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Journal ArticleDOI

Quantum well and resonance-band split off in a K monolayer on Cu(111)

25 Apr 2008-Physical Review B (American Physical Society)-Vol. 77, Iss: 15, pp 153410

Abstract: The potassium monolayer on Cu(111) defines the simplest metallic quantum well that confines a single $s$-like discrete level. The analysis of the metallization onset in such a K adlayer reveals, however, a subtle electronic structure. The metallic monolayer condensate is actually characterized by a pair of two-dimensional states that lie below the Fermi energy, namely, a quantum well state and a resonant band reminiscent of the Cu(111) surface state. All quantum well states, resonances, and Cu substrate bulk bands exhibit smooth K coverage dependence, suggesting that changes in the crystal potential upon K adsorption extend from the surface and/or interface inside the Cu substrate.
Topics: Fermi energy (58%), Monolayer (56%), Electronic structure (56%), Quantum well (55%), Fermi level (55%)

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  • F. Schiller,1 M. Corso,2 M. Urdanpilleta,1 T. Ohta,3 A. Bostwick,3 J. L. McChesney,3 E. Rotenberg,3 and J. E. Ortega1,2 1Departamento de Física Aplicada I, Universidad del País Vasco, Plaza Oñate 2, E-20018 San Sebastián, Spain 2Donostia International Physics Center and Centro Mixto de Materiales CSIC/UPV, Paseo Manuel Lardizabal 4, E-20018 San Sebastián, Spain 3Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Received 13 February 2008; published 25 April 2008.

