Nature of High Critical Current Density in Epitaxial Films of HTS YBCO Cuprate and Coated Conductors

Abstract

ABSTRACTCurrently a problem of crystal defects nanoengineering for pinning enhancement is extensively studied. A number of efforts were done to realize nanodot-like and particulate-dispersive pins to enhance pinning and critical current density in high-Tc cuprate films and coating. Sometimes some effect of Jc enhancement was achieved. However it is important to comprehend mechanisms of such an enhancement. It is known the ensemble of random point-like pins with size ro of order of coherence length, xab(T), can provide Jc(77 K) not to exceed 5§¹104 A/cm2. Estimations give the maximum pinning force of about for linear extended defects (if ). Here eo is the characteristic vortex energy. A model of vortex pinning and supercurrent limitation is developed and discussed on the base of measurements and analysis of magnetic feld and angle dependencies of Jc(H,¦È) in epitaxial c-oriented YBa2Cu3O7-δ (YBCO) films measured by the four-probe transport current technique, low-frequency ac magnetic susceptibility and SQUID magnetometry. Films nanostructure is studied by SEM, TEM, HREM and X-ray diffractometry. Rows of growth-induced out-of-plane edge dislocations (EDs), forming low angle subboundaries (LABs), are shown to play a key role in achievement of the highest critical current density Jc ¡Ý 2 106 A/cm2 at 77 K. The model takes into account the transparency of LABs for supercurrent as well as the pinning of vortex lattice on a network of LABs. Films defect structure parameters, such as a domain size distribution and a mean misorientation angle, are extracted from Jc(H||c)-curves as well as from X-ray diffraction data. Evolution of angle dependencies Jc(¦È) with H is shown to be consistent with the model, supposing dominant pinning on EDs. Strongly pinned vortices parallel to the c-axis exist in strongly tilted magnetic felds up to threshold feld Hp. Below Hp the magnetic induction within a film obeys a simple relation B = Hcos¦È. This feature is shown to explain the absence of the maximum in Jc(¦È)-plot, expecting at H||c in low applied feld. A peak-effect in Jc(H||ab)-dependencies and an angular hysteresis of Jc(¦È) observed in intermediate feld range, are discussed in terms of film thickness, surface quality and orientation of the applied feld. The effects observed are found to be consistent with the developed model. To our mind any nano-, micro- and macro-interfaces, emerging within films or coatings at the deposition process (e.g., nanodot-like and particulate dispersive inclusions) being coherently connected with a YBCO-matrix serve as a source of formation of a multitude of additional dislocations and as a result can promote the essential Jc-enhancement.