3,488 research outputs found
Physical condition and spin-resolved exchange correlation kernels in an inhomogeneous many electron system
We first exploit the spin symmetry relation for the exact exchange correlation
kernel in an inhomogeneous many electron system
with arbitrary spin polarization . The physical condition required to
satisfy the specific symmetry relation is derived and examined for simple
ferromagnetic-nonmagnetic structure by taking the electrochemical potential
into account. The condition is then applied to several composite systems useful
in spintronics applications such as the magnetic system with net spin
polarization.Comment: 5 pages, 1 figur
Long range order for lattice dipoles
We consider a system of classical Heisenberg spins on a cubic lattice in
dimensions three or more, interacting via the dipole-dipole interaction. We
prove that at low enough temperature the system displays orientational long
range order, as expected by spin wave theory. The proof is based on reflection
positivity methods. In particular, we demonstrate a previously unproven
conjecture on the dispersion relation of the spin waves, first proposed by
Froehlich and Spencer, which allows one to apply infrared bounds for estimating
the long distance behavior of the spin-spin correlation functions.Comment: 9 page
Theory of water and charged liquid bridges
The phenomena of liquid bridge formation due to an applied electric field is
investigated. A new solution for the charged catenary is presented which allows
to determine the static and dynamical stability conditions where charged liquid
bridges are possible. The creeping height, the bridge radius and length as well
as the shape of the bridge is calculated showing an asymmetric profile in
agreement with observations. The flow profile is calculated from the Navier
Stokes equation leading to a mean velocity which combines charge transport with
neutral mass flow and which describes recent experiments on water bridges.Comment: 10 pages 12 figures, misprints corrected, assumptions more
transparen
On the "Causality Paradox" of Time-Dependent Density Functional Theory
I show that the so-called causality paradox of time-dependent density
functional theory arises from an incorrect formulation of the variational
principle for the time evolution of the density. The correct formulation not
only resolves the paradox in real time, but also leads to a new expression for
the causal exchange-correlation kernel in terms of Berry curvature.
Furthermore, I show that all the results that were previously derived from
symmetries of the action functional remain valid in the present formulation.
Finally, I develop a model functional theory which explicitly demonstrates the
workings of the new formulation.Comment: 21 page
Universality of conductivity in interacting graphene
The Hubbard model on the honeycomb lattice describes charge carriers in
graphene with short range interactions. While the interaction modifies several
physical quantities, like the value of the Fermi velocity or the wave function
renormalization, the a.c. conductivity has a universal value independent of the
microscopic details of the model: there are no interaction corrections,
provided that the interaction is weak enough and that the system is at half
filling. We give a rigorous proof of this fact, based on exact Ward Identities
and on constructive Renormalization Group methods
Non-adiabatic electron dynamics in time-dependent density-functional theory
Time-dependent density-functional theory (TDDFT) treats dynamical exchange
and correlation (xc) via a single-particle potential, Vxc(r,t), defined as a
nonlocal functional of the density n(r',t'). The popular adiabatic
local-density approximation (ALDA) for Vxc(r,t) uses only densities at the same
space-time point (r,t). To go beyond the ALDA, two local approximations have
been proposed based on quantum hydrodynamics and elasticity theory: (a) using
the current as basic variable (C-TDDFT) [G. Vignale, C. A. Ullrich, and S.
Conti, Phys. Rev. Lett. 79, 4878 (1997)], (b) working in a co-moving Lagrangian
reference frame (L-TDDFT) [I. V. Tokatly, Phys. Rev. B 71, 165105 (2005)]. This
paper illustrates, compares, and analyzes both non-adiabatic theories for
simple time-dependent model densities in the linear and nonlinear regime, for a
broad range of time and frequency scales. C- and L-TDDFT are identical in
certain limits, but in general exhibit qualitative and quantitative differences
in their respective treatment of elastic and dissipative electron dynamics. In
situations where the electronic density rapidly undergoes large deformations,
it is found that non-adiabatic effects can become significant, causing the ALDA
to break down.Comment: 15 pages, 15 figure
Striped periodic minimizers of a two-dimensional model for martensitic phase transitions
In this paper we consider a simplified two-dimensional scalar model for the
formation of mesoscopic domain patterns in martensitic shape-memory alloys at
the interface between a region occupied by the parent (austenite) phase and a
region occupied by the product (martensite) phase, which can occur in two
variants (twins). The model, first proposed by Kohn and Mueller, is defined by
the following functional: where
is periodic in and almost everywhere.
