3,869 research outputs found
Transport in nanoscale systems: the microcanonical versus grand-canonical picture
We analyse a picture of transport in which two large but finite charged
electrodes discharge across a nanoscale junction. We identify a functional
whose minimisation, within the space of all bound many-body wavefunctions,
defines an instantaneous steady state. We also discuss factors that favour the
onset of steady-state conduction in such systems, make a connection with the
notion of entropy, and suggest a novel source of steady-state noise. Finally,
we prove that the true many-body total current in this closed system is given
exactly by the one-electron total current, obtained from time-dependent
density-functional theory.Comment: 6 pages, 1 figur
Nonconservative dynamics in long atomic wires
The effect of nonconservative current-induced forces on the ions in a
defect-free metallic nanowire is investigated using both steady-state
calculations and dynamical simulations. Non-conservative forces were found to
have a major influence on the ion dynamics in these systems, but their role in
increasing the kinetic energy of the ions decreases with increasing system
length. The results illustrate the importance of nonconservative effects in
short nanowires and the scaling of these effects with system size. The
dependence on bias and ion mass can be understood with the help of a simple pen
and paper model. This material highlights the benefit of simple preliminary
steady-state calculations in anticipating aspects of brute-force dynamical
simulations, and provides rule of thumb criteria for the design of stable
quantum wires.Comment: 20 pages, 8 figure
Length matters: keeping atomic wires in check
Dynamical effects of non-conservative forces in long, defect free atomic
wires are investigated. Current flow through these wires is simulated and we
find that during the initial transient, the kinetic energies of the ions are
contained in a small number of phonon modes, closely clustered in frequency.
These phonon modes correspond to the waterwheel modes determined from
preliminary static calculations. The static calculations allow one to predict
the appearance of non-conservative effects in advance of the more expensive
real-time simulations. The ion kinetic energy redistributes across the band as
non-conservative forces reach a steady state with electronic frictional forces.
The typical ion kinetic energy is found to decrease with system length,
increase with atomic mass, and its dependence on bias, mass and length is
supported with a pen and paper model. This paper highlights the importance of
non-conservative forces in current carrying devices and provides criteria for
the design of stable atomic wires.Comment: 6 pages, 5 figures, conference proceedings from 2014 MRS fall meetin
Magneto-mechanical interplay in spin-polarized point contacts
We investigate the interplay between magnetic and structural dynamics in
ferromagnetic atomic point contacts. In particular, we look at the effect of
the atomic relaxation on the energy barrier for magnetic domain wall migration
and, reversely, at the effect of the magnetic state on the mechanical forces
and structural relaxation. We observe changes of the barrier height due to the
atomic relaxation up to 200%, suggesting a very strong coupling between the
structural and the magnetic degrees of freedom. The reverse interplay is weak,
i.e. the magnetic state has little effect on the structural relaxation at
equilibrium or under non-equilibrium, current-carrying conditions.Comment: 5 pages, 4 figure
Are current-induced forces conservative?
The expression for the force on an ion in the presence of current can be
derived from first principles without any assumption about its conservative
character. However, energy functionals have been constructed that indicate that
this force can be written as the derivative of a potential function. On the
other hand, there exist compelling specific arguments that strongly suggest the
contrary. We propose physical mechanisms that invalidate such arguments and
demonstrate their existence with first-principles calculations. While our
results do not constitute a formal resolution to the fundamental question of
whether current-induced forces are conservative, they represent a substantial
step forward in this direction.Comment: 4 pages, 4 Figures, submitted to PR
Power dissipation in nanoscale conductors: classical, semi-classical and quantum dynamics
Modelling Joule heating is a difficult problem because of the need to introduce correct correlations between the motions of the ions and the electrons. In this paper we analyse three different models of current induced heating (a purely classical model, a fully quantum model and a hybrid model in which the electrons are treated quantum mechanically and the atoms are treated classically). We find that all three models allow for both heating and cooling processes in the presence of a current, and furthermore the purely classical and purely quantum models show remarkable agreement in the limit of high biases. However, the hybrid model in the Ehrenfest approximation tends to suppress heating. Analysis of the equations of motion reveals that this is a consequence of two things: the electrons are being treated as a continuous fluid and the atoms cannot undergo quantum fluctuations. A means for correcting this is suggested
On the Newtonian origin of the spin motive force in ferromagnetic atomic wires
We demonstrate numerically the existence of a spin-motive force acting on
spin-carriers when moving in a time and space dependent internal field. This is
the case of electrons in a one-dimensional wires with a precessing domain wall.
The effect can be explained solely by considering adiabatic dynamics and it is
shown to exist for both classical and quantum systems.Comment: 5 pages, 7 figures, added figure 7 and tex
Single-particle and Interaction Effects on the Cohesion and Transport and Magnetic Properties of Metal Nanowires at Finite Voltages
The single-particle and interaction effects on the cohesion, electronic
transport, and some magnetic properties of metallic nanocylinders have been
studied at finite voltages by using a generalized mean-field electron model.
The electron-electron interactions are treated in the self-consistent Hartree
approximation. Our results show the single-particle effect is dominant in the
cohesive force, while the nonzero magnetoconductance and magnetotension
coefficients are attributed to the interaction effect. Both single-particle and
interaction effects are important to the differential conductance and magnetic
susceptibility.Comment: 5 pages, 6 figure
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