Scanning tunneling spectroscopy on single crystal MgB2

We report on the results of scanning tunneling spectroscopy measurements on single crystals of MgB2. Tunneling was performed both parallel and perpendicular to the crystalline c-axis. In the first case, a single superconducting gap (Delta(pi) = 2.2 meV) associated with the pi-band is observed. Tunneling parallel to the ab-plane reveals an additional, larger gap (Delta(sigma) similar to 7 meV) originating in the highly 2D sigma-band. Vortex imaging in the pi-band was performed with the field and tunnel current parallel to the c-axis. The vortices have a large core size compared to estimates based on H-c2, and show an absence of localized states in the core. Furthermore, superconductivity between the vortices is rapidly suppressed by an applied field. A comparison to specific heat measurements is performed. (C) 2002 Elsevier Science B.V. All rights reserved.