Surface states at the „001… surface of CuAuI
R. G. Jordan
Alloy Research Center, Department of Physics, Florida Atlantic University, Boca Raton, Florida 33431-0991
G. Y. Guo
CCLRC Daresbury Laboratory, Warrington WA4 4AD, United Kingdom
Received 3 September 1996
We report calculations of the electronic structure in the neighborhood of the 001 surface of CuAuI using
the self-consistent-field, linear muffin-tin orbital method within the atomic sphere approximation, with a slab
geometry and including the surface dipole terms. We focus our attention on the vicinity of the M
¯
point in the
surface Brillouin zone and we identify the two Tamm-type surface states reported recently in photoemission
measurements. The overall agreement between the theoretically determined dispersions of these states and
those observed experimentally is very good and provides strong support for our calculational scheme. We show
also that measurements of the surface core level shifts should allow a determination of the termination.
S0163-18299702912-3
Recently, Xu and Jordan
1
reported the occurrence of two
Tamm-type surface states at the M
¯
point on the 001 and
100 surfaces of equiatomic CuAuI , with binding energies
in the ranges 1.3–1.6 and 6.1 eV, i.e., just above the
d -band continuum and in a gap within the Au-related
d -band complex, respectively. They measured the dispersion
relations but since there had been no previous theoretical
investigations of these particular surfaces — nor of any sur-
face of CuAuI for that matter — they limited their discussion
to the similarity of the surface states at the lower binding
energies to those previously observed at the M
¯
point of
Cu100Refs. 2–6 and Au100Ref. 7 and to a related
surface state on Cu
3
Au100.
8,9
These particular states are
split off from the top of the d -band continuum by a tenth of
an eV or so. In addition, through comparisons of the disper-
sions of the surface states with the larger binding energies,
Xu and Jordan
1
were able to draw some conclusions about
the atomic spacings on the 100 and 001 surfaces of CuAu
I .
At about the same time, Jordan et al.
10
demonstrated the
suitability of the self-consistent-field, linear muffin-tin or-
bital SCF-LMTO method within the atomic sphere ap-
proximation ASA, using a slab geometry and including the
surface dipole terms,
11
to the calculation of the electronic
structure in the vicinity of the 100 surface of Cu. They
showed, for example, that two surface states exist at the M
¯
point, one about 0.2 eV above the d -band continuum and the
other in a spin-orbit induced gap near the top of the d -band
continuum, in good agreement with experiments.
2–6
In addi-
tion, they showed that the layer-by-layer potential functions
generated by this method could be used in first-principles
photocurrent calculations, and the resulting spectra were in
excellent agreement with the photoemission measurements
of Kevan and Shirley.
4
Jordan et al.
10
concluded therefore
that their approach was reliable and realistic.
In this paper, we extend the calculational scheme de-
scribed in Ref. 10 to investigate the electronic structure at the
001 surface of CuAuI . Initially, we concentrate on surface
states at the M
¯
point of the surface Brillouin zone. We show
that surface states exist whose dispersions in k
||
space are in
very good agreement with those obtained by Xu and Jordan
1
in their photoemission measurements and we show that the
occurrence of the surface states depends critically on the
composition of the terminating i.e., surface layer. In addi-
tion, we examine the dependence of the surface core level
shifts on the termination.
Equiatomic CuAuI has the L 1
0
layered tetragonal struc-
ture and so the planes normal to the 001 direction comprise
alternate layers of Cu and Au atoms. Thus, in the absence of
segregation, the 001 surface is either Cu or Au terminated;
careful compositional analysis by Auger electron spectros-
copy AES revealed that the surface layer is essentially
100% Au.
12
Therefore, in the spirit of Ref. 10 we calculated
the electronic structure in the vicinity of the 001 surface of
CuAuI using the SCF-LMTO-ASA method and a 16-layer
slab consisting of 11 alternating Cu and Au metal layers
and five vacuum layers. With this arrangement there are a
total number of six inequivalent metal atoms and, hence,
layers — which we label M 1 the central layer of Cu atoms
through M 6 the surface layers of Au atoms — and three
inequivalent vacuum layers of ‘‘empty’’ spheres — labeled
V1 the layers in contact with the M 6 layer through V 3 the
central vacuum layer. The calculation was carried out for 56
k points in the tetragonal Brillouin zone with a Wigner-Seitz
radius of 2.88 au for each sphere — the equilibrium value for
CuAuI determined from a previous self-consistent, ‘‘bulk’’
calculation using the experimentally measured c / a ratio
0.9251.
Inspection of the charge transferred between the inner-
most Cu and Au atomic spheres in layers M 1 and M 2,
respectively, see Table I, indicates that we have included a
sufficient number of layers in the slab since the magnitudes
differ by 0.02% and are within 0.2% of the value from the
‘‘bulk’’ calculation. In addition, the amount of charge in the
middle vacuum layer, V 3, is negligibly small. Furthermore,
in another set of calculations with two fewer metal layers
PHYSICAL REVIEW B 15 MARCH 1997-I VOLUME 55, NUMBER 11
55 0163-1829/97/5511/72225/$10.00 7222 © 1997 The American Physical Society