Nature of High Critical Current Density in Epitaxial Films of HTS YBCO Cuprate and Coated Conductors
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0946-HH03-04
Nature of High Critical Current Density in Epitaxial Films of HTS YBCO Cuprate and Coated Conductors Vladimir M. Pan1, Yuriy V. Cherpak1, Ernst A. Pashitskii2, and Aleksey V. Semenov1,2 1 Department of Superconductivity, Institute for Metal Physics, Vernadsky Boulevard, 36, Kiev, 03142, Ukraine 2 Department of Nonideal Solids, Institute of Physics, Science Ave, 46, Kiev, 03028, Ukraine
ABSTRACT A model of vortex pinning and supercurrent limitation is discussed on the basis of analysis of the Jc(H,θ) magnetic field and angle dependencies in epitaxial c-oriented YBa2Cu3O7−δ (YBCO) films measured by the four-probe transport current technique, lowfrequency ac magnetic susceptibility and SQUID magnetometry. Rows of growth-induced outof-plane edge dislocations (EDs), forming low angle boundaries (LABs), are shown to play a key role in achievement of the highest critical current density Jc ≥ 2 ⋅106 A/cm2 at 77 K. Evolution of angle dependencies Jc(θ) with H is shown to be consistent with the model, supposing dominant pinning on EDs. Being pinned strongly on EDs, vortices remain to be parallel to film c-axis even in strongly tilted magnetic fields up to threshold field Hp. Below Hp the magnetic induction within a film obeys a simple relation B = H cosθ. This feature is shown to explain the absence of the maximum in Jc(θ)-plot, expecting for H||c at low applied field. A peak-effect in Jc(H||ab)dependencies and an angular hysteresis of Jc(θ) observed in intermediate field range are discussed in terms of film thickness, surface quality and orientation of the applied field. The effects observed are found to be consistent with the developed model. Any nano-, micro- and macro-interfaces, emerging within films or coatings at the deposition process (e.g., nanodot-like and particulate dispersive inclusions) and being coherently connected with a YBCO-matrix serve as sources of the formation of a multitude of additional dislocations and as a result can promote the essential Jc-enhancement. INTRODUCTION The highest critical current density Jc(77 K) for high-temperature supercondutors (HTS) is achieved in single-crystal YBa2Cu3O7-δ (YBCO) c-oriented epitaxial thin films [1]. In order to comprehend Jc behavior in YBCO films at applied magnetic field of different orientations a few models were proposed. They are to be distinguished by Abrikosov vortex pinning mechanism responsible for Jc(H,θ ), i.e., for their magnetic field and orientation dependencies (θ is the angle between magnetic field and c-axis). The most effective pinning centers were suggested to be (i) linear, extended crystal defects – cores of edge dislocations (EDs), i.e., non-superconducting threads with their cross-section radii approximately equal to coherence length, ξ ab [2,3], (ii) planar, two-dimensional defects (twins [4] and anti-phase boundaries [5]), (iii) nanodots (normal phase inclusions) [6-8], (iv) film surface and its irregularities [9]. On the basis of numerous experimental results a consistent model was developed in our previous works [10-12]
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