STM Studies of the Electronic Structure of Vortex Cores in Bi2Sr2CaCu2O8+d

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.