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Quantum well and resonance-band split off in a K monolayer on Cu(111)
F. Schiller,
1
M. Corso,
2
M. Urdanpilleta,
1
T. Ohta,
3
A. Bostwick,
3
J. L. McChesney,
3
E. Rotenberg,
3
and J. E. Ortega
1,2
1
Departamento de Física Aplicada I, Universidad del País Vasco, Plaza Oñate 2, E-20018 San Sebastián, Spain
2
Donostia International Physics Center and Centro Mixto de Materiales CSIC/UPV, Paseo Manuel Lardizabal 4,
E-20018 San Sebastián, Spain
3
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
Received 13 February 2008; published 25 April 2008
The potassium monolayer on Cu111 defines the simplest metallic quantum well that confines a single s-like
discrete level. The analysis of the metallization onset in such a K adlayer reveals, however, a subtle electronic
structure. The metallic monolayer condensate is actually characterized by a pair of two-dimensional states that
lie below the Fermi energy, namely, a quantum well state and a resonant band reminiscent of the Cu111
surface state. All quantum well states, resonances, and Cu substrate bulk bands exhibit smooth K coverage
dependence, suggesting that changes in the crystal potential upon K adsorption extend from the surface and/or
interface inside the Cu substrate.
DOI: 10.1103/PhysRevB.77.153410 PACS numbers: 73.20.r, 73.21.b, 79.60.Dp
Quantum well QW states of thin metal films and multi-
layers are of fundamental interest in present and future tech-
nology applications. Over the past few years, a number of
metallic QW systems have been grown with a high degree of
interface and thickness perfection. This allows the discovery
of exotic energy and structure interplay phenomena, such as
the so-called quantum growth,
1
or to unveil the role of the
electronic and geometric properties of the substrate in the
spectrum of QW levels.
2,3
The latter is of particular impor-
tance because, in the end, the exotic properties in metallic
QWs, such as the spin polarization in nonmagnetic films,
4
are the direct consequence of QW scattering at the interface,
which in turn opens the way to new applications through
interface engineering.
3,5
Alkali metal films on noble metal substrates of the 111
orientation, such as Cu111, are prototypes of metallic quan-
tum wells with s-like electrons confined by a substrate bulk
gap. On the other side, the alkali monolayer is a model QW
with a single s-like level. The alkali/Cu111 system is very
suitable to track the evolution of electronic states from the
low-density, adsorbed-atom phase, with ionic bonding with
the surface, to a more condensed phase near monolayer
completion, with metallic bonds inside alkali islands metal-
lic condensation. The monolayer growth on Cu111 has a
rich structural behavior,
6
which is indeed reflected on the
electronic structure.
712
For very small coverage, the Shock-
ley state of the clean Cu111 surface survives but shifts
down in energy. As coverage increases, the surface state is
pushed beyond the bulk band gap and inside the bulk con-
tinuum. At the same time, the work function decreases and
the unoccupied image state shifts toward the Fermi energy.
At a coverage of 0.6–0.8 ML monolayer, the alkali metal
condenses into monolayer thick islands, and the image state
smoothly drops below E
F
, transforming into the characteris-
tic QW state of the alkali metal overlayer.
9,12
At a full mono-
layer coverage, angle resolved photoemission shows a para-
bolic QW band with a relatively large effective mass
compared to the Cu111 surface state 0.8m
e
vs 0.41m
e
located 135 meV below the Fermi energy for Li,
10
127 meV
for Na,
11
100 meV for K,
8
and 25 meV for Cs.
12
Despite its simplicity, the alkali/Cu111 system exhibits
subtle properties that break the simple picture of the totally
confined QW state.
12,13
The alkali layer and the Cu substrate
possess different lattice constants, such that an effective
overlap with bulk states may actually occur via umklapp
with reciprocal superlattice vectors.
13
On the other hand, fi-
nite size gaps lead to a considerable penetration of the QW
wave function inside the substrate crystal, and hence QW
electronic states must exhibit both substrate and overlayer
lattice properties. This double substrate and adlayer period-
icity explains the presence of an extra resonant state in first
principles calculations.
12
Here, we present the experimental
evidence of this QW/resonance split off that defines the
K/ Cu111 monolayer. We observe that the resonant state
evolves from the clean surface state, but it only becomes a
strong feature in the metallic condensate phase and close to
the Fermi energy. The photoemission intensity reflects a
rapid spectral density variation across the surface Brillouin
zone, which is explained as connected to the wave function
properties of the resonant state.
12
Finally, we observe that not
only the alkali metal layer states QW and resonance but
also bulk states display a smooth transformation upon the
monolayer completion, proving that the adlayer affects the
crystal potential deep inside the Cu substrate.
The photoemission experiments have been performed
at beamline 7.0.1 of the Advanced Light Source at the
Lawrence Berkeley Laboratory by using a hemispherical Sci-
enta R4000 spectrometer and p-polarized photons of energies
h
=80 eV and h
=105 eV. Energy and angular resolutions
were set to 25 meV and 0.1°, respectively. We used a cylin-
drical Cu crystal with a smooth 15° miscut variation with
respect to the 111 direction. Such a curved surface opens
the possibility to study the influence of the steps on the elec-
tronic states of the K adlayer. The small spot size below
100
m allowed scanning of the x-ray beam on the surface
and hence selecting the appropriate crystal orientation. Such
a curved surface is cleaned with standard sputtering-
annealing cycles, after which it exhibits a sharp low energy
electron diffraction pattern, with a smoothly varying spot
splitting away from the 111 orientation. In order to ensure
surface diffusion and homogeneous layer formation, the K
adsorption is done at 300 K, with the 111 plane placed
PHYSICAL REVIEW B 77, 153410 2008
1098-0121/2008/7715/1534104 ©2008 The American Physical Society153410-1

