494 research outputs found
Bosonization solution of the Falicov-Kimball model
We use a novel approach to analyze the one dimensional spinless
Falicov-Kimball model. We derive a simple effective model for the occupation of
the localized orbitals which clearly reveals the origin of the known ordering.
Our study is extended to a quantum model with hybridization between the
localized and itinerant states; we find a crossover between the well-known
weak- and strong-coupling behaviour. The existence of electronic polarons at
intermediate coupling is confirmed. A phase diagram is presented and discussed
in detail.Comment: RevTex, 10 pages, 1 figur
Slave boson theory of the extended Falicov-Kimball model
The extended Falicov-Kimball model, with both an on-site hybridization
potential and dispersive narrow band, is examined within the saddle-point
approximation to the Kotliar-Ruckenstein slave boson theory. We first set the
hybridization potential to zero and find that the phase diagram depends
strongly upon the orbital structure: for degenerate orbitals, a
correlated-insulating state is found at sufficiently strong interaction
strengths, whereas a finite orbital energy difference can lead to discontinuous
valence transitions. The obtained phase diagram is very sensitive to the
presence of a finite hybridization potential. As in Hartree-Fock theory, we
find an enhancement of the hybridization by the inter-orbital Coulomb
repulsion. The more precise treatment of correlation effects, however, leads to
large deviations from the Hartree-Fock results. In the limit of vanishing
hybridization an excitonic insulator state is only found when the orbitals are
degenerate, which restricts this phase to a much smaller parameter space than
in other available mean-field theories.Comment: 23 pages, 10 figure
Competing Orderings in an Extended Falicov-Kimball Model
We present a Hartree-Fock study of the Falicov-Kimball model extended by both
on-site and non-local hybridization. We examine the interplay between excitonic
effects and the charge-density wave (CDW) instability known to exist at zero
hybridization. It is found that the CDW state remains stable in the presence of
finite hybridization; for on-site hybridization the Coulomb interaction
nevertheless strongly enhances the excitonic average above its value in the
noninteracting system. In contrast, for non-local hybridization, we observe no
such enhancement of the excitonic average or a spontaneous on-site
hybridization potential. Instead, we find only a significant suppression of the
excitonic correlations in the CDW state. A phenomenological Ginzburg-Landau
analysis is also provided to understand the interplay.Comment: RevTex, 5 pages, 4 figures; expanded and corrected, typos added,
references adde
Hostility and cellular aging in men from the Whitehall II cohort.
Hostility is associated with a significantly increased risk of age-related disease and mortality, yet the pathophysiological mechanisms involved remain unclear. Here we investigated the hypothesis that hostility might impact health by promoting cellular aging
Telecentric F-theta fisheye lens for space applications
A very wide angle lens with a field of view of 360°x180° - a fisheye lens - has been designed to be used in a space environment. As a case study, the lens is assumed to be mounted on a spinning probe passing through a comet’s tail. The lens, rotating with the probe passing through the comet coma, may map the entire sky as viewed from the interior tail, providing unprecedented data on the spatial distribution of plasma and dust. Considering the foreseen space applications for the lens, radiation hardened glass has been taken into account for the design. A key feature of the lens is the “angular scale” uniformity (F-theta) of the sky distribution map projected on the focal plane allowing to obtain a reliable whole sky reconstruction. Care has also been taken to obtain an almost telecentric design, in order to permit filters placed on the focal plane to work properly. A telecentric fisheye operating with a pixel-limited resolution in the waveband from 500 nm up to 770 nm and with an F-theta distortion is presented in this paper
Charge Order in the Falicov-Kimball Model
We examine the spinless one-dimensional Falicov-Kimball model (FKM) below
half-filling, addressing both the binary alloy and valence transition
interpretations of the model. Using a non-perturbative technique, we derive an
effective Hamiltonian for the occupation of the localized orbitals, providing a
comprehensive description of charge order in the FKM. In particular, we uncover
the contradictory ordering roles of the forward-scattering and backscattering
itinerant electrons: the latter are responsible for the crystalline phases,
while the former produces the phase separation. We find an Ising model
describes the transition between the phase separated state and the crystalline
phases; for weak-coupling we present the critical line equation, finding
excellent agreement with numerical results. We consider several extensions of
the FKM that preserve the classical nature of the localized states. We also
investigate a parallel between the FKM and the Kondo lattice model, suggesting
a close relationship based upon the similar orthogonality catastrophe physics
of the associated single-impurity models.Comment: 39 pages, 6 figure
