  We report on low temperature scanning tunneling microscopy (STM) studies of
the electronic structure of vortex cores in Bi2Sr2CaCu2O8+d (BSCCO). At the
vortex core center, an enhanced density-of-states (DOS) is observed at energies
near 7 meV. Spectroscopic imaging at these energies reveals an exponential
decay of these 'core states' with a decay length of 2.2(3) nm. The four-fold
symmetry sometimes predicted for d-wave vortices is not seen in spectroscopic
vortex images. A locally nodeless order parameter induced by the magnetic field
may be consistent with these measurements.
