Geometry Effects at Atomic-Size Aluminium Contacts
Geometry Effects at Atomic-Size Aluminium
Contacts
U. Schwingenschlögl ∗, C. Schuster
Institut für Physik, Universität Augsburg, 86135 Augsburg, Germany
Abstract
We present electronic structure calculations for aluminium nanocontacts. Address-
ing the neck of the contact, we compare characteristic geometries to investigate
the effects of the local aluminium coordination on the electronic states. We find
that the Al 3pz states are very sensitive against modifications of the orbital over-
lap, which has serious consequences for the transport properties. Stretching of the
contact shifts states towards the Fermi energy, leaving the system instable against
ferromagnetic ordering. By spacial restriction, hybridization is locally suppressed
at nanocontacts and the charge neutrality is violated. We discuss the influence of
mechanical stress by means of quantitative results for the charge transfer.
Key words: density functional theory, electronic structure, stretched nanocontact,
hybridization, charge neutrality
PACS: 71.20.-b, 73.20.-r, 73.20.At, 73.40.-c, 73.63.Rt
Atomic-size contacts can be prepared by means of scanning tunneling mi-
croscopy [1] or break junction techniques [2]. In each case, piezoelectric ele-
ments are used to stretch a wire with a precision of a few picometers until
finally a single atom configuration is reached. Such contacts have attracted
great attention over the last couple of years, in particular concerning their
electrical transport properties. Since transport is restricted to a small number
of atomic orbitals at the contact, conductance across nanocontacts strongly
depends on the local electronic structure. An atomic-size constriction acco-
modates only a small number of conducting channels, which is determined by
the number of valence orbitals of the contact atom. The transmission of each
channel likewise is fixed by the local atomic environment. For a review on the
quantum properties of atomic-size conductors see Agräıt et al. [3].
∗ Corresponding author. Fax: 49-821-598-3262
Email address: Udo.Schwingenschloegl@physik.uni-augsburg.de
(U. Schwingenschlögl).
Preprint submitted to Elsevier 1 November 2018
http://arxiv.org/abs/0704.0693v1
From the theoretical point of view, the electronic structure and conductance
of nanocontacts and nanowires has been studied by ab initio band structure
calculations. For aluminium contacts, investigations of the electronic states
have been reported in [4,5,6,7], and the conductance has been addressed in
[8,9,10,11,12,13,14,15,16]. In these studies various geometries have been used,
which are assumed to model the local atomic structure of the contact in an
adequate way. The breakage of an aluminium contact has been simulated by
means of molecular dynamics calculations in [17,18,19], i.e. on the basis of
realistic structural arrangements. However, despite such a large number of
investigations, the literature lacks satisfactory reflections about the interrela-
tions between the details of the crystal structure and the local electronic states
at the nanocontact. In the present letter we will deal with this point by com-
paring characteristic contact geometries, including stretched configurations.
In a previous work [20] we have demonstrated that hybridization between Al
3s und 3p states is strongly suppressed at aluminium nanocontacts due to
directed bonds at the neck of the contact. We therefore expect the system
to be very sensitive against modifications of the orbital overlap coming along
with the specific contact geometry. As a consequence, structural details are
important for the electrical transport, since hybridization effects can play a
critical role for transport properties of atomic-size contacts and interfaces
[21,22,23]. In particular, stretching of the nanocontact alterates the chemical
bonding and thus may lead to unexpected electronic features. We will show
that it is mandatory to account for the very details of the contact geometry
in order to obtain adequate results from electronic structure calculations.
The band structure results presented subsequently are obtained within density
functional theory and the generalized gradient approximation. We use the
WIEN2k program package, a state-of-the-art full-potential code based on a
mixed lapw and apw+lo basis [24]. In our calculations the charge density is
represented by ≈150000 plane waves and the exchange-correlation potential is
parametrized according to the Perdew-Burke-Ernzernhof scheme. Moreover,
the mesh for the Brillouin zone integration comprises between 75 and 102
points in the irreducible wedge. While Al 1s, 2s, and 2p orbitals are treated
as core states, the valence states comprise Al 3s and 3p orbitals. The radius
of the aluminium muffin-tin spheres amounts to 2.6 Bohr radii.
Our calculations rely on two characteristic contact geometries, which we in-
troduce in the following. On the one hand, we address a configuration where
a single Al atom is connected to planar Al units on both sides, each consisting
of seven atoms in a hexagonal arrangement with fcc [111] orientation. The
central sites of these planar units lie on top of the contact atom, thus giving
rise to linear σ-type Al-Al bonds along the z-axis. For this reason, we call the
first geometry under consideration the linear contact configuration. The finite
Al units are connected to Al ab-planes of infinite extension, which enables us
to apply periodic boundary conditions. We note that the contact Al site in
this linear geometry, due to its two nearest neighbours, resembles the essential
structural features of atoms in a monostrand nanowire [20].
