Tunneling Spectroscopy of Fe and Cr(001) Surface States
Tunneling spectroscopy with the STM offers the capability of probing the
energy dependence of the electron states in the sample with atomic-scale
resolution, inviting the possibility of atomic-scale chemical identification.
Historically, tunneling conductance spectra of metals, however, have lacked
unique spectroscopic features which could be used for this purpose. In
contrast, we find that the tunneling conductance spectra from both Cr and
Fe(001) surfaces display sharp peaks near the Fermi energy which can be used for
chemical identification, and possibly
for spin dependent tunneling experiments.
Figure 1 shows the tunneling conductance spectra of the Fe(001) surface
recorded at several tip-sample separations (a-f). All the spectra show a very
narrow feature 0.17 eV above the Fe Fermi level, defined as 0 V. The strength of
this feature increases as the tip-sample distance is reduced, as expected from
the exponential distance dependence of tunneling matrix elements. A similar
conductance peak is observed on Cr(001);
for Cr the peak is centered 0.05 eV below the Fermi energy.
Figure 1. Tunneling conductance versus sample voltage measurements of an Fe(001) surface obtained at
constant height above the Fe surface. Curves (a)-(f) correspond to different tunneling distances
between the tip and sample obtained by stabilizing the initial tunneling conditions with different
initial sample voltages of a) 3, b) 2.5, (c) 2.0, d) 1.7, e) 1.4, and f) 1.1 V. The tunneling
conductance data was obtained numerically from the current vs voltage measurements.
Michael Weinert at Brookhaven National Laboratory carried out band
structure calculations of the Fe and Cr(001) surfaces to identify the source of these
conductance peaks. The calculations show that the peaks are due to a
Shockley-like surface state, arising from a
highly-localized atomic orbital
of mainly dz2 symmetry, which can be found on many bcc(001) surfaces.
To compare the calculations to experiment, the total local density of states (LDOS) around
the center of the surface Brillouin zone is determined from the
calculations. The results for Fe(001) are shown in Figure 2. The LDOS peak at +0.2 eV in
the minority band agrees very well with the experimental observation. (The majority-band
peak at -1.8 eV is too far below the Fermi energy to significantly contribute to the
conductance spectra). The calculations for Cr(001) also show the presence of this
surface state near the Fermi energy in the minority band. The spin polarization of
the state for both Cr and Fe will be useful in future spin-polarized tunneling experiments.
Figure 2. Local density of states at various distances above the Fe(001) surface arising
from states in a region around the center of the surface Brillouin zone for a) majority, b) minority, a
nd c) both spins.
Tunneling Spectroscopy of bcc(001) Surface States
Atomic-scale Observations of Alloying at the Cr-Fe(001) Interface
Joseph A. Stroscio
Daniel T. Pierce
Robert J. Celotta
Angela Davies - University of North Carolina (Charlotte)
Michael Weinert - Brookhaven National Laboratory
Supported in part by the Office of Naval Research
Online: May 1996
Last Updated: February 2008
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