Direct Observation of Mode-Coupling Instability in Two-Dimensional Plasma Crystals

Abstract

Dedicated experiments on melting of 2D plasma crystals were carried out. The melting was always accompanied by spontaneous growth of the particle kinetic energy, suggesting a universal plasma-driven mechanism underlying the process. By measuring three principal dust-lattice (DL) wave modes simultaneously, it is unambiguously demonstrated that the melting occurs due to the resonance coupling between two of the DL modes. The variation of the wave modes with the experimental conditions, including the emergence of the resonant (hybrid) branch, reveals exceptionally good agreement with the theory of mode-coupling instability.PACS numbers: 52.27.Lw, 52.27.Gr Strongly coupled complex (dusty) plasmas play an important role in the existing hierarchy of soft matter states [1]. Along with complex fluids and granular media as prominent examples of "regular" soft matter, complex plasmas represent natural model systems which enable remarkably broad interdisciplinary research. The characteristic length-and timescales in such systems are dramatically stretched (e.g., in complex plasmas -to hundreds of microns and tens of milliseconds, respectively). Therefore, numerous generic processes occurring in classical fluids or solids can be studied in greatest detail, at the most fundamental "atomistic" level [1][2][3]. Especially important here are 2D systems, where the complete information about all particles can be obtained at each moment of time. The current experimental capabilities make 2D complex plasmas ideal for comprehensive experimental studies of the atomistic dynamics [1,2,4,5].The investigation of atomistic processes that trigger the melting and crystallization is of particular interest. It is well known that the mechanisms resulting in the destruction of the long-range crystalline order in 2D systems can be very different from those operating in 3D crystals: The classical Kosterlitz-Thouless-HalperinNelson-Young (KTHNY) theory of 2D melting [6] predicts two consecutive phase transitions (with an intermediate, so called "hexatic phase" in between). The alternative theory [7] relates 2D melting with the formation of dislocation chains ("grain boundaries") percolating the system. These are generic melting mechanisms which can operate in very different strongly coupled systems [8][9][10][11][12].When studying generic classical phenomena occurring in regular liquids and solids, the relevance of a model system (be it colloidal suspensions, granular media, or complex plasmas) becomes crucial. Of course, careful analysis is required in the context of a given phenomenon (or, class of phenomena), but one can certainly identify essential common principles. In particular, the applicability of the Hamiltonial approach for the analysis of atomistic dynamics (for instance, to investigate kinetics of phase transitions) is one of these basic principles. This implies that non-Hamiltonian (non-conservative) behavior peculiar to model systems should play minor role at the relevant timescales [13].Non-Hamiltonial behavior of particle ensembl...

Mode couplings and resonance instabilities in finite dust chains

“…Using a point charge model [8,9,[19][20][21][22] to simulate the ion wake, it has been shown that resonance instability can be triggered when the two coupled wave modes intersect. This phenomenon was recently observed experimentally [18,[23][24][25] in the thermal fluctuation spectrum illustrated by a high-energy-density region at the intersection between the DLWs. This enhanced energy density corresponds to the resonance instability and can result in melting of the plasma crystal at low neutral gas pressures [23][24][25][26].…”

Kinetics of the melting front in two-dimensional plasma crystals: Complementary analysis with the particle image and particle tracking velocimetries

“…However, for the measurements in this experiment, which are an extension of previously reported investigations by Couëdel et al [15], it is necessary to have simultaneous measurements of particles in both the crystalline and melted regions. To accomplish this, a key feature of this experiment is the use of Particle Image Velocimetry (PIV) techniques to make measurements of the particle velocities and to make a clear determination of the transition region between the melted and crystalline zones of the experiment.…”

“…The melting of the two-dimensional plasma crystal was triggered by the mode-coupling instability [13-16, 22, 23]. This instability is driven by the coupling between two branches of the dust-lattice waves, as was predicted theoretically by Ivlev and Morfill [13] and discovered experimentally by Couëdel et al [15]. The instability is typically induced by decreasing the neutral pressure of a gasprovided a rf discharge power is sufficiently low [16]: The reduced pressure ensures low neutral gas friction (i.e., reduced damping of the microparticles), while low dis- Lower panel: Snapshot of a crystal 16.4 s after the beginning of the experiment during the melting phase.…”

Energy transfer between degrees of freedom of a dusty plasma system

“…So phenomenon and mechanisms of an energy transfer between degrees of freedom of a dusty plasma system are of great interest in the field of dusty plasma. One of such mechanisms [6,[18][19][20][21][22] is based on parametric resonance [3-5, 16, 23]. The charged dust particles oscillate in the sheath of the gas discharge.…”