Combined microwave analysis of vortex pinning and HAADF-STEM structural characterization of YBCO films with columnar defects

We report on a systematic investigation of the effects of columnar defects induced by heavy ion irradiation in YBa 2 Cu 3 O 7 − δ thin films. The study encompasses their structural characterization by transmission electron microscopy imaging (HAADF-STEM) and their impact on the vortex-pinning parameters, determined in the microwave frequency range by a coplanar waveguide resonator (CPWR) technique. The films were irradiated with 1.15-GeV Pb ions, and the resulting columnar defects revealed a core-shell structure with an amorphous core surrounded by a strained crystalline shell. Statistical analysis (considering any cluster as an individual defect) yield a mean column radius of r = 6 nm , which is in remarkable agreement with the radius of the confinement region obtained from fitting the irreversibility line within the Bose-glass model, r 0 = 5.8 nm , providing strong experimental validation of the functional characterization through direct structural imaging. The core of the analysis lies in the determination of the main superconducting parameters (such as the penetration depth) and the vortex dynamics and pinning parameters – including the pinning constant, viscosity, depinning and crossover frequencies, creep factor, and pinning energy – through the CPWR microwave technique. These parameters are analyzed and discussed within the theoretical frameworks proposed by Gittleman–Rosenblum, Coffey–Clem, and Blatter et al. Overall, columnar defects were found to enhance the pinning efficiency and, consequently, the critical current density j c and its ratio to the depairing current density, j c / j 0 . To determine their absolute values, a comparative analysis was carried out by combining results from the different experimental techniques with a theoretical treatment of j c / j 0 as a function of the STEM-measured column radius. All results converge into a consistent framework, yielding j c ( 5 K , 1 T ) = 8.8 · 10 11 Am − 2 and j c / j 0 = 0.34 for the YBCO films with irradiation-induced columnar defects.