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

The variation of topological defect density and hexatic order parameter were measured over a range of micro-Tesla fields in a 2-D superconducting vortex glass. This was achieved through scanning SQUID microscopy of the vortex distribution in YBa 2 Cu 3 O 7-δ thin films under field-cooled conditions. It was discovered that, although the defect density decreased for increasing magnetic fields, giving the impression of a more lattice-like vortex distribution, the hexatic order parameter also decreased, showing that the distribution was less orientationally ordered.