262 research outputs found
Front propagation into unstable metal nanowires
Long, cylindrical metal nanowires have recently been observed to form and be
stable for seconds at a time at room temperature. Their stability and
structural dynamics is well described by a continuum model, the nanoscale
free-electron model, which predicts cylinders in certain intervals of radius to
be linearly unstable. In this paper, I study how a small, localized
perturbation of such an unstable wire grows exponentially and propagates along
the wire with a well-defined front. The front is found to be pulled, and forms
a coherent pattern behind it. It is well described by a linear marginal
stability analysis of front propagation into an unstable state. In some cases,
nonlinearities of the wire dynamics are found to trigger an invasive mode that
pushes the front. Experimental procedures that could lead to the observation of
this phenomenon are suggested.Comment: 6 pages, 4 figure
Jahn-Teller Distortions and the Supershell Effect in Metal Nanowires
A stability analysis of metal nanowires shows that a Jahn-Teller deformation
breaking cylindrical symmetry can be energetically favorable, leading to stable
nanowires with elliptic cross sections. The sequence of stable cylindrical and
elliptical nanowires allows for a consistent interpretation of experimental
conductance histograms for alkali metals, including both the shell and
supershell structures. It is predicted that for gold, elliptical nanowires are
even more likely to form since their eccentricity is smaller than for alkali
metals. The existence of certain metastable ``superdeformed'' nanowires is also
predicted
Comment on "Density Functional Simulation of a Breaking Nanowire"
In a recent Letter, Nakamura et al. [Phys. Rev. Lett. 82, 1538 (1999)]
described first principles calculations for a breaking Na nanocontact. Their
system consists of a periodic one-dimensional array of supercells, each of
which contains 39 Na atoms, originally forming a straight, crystalline wire
with a length of 6 atoms. The system is elongated by increasing the length of
the unit cell. At each step, the atomic configuration is relaxed to a new local
equilibrium, and the tensile force is evaluated from the change of the total
energy with elongation. Aside from a discontinuity of the force occuring at the
transition from a crytalline to an amorphous configuration during the early
stages of elongation, they were unable to identify any simple correlations
between the force and the number of electronic modes transmitted through the
contact. An important question is whether their model is realistic, i.e.,
whether it can be compared to experimental results obtained for a single
nanocontact between two macroscopic pieces of metal. In this Comment, we
demonstrate that with such a small unit cell, the interference effects between
neighboring contacts are of the same size as the force oscillations in a single
nanocontact.Comment: 1 pag
Fluctuational Instabilities of Alkali and Noble Metal Nanowires
We introduce a continuum approach to studying the lifetimes of monovalent
metal nanowires. By modelling the thermal fluctuations of cylindrical nanowires
through the use of stochastic Ginzburg-Landau classical field theories, we
construct a self-consistent approach to the fluctuation-induced `necking' of
nanowires. Our theory provides quantitative estimates of the lifetimes for
alkali metal nanowires in the conductance range 10 < G/G_0 < 100 (where
G_0=2e^2/h is the conductance quantum), and allows us to account for
qualitative differences in the conductance histograms of alkali vs. noble metal
nanowires
Comment on "Nonlinear current-voltage curves of gold quantum point contacts" [Appl. Phys. Lett. 87, 103104 (2005)]
In a recent Letter [Appl. Phys. Lett. 87, 103104 (2005)], Yoshida et al.
report that nonlinearities in current-voltage curves of gold quantum point
contacts occur as a result of a shortening of the distance between electrodes
at finite bias, presumably due to thermal expansion. For short wires, the
electrode displacement induces a thickening of the wire, as well as
nonlinearities of the IV curve, while the radius of long wires is left
unchanged, thus resulting in a linear IV curve. We argue here that electron
shell effects, which favor wires with certain "magic radii," prevent the
thickening of long wires under compression, but have little effect on wires
below a critical length.Comment: Version accepted for publication in Applied Physics Letter
Theory of metastability in simple metal nanowires
Thermally induced conductance jumps of metal nanowires are modeled using
stochastic Ginzburg-Landau field theories. Changes in radius are predicted to
occur via the nucleation of surface kinks at the wire ends, consistent with
recent electron microscopy studies. The activation rate displays nontrivial
dependence on nanowire length, and undergoes first- or second-order-like
transitions as a function of length. The activation barriers of the most stable
structures are predicted to be universal, i.e., independent of the radius of
the wire, and proportional to the square root of the surface tension. The
reduction of the activation barrier under strain is also determined.Comment: 5 pages, 3 figure
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