  We report scanning tunneling spectroscopy imaging of the vortex lattice in
single crystalline MgB2. By tunneling parallel to the c-axis, a single
superconducting gap (Delta = 2.2 meV) associated with the pi-band is observed.
The vortices in the pi-band have a large core size compared to estimates based
on Hc2, and show an absence of localized states in the core. Furthermore,
superconductivity between the vortices is rapidly suppressed by an applied
field. These results suggest that superconductivity in the pi-band is, at least
partially, induced by the intrinsically superconducting sigma-band.
