Quantum well states with nonvanishing parallel momentum in Cu/Co/Cu(100)

Using low-temperature scanning tunneling spectroscopy we study quantum well states in the topmost copper layer of a Cu/Co/Cu(100) system above the Fermi energy. The emergence and the energetic positions of QWSs within this layer crucially depend on the interface quality tailored by the sample preparation method. Samples deposited at room temperature show a rough interface and lead to the well-known QWSs with only a momentum perpendicular to the interface. Atomically smooth interfaces for samples grown at 80 K exhibit states caused by stationary points with a large nonvanishing parallel momentum. Simulations taking into account the different band structures allow QWSs to be modeled from both stationary points and an identification of a crossover between bound and resonance states.