Slow Plastic Creep of 2D Dusty Plasma Solids

Abstract

We report complex plasma experiments, assisted by numerical simulations, providing an alternative qualitative link between the macroscopic response of polycrystalline solid matter to small shearing forces and the possible underlying microscopic processes. In the stationary creep regime we have determined the exponents of the shear rate dependence of the shear stress and defect density, being α = 1.15 ± 0.1 and β = 2.4 ± 0.4, respectively. We show that the formation and rapid glide motion of dislocation pairs in the lattice are dominant processes.The direct in situ observation of dynamical processes in bulk condensed matter is not yet available with atomic resolution in both space and time. Femtosecond pumpprobe techniques can resolve atomic motion, atomic force microscopy can detect atoms on surfaces, diffraction methods provide information about the bulk structure, but as long no method can combine all the benefits of these techniques, we are limited to rely of phenomenological models and numerical simulations. Alternative experimental methods have already proven to be helpful for the qualitative understanding of classical collective phenomena. Charged colloids suspended in a liquid environment, and dusty plasmas (solid micron sized particles charged and levitated in gas discharge plasmas) are both interacting many-particle systems that show very similar properties to conventional atomic matter, but at time and distance scales easily and directly accessible with simple video microscopy techniques. Both methods provide insight into the microscopic (particle level) details of different phenomena. Colloid systems are characterized by over-damped dynamics, due to the liquid environment, which makes them well suited for structural and phase transition studies [1], while the weak damping in low pressure gas discharges makes dusty plasmas perfect for studies of wave-dynamics, instabilities, and other collective excitations [2].In material science and metallurgy, creep is the time dependent plastic strain at constant stress and temperature; therefore, it is a special type of plastic deformation of solid matter. In general it is a slow process driven by the thermally activated movement of dislocations (dislocation creep), vacancies (vacancy creep) or diffusion (Nabarro-Herring and Coble creep). The applied stresses are below the rapid yield stress resulting in atomic movements that are crystallographically organized. The applied temperatures are usually above 1 2 T m , where T m is the melting temperature. The time (t) evolution of the deformation (strain ε) at constant stress is often described by one of the empirical formulae(1)where ε 0 is the immediate strain and δ, ϑ and φ are creep coefficients [3]. After a short transient phase ("primary creep"), approximated as logarithmic (∼ δ ln t) or using Andrade's law (∼ ϑt 1/3 ), this describes a steady-state "secondary" creep, dominated by the last term, where the rate φ is determined by the balance of work hardening and thermal softening. Under such circumstances the ste...

TandFasymmetries inπ0photoproduction on the proton

“…The target asymmetry T agrees well with existing data [20], in particular with measurements of CBELSA/TAPS [4]. The F asymmetry was measured for the first time.…”

“…Therefore, larger discrepancies are not surprising. The other models used recent polarization data from CBELSA/TAPS [3][4][5][6] as well as our precise cross-section data [2] in their fits. Figure 8 shows the dependence on the center-ofmomentum energy W for selected angular bins and the comparison to the model calculations.…”

“…The differential cross sections for the γp → π 0 p reaction were obtained with unprecedented accuracy and full angular coverage at MAMI [2]. New measurements of beam and beam-target asymmetries were performed by the CBELSA/TAPS experiment at ELSA [3][4][5][6], the GRAAL experiment at the ESRF [7], and the CLAS experiment at JLab [8,9]. Measurements of the polarization of the recoiling proton were performed at JLab and MAMI [10,11].…”

Recent Results by the CBELSA/TAPS-Collaboration

“…Apart from the double polarization observable G presented here, the CBELSA/TAPS collaboration reported results on the polarization observables E [4], T , P, and H [5,6] as well which have been used in the new fits. A comparison of the multipoles E 0+ , E 2− and M 2− of the dominantly contributing resonances reveal the impact of the new data [26].…”

Analysis Tools for Next-Generation Hadron Spectroscopy Experiments

“…Another possibility is the analysis of data on meson production taken at relatively low energies where the number of contributing multipoles is restricted and only few observables are needed to construct the amplitudes. In [81], this method was exploited to determine the helicity amplitudes of the N (1520)3/2 − resonance from an energy-independent analysis of S-, P -, and D-waves in γp → pπ 0 . The second approach is the so-called energy-dependent partial-wave analysis.…”