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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
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
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
Micromagnetic Simulations of Ferromagnetic Rings
Thin nanomagnetic rings have generated interest for fundamental studies of
magnetization reversal and also for their potential in various applications,
particularly as magnetic memories. They are a rare example of a geometry in
which an analytical solution for the rate of thermally induced magnetic
reversal has been determined, in an approximation whose errors can be estimated
and bounded. In this work, numerical simulations of soft ferromagnetic rings
are used to explore aspects of the analytical solution. The evolution of the
energy near the transition states confirms that, consistent with analytical
predictions, thermally induced magnetization reversal can have one of two
intermediate states: either constant or soliton-like saddle configurations,
depending on ring size and externally applied magnetic field. The results
confirm analytical predictions of a transition in thermally activated reversal
behavior as magnetic field is varied at constant ring size. Simulations also
show that the analytic one dimensional model continues to hold even for wide
rings
Zero-Temperature Dynamics of Plus/Minus J Spin Glasses and Related Models
We study zero-temperature, stochastic Ising models sigma(t) on a
d-dimensional cubic lattice with (disordered) nearest-neighbor couplings
independently chosen from a distribution mu on R and an initial spin
configuration chosen uniformly at random. Given d, call mu type I (resp., type
F) if, for every x in the lattice, sigma(x,t) flips infinitely (resp., only
finitely) many times as t goes to infinity (with probability one) --- or else
mixed type M. Models of type I and M exhibit a zero-temperature version of
``local non-equilibration''. For d=1, all types occur and the type of any mu is
easy to determine. The main result of this paper is a proof that for d=2,
plus/minus J models (where each coupling is independently chosen to be +J with
probability alpha and -J with probability 1-alpha) are type M, unlike
homogeneous models (type I) or continuous (finite mean) mu's (type F). We also
prove that all other noncontinuous disordered systems are type M for any d
greater than or equal to 2. The plus/minus J proof is noteworthy in that it is
much less ``local'' than the other (simpler) proof. Homogeneous and plus/minus
J models for d greater than or equal to 3 remain an open problem.Comment: 17 pages (RevTeX; 3 figures; to appear in Commun. Math. Phys.
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