Ion wake induced mode coupling in a horizontal chain in complex plasmas

TL;DR

This study experimentally investigates ion wake induced mode coupling in a horizontal dust chain within complex plasmas, confirming simulation predictions and revealing resonance-driven instabilities and their relation to melting thresholds.

Contribution

It provides the first experimental verification of ion wake induced mode coupling and resonance instabilities in a horizontal dust chain, linking spectral features to physical phenomena.

Findings

Detection of a double spectral branch confirming mode coupling.

Observation of discrete instabilities at mode intersections.

Evidence that instabilities cause melting via the Lindemann criterion.

Abstract

Ion wake induced mode coupling is investigated experimentally for a horizontal dust chain formed in a complex plasma, verifying results from previous simulation. A double branch of faint spectral lines is detected in the mode spectra which verifies the predicted rule of mode coupling between the vertical z(j=i$\pm$1) modes and the longitudinal mode x(i). Discreet instabilities are observed as the branches of x- and z-modes intersect each other. The mode spectra in the vicinity of the instabilities exhibit enhanced energy density at specific coupled x and z modes, serving as direct evidence that these instabilities are caused by resonance between the coupled modes. The instability-induced melting threshold was found to obey the Lindemann criterion through analysis of the instantaneous relative interparticle distance fluctuation (IDF). The relation between mode spectra and dispersion relations was further studied by multiplying the mode spectra with a transition matrix connecting the bases of normal mode eigenvectors and Fourier series in k space. Typical dispersion relations corresponding to the longitudinal and out-of-plane transverse Dust Lattice Waves (DLWs) are obtained, which also exhibit characteristics unique to finite systems, including discrete bands and strong fluctuations in the energy density.