179 research outputs found
Mechanical, Electrical, and Magnetic Properties of Ni Nanocontacts
The dynamic deformation upon stretching of Ni nanowires as those formed with
mechanically controllable break junctions or with a scanning tunneling
microscope is studied both experimentally and theoretically. Molecular dynamics
simulations of the breaking process are performed. In addition, and in order to
compare with experiments, we also compute the transport properties in the last
stages before failure using the first-principles implementation of Landauer's
formalism included in our transport package ALACANT.Comment: 5 pages, 6 figure
Transport in magnetically ordered Pt nanocontacts
Pt nanocontacts, like those formed in mechanically controlled break
junctions, are shown to develop spontaneous local magnetic order. Our density
functional calculations predict that a robust local magnetic order exists in
the atoms presenting low coordination, i. e., those forming the atom-sized
neck. In contrast to previous work, we thus find that the electronic transport
can be spin-polarized, although the net value of the conductance still agrees
with available experimental information. Experimental implications of the
formation of this new type of nanomagnet are discussed.Comment: 4 pages, 3 figure
Modeling contact formation between atomic-sized gold tips via molecular dynamics
The formation and rupture of atomic-sized contacts is modelled by means of
molecular dynamics simulations. Such nano-contacts are realized in scanning
tunnelling microscope and mechanically controlled break junction experiments.
These instruments routinely measure the conductance across the nano-sized
electrodes as they are brought into contact and separated, permitting
conductance traces to be recorded that are plots of conductance versus the
distance between the electrodes. One interesting feature of the conductance
traces is that for some metals and geometric configurations a jump in the value
of the conductance is observed right before contact between the electrodes, a
phenomenon known as jump-to-contact. This paper considers, from a computational
point of view, the dynamics of contact between two gold nano-electrodes.
Repeated indentation of the two surfaces on each other is performed in two
crystallographic orientations of face-centred cubic gold, namely (001) and
(111). Ultimately, the intention is to identify the structures at the atomic
level at the moment of first contact between the surfaces, since the value of
the conductance is related to the minimum cross-section in the contact region.
Conductance values obtained in this way are determined using first principles
electronic transport calculations, with atomic configurations taken from the
molecular dynamics simulations serving as input structures.Comment: 6 pages, 4 figures, conference submissio
Absence of magnetically-induced fractional quantization in atomic contacts
Using the mechanically controlled break junction technique at low
temperatures and under cryogenic vacuum conditions we have studied atomic
contacts of several magnetic (Fe, Co and Ni) and non-magnetic (Pt) metals,
which recently were claimed to show fractional conductance quantization. In the
case of pure metals we see no quantization of the conductance nor
half-quantization, even when high magnetic fields are applied. On the other
hand, features in the conductance similar to (fractional) quantization are
observed when the contact is exposed to gas molecules. Furthermore, the absence
of fractional quantization when the contact is bridged by H_2 indicates the
current is never fully polarized for the metals studied here. Our results are
in agreement with recent model calculations.Comment: 4 pages, 3 figure
Formation of a Metallic Contact: Jump to Contact Revisited
The transition from tunneling to metallic contact between two surfaces does
not always involve a jump, but can be smooth. We have observed that the
configuration and material composition of the electrodes before contact largely
determines the presence or absence of a jump. Moreover, when jumps are found
preferential values of conductance have been identified. Through combination of
experiments, molecular dynamics, and first-principles transport calculations
these conductance values are identified with atomic contacts of either
monomers, dimers or double-bond contacts.Comment: 4 pages, 5 figure
Observation of a parity oscillation in the conductance of atomic wires
Using a scanning tunnel microscope or mechanically controlled break
junctions, atomic contacts of Au, Pt and Ir are pulled to form chains of atoms.
We have recorded traces of conductance during the pulling process and averaged
these for a large amount of contacts. An oscillatory evolution of conductance
is observed during the formation of the monoatomic chain suggesting a
dependence on even or odd numbers of atoms forming the chain. This behaviour is
not only present in the monovalent metal Au, as it has been previously
predicted, but is also found in the other metals which form chains suggesting
it to be a universal feature of atomic wires
Mechanical annealing of metallic electrodes at the atomic scale
The process of creating an atomically defined and robust metallic tip is
described and quantified using measurements of contact conductance between gold
electrodes and numerical simulations. Our experiments show how the same
conductance behavior can be obtained for hundreds of cycles of formation and
rupture of the nanocontact by limiting the indentation depth between the two
electrodes up to a conductance value of approximately in the case of
gold. This phenomenon is rationalized using molecular dynamics simulations
together with density functional theory transport calculations which show how,
after repeated indentations (mechanical annealing), the two metallic electrodes
are shaped into tips of reproducible structure. These results provide a crucial
insight into fundamental aspects relevant to nano-tribology or scanning probe
microscopies
Dynamic bonding of metallic nanocontacts: Insights from experiments and atomistic simulations
The conductance across an atomically narrow metallic contact can be measured by using scanning tunneling microscopy. In certain situations, a jump in the conductance is observed right at the point of contact between the tip and the surface, which is known as "jump to contact" (JC). Such behavior provides a way to explore, at a fundamental level, how bonding between metallic atoms occurs dynamically. This phenomenon depends not only on the type of metal but also on the geometry of the two electrodes. For example, while some authors always find JC when approaching two atomically sharp tips of Cu, others find that a smooth transition occurs when approaching a Cu tip to an adatom on a flat surface of Cu. In an attempt to show that all these results are consistent, we make use of atomistic simulations; in particular, classical molecular dynamics together with density functional theory transport calculations to explore a number of possible scenarios. Simulations are performed for two different materials: Cu and Au in a [100] crystal orientation and at a temperature of 4.2 K. These simulations allow us to study the contribution of short- and long-range interactions to the process of bonding between metallic atoms, as well as to compare directly with experimental measurements of conductance, giving a plausible explanation for the different experimental observations. Moreover, we show a correlation between the cohesive energy of the metal, its Young's modulus, and the frequency of occurrence of a jump to contactW. Dednam thanks Dr. A. E. Botha for helpful discussions and acknowledges support from the National Research Foundation of South Africa through the Scarce Skills Masters scholarship funding programme (Grant Unique Number 92138). This work is supported by the Generalitat Valenciana through Grant Reference PROMETEO2012/011 and MINECO under Grant No. FIS2013-47328, by European Union structural funds and the Comunidad de Madrid Programs S2013/MIT-3007 and P2013/MIT-2850. This work is also part of the research programme of the Foundation for Fundamental Research on Matter (FOM), which is financially supported by the Netherlands Organisation for Scientific Research (NWO
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