9,864 research outputs found
Influence of electron-ion collisions on Coulomb crystallization of ultracold neutral plasmas
While ion heating by elastic electron-ion collisions may be neglected for a
description of the evolution of freely expanding ultracold neutral plasmas, the
situation is different in scenarios where the ions are laser-cooled during the
system evolution. We show that electron-ion collisions in laser-cooled plasmas
influence the ionic temperature, decreasing the degree of correlation
obtainable in such systems. However, taking into account the collisions
increases the ion temperature much less than what would be estimated based on
static plasma clouds neglecting the plasma expansion. The latter leads to both
adiabatic cooling of the ions as well as, more importantly, a rapid decrease of
the collisional heating rate
Strongly Coupled Plasmas via Rydberg-Blockade of Cold Atoms
We propose and analyze a new scheme to produce ultracold neutral plasmas deep
in the strongly coupled regime. The method exploits the interaction blockade
between cold atoms excited to high-lying Rydberg states and therefore does not
require substantial extensions of current ultracold plasma experiments.
Extensive simulations reveal a universal behavior of the resulting Coulomb
coupling parameter, providing a direct connection between the physics of
strongly correlated Rydberg gases and ultracold plasmas. The approach is shown
to reduce currently accessible temperatures by more than an order of magnitude,
which opens up a new regime for ultracold plasma research and cold ion-beam
applications with readily available experimental techniques.Comment: 5 pages, 5 figure
Relaxation to non-equilibrium in expanding ultracold neutral plasmas
We investigate the strongly correlated ion dynamics and the degree of
coupling achievable in the evolution of freely expanding ultracold neutral
plasmas. We demonstrate that the ionic Coulomb coupling parameter increases considerably in later stages of the expansion, reaching the
strongly coupled regime despite the well-known initial drop of
to order unity due to disorder-induced heating. Furthermore, we formulate a
suitable measure of correlation and show th at calculated from
the ionic temperature and density reflects the degree of order in the system if
it is sufficiently close to a quasisteady state. At later times, however, the
expansion of the plasma cloud becomes faster than the relaxation of
correlations, and the system does not reach thermodynamic equilibrium anymore
Electromagnetically Induced Transparency in strongly interacting Rydberg Gases
We develop an efficient Monte-Carlo approach to describe the optical response
of cold three-level atoms in the presence of EIT and strong atomic
interactions. In particular, we consider a "Rydberg-EIT medium" where one
involved level is subject to large shifts due to strong van der Waals
interactions with surrounding Rydberg atoms. We find excellent agreement with
much more involved quantum calculations and demonstrate its applicability over
a wide range of densities and interaction strengths. The calculations show that
the nonlinear absorption due to Rydberg-Rydberg atom interactions exhibits
universal behavior
Discovery of an outflow of the very low-mass star ISO 143
We discover that the very young very low-mass star ISO143 (M5) is driving an
outflow based on spectro-astrometry of forbidden [SII] emission lines at 6716A
and 6731A observed in UVES/VLT spectra. This adds another object to the handful
of brown dwarfs and very low-mass stars (M5-M8) for which an outflow has been
confirmed and which show that the T Tauri phase continues at very low masses.
We find the outflow of ISO143 to be intrinsically asymmetric and the accretion
disk to not obscure the outflow, as only the red outflow component is visible
in the [SII] lines. ISO143 is only the third T Tauri object showing a stronger
red outflow component in spectro-astrometry, after RW Aur (G5) and ISO217
(M6.25). We show here that including ISO143 two out of seven outflows confirmed
in the very low-mass regime (M5-M8) are intrinsically asymmetric. We measure a
spatial extension of the outflow in [SII] of up to 200-300 mas (about 30-50 AU)
and velocities of up to 50-70 km/s. We furthermore detect line emission of
ISO143 in CaII (8498), OI (8446), HeI (7065), and weakly in [FeII] (7155).
