STM studies of the electronic structure of vortex cores in Bi2Sr2CaCu2O8+delta

We report on low temperature scanning tunneling microscopy (STM) studies of the electronic structure of vortex cores in Bi2Sr2CaCu2O8+delta. At the vo rtex core center, an enhanced density of states is observed at energies nea r Omega = +/-7 meV. Spectroscopic imaging at these energies reveals an expo nential decay of these "core states" with a decay length of 22 +/- 3 Angstr om. The fourfold symmetry sometimes predicted for d-wave vortices is not se en in spectroscopic vortex images. A locally nodeless order parameter induc ed by the magnetic field may be consistent with these measurements.