Scanning tunneling spectroscopy studies on vortices in YBa2Cu3Oy single crystals

Low-temperature scanning tunneling spectroscopy (STS) has been performed on YBa2Cu3Oy, (YBCO) in the magnetic field mu(0)H = 1 T. Clear vortex images are obtained on the surface chemically etched with 1% Br-ethanol. We measured on the twinned crystals and directly observed the effect of twin boundary on the vortex arrangement. Vortices close to the twin boundary align themselves parallel to it, suggesting that vortices are effectively trapped by the twin boundary. The vortices very close to the twin boundary form almost square lattice. Such tendency decreases with increasing distance and the vortices form a hexagonal lattice in the region far from the twin boundary. However, the orientation of hexagonal lattice is not consistent with the previous reports. Possible origins of such disagreement are described in this paper. We also show spectra observed in the scan region and discuss their features. (C) 2003 Elsevier B.V. All rights reserved.