arXiv subjects

M. Bonitz

Publications and source records attributed to M. Bonitz.

At least 109 records · Page 6Linked to original sources

arXiv2009

Interacting Bosons beyond the Gross-Pitaevskii Mean-Field

Systems consisting of cold interacting bosons show interesting collective phenomena such as Bose-Einstein condensation or superfluidity and are currently studied in condensed matter and atomic physics. Of particular interest are nonideal bosons which exhibit strong correlation and spatial localization effects. Here we analyse the ground-state of a two-dimensional Bose system with a Hartree-Fock type approximation that was first introduced by Romanovsky et al. [Phys. Rev. Lett. {\bf 93}, 230405 (2004)]. We apply this method to a one dimensional system of charged bosons and analyze the behaviour at strong coupling.

cond-mat.mes-hall↗

arXiv2008

Structures of quantum 2D electron-hole plasmas

We investigate structures of 2D quantum electron-hole (e-h) plasmas by the direct path integral Monte Carlo method (PIMC) in a wide range of temperature, density and hole-to-electron mass ratio. Our simulation includes a region of appearance and decay of the bound states (excitons and biexcitons), the Mott transition from the neutral e-h plasma to metallic-like clusters, formation from clusters the hexatic-like liquid and formation of the crystal-like lattice.

physics.plasm-ph↗

arXiv2008

Effective interaction potential and superfluid-solid transition of spatially indirect excitons

Using an adiabatic approximation we derive an effective interaction potentially for spatially indirect excitons. Using this potential and path integral Monte Carlo simulations we study exciton crystllization and the quantum melting phase transition in a macroscopic system of 2D excitons. Furthermore, the superfluid fraction is calculated as a function of density and shown to vanish upon crystallization. We show that the commonly used dipole model fails to correctly describe indirect excitons in quantum well structures.

cond-mat.str-el↗

arXiv2008

Spectral properties of spherically confined dusty plasma crystals

A combined theoretical and experimental analysis of the normal modes of three-dimensional spherially confined Yukawa clusters is presented. Particular attention is paid to the breathing mode and the existence of multiple monopole oscillations in Yukawa systems. Finally, the influence of dissipation on the mode spectrum is investigated.

physics.plasm-ph↗

arXiv2008

Scanning tunneling microscopy and kinetic Monte Carlo investigation of Cesium superlattices on Ag(111)

Cesium adsorption structures on Ag(111) were characterized in a low-temperature scanning tunneling microscopy experiment. At low coverages, atomic resolution of individual Cs atoms is occasionally suppressed in regions of an otherwise hexagonally ordered adsorbate film on terraces. Close to step edges Cs atoms appear as elongated protrusions along the step edge direction. At higher coverages, Cs superstructures with atomically resolved hexagonal lattices are observed. Kinetic Monte Carlo simulations model the observed adsorbate structures on a qualitative level.

cond-mat.mtrl-sci↗

arXiv2008

Thermodynamic properties and electrical conductivity of strongly correlated plasma media

We study thermodynamic properties and the electrical conductivity of dense hydrogen and deuterium using three methods: classical reactive Monte Carlo (REMC), direct path integral Monte Carlo (PIMC) and a quantum dynamics method in the Wigner representation of quantum mechanics. We report the calculation of the deuterium compression quasi-isentrope in good agreement with experiments. We also solve the Wigner-Liouville equation of dense degenerate hydrogen calculating the initial equilibrium state by the PIMC method. The obtained particle trajectories determine the momentum-momentum correlation functions and the electrical conductivity and are compared with available theories and simulations.

physics.plasm-ph↗

arXiv2008

Nonequilibrium Green's functions approach to strongly correlated few-electron quantum dots

