Spectral properties of spherically confined dusty plasma crystals

Spherically confined dust crystals have recently attracted large attention, see e.g. [1] and [2] for an overview. After the exploration of the ground state structure [3, 4, 5, 6] and of metastable states and their probability [7, 8, 9] a question of particular interest is the dynamical excitation spectrum. The normal modes are a key property describing the response of a finite system to external excitation and also the melting behavior, e.g. [10, 11]. Particularly interesting is the situation in dusty plasmas where the normal mode spectrum of two-dimensional crystals could be analyzed experimentally, e.g. [12, 13]. Here, we extend this analysis to three-dimensional spherical Yukwa crystals (Yukawa balls) and present experimental results. From the theory side, we recall the determination of the normal modes of confined systems. We concentrate on one of the key modes – the breathing mode (BM) describing uniform expansion and contraction of the whole system. Recently it was shown [14] that a “true” (i.e. uniform) BM does not exist in Yukawa systems. Instead, there may exist several similar modes which, however, all deviate either from uniform or radial motion of the particles. The brief derivation of Ref. [14] is reproduced here in a more detailed way. One of the important consequences – the possible existence of multiple monopole modes – is demonstrated. Finally, we take into account the effect of dissipation on the normal mode spectrum which is crucial for dusty plasmas.