SRC Users' Meeting - Synchrotron Radiation Center - University of ...

LOW-DIMENSIONAL ELECTRONS AT SILICON SURFACES
J. N. Crain 1 , J. L. McChesney 1 , Fan Zheng 1 , M. C. Gallagher 2 , P. C. Snijders 3 , M. Bissen 4 ,
C. Gundelach 4 , S. C. Erwin 5 , and F. J. Himpsel 1
1 Dept. of Physics, UW-Madison, 1150 University Ave., Madison, Wisconsin 53706
2 Dept. of Physics, Lakehead University, Thunder Bay, ON P7B-5E1 Canada
3 Dept. of NanoScience, Delft University of Technology,
Lorentzweg, 2628 CJ Delft, The Netherlands
4 Synchrotron Radiation Center, UW-Madison, 3731 Schneider Dr., Stoughton, WI 53589
5 Center for Computational Materials Science, Naval Research Laboratory,
Washington, DC 20375
Electrons become increasingly correlated as the dimensionality is reduced from 3D to
2D and 1D. Already in 2D one observes correlations between electrons and vortices in the
quantum Hall effect leading to fractional charge and statistics. Predictions for a 1D metal
anticipate a complete breakdown of the single electron concept and the separation of spin and
charge into two collective excitations, spinons and holons.
It has become possible to systematically engineer two- and one-dimensional surface
structures of metals on silicon that are metallic [1,2]. Electrons near the Fermi level are decoupled
from the substrate because there energy lies in the band gap. The metal atoms,
however, are rigidly tied to the silicon lattice in substitutional positions according to x-ray
diffraction [3] and first principles band calculations. That makes a Peierls transition to an
insulator unfavorable and creates an opportunity for observing exotic states predicted for onedimensional
metallic electrons.
Examples of two-dimensional metals are the 3x3 and 21x21 reconstructions of
Ag and Au on Si(111), which exhibit an intricate pattern of Fermi surfaces induced by the
superlattice [2]. Information about the Fourier components of the superlattice potential is
obtained from avoided band crossings.
One-dimensional atomic chains are created by growing Au and a variety of other
metals at vicinal Si(111) surfaces in the 1/5 monolayer coverage regime [1]. This appears to
be a rather universal phenomenon, and even the flat Si(111) surface breaks its three-fold
symmetry to accommodate chain structures. This wide-open territory of 1D structures is
explored in real and reciprocal space by scanning tunneling microscopy (STM) and angleresolved
photoemission. The resulting energy bands and Fermi surfaces can be tuned between
2D and 1D by increasing the chain spacing, with the intra-chain / inter-chain coupling ratio
varying from 10:1 to >70:1. Unexpected metallic bands are found, such as a pair of nearly
degenerate, half-filled bands, a quarter-filled band, and a fractional electron count of 8/3
electrons per 1x1 cell [1]. A possible explanation for the fractional electron count is
suggested, where extra Si atoms in a 2D cell “dope” the 1D chain embedded in it. This would
be analogous to the doping in HiT c materials, where a 3D cell dopes an embedded 2D plane.
References:
[1] Losio, et al., Phys. Rev. Lett. 86, 4632 (2001); Altmann et al. Phys. Rev. B 64, 035406
(2001); Crain et al., Phys Rev. Lett., in press (2003).
[2] Crain, et al., Phys. Rev. B 66, 205302 (2002).
[3] Robinson, et al., Phys. Rev. Lett. 88, 096104 (2002).