Tunneling spectroscopy of the superconducting gap in MgB2 -: art. no. 094504

The tunneling technique with a break junction has been applied to investigate the multiple superconducting gaps in MgB2. The observed gap features can be explained by combining inherent and induced order parameters. The largest gap of Delta = 9-10 meV corresponds to a very strong-coupling ratio 2Delta/k(B)T(c) = 5-6. Upon warming, this gap decreases in accordance with the BCS dependence, while the smaller gaps reduce faster than that but persist up to T-c. Therefore, the smallest gap of Delta = 2-2.5 meV gives 2Delta/k(B)T(c) = 1.2-1.5. The magnetic field can distinguish the behavior of the gap. The extrapolated highest gap-closing field for the largest gap agrees with the upper-critical field, thereby indicating that this gap is predominant to MgB2.