at the focus of the analyzer. Thus, thickness-dependent
K/ Cu111 photoemission spectra could be recorded while
the evaporation was performed, as shown in Fig. 1. The ML
is defined as the coverage at which the QW energy reaches
its maximum binding energy see below.
11
Upon saturation
with 1 ML, the crystal was quickly cooled down to 125 K.
Figure 1 shows the Fermi level intensity measured as a
function of the K coverage along the
¯
M
¯
symmetry direc-
tion. The two intense maxima observed prior to K deposition
at 0.2 Å
−1
are the Fermi crossings of the Cu111
Shockley surface state. The measurement reflects the smooth
transformation of this surface state into a resonance and the
appearance of the double resonant/QW band at the onset
of the K metallic condensation. The evaporation rate is
0.07 ML/ min, such that a single spectrum was recorded ev-
ery 0.025 ML step. The evolution of the different features is
better understood in the light of Figs. 2a2c, where we
show the band dispersion for the clean surface, 0.4 and 1
ML. In order to enhance the resonant state in both Figs. 1
and 2, the intensity scale is saturated in the QW and the Cu
surface state. A more accurate quantitative photoemission in-
tensity variation of the surface resonance is shown in Fig. 3.
Coming back to Fig. 1, we can clearly observe that the
Cu111 surface state smoothly shifts toward the edge of the
gap vertical lines in Fig. 1 as the coverage increases, over-
lapping with the continuum of bulk bands and getting almost
quenched at 0.5 ML. It is known that the K QW band
crosses E
F
when the K layer becomes a metallic condensate
around 0.7 ML.
8,9
In Fig. 1, we also show that, at the same
time, a second emission emerges outside of the copper band
gap. This K ML resonance appears to evolve dotted line
from the surface state, although it only strongly builds up
upon metallic condensation. The critical coverage for the re-
surgence of the resonance and the crossing of the QW band
across E
F
is 0.75 ML, as carefully determined in Fig. 3a.
Assuming a 4.4 Å lattice constant for the 1 ML condensate,
6
the critical 0.75 ML coverage corresponds to a 5.08 Å K
adatom lattice, i.e., to the 2 2 commensurate reconstruc-
tion of the copper substrate with lattice constant a
=3.605 Å. Therefore, three features, namely, the 2 2
overlayer arrangement, the QW Fermi level crossing, and the
resonance, characterize the metallic condensation of K on
Cu111.
Thin lines in Fig. 2 represent parabolic fits to the different
bands after separate analysis of individual spectra. The sur-
face state evolves from E
0
=−0.41 eV and m
*
=0.41m
e
in the
clean surface to E
0
=−0.755 eV and m
*
=0.36m
e
for the 0.4
ML surface resonance and to E
0
=−1.03 eV and m
*
=0.55m
e
in the strong split off K ML resonance. The latter
values are similar to the energy and the effective mass pre-
dicted for the 22 Cs layer on Cu111,
12
indicating the
same physical nature in both K and Cs ML resonances. Simi-
larly, the K ML QW state values E
0
=−0.11 eV and m
*
=0.81m
e
agree with data on Na metallic monolayers E
0
=
−0.127 eV, m
*
=0.7m
e
Ref. 11兲兴. On the other hand, note
that the bulk sp band low energy parabola smoothly shifts
down from Cu111 to the K covered substrate. The down-
shift behavior in both the QW state and the resonance can be
explained as due to an increasingly attractive average crys-
1.0
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.
6
k(Å)
||
-1
0.8
0.6
0.4
0.2
0.0
K layer thickness (ML)
Cu(111) band gap
shutter open
FIG. 1. Color online Fermi surface cut along the
¯
M
¯
direction
as a function of K coverage. The vertical lines mark the edge of the
projected band gap in Cu111. The low coverage regime is charac-
terized by the transformation of the Cu111 surface state into a
weak resonance, mostly quenched at 0.5 ML. Upon metallic con-
densation, the QW band shifts below the Fermi energy and the
resonant emission strongly picks up dotted line.
k(Å)
||
-1
E-E (eV)
F
(
a
)
(b)
(c)
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
m
*
=0.41
Cu(111)
bulk
surface
state
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
m
*
=0.36
0.4 ML
K/Cu(111)
bulk
surface
state
0.6
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
-0.6 -0.4 -0.2 0.0 0.2 0.4
m
*
=0.81
m
*
=0.55
1.0 ML
K/Cu(111)
QW
resonance
bulk
FIG. 2. Color online K QW band, surface states and reso-
nances, and Cu bulk bands for a Cu111, b 0.4 ML K/ Cu111,
and c 1MLK/ Cu111. The photon energy is 80 eV. All surface
and bulk bands exhibit coverage-dependent shifts, suggesting that K
adsorption affects the crystal potential not only at the surface but
also inside the substrate.
BRIEF REPORTS PHYSICAL REVIEW B 77, 153410 2008
153410-2

