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September 4, 2002 Structures of Quantum Wires in Gold/Silicon(557)I. Robinson1, P. Bennett2, and F. Himpsel3 Scientists working at NSLS beamline X16A have revealed the full three-dimensional structure of nanowires formed by deposition of gold on a stepped silicon surface using surface X-ray diffraction. The result is important because of the unusual band-dispersion properties previously reported for this system. The result is interesting because the gold atoms are neither imaged by STM, nor are involved directly in the electronic states responsible for the metallic behavior of the surface. A "quantum wire" is a one-dimensional wire created by arranging metal atoms that touch each other in an exact straight line. The wave functions describing the electrons present in such a wire are localized in the directions perpendicular to its axis but can form solitons – waves that propagate with little loss of energy and retain their shape and speed after colliding other such waves – in the direction along the wire. The Pauli exclusion principle forbids the free propagation of these solitons along the wire and leads to a one-dimensional metal with highly unusual properties.
We used two techniques. The first is the scanning tunneling microscope (STM), which uses electron tunneling to map the positions of individual atoms and defects in a surface. Typical defects are vacancies, steps and kinks. We also used surface x-ray diffraction, a traditional structure-determining method that is less useful for looking at defects, but provides accurate three-dimensional structures. We measured a large set of crystal truncation rods to solve a crystalline structure formed by evaporation of 20 percent of a monolayer of gold onto a silicon(557) surface using the surface x-ray diffraction facility at NSLS beamline X16A. The derived structure, shown in the figure, reveals four main features. First, the row of gold atoms sits in the middle of the terrace between adjacent steps in the substrate, while we expected that the metal would have directly lined the step edge instead. Second, an almost ideal substitution takes place in the upper bilayer, resulting in bond lengths between gold and silicon of 2.47 angstrom and 2.35 angstrom, which are indistinguishable from the bond length between two silicon atoms, which is 2.35 angstrom. A gold atom appears to substitute in the silicon lattice with relatively little strain. The third feature is that the step edge reconstructs to form a five-membered ring, but removes a dangling bond. Finally, Figure 1 shows a row of adatoms attached to dangling bonds.
The STM micrograph reveals two lines per unit cell that might be mistaken for wires. The X-ray structure has only a single gold per unit cell. Analysis of the inter-row distances shows that neither of the rows can be gold atoms, which are therefore not visible to STM. The rows correspond instead to the step-edge silicon and the adatom silicon features. Because STM is surface-sensitive, it detects only the outermost electron shells of the topmost layer, whereas surface x-ray diffraction sees the full three-dimensional arrangement of the atom cores. FUNDING PUBLICATION FOR MORE INFORMATION |
The National Synchrotron Light Source at Brookhaven National Laboratory in New York, is a national user research
facility funded by the U.S. Department of Energy's Office of Basic Energy Science. The NSLS operates two electron
storage rings: an X-Ray ring and a Vacuum UltraViolet (VUV) ring which provide intense light spanning the electromagnetic
spectrum from the infrared through x-rays. Each year over 2300 scientists from universities, industries and government
labs perform research at the NSLS.
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We have investigated a quantum wire in a candidate system for
spin-charge separation, due to the formation of a Luttinger liquid,
in which electrons have long-range interactions between them. 