arXiv subjects

Peter Hartmann

Publications and source records attributed to Peter Hartmann.

33 records · Page 2Linked to original sources

arXiv2014

On the metastability of the hexatic phase during the melting of two-dimensional charged particle solids

For two-dimensional many-particle systems first-order, second-order, single step continuous, as well as two-step continuous (KTHNY-like) melting transitions have been found in previous studies. Recent computer simulations, using particle numbers in the $\geq 10^5$ range, as well as a few experimental studies, tend to support the two-step scenario, where the solid and liquid phases are separated by a third, so called hexatic phase. We have performed molecular dynamics simulations on Yukawa (Debye-Hückel) systems at conditions earlier predicted to belong to the hexatic phase. Our simulation studies on the time needed for the equilibration of the systems conclude that the hexatic phase is metastable and disappears in the limit of long times. We also show that simply increasing the particle number in particle simulations does not necessarily result in more accurate conclusions regarding the existence of the hexatic phase. The increase of the system size has to be accompanied with the increase of the simulation time to ensure properly thermalized conditions.

cond-mat.mtrl-sci↗

arXiv2013

Magnetoplasmons in rotating dusty plasmas

A rotating dusty plasma apparatus was constructed to provide the possibility of experimental emulation of extremely high magnetic fields by means of the Coriolis force, observable in a co-rotating measurement frame. We present collective excitation spectra for different rotation rates with a magnetic induction equivalent of up to 3200 Tesla. We identify the onset of magnetoplasmon-equivalent mode dispersion in the rotating macroscopic two-dimensional single-layer dusty plasma. The experimental results are supported by molecular dynamics simulations of 2D magnetized Yukawa systems.

physics.plasm-ph↗

arXiv2013

Transport control of dust particles via the Electrical Asymmetry Effect: experiment, simulation, and modeling

The control of the spatial distribution of micrometer-sized dust particles in capacitively coupled radio frequency discharges is relevant for research and applications. Typically, dust particles in plasmas form a layer located at the sheath edge adjacent to the bottom electrode. Here, a method of manipulating this distribution by the application of a specific excitation waveform, i.e. two consecutive harmonics, is discussed. Tuning the phase angle θbetween the two harmonics allows to adjust the discharge symmetry via the Electrical Asymmetry Effect (EAE). An adiabatic (continuous) phase shift leaves the dust particles at an equilibrium position close to the lower sheath edge. Their levitation can be correlated with the electric field profile. By applying an abrupt phase shift the dust particles are transported between both sheaths through the plasma bulk and partially reside at an equilibium position close to the upper sheath edge. Hence, the potential profile in the bulk region is probed by the dust particles providing indirect information on plasma properties. The respective motion is understood by an analytical model, showing both the limitations and possible ways of optimizing this sheath-to-sheath transport. A classification of the transport depending on the change in the dc self bias is provided, and the pressure dependence is discussed.

physics.plasm-ph↗

arXiv2012

Single exposure 3D imaging of dusty plasma clusters

We have worked out the details of a single camera, single exposure method to perform three-dimensional imaging of a finite particle cluster. The procedure is based on the plenoptic imaging principle and utilizes a commercial Lytro light field still camera. We demonstrate the capabilities of our technique on a single layer particle cluster in a dusty plasma, where the camera is aligned inclined at a small angle to the particle layer. The reconstruction of the third coordinate (depth) is found to be accurate and even shadowing particles can be identified.

physics.plasm-ph↗

arXiv2012

Collective Modes in Two Dimensional Binary Yukawa Systems

We analyze via theoretical approaches and molecular dynamics simulations the collective mode structure of strongly coupled two-dimensional binary Yukawa systems, for selected density, mass and charge ratios, both in the liquid and crystalline solid phases. Theoretically, the liquid phase is described through the Quasi-Localized Charge Approximation (QLCA) approach, while in the crystalline phase we study the centered honeycomb and the staggered rectangular crystal structures through the standard harmonic phonon approximation. We identify "longitudinal" and "transverse" acoustic and optic modes and find that the longitudinal acoustic mode evolves from its weakly coupled counterpart in a discontinuous non-perturbative fashion. The low frequency acoustic excitations are governed by the oscillation frequency of the average atom, while the high frequency optic excitation frequencies are related to the Einstein frequencies of the systems.