On the other hand, we study an Al atom sandwiched between two pyramidal
Al electrodes in fcc [001] orientation, which we call the pyramidal contact
configuration. To be specific, the contact Al site has four crystallographically
equivalent nearest neighbours on both sides, which prohibits σ-type Al-Al
bonding via the 3pz orbitals along the z-axis. Whereas the second pyramidal
layer comprises nine atoms, the third layer off the contact extends infinitely
on account of periodic boundary conditions.
For both contact configurations, a convenient choice for the bond lengths and
bond angles is given by the bulk (fcc) aluminium values, therefore by inter-
atomic distances of 2.86 Å. Mechanical stress can increase this bond length at
the nanocontact, which we simulate by interatomic distances of 3.95 Å for the
linear and 3.62 Å for the pyramidal contact configuration. In both cases, only
the bond lengths between the contact Al site and its nearest neighbours are
changed with respect to the fcc setup. Structural relaxation of the electrodes
due to the elongated contact bonds plays a minor role.
For bulk aluminium it is well established that the formal Al 3s23p1 electronic
configuration is seriously interfered by hybridization effects, giving rise to a
prototypical sp-hybrid system. However, the situation changes dramatically
when covalent bonding is no longer isotropic but restricted to specific direc-
tions, as for an atomic-size contact. Partial Al 3s, 3pz, and 3px densities of
states (DOS) as calculated for the Al site at the neck of our linear contact
configuration are shown in figure 1. By symmetry, px and py states are degen-
erate. While most of the occupied states are of 3s type, the 3p states dominate
at energies above the Fermi level. Because hardly any contribution of 3s and
3p states is found at energies dominated by the other states, respectively, evo-
lution of hybrid orbitals and an interpretation in terms of sp-hybrid states
is precluded. Most 3p electrons occupy the pz orbital, which is oriented along
the principal axis of the contact and therefore mediates σ-type orbital overlap.
Because neither 3s nor 3px states give rise to significant contributions to the
DOS at the Fermi energy, chemical bonding is well characterized in terms of
directed 3pz bonds.
When the Al-Al bond length is stretched from 2.86 Å to 3.95 Å at the nanocon-
tact, the central Al site decouples from its neighbours. Its electronic states
hence become more atom-like, which is clearly visible for the 3s states in fig-
ure 1. Smaller band widths and sharper DOS structures likewise are obvious
for the 3px states. Finally, for the 3pz symmetry component we observe a
shift of states from lower energies to the Fermi level and from higher energies
to a new structure at about 2 eV. The DOS at the Fermi energy increases
d=3.95 Å
d=2.86 Å
Al 3s
20-2-4-6
Al 3p
20-2-4-6
Al 3p
E-EF(eV)
20-2-4-6
Fig. 1. Partial Al 3s, 3pz, and 3px densities of states for the contact aluminium site
in the linear contact configuration.
configuration bond length charge transfer
linear 2.86 Å 0.51
2.97 Å 0.54
3.08 Å 0.57
3.33 Å 0.59
3.58 Å 0.62
3.95 Å 0.63
pyramidal 2.86 Å 0.22
3.22 Å 0.53
3.62 Å 0.61
4.04 Å 0.66
Table 1
Net charge transfer off the contact aluminium site.
significantly.
Turning to the occupation of the valence orbitals, the decoupling of the contact
Al site comes along with a reduction of charge, amounting to 0.13 electrons.
In general, the px/py atomic orbital does not mediate chemical bonding due to
the spacial restriction of the crystal structure. Its occupation thus is strongly
reduced and we cannot expect local charge neutrality. Calculated values for
the net charge transfer off the contact Al site, as compared to bulk aluminium,
are given in table 1 for bond lengths between 2.86 Å and 3.95 Å.
Figure 2 shows partial Al 3s, 3pz, and 3px densities of states for the pyramidal
contact configuration. As compared to the linear case fundamental differences
are found. For an Al-Al bond length of 2.86 Å we now have a finite Al 3s DOS
at the Fermi energy. The same is true for the 3pz states, whereas the 3px DOS
almost vanishes. Again, px and py states are degenerate by symmetry. While
most of the occupied states still are of 3s type, contributions of the 3p states are
larger than for the linear contact configuration. Chemical bonding therefore is
more isotropic, which is reflected by larger band widths. As expected from the
contact geometry, σ-type bonding via the 3pz-orbital is significantly reduced.
