The Metallicity of Surfaces and Metal Thin Film Overlayers

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This research program will investigate why some materials exhibit nonmetallic behavior when deposited on metallic substrates. This unusual behavior has been confirmed recently by several different spectroscopies including experimental band structure mapping, resonant photoemission, core level spectroscopy and electron energy loss. Nonmetallic overlayers undergo a nonmetal to metal transition with increasing coverage and the nonmetal to metal transition is typically accompanied by a structural transition indicative of a first order transition. University-industry collaborations will assist in characterizing nonmetallic overlayers which exhibit an excitonic insulating behavior. Microscopic probes of structure will be correlated with the more conventional photoemission and resonant photoemission techniques for studying electronic transitions. The similarities between the nonmetal to metal transition in free clusters and at metal surfaces are remarkable, particularly for the metal mercury. The fundamental reasons for the similar variations in metallicity with coordination number, and cluster size (film coverage or thickness) can only be established with further electronic and structural studies. Divalent metal overlayers such as Mg, Ba, and Hg will be investigated and compared with other overlayers exhibiting no nonmetal to metal transition. %%% This research program addresses a very basic and fundamental question: are the surfaces of metals metallic? Conventional measurements to determine metallic character, such as conductivity measurements for example, are difficult to apply to a thin film of material one or two atoms thick deposited on a metal substrate. New approaches to establishing what it means for a metal to be a metal have been developed and can now be applied to materials normally considered metals. There is a need to explore why the sufaces of some material have such fundamentally different properties form those normally associated with that material. Increasing number of examples of this phenomenon of nonmetallic behavior at metal surfaces or on metal substrates have been observed; however, general rules or principles to quide our understanding remain elusive.