We observe a strong reduction of the field induced thin film surface resistance measured at high microwave frequency (ν =47.7 GHz) in YBa2Cu3O 7−δ thin films grown on SrTiO3 substrates, as a consequence of the introduction of sub-micrometric BaZrO3 particles. The field increase of the surface resistance is smaller by a factor of ∼3 in the film with BaZrO3 inclusions, while the zerofield properties are not much affected. Combining surface resistance and surface reactance data we conclude (a) that BaZrO3 inclusions determine very deep and steep pinning wells and (b) that the pinning changes nature with respect to the pure film.Increasing pinning of vortex lines is an essential achievement for useful applications of superconductors in general, and of cuprate superconductors in particular due to the wide range of the H − T phase diagram where vortices are free or nearly free to move. Columnar defects are best suited to this aim, 1 but the introduction of such defects requires rather sophisticated techniques which are not suitable for mass production. Recently it was shown 2 that inclusions of BaZrO 3 (BZO) particles of dimensions in the 10-1000 nm range could determine a significant improvement of the dc properties of YBa 2 Cu 3 O 7−δ (YBCO) thin films. In particular, the critical current density 2,3 could be raised up to values useful for large scale dc applications, and the irreversibility line could be shifted above 10 T at 77 K. 4 A major interest comes from the easy incorporation of such sub-µm particles directly into the targets used for film deposition, thus making feasible the mass growth of very low losses superconductors. Pinning of vortices at high frequencies is desirable not only for devices operating in dc magnetic fields, but also because power handling is limited by depinning of selfnucleated vortices. In fact, tailored profiles of columnar defects have been shown to largely extend the linear regime at low microwave frequencies. 5 However, when the operating frequency increases, the dissipation due to oscillating vortices becomes increasingly difficult to reduce: with increasing frequency the amplitude of the vortex oscillation decreases, and becomes so small that only interactions between single vortices and pinning centers determine the response. In this regime the steepness of the pinning wells affects the balance between the reactive (elastic) and resistive (viscous) response, and the depth affects the creep of vortices. Even columnar defects, when the driving current oscillates in the high microwave range, determine at most a reduction of the dissipation of only ∼15%, as seen from measurements at * corresponding author. e-mail: silva@fis.uniroma3.it ∼50 GHz, 6 while in some cases the microwave dissipation raised after heavy-ion irradiation. 7 It seems reasonable to state that, up to now, no known artificial pinning centers have shown a significant reduction of the dissipation at very high frequencies. Measuring the response at the high edge of the microwave spectrum is a very stringent test for ...
In order to study the vortex pinning determined by artificially introduced pinning centers in the small-vortex displacement regime, we measured the microwave surface impedance at 47.7 GHz in the mixed state of YBa2Cu3O7−δ thin films, where submicrometric BaZrO3 particles have been incorporated. As a function of the BaZrO3 content, we observed that the absolute losses slightly decrease up to a BaZrO3 content of 5%, and then increase. We found that the magnetic-field-induced losses behave differently in that they are not monotonic with increasing BaZrO3 concentration. At small concentration (2.5%) the field-induced losses increase, but large reduction in the losses themselves, by factors up to 3, is observed upon further increasing the BaZrO3 concentration in the target up to 7%. Using measurements of both surface resistance and surface reactance, we estimate vortex pinning-related parameters. We found that BaZrO3 inclusions introduce deep and steep pinning wells. In particular, the minimum height of the energy barrier for single vortices is raised. At larger BaZrO3 content (5% and 7%) the phenomenon is at its maximum, but it is unclear whether it shows a saturation or not, leaving room for further improvements.
We present measurements of the magnetic field dependent microwave surface resistance in laser-ablated YBa2Cu3O 7−δ films on SrTiO3 substrates. BaZrO3 crystallites were included in the films using composite targets containing BaZrO3 inclusions with mean grain size smaller than 1 µm. X-ray diffraction showed single epitaxial relationship between BaZrO3 and YBa2Cu3O 7−δ . The effective surface resistance was measured at 47.7 GHz for 60< T <90 K and 0< µ0H <0.8 T. The magnetic field had a very different effect on pristine YBa2Cu3O 7−δ and YBa2Cu3O 7−δ /BaZrO3, while for µ0H =0 only a reduction of Tc in the YBa2Cu3O 7−δ /BaZrO3 film was observed, consistent with dc measurements. At low enough T , in moderate fields YBa2Cu3O 7−δ /BaZrO3 exhibited an intrinsic thin film resistance lower than the pure film. The results clearly indicate that BaZrO3 inclusions determine a strong reduction of the field-dependent surface resistance. From the analysis of the data in the framework of simple models for the microwave surface impedance in the mixed state we argue that BaZrO3 inclusions determine very steep pinning potentials. arXiv: 0710.1953v1 [cond-mat.supr-con]
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