Evolution of a symmetry gap and synergetic quantum well states in ultrathin Ag films on Au(111) substrates

Using first-principles calculations within density functional theory (DFT) and high-resolution angle-resolved photoemission spectroscopy (ARPES), we carry out a comprehensive study of how a symmetry gap around the Fermi level develops towards its bulk value as the thickness of Ag films grown on an Au(111) substrate increases. We show that, contrary to prevailing assumptions, the symmetry gap in ultrathin Ag films can be substantially wider than the gap of the Au substrate along the [111] direction. As a result, the first and second quantum well states (QWS) are confined within the Ag films only when the film reaches a certain thickness. We further show that, when ultrathin Au films are grown on Ag(111) slabs of comparable thicknesses, synergetic QWS spanning across the whole (Au+Ag) systems are established.