Scanning tunneling spectroscopy in MgB2

We present scanning tunneling microscopy measurements of the surface of sup erconducting MgB2 with a critical temperature of 39 K. In zero magnetic fie ld the conductance spectra can be analyzed in terms of the standard BCS the ory with a smearing parameter Gamma. The value of the superconducting gap i s 5 meV at 4.2 K, with no experimentally significant variation across the s urface of the sample. The temperature dependence of the gap follows the BCS form, fully consistent with phonon-mediated superconductivity in this nove l superconductor. The application of a magnetic field induces strong pair b reaking as seen in the conductance spectra in fields up to 6 T.