3,049 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
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
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
Friction as Contrast Mechanism in Heterodyne Force Microscopy
The nondestructive imaging of subsurface structures on the nanometer scale
has been a long-standing desire in both science and industry. A few impressive
images were published so far that demonstrate the general feasibility by
combining ultrasound with an Atomic Force Microscope. From different excitation
schemes, Heterodyne Force Microscopy seems to be the most promising candidate
delivering the highest contrast and resolution. However, the physical contrast
mechanism is unknown, thereby preventing any quantitative analysis of samples.
Here we show that friction at material boundaries within the sample is
responsible for the contrast formation. This result is obtained by performing a
full quantitative analysis, in which we compare our experimentally observed
contrasts with simulations and calculations. Surprisingly, we can rule out all
other generally believed responsible mechanisms, like Rayleigh scattering,
sample (visco)elasticity, damping of the ultrasonic tip motion, and ultrasound
attenuation. Our analytical description paves the way for quantitative
SubSurface-AFM imaging.Comment: 7 pages main paper + 11 pages supplementary material
Many-body theory of excitation dynamics in an ultracold Rydberg gas
We develop a theoretical approach for the dynamics of Rydberg excitations in
ultracold gases, with a realistically large number of atoms. We rely on the
reduction of the single-atom Bloch equations to rate equations, which is
possible under various experimentally relevant conditions. Here, we explicitly
refer to a two-step excitation-scheme. We discuss the conditions under which
our approach is valid by comparing the results with the solution of the exact
quantum master equation for two interacting atoms. Concerning the emergence of
an excitation blockade in a Rydberg gas, our results are in qualitative
agreement with experiment. Possible sources of quantitative discrepancy are
carefully examined. Based on the two-step excitation scheme, we predict the
occurrence of an antiblockade effect and propose possible ways to detect this
excitation enhancement experimentally in an optical lattice as well as in the
gas phase.Comment: 12 pages, 8 figure
Coulomb crystallization in expanding laser-cooled neutral plasmas
We present long-time simulations of expanding ultracold neutral plasmas,
including a full treatment of the strongly coupled ion dynamics. Thereby, the
relaxation dynamics of the expanding laser-cooled plasma is studied, taking
into account elastic as well as inelastic collisions. It is demonstrated that,
depending on the initial conditions, the ionic component of the plasma may
exhibit short-range order or even a superimposed long-range order resulting in
concentric ion shells. In contrast to ionic plasmas confined in traps, the
shell structures are built up from the center of the plasma cloud rather than
from the periphery
Universal shape function for the double ionization cross section of negative ions by electron impact
It is shown that recently measured cross sections for double ionization of negative ions (, and ) possess a universal shape when plotted in suitable dimensionless units. The shape can be represented with a simple analytical function, following the same principles as it has been done in establishing a universal shape function for single ionization [Rost and Pattard 1997 Phys. Rev. A {\bf 55} R5]. Thereby, it is demonstrated that direct double ionization dominates the cross section for the targets considered
Breakdown of step-flow growth in unstable homoepitaxy
Two mechanisms for the breakdown of step flow growth, in the sense of the
appearance of steps of opposite sign to the original vicinality, are studied by
kinetic Monte Carlo simulations and scaling arguments. The first mechanism is
the nucleation of islands on the terraces, which leads to mound formation if
interlayer transport is sufficiently inhibited. The second mechanism is the
formation of vacancy islands due to the self-crossing of strongly meandering
steps. The competing roles of the growth of the meander amplitude and the
synchronization of the meander phase are emphasized. The distance between
vacancy islands along the step direction appears to be proportional to the
square of the meander wavelengthComment: 7 pages, 9 figure
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