Volume 58, Number 5, June 2002
|Page(s)||764 - 770|
|Section||Condensed matter: electronic structure, electrical, magnetic, and optical properties|
|Published online||01 May 2002|
Strong coupling and double-gap density of states in superconducting
Groupe de Physique des Solides, Universités
Paris 7 et Paris 6 - Paris, France
and Unité Mixte de Recherche CNRS (UMR
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
Accepted: 14 March 2002
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.
PACS: 74.20.-z – Theories and models of superconducting state / 74.50.+r – Proximity effects, weak links, tunneling phenomena, and Josephson effects / 74.70.Ad – Metals; alloys and binary compounds (including A15, Laves phases, etc.)
© EDP Sciences, 2002
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