Microscopic derivation of Ginzburg-Landau equations for coexistent states of superconductivity and magnetism
Ginzburg-Landau (GL) equations for the coexistent states of superconductivity
and magnetism are derived microscopically from the extended Hubbard model with
on-site repulsive and nearest-neighbor attractive interactions. In the derived
GL free energy a cubic term that couples the spin-singlet and spin-triplet
components of superconducting order parameters (SCOP) with magnetization
exists. This term gives rise to a spin-triplet SCOP near the interface between
a spin-singlet superconductor and a ferromagnet, consistent with previous
theoretical studies based on the Bogoliubov de Gennes method and the
quasiclassical Green's function theory. In coexistent states of singlet
superconductivity and antiferromagnetism it leads to the occurrence of
pi-triplet SCOPs.Comment: 18 page
The fisheye of the comet interceptor's EnVisS camera
Entire Visible Sky (EnVisS) camera is one of the payload proposed for the ESA selected F-Class mission Comet Interceptor. The main aim of the mission is the study of a dynamic new comet, or an interstellar object, entering the inner solar system for the first time. The Comet Interceptor mission is conceived to be composed of three spacecraft: a parent spacecraft A and two, spacecraft B1 and B2, dedicated to a close and risky fly-by. EnVisS will be mounted on spacecraft B2, which is foreseen to be spin-stabilized. The EnVisS camera is designed to capture the entire sky in some visible wavelength bands while the spacecraft pass through the comet's coma. EnVisS optical head is composed of a fisheye lens with a field of view of 180° x 40° coupled with an imaging detector equipped with both band-pass and polarimetric filters. The design of fisheye lenses requires to take into account some issues typical of very wide-angle lenses. The fundamental origin of the optical problems resides on the entrance pupil shift at large angle, where the paraxial approximation is no more valid: chief rays angles on the object side are not preserved passing through the optics preceding the aperture stop (fore-optics). This effect produces an anamorphic deformation of the image on the focal plane, i.e. the focal length is changing along the elevation angles. Tracing the rays appropriately requires some effort by the designer. It has to be considered that distortion, including anamorphism, is an aberration that does not affect the quality of a point source image, thus it can be present also in well corrected lenses. In this paper the optical design of the fisheye lens, that will be mounted on the EnVisS camera for the ESA F-class "Comet Interceptor" mission, will be presented together with the initial optical requirements and the final expected optical performances
Magnetic order in orbital models of the iron pnictides
We examine the appearance of the experimentally-observed stripe
spin-density-wave magnetic order in five different orbital models of the iron
pnictide parent compounds. A restricted mean-field ansatz is used to determine
the magnetic phase diagram of each model. Using the random phase approximation,
we then check this phase diagram by evaluating the static spin susceptibility
in the paramagnetic state close to the mean-field phase boundaries. The momenta
for which the susceptibility is peaked indicate in an unbiased way the actual
ordering vector of the nearby mean-field state. The dominant orbitally resolved
contributions to the spin susceptibility are also examined to determine the
origin of the magnetic instability. We find that the observed stripe magnetic
order is possible in four of the models, but it is extremely sensitive to the
degree of the nesting between the electron and hole Fermi pockets. In the more
realistic five-orbital models, this order competes with a strong-coupling
incommensurate state which appears to be controlled by details of the
electronic structure below the Fermi energy. We conclude by discussing the
implications of our work for the origin of the magnetic order in the pnictides.Comment: 19 pages, 19 figures; published version, typos corrected, references
adde
Interplay of ferromagnetism and triplet superconductivity in a Josephson junction
In this paper we extend our earlier analysis of the novel Josephson effect in
triplet superconductor--ferromagnet--triplet superconductor (TFT) junctions [B.
Kastening \emph{et al.}, Phys. Rev. Lett. {\bf{96}}, 047009 (2006)]. In our
more general formulation of the TFT junction we allow for potential scattering
at the barrier and an arbitrary orientation of the ferromagnetic moment.
Several new effects are found upon the inclusion of these extra terms: for
example, we find that a Josephson current can flow even when there is vanishing
phase difference between the superconducting condensates on either side of the
barrier. The critical current for a barrier with magnetization parallel to the
interface is calculated as a function of the junction parameters, and is found
to display strong non-analyticities. Furthermore, the Josephson current
switches first identified in our previous work are found to be robust features
of the junction, while the unconventional temperature-dependence of the current
is very sensitive to the extra terms in the barrier Hamiltonian.Comment: 24 pages, 15 figure
- …