Strong coupling and double-gap density of states in superconducting

¹ Groupe de Physique des Solides, Universités Paris 7 et Paris 6 - Paris, France and Unité Mixte de Recherche CNRS (UMR 75 88) 2 Place Jussieu, 75251 Paris Cedex 5, France
² Dipartimento di Fisica and Unità INFM, Università di Salerno via S. Allende, 84081 Baronissi (SA), Italy

Received: 11 December 2001
Accepted: 14 March 2002

Abstract

Using scanning tunneling spectroscopy at K, we perform simultaneously the topographic imaging and the quasiparticle density of states (DOS) mapping in granular MgB₂. We observe a new type of spectrum, showing a pronounced double gap, with the magnitudes of meV and meV, i.e. well below and well above the BCS limit. The largest gap value gives the ratio , which implies strong electron-phonon coupling. Other superconducting regions are found to have a characteristic BCS-shaped DOS. However, the variation of the spectral shape and lower gap widths, from 2.0 meV to 6.5 meV, indicate the importance of surface inhomogeneity and proximity effects in previously published tunneling data. Our finding gives no evidence for any important gap anisotropy. Instead, it strongly supports the multiple-gap scenario in MgB₂ in the clean limit, and the single-gap scenario in the dirty limit.