Vortex pinning by natural defects in thin films of YBa₂Cu₃O_7−δ

Abstract

Although vortex pinning in laser-ablated YBa 2 Cu 3 O 7−δ films on (100) SrTiO 3 is dominated by threading dislocations (Dam B et al (1999) Nature 399 439), many other natural pinning sites are present. To identify the contribution from twin planes, surface corrugations and point defects, we manipulate the relative densities of all defects by post-annealing films with various as-grown dislocation densities, n disl. While a universal magnetic field B dependence of the transport current density j s (B, T) is observed (independently of n disl , temperature T and the annealing treatment), the defect structure changes considerably. Correlating the microstructure to j s (B, T), it becomes clear that surface roughness, twins and point defects are not important at low magnetic fields compared to linear defect pinning. Transmission electron microscopy indicates that threading dislocations are not part of grain boundaries nor are they related to the twin domain structure. We conclude that j s (B, T) is essentially determined by pinning along threading dislocations, naturally induced during the growth process. Even in high magnetic fields, where the vortex density outnumbers n disl , it appears that linear defects stabilize the vortex lattice by means of the vortex-vortex interaction.

Strong pinning linear defects formed at the coherent growth transition of pulsed-laser-depositedYBa2Cu3Dam1, Huijbregtse2, Rector3 2002Phys. Rev. B

“…43 As a result, the dislocation density is proportional to the island density. The island density decreases at elevated substrate temperatures, due to the increased surface mobility at t c .…”

“…The reduction of the threading dislocation density with substrate temperature follows from the decrease in island density in the first stages of growth. 43 So far, we have neglected the nature of the threading dislocation and the effect of the growth technique on the dislocation formation. Typically, depending on the growth conditions, 44 in PLD the YBa 2 Cu 3 O 7Ϫ␦ islands grow in a two-dimensional nucleation and growth mode and we cannot distinguish between edge and screw dislocations.…”

Structural and flux-pinning properties of laser ablated YBa2Cu3O7−δ thin films: Effects of self-assembled CeO2 nanodots on LaAlO3 substrates

“…However, studies have shown that the majority of naturally occurring growth defects are either not effective enough to suppress thermal fluctuations or the spatial densities are not high enough to stop the vortex mobility by pinning them in order to maintain the necessary levels of high density electrical currents in high magnetic fields [2,3]. Therefore, it is important to improve further the flux-pinning properties of YBCO thin films by introducing defects.…”

Enhancement of flux pinning and critical currents in YBa2Cu3O7−δ films by nanoscale iridium pretreatment of substrate surfaces

“…However, studies have shown that the majority of these naturally occurring growth defects are either not effective enough to suppress thermal fluctuations or the densities not high enough to maintain the necessary levels of critical currents ͑I c ͒ in high magnetic fields. [2][3][4] Therefore, it is imperative to improve further the flux pinning properties of YBCO thin films in the field and temperature ranges targeted for potential applications, which would in turn yield an enhanced performance, increased efficiency, and cost reduction of practical HTS systems. In these endeavors, an important related consideration is the need to maintain a high J c level with respect to field orientation , since the coated conductors in HTS power systems experience magnetic fields varying over a distribution of directions.…”