Scanning tunneling microscopy and spectroscopy (STM/STS) experiments on a high-density sintered MgB2 surface were performed at 4.2 K, using a low temperature STM. The intrinsic superconducting density of states (DOS) was obtained in the tunneling spectra exhibiting the BCS-shaped characteristic with a metallic background. A double-gap structure, with the gap values of ΔL=9.5 meV and ΔS=4.0 meV, was clearly observed in the local tunneling spectra. The result supports the multiple-gap model in MgB2 in the clean limit. The proximity effect was observed in the present conductance spectra and experimentally confirmed to be due to the Ar+ ion sputtering. The origin of the varied energy gap values reported in surface-sensitive STS measurements on MgB2 was also discussed in detail.
Xu, M., Xiao, Z., Takano, Y., Hatano, T., & Fujita, D. (2004). Low temperature STM/STS studies on MgB2. Physica C: Superconductivity, 412–414, P, 283–287. https://doi.org/10.1016/j.physc.2004.01.052