11,951 research outputs found
Relationship between ferroelectricity and Dzyaloshinskii-Moriya interaction in multiferroics and the effect of bond-bending
We studied the microscopic mechanism of multiferroics, in particular with the
"spin current" model (Hosho Katsura, Naoto Nagaosa and Aleander V. Balatsky,
Phys. Rev. Lett. 95, 057205 (2005)). Starting from a system with helical spin
configuration, we solved for the forms of the electron wave functions and
analyzed their characteristics. The relation between ferroelectricity and
Dzyaloshinskii-Moriya interaction (I. Dzyaloshinskii, J. Phys. Chem. Solids 4,
241 (1958) and T. Moriya, Phys. Rev. 120, 91 (1960)) is clearly established.
There is also a simple relation between the electric polarization and the wave
vector of magnetic orders. Finally, we show that the bond-bending exists in
transition metal oxides can enhance ferroelectricity.Comment: 14 pages, 3 figures. acceptby Physical Review
Micellar Crystals in Solution from Molecular Dynamics Simulations
Polymers with both soluble and insoluble blocks typically self-assemble into
micelles, aggregates of a finite number of polymers where the soluble blocks
shield the insoluble ones from contact with the solvent. Upon increasing
concentration, these micelles often form gels that exhibit crystalline order in
many systems. In this paper, we present a study of both the dynamics and the
equilibrium properties of micellar crystals of triblock polymers using
molecular dynamics simulations. Our results show that equilibration of single
micelle degrees of freedom and crystal formation occurs by polymer transfer
between micelles, a process that is described by transition state theory. Near
the disorder (or melting) transition, bcc lattices are favored for all
triblocks studied. Lattices with fcc ordering are also found, but only at lower
kinetic temperatures and for triblocks with short hydrophilic blocks. Our
results lead to a number of theoretical considerations and suggest a range of
implications to experimental systems with a particular emphasis on Pluronic
polymers.Comment: 12 pages, 11 figures. Note that some figures are extremely low
quality to meet arXiv's file size limit
Modelling a Bistable System Strongly Coupled to a Debye Bath: A Quasiclassical Approach Based on the Generalised Langevin Equation
Bistable systems present two degenerate metastable configurations separated
by an energy barrier. Thermal or quantum fluctuations can promote the
transition between the configurations at a rate which depends on the dynamical
properties of the local environment (i.e., a thermal bath). In the case of
classical systems, strong system-bath interaction has been successfully
modelled by the Generalised Langevin Equation (GLE) formalism. Here we show
that the efficient GLE algorithm introduced in Phys. Rev. B 89, 134303 (2014)
can be extended to include some crucial aspects of the quantum fluctuations. In
particular, the expected isotopic effect is observed along with the convergence
of the quantum and classical transition rates in the strong coupling limit.
Saturation of the transition rates at low temperature is also retrieved, in
qualitative, yet not quantitative, agreement with the analytic predictions. The
discrepancies in the tunnelling regime are due to an incorrect sampling close
to the barrier top. The domain of applicability of the quasiclassical GLE is
also discussed.Comment: 21 pages, 5 figures. Presented at the NESC16 conference: Advances in
theory and simulation of non-equilibrium system
Applications of the Generalised Langevin Equation: towards a realistic description of the baths
The Generalised Langevin Equation (GLE) method, as developed in Ref. [Phys.
Rev. B 89, 134303 (2014)], is used to calculate the dissipative dynamics of
systems described at the atomic level. The GLE scheme goes beyond the commonly
used bilinear coupling between the central system and the bath, and permits us
to have a realistic description of both the dissipative central system and its
surrounding bath. We show how to obtain the vibrational properties of a
realistic bath and how to convey such properties into an extended Langevin
dynamics by the use of the mapping of the bath vibrational properties onto a
set of auxiliary variables. Our calculations for a model of a Lennard-Jones
solid show that our GLE scheme provides a stable dynamics, with the
dissipative/relaxation processes properly described. The total kinetic energy
of the central system always thermalises toward the expected bath temperature,
with appropriate fluctuation around the mean value. More importantly, we obtain
a velocity distribution for the individual atoms in the central system which
follows the expected canonical distribution at the corresponding temperature.
This confirms that both our GLE scheme and our mapping procedure onto an
extended Langevin dynamics provide the correct thermostat. We also examined the
velocity autocorrelation functions and compare our results with more
conventional Langevin dynamics.Comment: accepted for publication in PR
Effects of boundary conditions on the critical spanning probability
The fractions of samples spanning a lattice at its percolation threshold are
found by computer simulation of random site-percolation in two- and
three-dimensional hypercubic lattices using different boundary conditions. As a
byproduct we find in the cubic lattice.Comment: 8 pages Latex, To appear in Int. J. Mod. Phys.
Nonequilibrium processes from Generalised Langevin Equations: realistic nanoscale systems connected to two thermal baths
We extend the Generalised Langevin Equation (GLE) method [Phys. Rev. B 89,
134303 (2014)] to model a central classical region connected to two realistic
thermal baths at two different temperatures. In such nonequilibrium conditions
a heat flow is established, via the central system, in between the two baths.
The GLE-2B (GLE two baths) scheme permits us to have a realistic description of
both the dissipative central system and its surrounding baths. Following the
original GLE approach, the extended Langevin dynamics scheme is modified to
take into account two sets of auxiliary degrees of freedom corresponding to the
mapping of the vibrational properties of each bath. These auxiliary variables
are then used to solve the non-Markovian dissipative dynamics of the central
region. The resulting algorithm is used to study a model of a short Al nanowire
connected to two baths. The results of the simulations using the GLE-2B
approach are compared to the results of other simulations that were carried out
using standard thermostatting approaches (based on Markovian Langevin and
Nose-Hoover thermostats). We concentrate on the steady state regime and study
the establishment of a local temperature profile within the system. The
conditions for obtaining a flat profile or a temperature gradient are examined
in detail, in agreement with earlier studies. The results show that the GLE-2B
approach is able to treat, within a single scheme, two widely different thermal
transport regimes, i.e. ballistic systems, with no temperature gradient, and
diffusive systems with a temperature gradient.Comment: present version accepted for publication in Phys. Rev. B (Apr 2016
Power law tails of time correlations in a mesoscopic fluid model
In a quenched mesoscopic fluid, modelling transport processes at high
densities, we perform computer simulations of the single particle energy
autocorrelation function C_e(t), which is essentially a return probability.
This is done to test the predictions for power law tails, obtained from mode
coupling theory. We study both off and on-lattice systems in one- and
two-dimensions. The predicted long time tail ~ t^{-d/2} is in excellent
agreement with the results of computer simulations. We also account for finite
size effects, such that smaller systems are fully covered by the present theory
as well.Comment: 11 pages, 12 figure
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