The effect of electron-electron scattering on the equilibrium properties of few-electron quantum dots is investigated by means of nonequilibrium Green's functions theory. The ground and equilibrium state is self-consistently computed from the Matsubara (imaginary time) Green's function for the spatially inhomogeneous quantum dot system whose constituent charge carriers are treated as spin-polarized. To include correlations, the Dyson equation is solved, starting from a Hartree-Fock reference state, within a conserving (second order) self-energy approximation where direct and exchange contributions to the electron-electron interaction are included on the same footing. We present results for the zero and finite temperature charge carrier density, the orbital-resolved distribution functions and the self-consistent total energies and spectral functions for isotropic, two-dimensional parabolic confinement as well as for the limit of large anisotropy--quasi-one-dimensional entrapment. For the considered quantum dots with N=2, 3 and 6 electrons, the analysis comprises the crossover from Fermi gas/liquid (at large carrier density) to Wigner molecule or crystal behavior (in the low-density limit).

cond-mat.mes-hall↗

arXiv2008

Dynamics of strongly correlated ions in a partially ionized quantum plasma

A scheme which allows to compute the dynamics of strongly correlated classical ions embedded into a partiallzy ionized quantum plasma by first principles molecular dynamics is presented. The dynamically screened dust approach of Joyce and Lampe [Phys. Rev. Lett. {\bf 88}, 095006 (2002] ist generalized to quantum systems. The electrons are treated fully quantum-mechanically taking into account their dynamical screening of the ion-ion interaction in linear response on the basis of an extended Mermin formula. The scheme allows to include the effect of the electron dynamics, electron streaming, wake effects and electron magnetization.

physics.plasm-ph↗

arXiv2008

Angular distributions of atomic photoelectrons produced in the UV and XUV regimes

We present angular distributions of photoelectrons of atomic model systems excited by intense linearly polarized laser pulses in the VUV- and XUV-regime. We solve the multi-dimensional time-dependent Schrödinger equation for one particle on large spatial grids and investigate the direction dependence of the ionized electrons for isotropic s-states as well as p-states. Although the ponderomotive potential is small compared to the binding energy of the initially bound electron and the photon energy of the exciting laser field, richly structured photoelectron angular distributions are found which sensitively depend on the laser frequency and intensity as well as on the number of absorbed photons. The occuring shapes are explained in terms of scattering mechanisms.

physics.atom-ph↗

arXiv2008

Superdiffusion in quasi-two-dimensional Yukawa liquids

The emergence and vanishing of superdiffusion in quasi-two-dimensional Yukawa systems are investigated by molecular dynamics simulations. Using both the asymptotic behaviour of the mean-squared displacement of the particles and the long-time tail of the velocity autocorrelation function as indicators for superdiffusion, we confirm the existence of a transition from normal diffusion to superdiffusion in systems changing from a three-dimensional to a two-dimensional character. A connection between superdiffusion and dimensionality is established by the behaviour of the projected pair distribution function.

physics.plasm-ph↗

arXiv2008

Structural transitions of finite spherical Yukawa crystals

Small three-dimensional strongly coupled clusters of charged particles in a spherical confinement potential arrange themselves in nested concentric shells. If the particles are immersed into a background plasma the interaction is screened. The cluster shell configuration is known to be sensitive to the screening strength. With increased screening an increased population of the inner shell(s) is observed. Here, we present a detailed analysis of the ground state shell configurations and configuration changes in a wide range of screening parameters for clusters with particle numbers $N$ in the range of 11 to 60. We report three types of anomalous behaviors which are observed upon increase of screening, at fixed $N$ or for an increase of $N$ at fixed screening. The results are obtained by means of extensive first principle molecular dynamics simulations.

physics.plasm-ph↗

arXiv2008

Probability of metastable configurations in spherical three-dimensional Yukawa crystals

