Vorticity wave vortices through the superposition of normally propagating plane transverse waves in 2D harmonic crystals

Chen-Yu Siao^1*, Wei-Shuo Lo¹, Lin I¹

¹Department of Physics and Center for Complex Systems, National Central University, Taoyuan, Taiwan

* Presenter:Chen-Yu Siao, email:chenyusiao@g.ncu.edu.tw

The wave vortex is a localized coherent wave entity with a wave crest surface helically winding a screw dislocation filament with null amplitude and undefined phase. It has been widely observed in optical, acoustic, elastic, and quantum systems. In solids, the ordered structure can support the propagation of coherent transverse and longitudinal phonons, which can be thermally excited and deteriorated by stochastic thermal agitations. The recent experimental and numerical studies have demonstrate the direct observation of multiscale vortical phonons (vorticity waves, VWs) in the form of swirls with varying shapes and alternating signs of local vorticity, mainly contributed by the superposition of thermally excited transverse phonons. Vorticity wave vortices (VWVs) with vorticity wave crest helically winding around screw dislocation filament cores were also observed. Here, numerically using a simple system composed of two and four normally propagating periodic plane transverse waves in the two dimensional harmonic crystal, we try to lay a foundation to understand the experimental observations. By adjusting the wavelength and initial phase differences of different sets of these transverse waves, we uncover rich spatiotemporal dynamics and underlining mechanisms of VWVs, including pair generation, propagation, and pair annihilation of VWVs with opposite helicities. Similarly, when the transverse phonons are replaced by longitudinal phonons, divergence waves and aster phonon vortices emerge, whose dynamics are also discussed.

Keywords: wave vortex, phonon, screw dislocation