Local tunneling study of three-dimensional order parameter in the pi band of Al-doped MgB2 single crystals

We have performed an extensive study of the temperature and magnetic field dependencies of local tunneling spectra measured by means of variable temperature scanning tunneling spectroscopy on high quality Mg1-xAlxB2 single crystals. Single gap conductance spectra due to c-axis tunneling were measured, probing different amplitudes of the three-dimensional Delta(pi) as a function of Al content (i.e., as a function of the critical temperature T-C). We give clear evidence that the locally measured energy gap, as well as the upper critical field, reach maximum values Delta(pi)=2.4 meV and H-c2 >= 3 T around x similar or equal to 0.1 (T-C=33 K) regardless of the monotonous fall of T-C as a function of doping. Our results strongly suggest that different interband scattering occurs for different gap amplitudes.