344 research outputs found
A thick shell Casimir effect
We consider the Casimir energy of a thick dielectric-diamagnetic shell under
a uniform velocity light condition, as a function of the radii and the
permeabilities. We show that there is a range of parameters in which the stress
on the outer shell is inward, and a range where the stress on the outer shell
is outward. We examine the possibility of obtaining an energetically stable
configuration of a thick shell made of a material with a fixed volume
Photon Green's function and the Casimir energy in a medium
A new expansion is established for the Green's function of the
electromagnetic field in a medium with arbitrary and . The
obtained Born series are shown to consist of two types of interactions - the
usual terms (denoted ) that appear in the Lifshitz theory combined with
a new kind of terms (which we denote by ) associated with the changes
in the permeability of the medium. Within this framework the case of uniform
velocity of light () is studied. We obtain expressions
for the Casimir energy density and the first non-vanishing contribution is
manipulated to a simplified form. For (arbitrary) spherically symmetric
we obtain a simple expression for the electromagnetic energy density, and as an
example we obtain from it the Casimir energy of a dielectric-diamagnetic ball.
It seems that the technique presented can be applied to a variety of problems
directly, without expanding the eigenmodes of the problem and using boundary
condition considerations
Casimir energy of a dilute dielectric ball with uniform velocity of light at finite temperature
The Casimir energy, free energy and Casimir force are evaluated, at arbitrary
finite temperature, for a dilute dielectric ball with uniform velocity of light
inside the ball and in the surrounding medium. In particular, we investigate
the classical limit at high temperature. The Casimir force found is repulsive,
as in previous calculations.Comment: 15 pages, 1 figur
Effects of Accelerate Ageing and Low Temperatures on Germination of Range Grasses
Accelerated ageing and cold tests were used to determine the seed vigour of different valuable forage grasses from the temperate semiarid region of Argentina (Piptochaetium napostaense, Poa ligularis, Stipa longiglumis, Stipa tenuis, Digitaria californica, Pappophorum subbulbosum, Setaria leiantha, Sorghastrum pellitum, Trichloris crinita). In general, warm season species showed greater vigour than cool season species
Casimir energy of a dilute dielectric ball in the mode summation method
In the --approximation the Casimir energy of a
dilute dielectric ball is derived using a simple and clear method of the mode
summation. The addition theorem for the Bessel functions enables one to present
in a closed form the sum over the angular momentum before the integration over
the imaginary frequencies. The linear in contribution
into the vacuum energy is removed by an appropriate subtraction. The role of
the contact terms used in other approaches to this problem is elucidated.Comment: 14 pages, REVTeX, no figures, no tables; presentation is made better,
new references are adde
Full density matrix dynamics for large quantum systems: Interactions, Decoherence and Inelastic effects
We develop analytical tools and numerical methods for time evolving the total
density matrix of the finite-size Anderson model. The model is composed of two
finite metal grains, each prepared in canonical states of differing chemical
potential and connected through a single electronic level (quantum dot or
impurity). Coulomb interactions are either excluded all together, or allowed on
the dot only. We extend this basic model to emulate decoherring and inelastic
scattering processes for the dot electrons with the probe technique. Three
methods, originally developed to treat impurity dynamics, are augmented to
yield global system dynamics: the quantum Langevin equation method, the well
known fermionic trace formula, and an iterative path integral approach. The
latter accommodates interactions on the dot in a numerically exact fashion. We
apply the developed techniques to two open topics in nonequilibrium many-body
physics: (i) We explore the role of many-body electron-electron repulsion
effects on the dynamics of the system. Results, obtained using exact path
integral simulations, are compared to mean-field quantum Langevin equation
predictions. (ii) We analyze aspects of quantum equilibration and
thermalization in large quantum systems using the probe technique, mimicking
elastic-dephasing effects and inelastic interactions on the dot. Here, unitary
simulations based on the fermionic trace formula are accompanied by quantum
Langevin equation calculations
Casimir forces in a T operator approach
We explore the scattering approach to Casimir forces. Our main tool is the
description of Casimir energy in terms of transition operators, as presented in
Kenneth and Klich, Phys. Rev. Lett. 97, 160401 (2006). We study the convergence
properties of the formula and how to utilize it, together with scattering data
to compute the force. We illustrate the approach by describing the force
between scatterers in 1d and 3d,, and in particular show how it may be applied
in order to study the interaction between two spherical bodies at all
distances
Auxiliary fermion approach to the resonant inelastic x-ray scattering response in an underdoped cuprate
We describe a method for calculating the resonant inelastic x-ray scattering (RIXS) response—including the dynamics of the transient core hole—of many-body systems with nontrivial gap structure encoded in their single particle Green's function. Our approach introduces auxiliary fermions in order to obtain a form amenable to the determinant method of Benjamin et al., [Phys. Rev. Lett. 112, 247002 (2014)], and is applicable to systems where interactions are most strongly felt through a renormalization of the single particle propagator. As a test case we consider the Yang-Rice-Zhang ansatz for cuprate phenomena in the underdoped “pseudogap” regime, which remains a popular tool for interpreting the results of experimental probes. We show that taking the core hole dynamics into account for a system described by this ansatz pushes the RIXS peaks towards higher energy transfer, improving agreement with experiments
Distribution of velocities and acceleration for a particle in Brownian correlated disorder: inertial case
We study the motion of an elastic object driven in a disordered environment
in presence of both dissipation and inertia. We consider random forces with the
statistics of random walks and reduce the problem to a single degree of
freedom. It is the extension of the mean field ABBM model in presence of an
inertial mass m. While the ABBM model can be solved exactly, its extension to
inertia exhibits complicated history dependence due to oscillations and
backward motion. The characteristic scales for avalanche motion are studied
from numerics and qualitative arguments. To make analytical progress we
consider two variants which coincide with the original model whenever the
particle moves only forward. Using a combination of analytical and numerical
methods together with simulations, we characterize the distributions of
instantaneous acceleration and velocity, and compare them in these three
models. We show that for large driving velocity, all three models share the
same large-deviation function for positive velocities, which is obtained
analytically for small and large m, as well as for m =6/25. The effect of small
additional thermal and quantum fluctuations can be treated within an
approximate method.Comment: 42 page
Decoherence induced by interacting quantum spin baths
We study decoherence induced on a two-level system coupled to a
one-dimensional quantum spin chain. We consider the cases where the dynamics of
the chain is determined by the Ising, XY, or Heisenberg exchange Hamiltonian.
This model of quantum baths can be of fundamental importance for the
understanding of decoherence in open quantum systems, since it can be
experimentally engineered by using atoms in optical lattices. As an example,
here we show how to implement a pure dephasing model for a qubit system coupled
to an interacting spin bath. We provide results that go beyond the case of a
central spin coupled uniformly to all the spins of the bath, in particular
showing what happens when the bath enters different phases, or becomes
critical; we also study the dependence of the coherence loss on the number of
bath spins to which the system is coupled and we describe a
coupling-independent regime in which decoherence exhibits universal features,
irrespective of the system-environment coupling strength. Finally, we establish
a relation between decoherence and entanglement inside the bath. For the Ising
and the XY models we are able to give an exact expression for the decay of
coherences, while for the Heisenberg bath we resort to the numerical
time-dependent Density Matrix Renormalization Group.Comment: 18 pages, 20 figure
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