Strong Reduction of the Field-Dependent Microwave Surface Resistance in YBCO with BaZrO₃ Inclusions

N. Pompeo⁽¹⁾, V. Galluzzi⁽²⁾, A. Augieri⁽²⁾, F. Fabbri⁽²⁾, G. Celentano⁽²⁾,
T. Petrisor⁽³⁾, R. Rogai⁽¹⁾ and E. Silva^(1,4)

(1) Dipartimento di Fisica “E. Amaldi” and Unità CNISM, Università Roma Tre,
Via della Vasca Navale 84, I-00146, Roma, Italy
(2) ENEA-Frascati, Via Enrico Fermi 45, 00044 Frascati, Roma, Italy
(3) Department of Physics, Technical University of Cluj, 400020 Cluj-Napoca (Romania)
(4) E-mail: silva@fis.uniroma3.it

Abstract - 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 YBa₂Cu₃O7_-δ/BaZrO₃, while for μ₀H = 0 only a reduction of Tc in the YBa₂Cu₃O7_-δ/BaZrO₃ film was observed, consistent with dc measurements. At low enough T and in moderate fields, YBa₂Cu₃O_7-δ/BaZrO₃ exhibited an intrinsic thin-film resistance lower than that of the pure film. The results clearly indicate that BaZrO₃ inclusions cause 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 result in very steep pinning potentials.

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Manuscript received December 14, 2007; accepted January 8, 2008. Reference No. ST21, Category 2.
Paper submitted to Proceedings of EUCAS 2007; published in JPCS 98 (2008), paper # 012173