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On the close-packed surfaces of noble metals exists a surface state.
The electrons in the surface state form a two-dimensional electron
gas. Scattering of the electrons in the surface state forms the
well-known standing wave patterns. It is known that these surface
states survive as interface states below ordered adsorbate layers.
In the case of Xe/Cu(111) these interface states were imaged recently
with a scanning tunneling microscope at low temperatures.
We have grown ultrathin sodium chloride layers (two monolayers)
on a copper (111) surface and have observed the existence of an
interface state located at the NaCl/Cu interface, descending from
the original Cu(111) surface state. Several images of the same region
near a NaCl step edge, taken at various voltages, show that the
wavelength of the NaCl/Cu(111) interface state is larger than that
of the Cu(111) surface state (see images a-d). Detailed spectroscopic
experiments at 8 K determined the bottom of the interface band to
be about 225 meV below the Fermi level. A small increase of the
effective mass of the electrons is observed. The phase-accumulation
model provides a qualitative understanding of the observed changes
in the dispersion. Even more relevant, it reveals that the wave
functions of the surface electrons are barely modified upon adsorption
of NaCl, which makes a high transmission through NaCl step edges
promising.
The misalignment between the NaCl lattice and the underlying Cu
surface forms a Moiré pattern observed in atomically resolved
STM images. This additional periodicity produces a splitting in
the band structure of the interface state of about 50 meV at 20-400
meV above the Fermi level, depending on the specific rotation of
the NaCl domain. The electron density just above and just below
the band gap is strongly modulated as can be seen in corresponding
STM images. The strong standing plane waves just below the band
gap allows us to study the refraction of two-dimensional electron
waves at the NaCl island edges as shown. The Moiré-pattern-induced
band gap shows a new way to tailor the properties of a two-dimensional
electron gas for future applications. These possibilities can be
further extended by controlled sequential growth of different dielectric
materials. In contrast to surface states, interface states are inherently
protected by the dielectric adlayer and can even be studied under
ambient conditions.
References
M. F. Crommie, C. P. Lutz, and D. M. Eigler, Nature 363,
524 (1993); Y. Hasegawa and Ph. Avouris, Phys. Rev. Lett.
71, 1071 (1993).
A. Hotzel, G. Moos, K. Ishioka, M. Wolf, G. Ertl, Appl. Phys.
B 68, 615 (1999).
J.-Y. Park, U. D. Ham, S.-J. Kahng, Y. Kuk, K. Miyake, K. Hata,
and H. Shigekawa, Phys. Rev. B 62, R16341 (2000).
N. V. Smith, Phys. Rev. B 32, 3549 (1985).
J. Repp, G. Meyer, K. H. Rieder, Phys. Rev. Lett., 92(3)
036803 (2004).
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IBM Research
