Local study of the Mg1-xAlxB2 single crystals by scanning tunneling spectroscopy in magnetic field up to 3 Tesla

We have performed local tunneling spectroscopy on high quality Mg1-xAlxB2 single crystals by means of variable temperature scanning tunneling spectroscopy in magnetic field up to 3 Tesla. Single gap conductance spectra due to c-axis tunneling were extensively measured, probing different amplitudes of the three-dimensional Δπ as a function of Al content (i.e. as a function of the critical temperature TC). Temperature and magnetic field dependences of the conductance spectra were studied in S-I-N configuration: the effect of the doping resulted in a monotonous reduction of the locally measured TC down to 24 K for x = 0.2. The magnetic field dependence was studied in a local way: An estimation for upper critical field Hc2 was inferred from the evolution of the tunneling spectra with the field perpendicular 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 Hc2≅3T was found for samples with TC=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 π-band superconductivity for 0 T < B < 0.5 T. At high fields (0.8 T < B < 3 T) the superconductivity in the π-band survives uniquely due to the coupling to the σ-band. The shape of tunneling spectra suggests an important role played by the quasiparticle inter-band scattering