STM/STS studies on vortex and electronic state in YBa2Cu3Oy

Low temperature scanning tunneling microscopy (LT-STM) studies have been performed on the LT-cleaved and the chemically etched surface of YBa2Cu3Oy single crystals. We find that the vortex structure can be observed on the chemically etched surface of YBa2Cu3Oy as a function of temperature and magnetic field. At low magnetic fields, we observed a slightly distorted triangular lattice, which is attributed to the Bragg-glass phase. The triangular lattice transforms into a disordered structure in high magnetic fields above the order–disorder transition H∗(T). We observed that the disordered vortices form small clusters comprising 5- and 7-fold coordination pairs. These microscopically determined vortex structures are in good agreement with the vortex matter phase diagram derived from the macroscopic measurements, thus providing the evidence of the field-driven transition in the vortex solid phase of YBa2Cu3Oy.