Conti proved that if then the minimal specific
energy scales like ,
as . In the regime , we improve Conti's results, by computing exactly the
minimal energy and by proving that minimizers are periodic one-dimensional
sawtooth functions.Comment: 29 pages, 3 figure
Froth-like minimizers of a non local free energy functional with competing interactions
We investigate the ground and low energy states of a one dimensional non
local free energy functional describing at a mean field level a spin system
with both ferromagnetic and antiferromagnetic interactions. In particular, the
antiferromagnetic interaction is assumed to have a range much larger than the
ferromagnetic one. The competition between these two effects is expected to
lead to the spontaneous emergence of a regular alternation of long intervals on
which the spin profile is magnetized either up or down, with an oscillation
scale intermediate between the range of the ferromagnetic and that of the
antiferromagnetic interaction. In this sense, the optimal or quasi-optimal
profiles are "froth-like": if seen on the scale of the antiferromagnetic
potential they look neutral, but if seen at the microscope they actually
consist of big bubbles of two different phases alternating among each other. In
this paper we prove the validity of this picture, we compute the oscillation
scale of the quasi-optimal profiles and we quantify their distance in norm from
a reference periodic profile. The proof consists of two main steps: we first
coarse grain the system on a scale intermediate between the range of the
ferromagnetic potential and the expected optimal oscillation scale; in this way
we reduce the original functional to an effective "sharp interface" one. Next,
we study the latter by reflection positivity methods, which require as a key
ingredient the exact locality of the short range term. Our proof has the
conceptual interest of combining coarse graining with reflection positivity
methods, an idea that is presumably useful in much more general contexts than
the one studied here.Comment: 38 pages, 2 figure
To what extent can dynamical models describe statistical features of turbulent flows?
Statistical features of "bursty" behaviour in charged and neutral fluid
turbulence, are compared to statistics of intermittent events in a GOY shell
model, and avalanches in different models of Self Organized Criticality (SOC).
It is found that inter-burst times show a power law distribution for turbulent
samples and for the shell model, a property which is shared only in a
particular case of the running sandpile model. The breakdown of self-similarity
generated by isolated events observed in the turbulent samples, is well
reproduced by the shell model, while it is absent in all SOC models considered.
On this base, we conclude that SOC models are not adequate to mimic fluid
turbulence, while the GOY shell model constitutes a better candidate to
describe the gross features of turbulence.Comment: 14 pages, 4 figures, in press on Europhys. Lett. (may 2002
Techno economic and environmental assessment of Flettner rotors for marine propulsion
Wind energy is a mature renewable energy source that offers significant potential for near-term (2020) and long-term (2050) greenhouse gas (GHG) emissions reductions. Similar to all sectors of the transportation industry, the marine industry is also focused towards reduction of environmental emissions. A direct consequence of this being is a renewed interest in utilising wind as supplementary energy source for propulsion on cargo/merchant ships.
This research utilises a techno economic and environmental analysis approach to assess the possibility and benefits of harnessing wind energy, with an aim to establish the potential role of wind energy in reducing GHG emissions during conventional operation of marine vessels. The employed approach enables consistent assessment of different competing traditional propulsion systems when operated in conjunction with a novel environmental friendly technology, in this instance being the Flettner rotor technology. The assessment specifically focuses on quantifying the potential and relative reduction in fuel consumption and pollutant emissions that may be accrued while operating on typical Sea Lines of Communication.
The results obtained indicate that the implementation of Flettner towers on commercial vessels could result in potential savings of up to 20% in terms of fuel consumption, and similar reductions in environmental emissions
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