•J. Geerk, R. Schneider, A. Zaitsev, R. Heid und G. Linker
Forschungszentrum Karlsruhe, Institut für Festkörperphysik

We have prepared tunnel junctions of the sandwich-type on superconducting thin films of MgB₂. The films were prepared by physical vapor deposition from an evaporation source for magnesium and a sputtering source for boron with T_c values near 33 K. The tunnel junctions showed an energy gap between 2.5 and 3.0 meV and phonon structures in the reduced density of states with a strength of about 0.8%. The inversion of the tunnel data using standard single-band Eliashberg equations revealed an Eliashberg function with 3 distinct peaks at 40, 60 and 90 meV. It can be deduced that the boron-like phonons (60 and 90 meV) couple about 3 times stronger than the magnesium like phonons (40 meV). The strong peak due to the E_2g-mode near 70 meV which is predicted by several theoretical calculations could not be detected in the tunnel data.