Nanoscale spatial non-homogeneity of 3D in Delta pi Mg0.9Al0.1B2 single crystals

We have performed I(V) and dI/dV(V) measurements on high quality Mg0.9Al0.1B2 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 similar to 3 T, i.e., a 25% raising with respect to what observed in the pure crystal. (C) 2007 Elsevier B.V. All rights reserved.