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 Δ of about 2 meV, and this corresponds to a 2Δ/kBTcvalue 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 temperature dependence of both gaps is BCS-like, and start to open up at temperatures just below Tc (39.5 K). This confirms that these gaps are indeed bulk properties of MgB2. The junction is stable only up to a field of 3.2 T then 'collapsed' into Josephson tunneling for higher fields