Electronic structure of individual flux lines in YBa₂Cu₃O_7-delta

We present images of the flux line lattice and spectroscopic measurements of the vortex core region on the as grown (001) surface of an YBa2Cu3O7-(delta ) single crystal at 4.2 Kelvin. These measurements were done in a magnetic field of 6 Tesla applied parallel to the c-axis using scanning tunneling microscopy. We find an oblique vortex lattice with nearly equal primitive vectors at an angle of 77 degrees. The vortex cores are ellipsoidal, consistent with the ab-plane anisotropy of YBCO. These characteristics can be fit to a distorted square lattice rotated 45 degrees with respect to the YBCO crystal lattice. The tunneling spectroscopy into the vortex core region reveals two bound quasiparticle energy levels with a gap of 11 meV at the Fermi level. Away from the vortex core, the tunneling characteristics are very similar to the zero-field spectra and show a multiple gap-like structure and a large zero-bias conductance