2,312 research outputs found
Chaotic Orbits in Thermal-Equilibrium Beams: Existence and Dynamical Implications
Phase mixing of chaotic orbits exponentially distributes these orbits through
their accessible phase space. This phenomenon, commonly called ``chaotic
mixing'', stands in marked contrast to phase mixing of regular orbits which
proceeds as a power law in time. It is operationally irreversible; hence, its
associated e-folding time scale sets a condition on any process envisioned for
emittance compensation. A key question is whether beams can support chaotic
orbits, and if so, under what conditions? We numerically investigate the
parameter space of three-dimensional thermal-equilibrium beams with space
charge, confined by linear external focusing forces, to determine whether the
associated potentials support chaotic orbits. We find that a large subset of
the parameter space does support chaos and, in turn, chaotic mixing. Details
and implications are enumerated.Comment: 39 pages, including 14 figure
Tuning p-wave interactions in an ultracold Fermi gas of atoms
We have measured a p-wave Feshbach resonance in a single-component, ultracold
Fermi gas of potassium atoms. We have used this resonance to enhance the
normally suppressed p-wave collision cross-section to values larger than the
background s-wave cross-section between potassium atoms in different
spin-states. In addition to the modification of two-body elastic processes, the
resonance dramatically enhances three-body inelastic collisional loss.Comment: 4 pages, 5 figure
Fluctuations Do Matter: Large Noise-Enhanced Halos in Charged-Particle Beams
The formation of beam halos has customarily been described in terms of a
particle-core model in which the space-charge field of the oscillating core
drives particles to large amplitudes. This model involves parametric resonance
and predicts a hard upper bound to the orbital amplitude of the halo particles.
We show that the presence of colored noise due to space-charge fluctuations
and/or machine imperfections can eject particles to much larger amplitudes than
would be inferred from parametric resonance alone.Comment: 13 pages total, including 5 figure
Loss of molecules in magneto-electrostatic traps due to nonadiabatic transitions
We analyze the dynamics of a paramagnetic, dipolar molecule in a generic
"magneto-electrostatic'' trap where both magnetic and electric fields may be
present. The potential energy that governs the dynamics of the molecules is
found using a reduced molecular model that incorporates the main features of
the system. We discuss the shape of the trapping potentials for different field
geometries, as well as the possibility of nonadiabatic transitions to untrapped
states, i.e., the analog of Majorana transitions in a quadrupole magnetic
atomic trap. Maximizing the lifetime of molecules in a trap is of great concern
in current experiments, and we assess the effect of nonadiabatic transitions on
obtainable trap lifetimes.Comment: 13 pages, 6 figure
Ultracold collisions of oxygen molecules
Collision cross sections and rate constants between two ground- state oxygen
molecules are investigated theoretically at translational energies below K and in zero magnetic field. We present calculations for elastic and spin-
changing inelastic collision rates for different isotopic combinations of
oxygen atoms as a prelude to understanding their collisional stability in
ultracold magnetic traps. A numerical analysis has been made in the framework
of a rigid- rotor model that accounts fully for the singlet, triplet, and
quintet potential energy surfaces in this system. The results offer insights
into the effectiveness of evaporative cooling and the properties of molecular
Bose- Einstein condensates, as well as estimates of collisional lifetimes in
magnetic traps. Specifically, looks like a good candidate for
ultracold studies, while is unlikely to survive evaporative
cooling. Since is representative of a wide class of molecules that
are paramagnetic in their ground state we conclude that many molecules can be
successfully magnetically trapped at ultralow temperatures.Comment: 15 pages, 9 figure
A Dielectric Superfluid of Polar Molecules
We show that, under achievable experimental conditions, a Bose-Einstein
condensate (BEC) of polar molecules can exhibit dielectric character. In
particular, we derive a set of self-consistent mean-field equations that couple
the condensate density to its electric dipole field, leading to the emergence
of polarization modes that are coupled to the rich quasiparticle spectrum of
the condensate. While the usual roton instability is suppressed in this system,
the coupling can give rise to a phonon-like instability that is characteristic
of a dielectric material with a negative static dielectric function.Comment: Version published in New Journal of Physics, 11+ pages, 4 figure
Writing for Intercultural Growth on Study Abroad in Australia
For students participating in study abroad programs in seemingly familiar environs, ongoing cultural mentoring is critically important. This study looks at intercultural development using both the Intercultural Development Inventory (IDI) and qualitative analysis of reflective writing assignments. U.S. participants in a semester-long faculty-led program in Australia experienced significant intercultural growth as measured by the IDI. Four writing prompts were analyzed in terms of how well they correlated with student IDI Developmental Orientation (DO) scores. Written reflection reveals how students write about intercultural issues at different developmental stages. Some writing prompts were found to facilitate intercultural development, allowing students to ‘write beyond’ their DO scores. While sojourners effectively wrote about cross-cultural issues pertaining to the host environment, more ethnocentric thinking was revealed when asked to reflect on cultural conflicts in their home community. The findings illustrate that significant intercultural growth is achievable in a culturally and linguistically similar host country, and that carefully designed reflective writing prompts can both illustrate and facilitate meaningful intercultural growth
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