Vortex dynamics at subcritical currents at microwave frequencies inDyBa2Cu3O

Abstract

We have investigated the dynamics of vortices at subcritical microwave currents in dc magnetic fields ͑up to 0.8 T͒ in epitaxial DyBa 2 Cu 3 O 7Ϫ␦ ͑DBCO͒ thin films. Microwave measurements were performed using microstrip resonators as test vehicles at 4.88 GHz and 9.55 GHz on laser ablated DBCO thin films in the thickness range 1800-3800 Å. Experimental evidence indicates that the peak effect ͑PE͒ observed in surface resistance vs temperature (R s vs T) plots in applied dc magnetic fields up to 0.8 T is primarily due to the extended defects in thinner films ͑1800 Å͒ such as twin boundaries at the substrate(LaAlO 3 )-film interface; whereas, the high density of point defect disorder in thicker (у3000 Å) films is responsible for low R s and high depinning frequency p . This has been confirmed by generation of columnar defects using 200 MeV Ag ion irradiation which showed that even thicker DBCO films show PE in R s after the introduction of columnar defects. Further, DBCO films grown on low-twinned LaAlO 3 substrates ͑which cause low density of substrate-related extended defects in the film͒ have shown PE only

The microwave response of thin superconducting films with trapped magnetic flux

“…its thickness d and width 2w. As it is seen from figure 1(A), the most pronounced peaks in the curve P dis (S 0 ) are located in the region of tens of S 0 ; thus, only if the dimensions of a resonator lie in the range corresponding to 5 ≤ S 0 ≤ (20 − 30) for the rather low frequency, the multi-peak picture similar to the ones obtained in [6,7,8] can be experimentally observed.…”

“…In the current report the theoretical model of the linear response of HTS thin films with trapped vortices developed in [9] is used to model the dc magnetic field and the temperature dependence of energy losses P dis (B, T ) connected with the oscillation of Abrikosov vortices in thin SC films cooled down in (FC) and without (ZFC) external dc magnetic field. It is shown that, under the general assumptions on the magnetic field and temperature behavior of J c , α L and η, the quasi-periodic form of P dis (T ) dependence neatly corresponds to the ones obtained in [6,7,8]. By varying the shape of J c (B, T ), α L (B, T ) and η(B, T ) dependences, the conditions for the appearance of nonmonotonic parts on P dis (B, T ) curve is also clarified.…”

“…The original critical state model is modified in the following way: B(y) = B remn for |y| < b, and |J(y)| = J c for b < |y| < w with b = w/ cosh(πB dc /µ 0 J c ) and B remn ≪ B dc being some remnant field. This modification allows us to use the foregoing approach to model the ac response of the film in ZFC state and, as long as B remn ≪ B dc , it does not affect the results; on the other hand, such modification is justified by the fact that in a lot of experiments some unscreened magnetic field of order of several oersted is always present [8]. In both cases, the following temperature [15] and the dc magnetic field [16] dependence of α L and η is supposed:…”

“…However, a number of experiments shows unusual quasi-periodic dependence of the surface impedance of HTS films on the temperature or the external dc magnetic field [6,7,8], which cannot be unambiguously explained within the frameworks of the existing theoretical models.…”

Analysis of the Measurements of Anisotropic AC Vortex Resistivity in Tilted Magnetic Fields

“…In this case, the change in the response due to the application of the magnetic field is due to the vortex motion: superfluid and quasiparticle contributions give a nearly field-independent contribution that can be easily subtracted out. Moreover, we assume the superconducting material to be a thin film, a sample geometry widely used in microwave experiments [3,19,31,53,54,59]. In these conditions it can be shown 1 that ρ v , being essentially proportional to the surface impedance tensor, can be taken as the actually measured quantity.…”