Recently the occurrence probabilities of ground- and metastable states of three-dimensional Yukawa clusters with 27 and 31 particles have been analyzed in dusty plasma experiments [Block et al., Physics of Plasmas 15, 040701 (2008)]. There it was found that, in many cases, the ground state appeared substantially less frequently than excited states. Here we analyze this question theoretically by means of molecular dynamics (MD) and Monte Carlo simulations and an analytical method based on the canonical partition function. We confirm that metastable states can occur with a significantly higher probability than the ground state. The results strongly depend on the screening parameter of the Yukawa interaction and the damping coefficient used in the MD simulations. The analytical method allows one to gain insight into the mechanisms being responsible for the occurrence probabilities of metastable states in strongly correlated finite systems.

physics.plasm-ph↗

arXiv2008

On the Coulomb-dipole transition in mesoscopic classical and quantum electron-hole bilayers

We study the Coulomb-to-dipole transition which occurs when the separation $d$ of an electron-hole bilayer system is varied with respect to the characteristic in-layer distances. An analysis of the classical ground state configurations for harmonically confined clusters with $N\leq30$ reveals that the energetically most favorable state can differ from that of two-dimensional pure dipole or Coulomb systems. Performing a normal mode analysis for the N=19 cluster it is found that the lowest mode frequencies exhibit drastic changes when $d$ is varied. Furthermore, we present quantum-mechanical ground states for N=6, 10 and 12 spin-polarized electrons and holes. We compute the single-particle energies and orbitals in self-consistent Hartree-Fock approximation over a broad range of layer separations and coupling strengths between the limits of the ideal Fermi gas and the Wigner crystal.

cond-mat.str-el↗

arXiv2008

Existence and Vanishing of the Breathing Mode in Strongly Correlated Finite Systems

One of the fundamental eigenmodes of finite interacting systems is the mode of {\em uniform radial expansion and contraction} -- the ``breathing'' mode (BM). Here we show in a general way that this mode exists only under special conditions: i) for harmonically trapped systems with interaction potentials of the form $1/r^γ$ $(γ\in\mathbb{R}_{\neq0})$ or $\log(r)$, or ii) for some systems with special symmetry such as single shell systems forming platonic bodies. Deviations from the BM are demonstrated for two examples: clusters interacting with a Lennard-Jones potential and parabolically trapped systems with Yukawa repulsion. We also show that vanishing of the BM leads to the occurence of multiple monopole oscillations which is of importance for experiments.

physics.plasm-ph↗

arXiv2008

Classical and quantum Coulomb crystals

Strong correlation effects in classical and quantum plasmas are discussed. In particular, Coulomb (Wigner) crystallization phenomena are reviewed focusing on one-component non-neutral plasmas in traps and on macroscopic two-component neutral plasmas. The conditions for crystal formation in terms of critical values of the coupling parameters and the distance fluctuations and the phase diagram of Coulomb crystals are discussed.

physics.plasm-ph↗

arXiv2007

Mesoscopic Coulomb Supersolid

When a few tens of charged particles are trapped in a spherical electrostatic potential at low temperature they form concentric shells resembling atoms. These ``artificial atoms'' can be easily controlled by varying the confinement strength. We analyze such systems for the case that the particles are bosons and find superfluid behavior which even persists in the solid state. This novel state of matter is a mesoscopic supersolid.

cond-mat.stat-mech↗

arXiv2007

Melting of trapped few particle systems

In small confined systems predictions for the melting point strongly depend on the choice of quantity and on the way it is computed, even yielding divergent and ambiguous results. We present a very simple quantity which allows to control these problems -- the variance of the block averaged interparticle distance fluctuations.

cond-mat.stat-mech↗

arXiv2007

Ground state of a confined Yukawa plasma including correlation effects

The ground state of an externally confined one-component Yukawa plasma is derived analytically using the local density approximation (LDA). In particular, the radial density profile is computed. The results are compared with the recently obtained mean-field (MF) density profile \cite{henning.pre06}. While the MF results are more accurate for weak screening, LDA with correlations included yields the proper description for large screening. By comparison with first-principle simulations for three-dimensional spherical Yukawa crystals we demonstrate that both approximations complement each other. Together they accurately describe the density profile in the full range of screening parameters.

physics.plasm-ph↗