physics.plasm-ph↗

arXiv2012

Consequences of an attractive force on collective modes and dust structures in a strongly coupled dusty plasma

We present an investigation of the combined effects of the Debye-Huckel (DH) repulsive and overlaping Debye spheres (ODS) attractive interaction potentials around charged dust particles on collective modes, phase separation and ordered dust structures in a strongly coupled dusty plasma. We obtain static and dynamical information via Molecular Dynamics simulations in the liquid and crystallized phases and identify the onset of an instability in the transverse collective mode, by using (zero-temperature) lattice summation method. The present results are useful for understanding the origin of coagulation/agglomeration of charged dust particles and the formation of ordered dust structures in low-temperature laboratory and space dusty plasmas.

physics.plasm-ph↗

arXiv2011

Static and dynamic shear viscosity of a single layer complex plasma

We measured the static and dynamic (complex) shear viscosity of a single layer complex plasma by applying, respectively, a stationary and a periodically modulated shear stress induced by the light pressure of manipulating laser beams. Under static conditions the shear viscosity reproduced the numerically predicted shear rate dependence, the so called shear-thinning effect. Under oscillating shear both the magnitude and the ratio of the dissipative and elastic contributions to the complex viscosity show strong frequency dependence. Accompanying molecular dynamics simulations explain and support the experimental observations.

physics.plasm-ph↗

arXiv2011

Strong Coupling Effects in Binary Yukawa Systems

We analyze the acoustic collective excitations in two- and three-dimensional binary Yukawa systems, consisting of two components with different masses. Theoretical analysis reveals a profound difference between the weakly and strongly correlated limits: at weak coupling the two components interact via the mean field only and the oscillation frequency is governed by the light component. In the strongly correlated limit the mode frequency is governed by the combined mass, where the heavy component dominates. Computer simulations in the full coupling range extend and confirm the theoretical results.

physics.plasm-ph↗

arXiv2011

Ground state structures of superparamagnetic 2D dusty plasma crystals

Ground state structures of finite, cylindrically confined two-dimensional Yukawa systems composed of charged superparamagnetic dust grains in an external magnetic field are investigated numerically, using molecular dynamic simulations and lattice summation methods. The ground state configuration of the system is identified using, as an approximation, the experimentally obtained shape of the horizontal confinement potential in a classical single layer dusty plasma experiment with non-magnetic grains. Results are presented for the dependence of the number density and lattice parameters of the dust layer on (1) the ratio of the magnetic dipole-dipole force to electrostatic force between the grains and (2) the orientation of the grain magnetic moment with respect to the layer.

physics.plasm-ph↗

arXiv2010

Crystallization dynamics of a single layer complex plasma

We report a series of complex (dusty) plasma experiments, aimed at the study of the detailed time evolution of the re-crystallisation process following a rapid quench of a two dimensional dust liquid. The experiments were accompanied by large-scale (million particle) molecular dynamics simulations, assuming Yukawa type inter-particle interaction. Both experiment and simulation show a $\propto t^α$ (power law) dependence of the linear crystallite domain size as measured by the bond-order correlation length, translational correlation length, dislocation (defect) density, and a direct size measurement algorithm. The results show two stages of order formation: on short time-scales individual particle motion dominates; this is a fast process characterized by $α=0.93\pm0.1$. At longer time-scales, small crystallites undergo collective rearrangement, merging into bigger ones, resulting in a smaller exponent $α=0.38\pm0.06$.

physics.plasm-ph↗

arXiv2010

Dynamics of two-dimensional dipole systems

Using a combined analytical/molecular dynamics (MD) approach, we study the current fluctuation spectra and longitudinal and transverse collective mode dispersions of the classical two-dimensional (point) dipole system (2DDS) characterized by the $ϕ_{D}(r)=μ^2/r^3$ repulsive interaction potential; $μ$ is the electric dipole strength. The interest in such two-dimensional dipole systems (2DDS) is twofold. First, the quasi-long range $1/r^{3}$ interaction makes the system a unique classical many body system, with a remarkable collective mode behavior. Second, the system may be a good model for a closely spaced semiconductor electron-hole bilayer, a system that is in the forefront of current experimental interest. The longitudinal collective excitations, which are of primary interest for the liquid phase, are acoustic at long wavelengths. At higher wave numbers and for sufficiently high coupling strength, we observe the formation of a deep minimum in the dispersion curve preceded by a sharp maximum; this is identical to what has been observed in the dispersion of the zero temperature bosonic dipole system, which in turn emulates so called roton-maxon excitation spectrum of the superfluid $^4$He. The analysis we present gives an insight into the emergence of this apparently universal structure, governed by strong correlations. We study both the liquid and the crystalline solid state. We also observe the excitation of combination frequencies, resembling the roton-roton, roton-maxon, etc. structures in $^4$He.

