Scanning tunneling microscopy and spectroscopy (STM/STS) experiments on a high-density sintered MgB2surface 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 MgB2in 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 MgB2was also discussed in detail. © 2004 Elsevier B.V. All rights reserved.
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