ScienceWeek

CHEMICAL PHYSICS: ON THE ELECTRONIC STRUCTURE OF LIQUID LEAD

The following points are made by Yves Petroff (Science 2004 306:2200):

1) Crystalline metals have been studied intensively over the past 40 years, and sophisticated theoretical models and experimental tools have resulted in a generally very good understanding of these materials. In contrast, the atomic and electronic structure of liquid metals is poorly understood. In a liquid metal, the atomic structure varies in both time and space, and the only information that can be obtained is averaged. The lack of periodicity makes it also very difficult to determine whether the electrons are bound to individual atoms or delocalized over the entire liquid, because the band structure (which determines the electronic properties) can no longer be measured.

2) Baumberger et al [1] report the first direct measurements of the band structure of liquid lead at the lead/copper interface. They use angular resolved photoemission to show that the Fermi surface (which separates the occupied electronic states from the empty ones) persists in the liquid phase and that the localization of the electronic wave function depends strongly on the symmetry of the two p(sub[x,y]) bands of lead.

3) In the year 2000, Reichart et al [2] introduced a trick to enable them to study the atomic structure of liquid lead. It has been predicted [3,4] that in monatomic three-dimensional liquids such as lead, atoms should cluster to form icosahedrons. Reichart et al argued that at the interface of liquid lead with a silicon (001) surface, the potential of the silicon surface cannot cause any long-range ordering in the lead, but that it can break the icosahedrons into pentagonal halves, which can be captured at the silicon surface in a preferred orientation. They therefore measured the scattering of totally reflected (evanescent) x-rays, which are sensitive only to the liquid structure at the interface, from a liquid lead layer supported on Si(001). They detected a five-fold local symmetry and obtained experimental evidence for the predicted icosahedral fragments.

4) Baumberger et al [1] studied the electronic properties of a liquid lead film on a copper surface. They performed angular resolved photoemission spectroscopy to obtain the band structure E(k) of liquid lead. To do so, they investigated a lead monolayer supported on a copper (111) surface as the temperature is raised through the melting transition (at 568 K) of the film. Lead films on Cu(111) grow layer by layer with a defined orientation (they form "epitaxial films") [5]. Because of the proximity of the Cu(111) substrate, information about the momentum of the electronic states of the liquid phase can be retrieved.

References (abridged):

1. F. Baumberger, W. Auwärter, T. Greber, J. Osterwalder, Science 306, 2221 (2004)

2. H. Reichart et al., Nature 408, 839 (2000)

3. F. C. Frank, Proc. R. Soc. London A 215, 43 (1952)

4. J. D. Bernal, Proc. R. Soc. London A 280, 299 (1952)

5. F. Baumberger et al., Surf. Sci 532, 82 (2003)

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