2,042 research outputs found
A simple interpretation of quantum mirages
In an interesting new experiment the electronic structure of a magnetic atom
adsorbed on the surface of Cu(111), observed by STM, was projected into a
remote location on the same surface. The purpose of the present paper is to
interpret this experiment with a model Hamiltonian, using ellipses of the size
of the experimental ones, containing about 2300 atoms. The charge distribution
for the different wavefunctions is analyzed, in particular, for those with
energy close to the Fermi energy of copper Ef. Some of them show two symmetric
maxima located on the principal axis of the ellipse but not necessarily at the
foci. If a Co atom is adsorbed at the site where the wavefunction with energy
has a maximum and the interaction is small, the main effect of the
adsorbed atom will be to split this particular wavefunction in two. The total
charge density will remain the same but the local density of states will
present a dip at Ef at any site where the charge density is large enough. We
relate the presence of this dip to the observation of quantum mirages. Our
interpretation suggests that other sites, apart from the foci of the ellipses,
can be used for projecting atomic images and also indicates the conditions for
other non magnetic adsorbates to produce mirages.Comment: 3 pages, 3 Fig
Capture of particles of dust by convective flow
Interaction of particles of dust with vortex convective flows is under
theoretical consideration. It is assumed that the volume fraction of solid
phase is small, variations of density due to nonuniform distribution of
particles and those caused by temperature nonisothermality of medium are
comparable. Equations for the description of thermal buoyancy convection of a
dusty medium are developed in the framework of the generalized Boussinesq
approximation taking into account finite velocity of particle sedimentation.
The capture of a cloud of dust particles by a vortex convective flow is
considered, general criterion for the formation of such a cloud is obtained.
The peculiarities of a steady state in the form of a dust cloud and backward
influence of the solid phase on the carrier flow are studied in detail for a
vertical layer heated from the sidewalls. It is shown that in the case, when
this backward influence is essential, a hysteresis behavior is possible. The
stability analysis of the steady state is performed. It turns out that there is
a narrow range of governing parameters, in which such a steady state is stable.Comment: 14 pages, 10 figures, published in Physics of Fluid
Control of axisymmetric vortex breakdown in a constricted pipe: Nonlinear steady states and weakly nonlinear asymptotic expansions
The UK's Global Health Respiratory Network: Improving respiratory health of the world's poorest through research collaborations.
Respiratory disorders are responsible for considerable morbidity, health care utilisation, societal costs and approximately one in five deaths worldwide [1-4]. Yet, despite this substantial health and societal burden – which particularly affects the world’s poorest populations and as such is a major contributor to global health inequalities – respiratory disorders have historically not received the
policy priority they warrant. For example, despite causing an estimated 1000 deaths per day, less than half of the world’s countries collect data on asthma prevalence (http://www.globalasthmareport.org/). This
is true for both communicable and non-communicable respiratory disorders, many of which are either amenable to treatment or preventable
Continuum limit of amorphous elastic bodies: A finite-size study of low frequency harmonic vibrations
The approach of the elastic continuum limit in small amorphous bodies formed
by weakly polydisperse Lennard-Jones beads is investigated in a systematic
finite-size study. We show that classical continuum elasticity breaks down when
the wavelength of the sollicitation is smaller than a characteristic length of
approximately 30 molecular sizes. Due to this surprisingly large effect
ensembles containing up to N=40,000 particles have been required in two
dimensions to yield a convincing match with the classical continuum predictions
for the eigenfrequency spectrum of disk-shaped aggregates and periodic bulk
systems. The existence of an effective length scale \xi is confirmed by the
analysis of the (non-gaussian) noisy part of the low frequency vibrational
eigenmodes. Moreover, we relate it to the {\em non-affine} part of the
displacement fields under imposed elongation and shear. Similar correlations
(vortices) are indeed observed on distances up to \xi~30 particle sizes.Comment: 28 pages, 13 figures, 3 table
Elastic Constants of Quantum Solids by Path Integral Simulations
Two methods are proposed to evaluate the second-order elastic constants of
quantum mechanically treated solids. One method is based on path-integral
simulations in the (NVT) ensemble using an estimator for elastic constants. The
other method is based on simulations in the (NpT) ensemble exploiting the
relationship between strain fluctuations and elastic constants. The strengths
and weaknesses of the methods are discussed thoroughly. We show how one can
reduce statistical and systematic errors associated with so-called primitive
estimators. The methods are then applied to solid argon at atmospheric
pressures and solid helium 3 (hcp, fcc, and bcc) under varying pressures. Good
agreement with available experimental data on elastic constants is found for
helium 3. Predictions are made for the thermal expectation value of the kinetic
energy of solid helium 3.Comment: 9 pages doublecolumn, 6 figures, submitted to PR
Elasticity of Gaussian and nearly-Gaussian phantom networks
We study the elastic properties of phantom networks of Gaussian and
nearly-Gaussian springs. We show that the stress tensor of a Gaussian network
coincides with the conductivity tensor of an equivalent resistor network, while
its elastic constants vanish. We use a perturbation theory to analyze the
elastic behavior of networks of slightly non-Gaussian springs. We show that the
elastic constants of phantom percolation networks of nearly-Gaussian springs
have a power low dependence on the distance of the system from the percolation
threshold, and derive bounds on the exponents.Comment: submitted to Phys. Rev. E, 10 pages, 1 figur
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