Based on a line profile analysis and decomposition we demonstrate that (i) the
CaII emission can be attributed to chromospheric activity, a variable wind, and
the magnetospheric infall zone, (ii) the OI emission mainly to
accretion-related processes but also a wind, and (iii) the HeI emission to
chromospheric or coronal activity. We estimate a mass outflow rate of ISO143 of
~10^{-10} Msol/yr and a mass accretion rate in the range of ~10^{-8} to
~10^{-9} Msol/yer. These values are consistent with those of other brown dwarfs
and very low-mass stars. The derived Mout/Macc ratio of 1-20% is not supporting
previous findings of this number to be very large (>40%) for very low-mass
objects.Comment: Accepted for publication at A&A; 9 pages, 5 figures. Minor changes
due to language editin
Ultracold Neutral Plasmas
Ultracold neutral plasmas, formed by photoionizing laser-cooled atoms near
the ionization threshold, have electron temperatures in the 1-1000 kelvin range
and ion temperatures from tens of millikelvin to a few kelvin. They represent a
new frontier in the study of neutral plasmas, which traditionally deals with
much hotter systems, but they also blur the boundaries of plasma, atomic,
condensed matter, and low temperature physics. Modelling these plasmas
challenges computational techniques and theories of non-equilibrium systems, so
the field has attracted great interest from the theoretical and computational
physics communities. By varying laser intensities and wavelengths it is
possible to accurately set the initial plasma density and energy, and
charged-particle-detection and optical diagnostics allow precise measurements
for comparison with theoretical predictions. Recent experiments using optical
probes demonstrated that ions in the plasma equilibrate in a strongly coupled
fluid phase. Strongly coupled plasmas, in which the electrical interaction
energy between charged particles exceeds the average kinetic energy, reverse
the traditional energy hierarchy underlying basic plasma concepts such as Debye
screening and hydrodynamics. Equilibration in this regime is of particular
interest because it involves the establishment of spatial correlations between
particles, and it connects to the physics of the interiors of gas-giant planets
and inertial confinement fusion devices.Comment: 89 pages, 54 image
Dynamical Crystallization in the Dipole Blockade of Ultracold Atoms
We describe a method for controlling many-body states in extended ensembles
of Rydberg atoms, forming crystalline structures during laser excitation of a
frozen atomic gas. Specifically, we predict the existence of an excitation
number staircase in laser excitation of atomic ensembles into Rydberg states.
Each step corresponds to a crystalline state with a well-defined of regularly
spaced Rydberg atoms. We show that such states can be selectively excited by
chirped laser pulses. Finally, we demonstarte that, sing quantum state transfer
from atoms to light, such crystals can be used to create crystalline photonic
states and can be probed via photon correlation measurements
Charged Current Neutrino Nucleus Interactions at Intermediate Energies
We have developed a model to describe the interactions of neutrinos with
nucleons and nuclei, focusing on the region of the quasielastic and Delta(1232)
peaks. We describe neutrino nucleon collisions with a fully relativistic
formalism which incorporates state-of-the-art parametrizations of the form
factors for both the nucleon and the N-Delta transition. The model has then
been extended to finite nuclei, taking into account nuclear effects such as
Fermi motion, Pauli blocking (both within the local density approximation),
nuclear binding and final state interactions. The in-medium modification of the
Delta resonance due to Pauli blocking and collisional broadening have also been
included. Final state interactions are implemented by means of the
Boltzmann-Uehling-Uhlenbeck (BUU) coupled-channel transport model. Results for
charged current inclusive cross sections and exclusive channels as pion
production and nucleon knockout are presented and discussed.Comment: 26 pages, 24 figures; v2: 2 figures and discussion added, version
accepted for publication in Phys. Rev.
Strong-coupling effects in the relaxation dynamics of ultracold neutral plasmas
We describe a hybrid molecular dynamics approach for the description of
ultracold neutral plasmas, based on an adiabatic treatment of the electron gas
and a full molecular dynamics simulation of the ions, which allows us to follow
the long-time evolution of the plasma including the effect of the strongly
coupled ion motion. The plasma shows a rather complex relaxation behavior,
connected with temporal as well as spatial oscillations of the ion temperature.
Furthermore, additional laser cooling of the ions during the plasma evolution
drastically modifies the expansion dynamics, so that crystallization of the ion
component can occur in this nonequilibrium system, leading to lattice-like
structures or even long-range order resulting in concentric shells
Creating Non-Maxwellian Velocity Distributions in Ultracold Plasmas
We present techniques to perturb, measure and model the ion velocity
distribution in an ultracold neutral plasma produced by photoionization of
strontium atoms. By optical pumping with circularly polarized light we promote
ions with certain velocities to a different spin ground state, and probe the
resulting perturbed velocity distribution through laser-induced fluorescence
spectroscopy. We discuss various approaches to extract the velocity
distribution from our measured spectra, and assess their quality through
comparisons with molecular dynamic simulationsComment: 13 pages, 8 figure
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