Local study of the Mg 1x Al x B 2 single crystals by scanning tunneling spectroscopy in magnetic field up to 3 Tesla F. Giubileo a, * , F. Bobba a , A. Scarfato a , A.M. Cucolo a , A. Kohen b , D. Roditchev b , N.D. Zhigadlo c , J. Karpinski c a CNR-INFM Laboratorio Regionale SUPERMAT, 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 Accepted 30 November 2007 Available online 7 March 2008 Abstract We have performed local tunneling spectroscopy on high quality Mg 1x Al x B 2 single crystals by means of variable temperature scan- ning tunneling spectroscopy in magnetic field up to 3 Tesla. Single gap conductance spectra due to c-axis tunneling were extensively mea- sured, probing different amplitudes of the three-dimensional D p as a function of Al content (i.e. as a function of the critical temperature T C ). Temperature and magnetic field dependences of the conductance spectra were studied in S–I–N configuration: the effect of the dop- ing resulted in a monotonous reduction of the locally measured T C down to 24 K for x = 0.2. The magnetic field dependence was studied in a local way: An estimation for upper critical field H c2 was inferred from the evolution of the tunneling spectra with the field perpen- dicular to the sample surface, for different doping levels. The high spatial resolution of the STS technique allowed us to evidence possible non-homogeneities of the superconducting properties on the sample surface with variation of in the same sample depending on different local levels of doping. The locally measured upper critical field resulted to vary for different dopings, and the maximum value H c2 ’ 3T was found for samples with T C ¼ 33 K. The evolution of the density of states (DOS) was found to be characterized by two distinct regimes separated by a crossover region. Our results indicate a rapid suppression of the intrinsic term in p-band superconductivity for 0 T < B < 0.5 T. At high fields (0.8 T < B < 3 T) the superconductivity in the p-band survives uniquely due to the coupling to the r-band. The shape of tunneling spectra suggests an important role played by the quasiparticle inter-band scattering. Ó 2008 Elsevier B.V. All rights reserved. PACS: 74.50+r; 74.70.Ad Keywords: MgB 2 ; Scanning tunneling spectroscopy 1. Introduction The existence of two distinct superconducting energy gaps in the simple binary compound MgB 2 with a relatively high T C of 40 K offers a unique opportunity to study the electrodynamics of a multigap superconductor. The effects of inter-band coupling and scattering on the superconduc- ting order parameter and T C were discussed since long time ago [1], but only after the discovery of superconductivity in magnesium diboride [2] there has been a real opportunity to experimentally study the phenomenon. Two-gap super- conductivity in MgB 2 is now evidenced by numerous exper- imental results [3–11]. Chemical substitution effect in MgB 2 is of particular interest since it allows to modify its superconducting prop- erties in a systematic way. Substitution of C for B-site and Al or Mn for Mg-site are possible [12–16]. In all cases the doping produces a suppression of T C . Despite theoretical calculations predict the two-gaps collapsing into one gap 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.11.079 * Corresponding author. E-mail address: giubileo@sa.infn.it (F. Giubileo). www.elsevier.com/locate/physc Available online at www.sciencedirect.com Physica C 468 (2008) 828–831