Apparent quantum number paradox in Ag quantum wells on Si(111)

Angle-resolved photoemission studies of the quantum well electronic structure in atomically uniform Ag films grown on Si(111)-(7×7) reveal an anomalous bifurcation of one of the subbands as it disperses toward the Fermi level. This bifurcation leads to an apparent quantum number paradox, since subbands must be associated with consecutive integer quantum numbers. Evident only in films thicker than 15 monolayers, the bifurcation migrates upon annealing from subband to subband toward the zone center and ultimately vanishes. Various tests indicate that this perplexing behavior arises from transverse resonances in the film electronic structure caused by the reconstruction at the interface.