标题：Directobservation of melting in a two-dimensional driven granular system
作者：XiaoyanSun1, Yang Li1, Yuqiang Ma1,2 & ZexinZhang1,3
Melting is consideredto be one of the most fundamental problems in physical science. Generally,dimensionality plays an important role in melting. In three-dimension, it'swell known that a crystal melts directly into a liquid via a first-ordertransition. In two-dimensional (2D), however, the melting process has beenwidely debated whether it is a first-order transition or a two-step transitionwith an intermediate hexatic phase. Experimentally 2D melting has beenintensively studied in equilibrium systems such as molecular and colloidalcrystals, but rarely been explored in non-equilibrium system such as granularmaterials. In this paper, we experimentally studied the 2D melting in a drivengranular model system at single particle level using video recording andparticle tracking techniques. Measurements of orientational/translationalcorrelation functions show evidences that the melting is a two-step transition.A novel concept of orientational/translational susceptibilities enable us toclearly resolve the intermediate hexatic phase. Our results are in excellentagreement with the two-step melting scenario predicted by KTHNY theory, anddemonstrate that the KTHNY melting scenario can be extended to non-equilibriumsystems.