Direct Vortex Lattice Imaging and Tunneling Spectroscopy of Flux Lines onYBa2Cu3O7−δ

We report the observation of the flux line lattice in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ by scanning tunneling microscopy. The measurements were carried out at 4.2 K and in a magnetic field of 6 T applied along the $c$ axis. The vortices appear arranged in an oblique lattice in which the primitive vectors are nearly equal and form an angle of approximately 77°. We also report local tunneling spectroscopy into a vortex core which reveals two peaks separated by about 11 meV. The zero-field spectra are reproducible over large areas of the sample and show a multiple peak structure.