physics.plasm-ph↗

arXiv2008

Time correlation functions and transport coefficients of two-dimensional Yukawa liquids

The existence of coefficients for diffusion, viscosity and thermal conductivity is examined for two-dimensional (2D) liquids. Equilibrium molecular dynamics simulations are performed using a Yukawa potential, and the long-time behavior of autocorrelation functions is tested. Advances reported here as compared to previous 2D Yukawa liquid simulations include an assessment of the thermal conductivity, using a larger system size to allow meaningful examination of longer times, and development of improved analysis methods. We find that the transport coefficient exists for diffusion at high temperature, and viscosity at low temperature, but not in the opposite limits. The thermal conductivity coefficient does not appear to exist at high temperature. Further advances in computing power could improve these assessments by allowing even larger system sizes and longer time series.

physics.flu-dyn↗

arXiv2008

Dynamical correlations and collective excitations of Yukawa liquids

In dusty (complex) plasmas, containing mesoscopic charged grains, the grain-grain interaction in many cases can be well described through a Yukawa potential. In this Review we summarize the basics of the computational and theoretical approaches capable of describing many-particle Yukawa systems in the liquid and solid phases and discuss the properties of the dynamical density and current correlation spectra of three- and two-dimensional strongly coupled Yukawa systems, generated by molecular dynamics simulations. We show details of the $ω(k)$ dispersion relations for the collective excitations in these systems, as obtained theoretically following the quasilocalized charge approximation, as well as from the fluctuation spectra created by simulations. The theoretical and simulation results are also compared with those obtained in complex plasma experiments.

physics.plasm-ph↗

arXiv2008

Acoustic dispersion in a two-dimensional dipole system

We calculate the full density response function, and from it the long-wavelength acoustic dispersion for a two-dimensional system of strongly coupled point dipoles interacting through a 1/r^3 potential at arbitrary degeneracy. Such a system has no RPA limit and the calculation has to include correlations from the outset. We follow the Quasi-Localized Charge (QLC) approach, accompanied by Molecular Dynamics (MD) simulations. Similarly to what has been recently reported for the closely spaced classical electron-hole bilayer [G. J. Kalman et al. Phys. Rev. Lett. 98, 236801 (2007)] and in marked contrast to the RPA, we report a long-wavelength acoustic phase velocity that is wholly maintained by particle correlations and varies linearly with the dipole moment p. The oscillation frequency, calculated both in an extended QLC approximation and in the Singwi-Tosi-Land-Sjolander approximation, is invariant in form over the entire classical to quantum domains all the way down to zero temperature. Based on our classical MD-generated pair distribution function data and on ground-state energy data generated by recent quantum Monte Carlo simulations on a bosonic dipole system [Astrakharchik et al, Phys. Rev. Lett. 98, 060405 (2007)], there is a good agreement between the QLCA kinetic sound speeds and the standard thermodynamic sound speeds in both the classical and quantum domains.

cond-mat.other↗

arXiv2006

Collective excitations in electron-hole bilayers

We report a combined analytic and Molecular Dynamics analysis of the collective mode spectrum of an electron-hole (bipolar) bilayer in the strong coupling quasi-classical limit. A robust, isotropic energy gap is identified in the out-of-phase spectra, generated by the combined effect of correlations and of the excitation of the bound dipoles; the in-phase spectra exhibit a correlation governed acoustic dispersion for the longitudinal and transverse modes. Strong nonlinear generation of higher harmonics of the fundamental dipole oscillation frequency and the transfer of harmonics between different modes is observed. The mode dispersions in the liquid state are compared with the phonon spectrum in the crystalline solid phase, reinforcing a coherent physical picture.

cond-mat.other↗