Nanoscale spatial non-homogeneity of 3D in D p Mg 0.9 Al 0.1 B 2 single crystals F. Giubileo a, * , F. Bobba a , A. Scarfato a , D. Roditchev b , N. Zhigadlo c , J. Karpinski c , A.M. Cucolo a a CNR-INFM Laboratorio Regionale SUPERMAT e Dipartimento di Fisica ‘‘E.R. Caianiello’’, Universita ` degli Studi di Salerno, via Salvador Allende, 84081 Baronissi (SA), Italy b Institut des Nanosciences de Paris, INSP, Universite ´ P. et M.Curie Paris 6, CNRS, UMR 75-88, Paris, France c Solid State Physics Laboratory, ETH Zurich, CH-8093 Zurich, Switzerland Available online 19 April 2007 Abstract We have performed I(V) and dI/dV(V) measurements on high quality Mg 0.9 Al 0.1 B 2 single crystals by means of a variable temperature scanning tunneling spectroscopy (STS) working in magnetic field up to 7 T. c-axis tunneling showed a single gap, probing the three- dimensional Dp that appeared highly non-homogeneous in its spatial distribution on nanometer scale, with an amplitude between 1.5 meV and 2.3 meV. Temperature and magnetic field dependence of the conductance spectra were studied in S–I–N configuration as well as in S–I–S configuration, after pushing the Pt/Ir tip in the sample to capture a superconducting grain at the very apex of the tip. For the largest energy gap (2.3 meV), we found H c2  3 T, i.e., a 25% raising with respect to what observed in the pure crystal. Ó 2007 Elsevier B.V. All rights reserved. 1. Introduction The discovery of the superconductivity at T  40 K in the simple MgB 2 compound [1] has given rise to a consid- erable interest in the scientific community. Intensive studies of its electronic structure revealed that this compound is a two-gap multiband superconductor with two-dimensional (2D) p-bands and three-dimensional (3D) r-bands [2–5]. Golubov and Mazin [6] pointed out that the two-gap superconductivity must be strongly affected by disorder. In particular, inter-band scattering by non-magnetic impu- rities is expected to induce pair breaking which suppresses the critical temperature T C . The pair breaking mechanism is expected to induce the increase of the small gap and the decrease of the large one, so these are to merge to the BCS value once a complete isotropization has taken place. Of several attempted chemical substitutions, only Al and C resulted successful. For Al doping, T C (x) falls with increas- ing Al concentration x, reaching zero for x between 0.5 and 0.6 [7]. In this paper we report a detailed study by means of scanning tunneling spectroscopy (STS) on high quality Mg 0.9 Al 0.1 B 2 single crystals. Directional tunneling along c-axis allowed to probe selectively the p-band energy gap. 2. Scanning tunneling spectroscopy experiment Scanning tunneling spectroscopy (STS) appears as the major tool with its high spatial and energy resolution allowing a direct measure of the local quasiparticle DOS. The experiments were carried out by means of ultra-high vacuum variable temperature STM/STS at the Physics Department of the University of Salerno. Single crystals of Mg 1x Al x B 2 were grown by a high pressure method described elsewhere [8]. The tunneling junctions were achieved by approaching mechanically cut Pt/Ir tips to the c-axis oriented surface of the crystals. Due to the elec- tronic structure of MgB 2 , c-axis tunnelling allows to probe mainly the quasiparticle density of states (DOS) in the p- band [9,10]. 0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.04.136 * Corresponding author. Tel.: +39 089 965285; fax: +39 089 965275. E-mail address: giubileo@sa.infn.it (F. Giubileo). www.elsevier.com/locate/physc Physica C 460–462 (2007) 585–586