Electron tunneling into superconducting ZrN

We have prepared tunnel junctions on superconducting ZrN thin films using the natural oxide as a tunnel barrier. In the superconducting density of states we observed the phonon-induced structures due to the acoustic and optical phonons. The tunnel data could be analyzed by the McMillan-Rowell procedure without the need to correct for proximity effects. From the resulting Eliashberg function it was concluded that the optical phonons are coupled slightly more strongly to the electrons than the acoustic phonons