Electron tunneling spectroscopy on superconducting Al-doped MgB₂ thin films: energy gap and Eliashberg function

Superconducting thin films of composition Mg_1−xAl_xB₂ with 0x<0.5 were prepared in situ by sublimation of Mg combined with B and Al magnetron sputtering. The critical temperature T_c decreased linearly with x up to 0.4. For 0.4<x<0.5 the formation of a plateaulike feature at a T_c12  K was observed. This effect is supposed to be due to the incipient formation of the superstructure MgAlB₄ with ordered alternating Mg and Al planes separated by B planes. To detailedly study the influence of Al doping on the electron-phonon coupling in the polycrystalline films with a preferred c-axis texture quasiparticle tunneling experiments were performed on planar tunnel junctions with natural thermal oxide or artificial aluminum oxide tunnel barriers. Differential conductance measurements at low-bias voltage and low temperature of superconductor-insulator-superconductor tunnel junctions allowed the direct determination of the energy gap of the Fermi surface sheet. The energy gap decreased linearly with decreasing T_c of the films in agreement with the model of band filling. Whereas all the tunneling studies published so far mainly revealed features of the two energy gaps, the observation of phonon-induced structures in the differential conductance measurements at high bias voltage, i.e., in the phonon region, in this study enabled the important determination of the energy-dependent Eliashberg function ²F of the Fermi surface sheet for various Al doping levels. Compared to the undoped MgB₂, significant changes in ²F could be observed that were confirmed by first-principles calculations. ©2010 The American Physical Society