A. L. Kasatkin

Microwave impedance of YBa2Cu3O7−δ high-temperature superconductor films in a magnetic field

The temperature, field, and intensity (amplitude) dependences of the surface impedance of magnetron-sputtered YBa2Cu3O7−δ quasi-single-crystal films on a sapphire substrate with a CeO2 buffer layer were measured. The measurements were performed with a coplanar resonator at 5.25 GHz in a weak constant magnetic field 0<B<12 mT in the temperature range 13 K <T<80 K. They made it possible to obtain the surface resistance and penetration depth versus the temperature and the magnetic field strength and to determine the contribution of Abrikosov vortices to the impedance. The cases with a frozen magnetic field and zero-field cooling with the field subsequently switched on at temperatures below the critical value were investigated separately. A substantial difference in the behavior of the high-frequency response of the vortices in these two cases was found. The Coffey-Clem and Brandt theories were used to study the linear response and the critical state model the nonlinear response of the vortices.

Vortex transfer mechanisms in C-oriented YBCO films with small-angle-boundaries

The mechanism of vortex depinning and motion along the row of parallel equidistant linear pins in an anisotropic type-II superconductor is considered, The essential role of the surface effects for these processes is emphasized, The activation energy of vortex motion and the critical current value are calculated for the current flow perpendicular to this row. We suppose this model to be applicable for description of resistive properties and the critical current dependence on the azimuthal misorientation angle between neighboring grains for YBCO bicrystals and rather thick c-oriented films with low-angle tilt grain boundaries, containing edge dislocations, which are parallel to the c-axis.

Vortex pinning/dynamics in YBCO films with J{sub c} (77 K) {gt} 3{center{underscore}dot}10{sup 6} A/cm{sup 2} studied by direct transport measurements

Critical current density, Jc, is studied for highly biaxially-oriented YBCO thin films with Jc(77 K) > 3 MA/cm2in a range of magnetic fields, temperatures, angles between magnetic field vector and film c-axis. Jc(H, θ) is shown to be dominated by vortex interaction with high density (> 1011lines/cm2) of linear pins — edge dislocations. A model is developed to describe different vortex arrays behavior in the presence of two-dimensional square network of linear pins.

Nonlinear Microwave Properties of High-Temperature Superconducting YBa 2 Cu 3 O 7-δ Single Crystal Thin Films

This work establishes the possible sources of nonlinear microwave response in perfect epitaxially-grown single-crystal HTS YBa2Cu3O7-δ (YBCO) thin films, their origin and conditions at which they become essential, as well as possibilities for their mitigation. Microwave measurements of YBCO thin film coplanar and strip-line resonators are performed at different experimental conditions. The nonlinear surface resistance of these resonators are studied as a function of the rf current amplitude at different temperatures and dc magnetic field for field-cooled experiments.

The Nonlinear Microwave Surface Impedance of HTS Cuprate YBA 2 CU 3 O 7-δ Thin Film Patterned as a Stripline Resonator

Our measurements performed at low input power levels (P_in ≪ P_pit) have revealed the nonlinear character of the surface resistance R_s(I₀) of the YBCO film under study. It is shown that at low enough amplitudes I₀ of the microwave current in the resonator R_s(I₀) ˜ log(I₀), while at high enough I₀ amplitudes (which are less than the critical current value I_c for the constriction) the R_s(I₀) dependence is quadratic: R_s(I₀) = α(ω,j_cw,d)I₀₂ with a proportionality coefficient α, depending on the microwave frequency ω, critical current density j_c, resonator width w and thickness of the film d: α(ω, j_c, w, d) ˜ ω(j_c ² d²w). This type of nonlinear Rs(I₀) dependence (at least at high enough I₀ values) allows to assume that the origin of nonlinearity is related to the entry of microwave induced Abrikosov vortices through the film edges.

Nonlinear flux diffusion in superimposed weak AC magnetic field

The influence of superimposed weak ac magnetic field on flux creep and relaxation phenomena is studied theoretically for a superconducting slab in superimposed dc and weak ac magnetic fields parallel to the slab surface. Nonlinear flux diffusion equations are solved with appropriate periodical boundary conditions, which describe the influence of the ac field component. The flux creep rate and magnetization decay are enhanced by the ac field as a result of rather strong nonlinearity of flux diffusion. The long-time relaxation profiles of electric field and shielding current change significantly as compared to the well-known stationary solutions in the absence of ac field. These results reflect rather general properties of nonlinear systems, namely the possibility of the large effect of small perturbations on the long-time behavior of a nonlinear system.