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