3,297 research outputs found
Tuning the interactions of spin-polarized fermions using quasi-one-dimensional confinement
The behavior of ultracold atomic gases depends crucially on the two-body
scattering properties of these systems. We develop a multichannel scattering
theory for atom-atom collisions in quasi-one-dimensional (quasi-1D) geometries
such as atomic waveguides or highly elongated traps. We apply our general
framework to the low energy scattering of two spin-polarized fermions and show
that tightly-confined fermions have infinitely strong interactions at a
particular value of the 3D, free-space p-wave scattering volume. Moreover, we
describe a mapping of this strongly interacting system of two quasi-1D fermions
to a weakly interacting system of two 1D bosons.Comment: Submitted to Phys. Rev. Let
Light transmission assisted by Brewster-Zennek modes in chromium films carrying a subwavelength hole array
This work confirms that not only surface plasmons but many other kinds of
electromagnetic eigenmodes should be considered in explaining the values of the
transmittivity through a slab bearing a two-dimensional periodic corrugation.
Specifically, the role of Brewster-Zennek modes appearing in metallic films
exhibiting regions of weak positive dielectric constant. It is proposed that
these modes play a significant role in the light transmission in a thin
chromium film perforated with normal cylindrical holes, for appropriate lattice
parameters.Comment: 5 pages, 4 figures. Published versio
Plaqueta decorada de la cueva de El Horno (Ramales de la Victoria, Cantabria) en su contexto local y regional
En la actualidad, uno de los retos más relevantes de la investigación sobre el PaleolÃtico es el de superar definitivamente el paradigma del yacimiento único, abordando el estudio de las sociedades de cazadores desde una perspectiva regional, tal y como demanda el carácter móvil de las mismas. De hecho, la movilidad deberÃa ser considerada siempre en nuestras estrategias de investigación acerca de los grupos paleolÃticos, y no sólo al abordar el estudio de materiales más o menos exóticos. El arte mueble, estudiado desde el punto de vista de las cadenas operativas, aporta datos relevantes para la compresión de las sociedades de cazadores en su territorio. Los resultados del análisis de una plaqueta de arenisca procedente del nivel 2 de El Horno (Ramales de la Victoria, Cantabria), atribuido al Magdaleniense superior-final, constituyen un buen ejemplo de lo comentado. La cara A de la plaqueta presenta el grabado de una cabeza de probable cierva, un tema poco común a finales del Magdaleniense, cuyo estudio tecnológico ha revelado que se trata de la obra de un artista inexperto. La cara B está grabada y pintada, y los estigmas identificados sugieren la utilización de la plaqueta como mortero para machacar algún material, quizá el propio ocre. En ese sentido, el material presentado aporta elementos de juicio a la reflexión actual acerca de la tradicional dicotomÃa establecida en el arte mobiliar entre los soportes utilitarios y no utilitarios
Unscreened Coulomb repulsion in the one dimensional electron gas
A tight binding model of electrons interacting via bare Coulomb repulsion is
numerically investigated by use of the Density Matrix Renormalization Group
method which we prove applicable also to very long range potentials. From the
analysis of the elementary excitations, of the spin and charge correlation
functions and of the momentum distribution, a picture consistent with the
formation of a one dimensional "Wigner crystal" emerges, in quantitative
agreement with a previous bosonization study. At finite doping, Umklapp
scattering is shown to be ineffective in the presence of long range forces.Comment: RevTex, 5 pages with 8 eps figures. To be published on Phys. Rev.
