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Probing electron confinement in ultrathin metal films: From spin-polarized quantum-well states to surface resonances

The unoccupied electron states of the sandwich structure Cu/Co/Cu(100) have been investigated by spin- and angle-resolved inverse photoemission. Different types of electron states are clearly identified based on the spin-resolved data: spin-polarized quantum-well states within the Cu layer, surface states and a surface resonance at the copper/vacuum interface, and bulk-like minority d contributions from the underlying ferromagnetic Co layer, which are accessible within the experimental probing depth. The quantum-well states are shown to develop into a surface resonance as a function of the electron momentum. Experimental evidence of this change in electron confinement is a vanishing exchange splitting. The loss of spin polarization is caused by a decoupling of the electron wave function from the ferromagnetic Co at the inner interface.