Local density of electronic states in MgB2 studied by low temperature STM and STS: direct evidence for a multiple-gap superconductor

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. These values give a BCS ratio (2Δ/kBTC) of 5.6 and 2.4, respectively, well above and well below the BCS limit. The result supports the multiple-gap model in MgB2 in the clean limit. The proximity effect observed in the conductance spectra and the origin of the varied reported energy gap values of MgB2 were also discussed in this study.