Experimental observation of a first-order phase transition in a complex plasma monolayer crystal

Abstract

The formation and melting of a mono-layered charged dust particle crystal in a DC glow discharge Argon plasma is studied. The nature of the melting/formation process is established as a first order phase transition from the nature of the variations in the Coulomb coupling parameter, the dust temperature, the structural order parameter and from the existence of a hysteresis behavior. Our experimental results are distinctly different from existing theoretical predictions for 2D crystals based on the KTHNY mechanism or the Grain boundary induced melting and indicate a novel mechanism that is akin to a fluctuation induced first order phase transition that has not been observed before in complex plasmas. PACS numbers: 52.27.Lw, 52.35.Fp, 52.35.Sb Introduction.-The phase behaviour of two dimensional structures, particularly the nature of their melting transition, has long been a subject of theoretical and experimental interest and also the source of some controversy. Some commonly studied two dimensional structures are molecular monolayers formed by surfactants spread on a water layer [1], electrons on the surface of liquid Helium [2], colloidal suspensions of charged sub-micron spheres [3] and more recently single layer crystalline structures of charged micro-particles (dust) suspended in the electric sheath of a plasma [4][5][6][7]. Two dimensional structures have also been the subject of many computer simulations based on simple theoretical model systems [8][9][10][11]. The most well known theory for two dimensional melting is the one proposed by KTHNY [12] which describes the melting as a two stage transition process with an intermediate hexatic phase. The melting begins by forming dislocations and disclinations in the 2D crystalline structure and as a result the long range translational order breaks down and leads to a hexatic phase. The transitions from the solid to the hexatic and then on to the liquid phase are both continuous in nature constituting second order transitions. Other competing mechanisms of melting consist of the Grain-boundary induced (GBI) melting theory [13,14], the density wave theory [15] and the instability triggered theory [16]. The GBI melting [13,14] proceeds through long arrays of dislocations at the lattice boundaries that drive a first order phase transition from the crystalline phase directly to the liquid phase. Density wave theory [15] deals with the change of entropy and structure factor variations, but it fails to establish the order of the phase transition. Likewise simulation studies to date do not provide a definitive picture of the nature of the 2D melting phase transition and the question still remains open.The advent of dusty plasma crystals in recent times has provided a strong impetus to the experimental study of 2D melting since the process can be well diagnosed using non-perturbative techniques and a number of such studies have addressed this problem [4,6,7,[17][18][19]. In

Self-sustained non-equilibrium co-existence of fluid and solid states in a strongly coupled complex plasma system

“…The value of the effective coupling parameter in the ordered peripheral region is estimated to be 210, while in the disordered structure at the center, the value turns out to be 35. Both the values and the statement of phase coexistence in the observed system are in agreement with the threshold values obtained by Vaulina and Khrapak 44 and also with our earlier experiments 38 , 39 . The crystal-fluid phase coexistence of the complex plasma system is further confirmed by the Voronoi diagram based structural analysis.…”

“…As a result, the confinement area for the dust particles shrinks, which causes a few particles at the center of the mono-layer to move below the mono-layer as shown in Fig. 2 b 38 . These particles then interact strongly with the particles that reside in the mono-layer and form a fluid-like structure in the central region.…”

“…However, as soon as the threshold for the instability onset in dusty plasmas is density dependent in most cases, phase coexistence can be expected if the collection of particles is spatially nonuniform 37 . Spatial nonuniformity of structural and dynamical characteristics reveals in conditions of many experiments with dusty plasmas due to the action of central electrostatic confinement which keeps the system stable [38][39][40] . Such central trap can only be compensated by an inter-particle interaction force gradient and subsequent density gradient.…”

DPEx-II: A New Dusty Plasma Device Capable of Producing Large Sized DC Coulomb Crystals

“…A dusty plasma crystal having few dust particles doesn't support the excitation and propagation of the wave and hence a dust crystal of reasonably large dimension is indispensable to accommodate few wavelengths of the excited wave [26][27][28] . Additionally, studies like, void formation [14][15][16] , phase co-existence 29 , phase transition 7,30 , effect of spatial inhomogeneity of dust density on the dynamics of dust particles etc. cannot be investigated with few dust particles in the crystal 31,32 .…”

Shocks propagate in a 2D dusty plasma with less attenuation than that due to gas friction alone

“…Because of that large charge, the microparticles can behave as a strongly coupled plasma component, organizing in a crystalline microstructure. [9][10][11][12][13][14][15][16] Also because of that large charge, large amplitudes are easily attained when compressional waves propagate through a dusty plasma. Wave amplitudes are often great enough for the wave to become nonlinear, and in some cases they can have properties of a shock.…”