Two-gap interplay in MgB 2: a tunneling spectroscopy study

Abstract

Tunneling spectroscopy on various samples of MgB 2 was performed. The first direct evidence for the two-gap superconductivity was given with an inverted junction setup, in which a small crystal of MgB 2 was used as the STM tip and 2H-NbSe 2 as the sample. This technique allowed to show that both gaps close at the critical temperature of the bulk material and thus are intimately related to the superconductivity. The experiments performed in the standard N-I-S geometry evidenced for two strongly coupled gaps Δ L=7.0±1.0 meV and Δ S=3.0±1.0 meV at 4.2 K. STS on as-grown c-axis oriented thin films yielded only small gap which confirmed the identification of this gap as originating from 3D-like π-band and, by exclusion, that of the large gap from 2D-like σ-band. The low gap values Δ S=2.2±0.3 meV were attributed to the degraded film surface. After chemical etching, the gap increased to Δ S=2.8±0.3 meV. The c-axis tunneling spectra are better fitted considering anisotropic superconductivity inside the π-band. The issues of our findings are discussed in terms of two-band superconductivity.