28 research outputs found
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
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
Inelastic electron injection in a water chain
Irradiation of biological matter triggers a cascade of secondary particles
that interact with their surroundings, resulting in damage. Low-energy
electrons are one of the main secondary species and electron-phonon interaction
plays a fundamental role in their dynamics. We have developed a method to
capture the electron-phonon inelastic energy exchange in real time and have
used it to inject electrons into a simple system that models a biological
environment, a water chain. We simulated both an incoming electron pulse and a
steady stream of electrons and found that electrons with energies just outside
bands of excited molecular states can enter the chain through phonon emission
or absorption. Furthermore, this phonon-assisted dynamical behaviour shows
great sensitivity to the vibrational temperature, highlighting a crucial
controlling factor for the injection and propagation of electrons in water
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
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
Inelastic quantum transport: the self-consistent Born approximation and correlated electron-ion dynamics
A dynamical method for inelastic transport simulations in nanostructures is
compared with a steady-state method based on non-equilibrium Green's functions.
A simplified form of the dynamical method produces, in the steady state in the
weak-coupling limit, effective self-energies analogous to those in the Born
Approximation due to electron-phonon coupling. The two methods are then
compared numerically on a resonant system consisting of a linear trimer weakly
embedded between metal electrodes. This system exhibits enhanced heating at
high biases and long phonon equilibration times. Despite the differences in
their formulation, the static and dynamical methods capture local
current-induced heating and inelastic corrections to the current with good
agreement over a wide range of conditions, except in the limit of very high
vibrational excitations, where differences begin to emerge.Comment: 12 pages, 7 figure
Towards temperature-induced topological phase transition in SnTe: A first principles study
The temperature renormalization of the bulk band structure of a topological
crystalline insulator, SnTe, is calculated using first principles methods. We
explicitly include the effect of thermal-expansion-induced modification of
electronic states and their band inversion on electron-phonon interaction. We
show that the direct gap decreases with temperature, as both thermal expansion
and electron-phonon interaction drive SnTe towards the phase transition to a
topologically trivial phase as temperature increases. The band gap
renormalization due to electron-phonon interaction exhibits a non-linear
dependence on temperature as the material approaches the phase transition,
while the lifetimes of the conduction band states near the band edge show a
non-monotonic behavior with temperature. These effects should have important
implications on bulk electronic and thermoelectric transport in SnTe and other
topological insulators.Comment: 10 pages, 8 figures. Accepted for publication in Phys. Rev. B on June
8, 202