673 research outputs found
Momentum average approximation for models with boson-modulated hopping: Role of closed loops in the dynamical generation of a finite quasiparticle mass
We generalize the momentum average approximation to study the properties of
single polarons in models with boson affected hopping, where the fermion-boson
scattering depends explicitly on both the fermion's and the boson's momentum.
As a specific example, we investigate the Edwards fermion-boson model in both
one and two dimensions. In one dimension, this allows us to compare our results
with exact diagonalization results, to validate the accuracy of our
approximation. The generalization to two-dimensional lattices allows us to
calculate the polaron's quasiparticle weight and dispersion throughout the
Brillouin zone and to demonstrate the importance of Trugman loops in generating
a finite effective mass even when the free fermion has an infinite mass.Comment: 15 pages, 14 figure
Grazing Animal Production Systems and Grazing Land Characteristics in a Semi-Arid Region of Greece
Rough grazing in Greece cover about 40% of the total land area, is publicly owned and managed extensively (Hadjigeorgiou et al., 2002). The Prefecture of Larisa is in the centre of Greece, and has 212,000 ha of rough grazing land, with a variable topography ranging from sea level up to 3,000 m a.s.l. This area is utilized by a total population of 135,000 LU (mainly sheep, goats and some suckler cows), which consumes annually an appreciable fraction of their total nutrient requirements from rough grazing
Controlled engineering of extended states in disordered systems
We describe how to engineer wavefunction delocalization in disordered systems
modelled by tight-binding Hamiltonians in d>1 dimensions. We show analytically
that a simple product structure for the random onsite potential energies,
together with suitably chosen hopping strengths, allows a resonant scattering
process leading to ballistic transport along one direction, and a controlled
coexistence of extended Bloch states and anisotropically localized states in
the spectrum. We demonstrate that these features persist in the thermodynamic
limit for a continuous range of the system parameters. Numerical results
support these findings and highlight the robustness of the extended regime with
respect to deviations from the exact resonance condition for finite systems.
The localization and transport properties of the system can be engineered
almost at will and independently in each direction. This study gives rise to
the possibility of designing disordered potentials that work as switching
devices and band-pass filters for quantum waves, such as matter waves in
optical lattices.Comment: 14 pages, 11 figure
Molecular simulation of absolute hydration Gibbs energies of polar compounds
In this work, we present simulation-based predictions of the absolute hydration energy for several simple polar molecules with different functional groups, as well as for more complex multifunctional molecules. Our calculations were performed using the thermodynamic integration methodology where electrostatic and non-polar interactions were treated separately, allowing for a stable transition path between the end-points of the integration. An appropriate methodology for the analytical integration of the simulation data was applied. We compare the performance of three popular molecular mechanics force fields: TraPPE. Gromos and OPLS-AA for the description of solute atoms in MSPC/E water. It is observed that these force fields generally perform well for the simpler molecules, but are less accurate when multifunctional molecules are considered
Controlling the nuclear polarization in quantum dots using optical pulses with a modest bandwidth
We show that detuned optical pulse trains with a modest spectral width can
polarize nuclear spins in InAs quantum dots. The pulse bandwidth is large
enough to excite a coherent superposition of both electron spin eigenstates in
these negatively charged dots but narrow enough to give partial spectral
selectivity between the eigenstates. The coherent precession of electron spin
states and periodic excitation focuses the nuclear spin distribution, producing
a discrete set of precession modes. The spectral selectivity generates a net
nuclear polarization, through a mechanism that relies on optical spin rotations
rather than electron spin relaxation.Comment: 7 pages, 7 figure
Multivariate Hierarchical Modelling of Household Air Pollution
This is the author accepted manuscript. The final version is available from the Statistical Modelling Society via the link in this recordExposure to household air pollution has been attributed to an estimated 3.8 million deaths per year. A major contributor to this exposure is the reliance on various polluting fuels for cooking by almost half of all households in low and middle-income countries. We present a multivariate hierarchical model for surveys of the proportion of people relying on each fuel type, for the period 1990-2017, addressing several challenges with modelling the data including incomplete surveys and sampling bias.Natural Environment Research Council (NERC)World Health Organizatio
A mineralogical study of pigments used in two Iakovidis paintings: Verification of artwork authenticity using Raman micro-spectroscopy method.
For the purpose of the current study, we examined two paintings, an original and a fake one, entitled "Still life with grapes", and claimed both to be created from the Greek Painter G. Iakovidis. The current Research Project has been carried out at the Centre Nikias, an innovative Research Centre specialised on certification, maintenance and restoration of art works. Raman spectroscopic analysis has been carried to verify the authenticity of the used pigments and also the originality of the two paintings. The Raman spectra acquired confirmed four different colours in both painting`s pigments: red, blue, white and yellow. For the first painting Cinnabar for the red pigment, Ultramarine for the blue pigment, White earths for the white pigment and Yellow ochre for the yellow pigment. In the second painting the colours used verified as synthetic pigments. We identified the presence of Cadmium red for the red colour, Cobalt blue for the blue pigment, Zinc white for the white and Cadmium yellow for the yellow one
Tunneling out of a time-dependent well
Solutions to explicit time-dependent problems in quantum mechanics are rare.
In fact, all known solutions are coupled to specific properties of the
Hamiltonian and may be divided into two categories: One class consists of
time-dependent Hamiltonians which are not higher than quadratic in the position
operator, like i.e the driven harmonic oscillator with time-dependent
frequency. The second class is related to the existence of additional
invariants in the Hamiltonian, which can be used to map the solution of the
time-dependent problem to that of a related time-independent one.
In this article we discuss and develop analytic methods for solving
time-dependent tunneling problems, which cannot be addressed by using quadratic
Hamiltonians. Specifically, we give an analytic solution to the problem of
tunneling from an attractive time-dependent potential which is embedded in a
long-range repulsive potential.
Recent progress in atomic physics makes it possible to observe experimentally
time-dependent phenomena and record the probability distribution over a long
range of time. Of special interest is the observation of macroscopical
quantum-tunneling phenomena in Bose-Einstein condensates with time-dependent
trapping potentials. We apply our model to such a case in the last section.Comment: 11 pages, 3 figure
Kinetic and Transport Equations for Localized Excitations in Sine-Gordon Model
We analyze the kinetic behavior of localized excitations - solitons,
breathers and phonons - in Sine-Gordon model. Collision integrals for all type
of localized excitation collision processes are constructed, and the kinetic
equations are derived. We analyze the kinetic behavior of localized excitations
- solitons, breathers and phonons - in Sine-Gordon model. Collision integrals
for all type of localized excitation collision processes are constructed, and
the kinetic equations are derived. We prove that the entropy production in the
system of localized excitations takes place only in the case of inhomogeneous
distribution of these excitations in real and phase spaces. We derive transport
equations for soliton and breather densities, temperatures and mean velocities
i.e. show that collisions of localized excitations lead to creation of
diffusion, thermoconductivity and intrinsic friction processes. The diffusion
coefficients for solitons and breathers, describing the diffusion processes in
real and phase spaces, are calculated. It is shown that diffusion processes in
real space are much faster than the diffusion processes in phase space.Comment: 23 pages, latex, no figure
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