Low Temperature Physics: 32, 783 (2006); https://doi.org/10.1063/1.2219500 (19 pages)
Fizika Nizkikh Temperatur: Volume 32, Number 8-9 (August 2006), p. 1029-1054    ( to contents , go back )

Supercurrent density above 106 А/см2 at 77 K in a single-crystal films conductor of HTS YBa2Cu3O7-d cuprate -dream or reality?

V.M. Pan

Institute of Metal Physics of the National Academy of Sciences of Ukraine, Kiev 03142, Ukraine
E-mail: pan@imp.kiev.ua
pos Анотація:

Received April 10, 2006, revised May 3, 2006

Abstract

The supercurrent transport phenomena in epitaxial с-axis-oriented thin films of HTS cuprate YBa2Cu3O7-d (YBCO) with high Jc(77 К) ≥ 2·106 А/см2 are studied by the four-probe transport current technique, low-frequency ac magnetic susceptibility and SQUID magnetometry. The films under study are deposited onto r-cut sapphire substrates buffered with a CeO2 layer or LaAlO3 (100) by the off-axis dc magnetron sputtering or by the pulse-laser ablation. A consistent model of vortex pinning and supercurrent limitation is developed. Its predictions are compared with the experimental data on transport current and magnetic field dependences, as well as the data on nanostructure obtained by high-resolution transmission electron microscopy and electron back scattering diffraction. Low angle subboundaries (LABs) between slightly inplane misaligned domains, formed in the process of film growth are shown to play a key role in the phenomena detected at the supercurrent flow. Tilt LABs can be presented as equidistant ordered rows of edge dislocations with nonsuperconducting cores of about 3–4 nm in diameter. The dislocation lines are parallel to each others and perpendicular to the film plane. A mean area density of dislocations depends on statistical parameters of the random domain boundaries network and may reach 1011 см-2. As the diameter of «normal» dislocation core is close to the diameter of Abrikosov vortex core the elementary pinning force appears to go up to its maximum value. Our model takes into account both the transparency of LABs for supercurrent as well as the pinning of vortex lattice on the network of LABs and allows to extract the main statistical parameters of the film nanostructure such as domain size distribution and mean misorientation angle from the Jc(Н) curves measured at magnetic fields, Н, parallel to the c axis. The model provides a possibility to comprehend all the experimental results: 1) the achievement of the highest Jc(77 К) ≥ 2·106 А/см2 and Jc(20 К) > 107 А/см2 just in epitaxial cuprate films and conductors; 2) the existence of a «plateau» Jc(Н) = const in the Jc(Н), dependencies at Н < Hm; 3) the logarithmic of Jc(Н) fall at Н > Hm, due to the transition from the single-vortex pinning regime to the «collective» one; 4) the existence of the characteristic threshold field Нр, below which the vortices within a thin film (dl) remain straight and perpendicular to the film surface even in strongly inclined field, making clear the absence of the expected maximum at H || c for Jc(Q) ; 5) the evolution of the angle dependences Jc(Q) with H variation is shown to be consistent with the dominating pinning on edge dislocations. The observed effects are found to be consistent with the model developed. A new «peak-effect», i.e., an increase in Jc(H || ab) with applied longitudinal dc magnetic field, emerging due to the additional electromagnetic pinning, was detected for the first time in YBCO films with a very smooth surface.

PACS:
74.25.Sv - Critical currents
74.72.Bk - Y-based cuprates
74.78.Bz - High-Tc films

Key words: high-temperature superconducting cuprates, epitaxial thin YBa2Cu3O7-d films, critical currents, Abrikosov vortex pinning, growth-induced edge dislocations, low-angle domain subboundaries.

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