Two-gap interplay in MgB 2 : a tunneling spectroscopy study D. Roditchev a , F. Giubileo b, * , F. Bobba b , R. Lamy a , E.-M. Choi c , H.-J. Kim c , W.N. Kang c , S. Miraglia d , J. Marcus d , W. Sacks a , J. Klein a , A.M. Cucolo b , S.-I. Lee c , D. Fruchart d a GPS, UMR75-88 au CNRS, Univ. Paris 6 et 7, Paris, France b Department of Physics and INFM Unit, University of Salerno, via S. Allende, 84081 Baronissi, SA, Italy c NCRICS, Department of Physics, Pohand University, Pohang, South Korea d Laboratoire de Cristallographie et LEPES, CNRS, Grenoble, France Abstract Tunneling spectroscopy on various samples of MgB 2 was performed. The first direct evidence for the two-gap su- perconductivity 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 D L ¼ 7:0  1:0 meV and D S ¼ 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 p-band and, by exclusion, that of the large gap from 2D-like r-band. The low gap values D S ¼ 2:2  0:3 meV were attributed to the degraded film surface. After chemical etching, the gap increased to D S ¼ 2:8  0:3 meV. The c-axis tunneling spectra are better fitted considering anisotropic superconductivity inside the p-band. The issues of our findings are discussed in terms of two-band superconductivity. Ó 2004 Elsevier B.V. All rights reserved. PACS: 74.50.+r; 74.76.)w; 74.70.Ad Keywords: MgB 2 ; Tunneling phenomena; Thin films 1. Introduction Since its discovery in January 2001 [1], the super- conductivity in MgB 2 has stimulated an important effort in the condensed matter physics community. Indeed, the rather high critical temperature, unexpected for a con- ventional superconductor, has attracted much attention to this simple binary compound. Two years after the discovery, it has already been established that the su- perconductivity in MgB 2 is due to the electron–phonon interaction and, from this point of view, it is a conven- tional superconductor, fundamentally different from the high-T C superconducting cuprates. Detailed calculations of the electronic band structure of MgB 2 , and the the- oretical study of its phonon spectrum, led Liu et al. [2] to suggest that the compound could be a two-gap super- conductor. In fact, the electronic structure of the mate- rial is rather complex, and the Fermi surface is composed of different portions having a 3D character, arising from the p-bonds, and 2D-cylinders, arising from r-bonds [3]. The latter are responsible for the ab-plane metallic hole type conductivity. In [2] it was suggested that in the clean limit (2pn  k, where n is the coherence length, k the mean free path) the electron–phonon cou- pling (EPC) at low temperature may lead to a super- conducting state in which two distinct gaps open, each corresponding to a particular sheet of the Fermi surface. These predictions were experimentally checked in numerous reports. The results of some ARPES [4] and specific heat [5,6] experiments were consistent with the * Corresponding author. E-mail address: filgiu@sa.infn.it (F. Giubileo). 0921-4534/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2004.04.005 www.elsevier.com/locate/physc Physica C 408–410 (2004) 768–772