Electron Tunneling Spectroscopic Measurements on Al-doped MgB2 Thin Films

The effect of Al-doping on the electron-phonon coupling in magnesium diboride films was studied by electron tunneling spectroscopy on planar sandwich-type Mg1-xAlxB2-oxide-indium junctions. The superconducting Mg1-xAlxB2 films (0 ≤ x < 0.5) were prepared in situ by simultaneous sublimation of Mg and sputtering of B and Al. The oxide barrier was either natural or artificial (aluminium oxide). The high junction quality allowed precise measurements of the differential conduction up to high bias-voltages of 120 mV. In the low bias range the small energy gap Δπ was measured: it decreased linearly with increasing Al-doping in agreement with the band filling model. The measurements at higher energies revealed the phonon induced features in the electron density of states. Although the spectra were influenced by the proximity effect below ∼20meV, they enabled the determination of the effective Eliashberg function α2F. For the same film compositions α2F data were obtained theoretically from first principles calculations. The progressive loss of superconductivity with increasing Al doping was reflected in both the experiment and calculations by the evolution of α2F. The dominating π-σ interband pairing interaction, which contributes most to the superconductivity on the π sheet, was gradually weakened in correlation with the band structure calculations.