Analysis of low-field isotropic vortex glass containing vortex groups in YBa2Cu3O7−x thin films visualized by scanning SQUID microscopy

Abstract

The glass-like vortex distribution in pulsed laser deposited YBa2Cu3O7 − x thin films is observed by scanning superconducting quantum interference device microscopy and analysed for ordering after cooling in magnetic fields significantly smaller than the Earth's field. Autocorrelation calculations on this distribution show a weak short-range positional order, while Delaunay triangulation shows a near-complete lack of orientational order. The distribution of these vortices is finally characterised as an isotropic vortex glass. Abnormally closely spaced groups of vortices, which are statistically unlikely to occur, are observed above a threshold magnetic field. The origin of these groups is discussed, but will require further investigation.

Dynamic magneto-optical imaging of superconducting thin films

“…Magnetic flux penetration into superconducting films occurs on time-scales much smaller than those measurable using many magnetic imaging techniques such as scanning SQUID microscopy, which may require minutes or hours to create a magnetic profile of a sample with * Corresponding author's email: pan@uow.edu.au resolution approaching the vortex scale [26]. High-speed magneto-optical imaging (MOI) overcomes this limitation of other magnetic microscopy techniques as it does not require any scanning, instead relying on the Faraday effect and using visible optics to examine magnetic fields.…”

Direct Measurements of Field-Dependent Ordering in a Low-Field Vortex Glass State

“…The dark area at the left is outside the sample. The vortices in this image appear larger than their actual size, and show an apparent asymmetry due to the scan height and angle of the SQUID magnetometer [5].…”

“…Analyses of the properties of vortex glass states by direct measurement are less common, but may provide a more fundamental understanding of the nature of these distributions of vortices [5].…”

“…In our previous work [5], YBa 2 Cu 3 O 7−δ (YBCO) thin films under very low field conditions were directly observed and analysed using Scanning Superconducting Quantum Interference Device (SQUID) microscopy (SSM) [8], [9], and the vortices were found to be in an isotropic vortex glass state. Glass states such as these are expected to dominate the phase diagram in YBCO thin films grown by pulsed laser deposition (PLD) due to the high number of strong pinning sites [10]- [13].…”

“…Field-cooling was repeated for several applied field values in the low field vortex-glass region of the phase diagram (between 0.05 and 5.47 µT). A single high-resolution scan of the vortex distribution was taken at each field using the scanning SQUID microscopy (SSM) technique described in [5]. The SQUID loop has size 3 µm × 5 µm, and nominal scan height of 5 µm.…”