Coherently coupled quantum-well states 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. Oscillatory features in these properties are often observed and attributed to the energy variations of these quantum-well states near the Fermi level. Modern film growth capability has made it possible to create composite metal thin films in which one metal thin film is stacked on top of a dissimilar one with precise control of individual thickness. How the original quantum-well states evolve in the composite film becomes a critical issue in understanding the electronic structure of these new complex thin-film systems. In this paper we present first-principles calculations and measurements by angle-resolved photoemission spectroscopy for electronic states in a bimetallic film composed of ten layers of Pb and nine 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 a new set of 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 has modified effective masses and energy values compared with the parent ones. Even though the Pb/Ag interface is incommensurate, the coherently coupled electronic states across the whole bimetallic film are 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 in this bimetallic film confirms the physical picture proposed in this work, which could form the basis for exploring the electronic structure in multiple stacked thin films with more complicated designs.