We report measurements of the tunneling density of states (DOS) of ultrathin Al films in which superconductivity is quenched by a parallel magnetic field, $H_{\parallel }>48\mathrm{}\mathrm{kG}$. The normal state DOS not only displays the usual logarithmic zero bias anomaly (ZBA) but also a new anomaly, superimposed on the ZBA, that appears at bias voltages $V\sim 2{\mu }_B{H}_{\parallel }/e$, the electron Zeeman splitting. Measurements in tilted fields reveal that the Zeeman feature collapses with increasing perpendicular field component, suggesting that it is associated with the Cooper electron-electron interaction channel.

• Received 13 March 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.1139