tal potential V
0
. Thus, a downward shift of the bulk sp band
in Fig. 2 indicates that changes in V
0
extend to the bulk
crystal at least within the sampling depth of the photoemis-
sion experiment. This could, in turn, be related to the pres-
ence of a substrate rumpling, which is found in the
K/ Al111 and Cs/ Al111 systems
14
and suggested in
Na/ Cu111.
15
In Fig. 2, we observe a strong k
dependence of the inten-
sity in the K ML resonance band, exhibiting negligible emis-
sion at the band center and maximum intensity at E
F
. Such
dependence contrasts with the much weaker variation of the
Cu surface state in the clean and the 0.4 ML covered surface,
as depicted in Fig. 3b. The strong k
dependence of the K
ML resonance can be traced to the qualitative variation of its
wave function from bulklike lower photoemission intensity
to thin-film-like higher photoemission intensity along
¯
M
¯
,
as predicted for the Cs 2 2 metallic condensate in theo-
retical calculations.
12
At
¯
, the electron probability uni-
formly extends inside the bulk crystal, with a relatively low
weight in the surface plane. In contrast, close to E
F
, the
spatial variation of the wave function of the K ML resonance
is similar to that of the QW state, i.e., a maximum probabil-
ity at the K layer and a strong damping toward the bulk.
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
k(Å)
||
-1
E-E (eV)
F
80 eV
105 eV
h =105 eV
bulk
resonance
100
80
60
40
Intensity (arb.units)
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.
6
E=E
F
(
a
)
(
b)
quantum well
FIG. 4. Color online兲共a Band dispersion and b photoemis-
sion intensity at the Fermi energy for the K monolayer measured at
h
=105 eV. Solid lines are parabolic fits to the data, and dashed
lines represent the same bands measured at h
=80 eV extracted
from Fig. 2. The two vertical lines in b mark the Fermi level
crossings of the K resonance.
(
b)
Tilt an
g
le
(
de
g)
E-E (eV)
F
-1.10
-1.05
-1.00
-0.95
-0.90
12840
Surf. Res.
position
Quantum well
position
-0.10
-0.05
0.00
0.05
0.10
12840
14
12
10
8
6
4
2
0
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.
6
(
a)
Tilt angle (deg)
k(Å)
||,y
-1
E=E
F
FIG. 5. Color online兲共a Cut through the Fermi surface at
k
,x
=0 as a function of the tilt angle
and the wave vector k
,y
, with
k
,y
=0 being the symmetry center perpendicular to the steps. b
QW left and resonance right band bottom energies as a function
of the orientation of the substrate with respect to the 111 direction.
The measurements have been done at h
=80 eV and by scanning
the photon beam across a cylindrical crystal.
1.0
0.8
0.6
0.4
0.2
0.0
I/I
max
1.
0
0.80.60.40.20.0
k/k
F
Surf. St. Cu(111)
Surf. St. 0.4 ML K/Cu(111)
Resonance 1 ML K/Cu(111)
4000
3000
2000
1000
0
1.00.90.80.70.60.5
K coverage (ML)
-0.10
-0.08
-0.06
-0.04
-0.02
0.00
0.02
E-E
F
(eV)
QW band bottom
Surface
resonance
intensity at E
F
Fermi level intensity (arb.units)
(
a
)
(
b)
FIG. 3. Color online兲共a Band bottom position of the K QW
and intensity variation of the K ML resonance near metallic con-
densation dotted line in Fig. 1 vs coverage. b Normalized pho-
toemission intensity for the Cu surface state, the 0.5 ML K covered
surface state, and the K ML resonance as a function of k
. A strong
variation is observed for the K ML resonance, as expected from the
wave function properties see the text.
BRIEF REPORTS PHYSICAL REVIEW B 77, 153410 2008
153410-3

