Quantum-well states and electronic coherence in bimetallic Pb/Ag thin films

It has been well established that the presence of quantum-well states in metal thin films plays an essential role in determining the thickness dependence of many physical properties. Here we present first-principles calculations and measurements by angle-resolved photoemission spectroscopy for electronic states in a bimetallic film composed of 10 layers of Pb and 9 layers of Ag in the (111) direction on a Si substrate. It is found that the original quantum-well states in individual Pb and Ag films evolve into new states in the bimetallic film by extending into the additional space, instead of directly coupling with each other as one would have expected. The new set of quantum-well states therefore have modified effective masses and energy values compared with the parent ones. Even though the Pb/Ag interface is incommensurate, the coherence of the electronic states across the whole bimetallic film is verified by supercell configurations with different rotational arrangements in the calculation. The excellent agreement between theory and experiment in the energy dispersion of the quantum-well states confirms the physical picture proposed in this work, which could form the basis in exploring the electronic structure in multiple stacked thin films.