the electron scattering by nonmagnetic impurities in a magnetic field in the potential produced by a nonuniform distribution of the order parameter in a mixed state is investigated. The magnitude of the order parameter and quasiparticle density of states are determined from the solution of the system of Gor’kov equations. It is shown that in the presence of even a small amount of impurities, the superconducting state near the upper critical field is gapless. In this region, the oscillatory (in the magnetic field) contribution to the density of states and the characteristic damping of the amplitude of the magnetization oscillations in the superconducting state are found. Zh. éksp. Teor. Fiz. 112, 1873–1892 (November 1997)     

Theory for the calculation of the force characteristics of an electromagnetic suspension of a superconducting body

A theory based on the method of secondary sources is developed for the calculation of the magnetic field and the force characteristics of electromagnetic suspension. This method leads to a system of Fredholm vector integral equations of the second kind in the density of the secondary sources, whose solution gives the surface currents in the superconducting bodies and then by a simple integration, the magnetic field and the force characteristics of the suspension. It is shown how the problem can be reformulated to apply it to the determination of the scalar secondary sources (magnetic charges), leading to integral equations of lower dimension. Examples are given for the calculation of scalar secondary sources for a superconducting half-space and a cylinder. Zh. Tekh. Fiz. 67, 3–9 (January 1997)     

Intrinsic and mutual inductance of apertures in parallel superconducting films

Analytic expressions are derived to calculate the inductance of apertures in parallel superconducting films using the current distribution along the edge of the aperture in the film. The relations obtained show good agreement with the experimental results and can be used to model rf squids. Zh. Tekh. Fiz. 68, 75–79 (December 1998)