Correlation between vortex distribution and temperature in YBCO films and YBCO-QMG samples

We observed vortices with a modified Bitter method by evaporating nickel as the medium of decoration, in film and QMG (which are made with the quench-and-melt-growth method) YBa₂Cu₃O_7-δ (YBCO) samples. The temperature range of observation was from 25 K to 90 K, and the external magnetic field was 2 mT. Consequently, we found that the vortex structures of these samples were stable from 25 K to near the critical temperature (∼90 K). The vortex density was almost three times larger than that estimated from the external magnetic field. This suggests that the vortex motion may be hopping during nickel evaporation.