Due to increased hybridization between the 3s and the three 3p orbitals, all
these states can participate in the electrical transport. Figure 3 illustrates the
differences in the electronic structure at the linear and pyramidal contact by
means of electron density maps. In each case, the density map covers the plane
of the central Al site, where the principal axis of the contact runs from left to
d=3.62 Å
d=2.86 Å
Al 3s
20-2-4-6
Al 3p
20-2-4-6
Al 3p
E-EF(eV)
20-2-4-6
Fig. 2. Partial Al 3s, 3pz, and 3px densities of states for the contact aluminium site
in the pyramidal contact configuration.
pyramidal
linear
Fig. 3. Electron density maps for the neck of the linear and pyramidal contact.
right.
Stretching of the contact by increasing the Al-Al bond length from 2.86 Å to
3.62 Å has similar effects as for the linear contact configuration. The decou-
pling of the central Al site from the pyramidal electrodes results in smaller
band widths and sharper DOS structures for the 3s, 3pz, as well as 3px states,
see figure 2. In addition, the 3pz und 3px states shift to higher energies, giving
rise to pronounced DOS structures near 2 eV. Due to the reduced band width,
the 3s and 3pz DOS disappears almost completely in the vicinity of the Fermi
energy. As concerns the 3pz states, the pyramidal geometry thus shows the
opposite behaviour than the linear geometry. Since the Al 3pz states mediate
the main part of the orbital overlap across the nanocontact, they are very
sensitive against changes in the crystal structure. Structural rearrangement
during the breakage of an aluminium nanocontact or nanowire consequently
should have serious effects on the electrical transport, such as modulation of
the conductance.
Elongated bonds again are accompanied by a decline of charge at the contact.
However, the value of 0.39 electrons net charge transfer off the central Al site
is larger than in the linear case. This traces back to the fact that reduction
of hybridization on stretching is more efficient for the pyramidal geometry,
since for the linear geometry hardly any hybridization is left right from the
Al 3p
E-EF(eV)
20-2-4-6
Fig. 4. Spin majority and minority Al 3pz densities of states for the contact alu-
minium site in the stretched linear contact configuration (d = 3.95 Å).
beginning. Accordingly, smaller values for the net charge transfer are found in
the pyramidal case, see table 1.
We next show that the remarkable increase of the Al 3pz DOS at the Fermi
energy on stretching the linear contact configuration leads to an instability
against ferromagnetic ordering. A spontaneous magnetization of simple metal
nanowires has been predicted by Zabala et al. [4]. These authors have demon-
strated the instability in explicit calculations for an aluminium nanowire, using
a stabilized jellium model. For Al-Al bond lengths of d = 3.95 Å at the neck
of our contact, we hence have performed spin polarized electronic structure
calculations, yielding a stable ferromagnetic solution. Figure 4 displays the
corresponding spin majority and minority densities of states for the contact
Al site. The local spin splitting of about 0.2 eV is connected to an energy gain
of 5.5mRyd. Moreover, the magnetic moment amounts to 0.1µB, which is
significantly smaller than the 0.68µB reported by Zabala et al. [4] for an infi-
nite nanowire. However, Delin et al. [6] have shown for Pd nanowires that the
spontaneous magnetization decreases rapidly for short chains. Only the linear
contact configuration is subject to a magnetic instability on stretching. For
the pyramidal geometry, of course, magnetism cannot be expected, compare
the spin degenerate DOS curves in figure 2.
In conclusion, we have studied the electronic structure of aluminium nanocon-
tacts by means of band structure calculations within density functional theory.
Taking into account the details of the crystal structure, we have discussed the
electronic features of prototypical contact geometries (linear versus pyrami-
dal). Our calculations result in two largely different scenarios. In particular, the
Al 3pz states are strongly affected by modifications of the chemical bonding.
If σ-type bonding via the 3pz orbitals is dominant because of direct orbital
overlap across the contact, sp-type hybridization is almost completely sup-
pressed and only Al 3pz states remain at the Fermi energy [20]. Otherwise, if
the bonding is more isotropic for geometrical reasons, the Al 3s states likewise
have to be taken into account. The divers behaviour of the linear and pyrami-
dal geometry becomes even more pronounced when the contact is stretched.
Whereas in the linear case the Al 3pz DOS at the Fermi energy increases,
which even yields a ferromagnetic instability, it vanishes in the pyramidal
case, leaving the system insulating. As a consequence, the structural details of
the contact are expected to strongly influence the electrical transport. Because
of structural rearrangements, they are particularly relevant for the breakage
of a nanocontact or nanowire.
Acknowledgements
We thank U. Eckern and P. Schwab for helpful discussions and the Deutsche
Forschungsgemeinschaft for financial support (SFB 484).
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