Temperature and field dependence of MgB2 energy gaps from tunneling spectra

We have synthesized MgB2/Pb planar junctions to study the temperature and field dependence of the superconducting energy gap of MgB2. The major peak occurs at Delta of about 2 meV, and this corresponds to a 2Delta/kBTc value of 1.18. While this is significantly smaller than the BCS weak coupling value, there are features in the tunneling spectra indicating the possibility of another larger gap. By fitting the dI/dV curves with a simple model, the larger gap is estimated to be about 4.5 times the smaller gap. The study of the effect of magnetic field on the junctions shows a stability only up to a field of 3.2T then junctions "collapsed" into Josephson tunneling for higher fields. Estimation of the major energy gap from Josephson tunneling is consistent with that from quasiparticle tunneling.