897 research outputs found
Polaron Dissociation at the Insulator-to-Metal Transition
Considering the long range Coulomb interactions between large polarons in
dielectrics, we propose a model for their crystallization when no bipolarons
are formed. As the density increases, the melting is examined at . One
possibility is the delocalization towards a liquid state of polarons. However,
we show that this cannot happen if the electron-phonon coupling is larger than
some critical value. The other competing mechanism is the dissociation of the
polarons themselves, favored owing to their large mass at strong coupling.
Finally, we propose a phase diagram for the insulator-to metal transition as a
function of the density and electron-phonon coupling.Comment: 5 pages, 3 figures, to be published Mod. Phys. Lett. B; added 1
figure, references and minor change
Is the Quantum Melting of a Polaron Wigner Crystal an Insulator-to-Superconductor transition ?
On examining the stability of a Wigner Crystal (WC) in an ionic dielectric,
two competitive effects due to Polaron formation are found to be important: (i)
the screening of the Coulomb forces which destabilizes the crystal, compensated
by (ii) the increase of the carrier mass (polaron mass). The quantum melting of
the Polaron Wigner Crystal (PWC) is examined. By calculating the quantum
fluctuations of both the electrons and the polarization, we show that there is
a competition between the dissociation of the Polarons at the
insulator-to-metal transition (IMT), and a melting towards a polaron liquid. We
find that at strong coupling (), a liquid state of polarons
cannot exist, and the IMT is driven by polaron dissociation. Next, we show that
the dipolar interactions between localized polarons are responsible for a
phonon instability of the PWC as the density increases. This provides a new
mechanism for the IMT in doped dielectrics. Examining the sign of the
dielectric constant of the PWC, we conjecture that such an instability could
yield an Insulator-to-Superconductor transition.Comment: 4 Pages, 2 Figures included, Int. Conf. M2S-HTSC-VI (Houston 2000) to
be published in Physica
On the stability of hole crystals in layered cuprates
Recent STM measurements have revealed the existence of periodic charge
modulations at the surface of certain cuprate superconductors. Here we show
that the observed patterns are compatible with the formation of a
three-dimensional crystal of doped holes, with space correlations extending
between different Cu-O layers. This puts severe constraints on the dynamical
stability of the crystallised hole structure, resulting in a close relationship
between the periodicity of the electronic modulation and the interlayer
distance.Comment: completed reference list, fig. 3 corrected; accepted for publication
in Eur. Phys. J. B, Rapid Note
Bose-Fermi mixtures in the molecular limit
We consider a Bose-Fermi mixture in the molecular limit of the attractive
interaction between fermions and bosons. For a boson density smaller or equal
to the fermion density, we show analytically how a T-matrix approach for the
constituent bosons and fermions recovers the expected physical limit of a
Fermi-Fermi mixture of molecules and atoms. In this limit, we derive simple
expressions for the self-energies, the momentum distribution function, and the
chemical potentials. By extending these equations to a trapped system, we
determine how to tailor the experimental parameters of a Bose-Fermi mixture in
order to enhance the 'indirect Pauli exclusion effect' on the boson momentum
distribution function. For the homogeneous system, we present finally a
Diffusion Monte Carlo simulation which confirms the occurrence of such a
peculiar effect.Comment: 13 pages, 7 figures; final versio
Polaron Crystallization and Melting: Effects of the Long-Range Coulomb Forces
On examining the stability of a Wigner crystal in an ionic dielectric, two
competitive effects due to the polaron formation are found to be important: (i)
the screening of the Coulomb force, which destabilizes the crystal, compensated
by (ii) the increase of the carrier mass (polaron mass). The competition
between the two effects is carefully studied, and the quantum melting of the
polaronic Wigner crystal is examined by varying the density at zero
temperature. By calculating the quantum fluctuations of both the electron and
the polarization, we show that there is a competition between the dissociation
of the polarons at the insulator-to-metal transition (IMT), and a melting
towards a polaron liquid. We find that at strong coupling, a liquid state of
dielectric polarons cannot exist, and the IMT is driven by the polaron
dissociation. Next, taking into account the dipolar interactions between
localized carriers, we show that these are responsible for an instability of
the transverse vibrational modes of the polaronic Wigner crystal as the density
increases. This provides a new mechanism for the IMT in doped dielectrics,
which yields interesting dielectric properties below and beyond the transition.
An optical signature of such a mechanism for the IMT is provided.Comment: 10 pages, 3 figures, to be published in Int.J.Mod.Phys.
Quantum Monte Carlo Study of a Resonant Bose-Fermi Mixture
We study a resonant Bose-Fermi mixture at zero temperature by using the
fixed-node diffusion Monte Carlo method. We explore the system from weak to
strong boson-fermion interaction, for different concentrations of the bosons
relative to the fermion component. We focus on the case where the boson density
is smaller than the fermion density , for which a first-order
quantum phase transition is found from a state with condensed bosons immersed
in a Fermi sea, to a Fermi-Fermi mixture of composite fermions and unpaired
fermions. We obtain the equation of state and the phase diagram, and we find
that the region of phase separation shrinks to zero for vanishing .Comment: 5 pages, 3 figures, published versio
Quantized form factor shift in the presence of free electron laser radiation
In electron scattering, the target form factors contribute significantly to
the diffraction pattern and carry information on the target electromagnetic
charge distribution. Here we show that the presence of electromagnetic
radiation, as intense as currently available in Free Electron Lasers, shifts
the dependence of the target form factors by a quantity that depends on the
number of photons absorbed or emitted by the electron as well as on the
parameters of the electromagnetic radiation. As example, we show the impact of
intense ultraviolet and soft X-ray radiation on elastic electron scattering by
Ne-like Argon ion and by Xenon atom. We find that the shift brought by the
radiation to the form factor is in the order of some percent. Our results may
open up a new avenue to explore matter with the assistance of laser
2D-3D registration of CT vertebra volume to fluoroscopy projection: A calibration model assessment (doi:10.1155/2010/806094)
This study extends a previous research concerning intervertebral motion registration by means of 2D dynamic fluoroscopy to obtain a more comprehensive 3D description of vertebral kinematics. The problem of estimating the 3D rigid pose of a CT volume of a vertebra from its 2D X-ray fluoroscopy projection is addressed. 2D-3D registration is obtained maximising a measure of similarity between Digitally Reconstructed Radiographs (obtained from the CT volume) and real fluoroscopic projection. X-ray energy correction was performed. To assess the method a calibration model was realised a sheep dry vertebra was rigidly fixed to a frame of reference including metallic markers. Accurate measurement of 3D orientation was obtained via single-camera calibration of the markers and held as true 3D vertebra position; then, vertebra 3D pose was estimated and results compared. Error analysis revealed accuracy of the order of 0.1 degree for the rotation angles of about 1?mm for displacements parallel to the fluoroscopic plane, and of order of 10?mm for the orthogonal displacement.<br/
Analytical evaluation of atomic form factors: application to Rayleigh scattering
Atomic form factors are widely used for the characterization of targets and
specimens, from crystallography to biology. By using recent mathematical
results, here we derive an analytical expression for the atomic form factor
within the independent particle model constructed from nonrelativistic screened
hydrogenic wavefunctions. The range of validity of this analytical expression
is checked by comparing the analytically obtained form factors with the ones
obtained within the Hartee-Fock method. As an example, we apply our analytical
expression for the atomic form factor to evaluate the differential cross
section for Rayleigh scattering off neutral atoms.Comment: 7 pages, 1 figur
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