Tunnelling properties of Al doped MgB2 thin film junctions

Superconducting thin films with composition Mg1-xAlxB2 (0 ≤ x ≤ 0.5) were prepared in situ by simultaneous thermal evaporation of magnesium, rf sputtering of boron and dc sputtering of aluminium. The superconducting transition temperature decreased gradually with increasing doping from 34 K (undoped MgB2) down to 7 K (Mg0.53Al0.47B2). The films allowed a successful preparation of sandwich-type crossed-strip tunnel junctions suitable for reproducible low-noise tunnel measurements. The junctions were prepared using both artificial aluminium oxide barriers and natural barriers resulting from oxidation of the film surface. As counter-electrodes evaporated indium films were used. Differential conductance measurements at low-bias voltage allowed a direct determination of the energy gap on the Fermi surface π sheet. Conductance measurements at high-bias voltage revealed the energies of the electron-coupled phonons, which are responsible for the superconductivity in Mg1-xAlxB2. Compared to the undoped MgB2 significant changes in the phonon spectrum could be observed, e.g. a renormalisation of the optical phonons at the high-energy end, which is in agreement with the measurements on bulk samples by inelastic neutron scattering.