Superconductor-insulator-superconductor tunnel junctions have been fabricated on MgB2 that display Josephson and quasiparticle currents. These junctions exhibit a gap magnitude, ∆ ∼ 2.5 meV, that is considerably smaller than the BCS value, but which clearly and reproducibly closes near the bulk Tc. In conjunction with fits of the conductance spectra, these results are interpreted as direct evidence of two-band superconductivity.PACS numbers: 73.40. Gk, 74.50.+r, 74.70.Ad, 74.80.Fp The discovery of superconductivity in MgB 2 has led to intense research activity, but the nature of the energy gap, ∆, has been elusive. Tunneling spectroscopy, which is the most direct measure of this quantity, has revealed a large spread of ∆ values and considerable variation in its spectral shape. Sharp, BCS-like tunneling spectra have been observed in scanning tunneling microscopy (STM) with a surprisingly small ∆ = 2.0 meV [1]. Other STM and point-contact studies revealed double-peaked spectra at low temperatures [2,3] that were interpreted as evidence for two-gap superconductivity. A provocative suggestion is that multiple gaps are a consequence of the coupling of distinct electronic bands [4]. Our ability to fabricate superconductor-insulatorsuperconductor (SIS) break junctions has led to unique observations and we have gone beyond these initial reports to present more compelling evidence that MgB 2 is one of the rare examples of two-band superconductivity. In addition, our identification of a weak higher-bias spectral feature has provided important insight into the nature of the inter-band coupling.The simplicity of the crystalline structure in MgB 2 allows for ab-initio calculations of its electronic properties [6], from which it is known that the Fermi surface consists of four sheets, two being two dimensional (2d) bonding σ-bands and two being three dimensional (3d) bonding and antibonding π-bands. An and Pickett [7] propose superconductivity to be driven by the 2d σ-bands, where electrons are strongly coupled primarily to the E 2g phonon mode. This raises important questions of how superconductivity would manifest itself on the 3d sheets and how the tunneling density of states (DOS) would depend on the crystallographic orientation.A more recent work [4] treated the problem by reducing it to two distinct bands which, in the clean limit, leads to the appearance of two isotropic gaps, ∆ 2 ∼ 7.2 meV and ∆ 1 ∼ 2.4 meV, associated with the 2d and 3d bands respectively. The small gap, ∆ 1 , on the 3d sheets is enhanced above its intrinsic value due to virtual phonon exchange (pair transfer) with the 2d sheets and should persist up to the bulk T c . The results of our tunneling study address these issues in the following ways. First, the small gap feature is unambiguously tracked to high temperatures where it is still visible in the raw data, a key observation supporting two-band superconductivity. These junctions only probe the band with the small, induced gap suggesting the SIS configuration strongly favors tunneling between t...

Point-contact tunnel junctions using a Au tip on sintered MgB2 pellets reveal a sharp superconducting energy gap that is confirmed by subsequent metallic Sharvin contacts made on the same sample. The peak in the tunneling conductance and the Sharvin contact conductance follow the BCS form, but the gap values of 4.3 meV are less than the weak-coupling BCS value of 5.9 meV for the bulk Tc of 39 K. The low value of ∆ compared to the BCS value for the bulk Tc is possibly due to chemical reactions at the surface.