Low-frequency modes in two-dimensional Debye-Yukawa plasma crystals

In a Debye-Yukawa system, charged micro-particles interact with each other through a screened Coulomb potential phi = (Q/r)exp{-r /lambda (D)} where Q is the charge and lambda (D) is the screening length. In a strongly couple d Debye-Yukawa system (where Q is much larger than the magnitude of an elec tron charge), a plasma crystal may be formed. One important feature of such a strongly coupled system is the existence of transverse waves (or shear w aves) in low frequency modes. In this paper we present an analytic treatmen t of longitudinal (or compressional) and transverse low frequency modes in two-dimensional plasma crystals. Dispersion relations of the modes are obta ined for two-dimensional triangular lattices. It is also found that the mod es (the compressional and the transverse) are strongly damped near the 'neg ative dispersion' regime. Theoretical predictions agree quantitatively with recent experiments, and the effects of neutral gas damping are also discus sed. (C) 2001 Elsevier Science B.V. All rights reserved.