Transverse excitations of ultracold matter waves upon propagation past abrupt waveguide changes
The propagation of ultracold atomic gases through abruptly changing waveguide
potentials is examined in the limit of non-interacting atoms. Time-independent
scattering calculations of microstructured waveguides with discontinuous
changes in the transverse harmonic binding potentials are used to mimic
waveguide perturbations and imperfections. Three basic configurations are
examined: step-like, barrier-like and well-like with waves incident in the
ground mode. At low energies, the spectra rapidly depart from single-moded,
with significant transmission and reflection of excited modes. The high-energy
limit sees 100 percent transmission, with the distribution of the transmitted
modes determined simply by the overlap of the mode wave functions and
interference.Comment: 20 pages, 7 figures, under review PR
Mesoscopic continuous and discrete channels for quantum information transfer
We study the possibility of realizing perfect quantum state transfer in
mesoscopic devices. We discuss the case of the Fano-Anderson model extended to
two impurities. For a channel with an infinite number of degrees of freedom, we
obtain coherent behavior in the case of strong coupling or in weak coupling
off-resonance. For a finite number of degrees of freedom, coherent behavior is
associated to weak coupling and resonance conditions
Identification of the Beutler-Fano formula in eigenphase shifts and eigentime delays near a resonance
Eigenphase shifts and eigentime delays near a resonance for a system of one
discrete state and two continua are shown to be functionals of the Beutler-
Fano formulas using appropriate dimensionless energy units and line profile
indices. Parameters responsible for the avoided crossing of eigenphase shifts
and eigentime delays are identified. Similarly, parameters responsible for the
eigentime delays due to a frame change are identified. With the help of new
parameters, an analogy with the spin model is pursued for the S matrix and time
delay matrix. The time delay matrix is shown to comprise three terms, one due
to resonance, one due to a avoided crossing interaction, and one due to a frame
change. It is found that the squared sum of time delays due to the avoided
crossing interaction and frame change is unity.Comment: 17 pages, 3 figures, RevTe
Long range scattering resonances in strong-field seeking states of polar molecules
We present first steps toward understanding the ultracold scattering
properties of polar molecules in strong electric field-seeking states. We have
found that the elastic cross section displays a quasi-regular set of potential
resonances as a function of the electric field, which potentially offers
intimate details about the inter-molecular interaction. We illustrate these
resonances in a ``toy'' model composed of pure dipoles, and in more physically
realistic systems. To analyze these resonances, we use a simple WKB
approximation to the eigenphase, which proves both reasonably accurate and
meaningful. A general treatment of the Stark effect and dipolar interactions is
also presented
A multi-photon Stokes-parameter invariant for entangled states
We consider the Minkowskian norm of the n-photon Stokes tensor, a scalar
invariant under the group realized by the transformations of stochastic local
quantum operations and classical communications (SLOCC). This invariant is
offered as a candidate entanglement measure for n-qubit states and discussed in
relation to measures of quantum state entanglement for certain important
classes of two-qubit and three-qubit systems. This invariant can be directly
estimated via a quantum network, obviating the need to perform laborious
quantum state tomography. We also show that this invariant directly captures
the extent of entanglement purification due to SLOCC filters.Comment: 9 pages, 0 figures, Accepted for publication in Physical Review
Trouble with the Lorentz law of force: Incompatibility with special relativity and momentum conservation
The Lorentz law of force is the fifth pillar of classical electrodynamics,
the other four being Maxwell's macroscopic equations. The Lorentz law is the
universal expression of the force exerted by electromagnetic fields on a volume
containing a distribution of electrical charges and currents. If electric and
magnetic dipoles also happen to be present in a material medium, they are
traditionally treated by expressing the corresponding polarization and
magnetization distributions in terms of bound-charge and bound-current
densities, which are subsequently added to free-charge and free-current
densities, respectively. In this way, Maxwell's macroscopic equations are
reduced to his microscopic equations, and the Lorentz law is expected to
provide a precise expression of the electromagnetic force density on material
bodies at all points in space and time. This paper presents incontrovertible
theoretical evidence of the incompatibility of the Lorentz law with the
fundamental tenets of special relativity. We argue that the Lorentz law must be
abandoned in favor of a more general expression of the electromagnetic force
density, such as the one discovered by A. Einstein and J. Laub in 1908. Not
only is the Einstein-Laub formula consistent with special relativity, it also
solves the long-standing problem of "hidden momentum" in classical
electrodynamics.Comment: 7 pages, 1 figur
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