Andreev reflection spectroscopy evidence for multiple gaps in MgB2 -: art. no. 064513

Andreev reflection tunneling spectroscopy on MgB2/PtIr point contacts have systematically been investigated. By fitting tunneling conductance to Blonder-Tinkham-Klapwijk theory, s-wave superconducting gaps were obtained. The distribution of the obtained gaps unequivocally shows two groups: one centered at 9.8 meV and the other peaked at 2.8 meV with a gap ratio of 3.5. For two typical gaps Delta(1)=1.92 meV and Delta(2)=3.45 meV their temperature-dependent behavior display that Delta(1)(T) unambiguously persists to the bulk T-c, 39 K, and Delta(2)(T) closes at 33 K. The results support multigap scenarios; in addition our experiments yield a much higher value of the large gap than the previous reports and theoretical predictions, indicating there may exist even stronger electron-phonon coupling in MgB2.