The surfacelike and bulklike character of the K ML QW
and resonance bands, respectively, is further probed in Fig. 4,
where we test the photon energy dependence. On top, we
show the 1 ML bands measured with h
=105 eV marked by
solid lines, which are compared to the bands extracted from
Fig. 2 measured with h
=80 eV dashed lines. By increas-
ing the photon energy the QW remains unchanged, whereas
the resonance and the copper bulk bands shift by 0.28 and
0.4 eV, respectively, as expected for bulklike bands with k
dispersion. For the higher photon energy 105 eV, the K
resonance band loses intensity and is only observed at ener-
gies close to the Fermi energy see Fermi level crossings in
Fig. 4b .
In Fig. 5, we show the evolution of the quantum well and
the K resonance as a function of the surface orientation tilt
angle
away from the 111 direction. Figure 5a is a cut
through the Fermi surface with the wave vector perpendicu-
lar to the steps as a function of
, while Fig. 5b denotes the
position of the QW and the resonance-band bottom energies.
These data are taken in a straightforward way by scanning
the beam across the cylindrical surface. Moving away from
the 111 direction, we select substrate areas with an increas-
ing density of steps. The QW exhibits an upward energy shift
as a function of this tilt angle, reflecting the repulsive elec-
tron scattering at step edges, which leads to a partial confine-
ment within terraces of decreasing size.
16
The ML resonance,
in contrast, shows a negligible energy shift, as expected for a
wave function with a significant weight in the bulk of the
crystal and hence less sensitive to surface steps.
In summary, we have found the experimental evidence for
the split off resonance predicted in metallic alkali monolay-
ers on Cu111. Such split off characterizes the K overlayer
transformation from an ionic adsorbate to a metallic conden-
sate at 0.75 ML. The bulk resonant character of the reso-
nance state manifests in its photon energy dependence and its
sensitivity to surface defects steps. We also observe that Cu
substrate bands, as K quantum well and resonant states, dis-
play coverage dependence, suggesting that changes in the
crystal potential extend inside the bulk crystal.
Fruitful discussions with Slava Silkin are acknowledged.
The work is supported through projects of the University of
the Basque Country and the Basque Government IT-257-07
and the Spanish Ministerio de Educacion y Ciencia
MAT2007-63083.
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153410-4
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Journal ArticleDOI
Renee D. Diehl1, Ronan McGrath2Institutions (2)
Abstract: The study of the adsorption and the coadsorption of alkali metals on single-crystal metal surfaces is a very active sub-field of surface science, partly because of the importance of technological applications (promotion of catalytic reactions, enhanced oxidation, increases in electron emission rates), but also because of the fundamental interest in how these “simple” adsorbates modify the surface. In the past few years there has been a marked increase in the number of structural studies of alkali adsorption and coadsorption systems, motivated in part by developments in theoretical models of adsorption. This article reviews recent studies of alkali adsorption and coadsorption using specifically structural techniques, with the intention of highlighting recent developments, providing a useful reference base to the community, and drawing attention to some unifying concepts.

258 citations


Journal ArticleDOI
Wolfgang Kuch1, Wolfgang Kuch2, Li Chelaru1, Francesco Offi1  +3 moreInstitutions (2)
TL;DR: Atomic-level control of the interface morphology is achieved by systematically varying the thicknesses of the bottom ferromagnetic and the antiferromagnetic layer, and it is found that the magnetic coupling across the interface is mediated by step edges of single-atom height, whereas atomically flat areas do not contribute.
Abstract: Characterization and control of the interface structure and morphology at the atomic level is an important issue in understanding the magnetic interaction between an antiferromagnetic material and an adjacent ferromagnet in detail, because the atomic spins in an antiferromagnet change direction on the length scale of nearest atomic distances. Despite its technological importance for the development of advanced magnetic data-storage devices and extensive studies, the details of the magnetic interface coupling between antiferromagnets and ferromagnets have remained concealed. Here we present the results of magneto-optical Kerr-effect measurements and layer-resolved spectro-microscopic magnetic domain imaging of single-crystalline ferromagnet-antiferromagnet- ferromagnet trilayers. Atomic-level control of the interface morphology is achieved by systematically varying the thicknesses of the bottom ferromagnetic and the antiferromagnetic layer. We find that the magnetic coupling across the interface is mediated by step edges of single-atom height, whereas atomically flat areas do not contribute.

133 citations


Journal ArticleDOI
TL;DR: The adsorption of potassium on Al(111) at 90 K and at 300 K has been investigated by low-energy electron diffraction (LEED) and the results are discussed in the light of recent density-functional-theory calculations.
Abstract: The adsorption of potassium on Al(111) at 90 K and at 300 K has been investigated by low-energy electron diffraction (LEED). Although a (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} structure is formed at each temperature, a detailed LEED analysis has revealed that the adsorbate positions are quite different and unusual in each case. At 90 K the adatoms occupy on-top sites and at 300 K they occupy substitutional sites. An irreversible phase transformation from the former to the latter structure occurs on warming to 300 K. These results are discussed in the light of recent density-functional-theory calculations.

108 citations


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