title: Direct observation of melting in a two-dimensional driven granular system.
abstract: Melting is considered to be one of the most fundamental problems in physical science. Generally, dimensionality plays an important role in melting. In three-dimension, it's well known that a crystal melts directly into a liquid via a first-order transition. In two-dimension (2D), however, the melting process has been widely debated whether it is a first-order transition or a two-step transition with an intermediate hexatic phase. Experimentally 2D melting has been intensively studied in equilibrium systems such as molecular and colloidal crystals, but rarely been explored in non-equilibrium system such as granular materials. In this paper, we experimentally studied the 2D melting in a driven granular model system at single particle level using video recording and particle tracking techniques. Measurements of orientational/translational correlation functions show evidences that the melting is a two-step transition. A novel concept of orientational/translational susceptibilities enable us to clearly resolve the intermediate hexatic phase. Our results are in excellent agreement with the two-step melting scenario predicted by KTHNY theory, and demonstrate that the KTHNY melting scenario can be extended to non-equilibrium systems.
authors.affiliation: Centre for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China.
authors.forename: Xiaoyan
authors.initials: X
authors.lastname: Sun
authors.affiliation: Centre for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China.
authors.forename: Yang
authors.initials: Y
authors.lastname: Li
authors.affiliation: Centre for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China.|National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China.
authors.forename: Yuqiang
authors.initials: Y
authors.lastname: Ma
authors.affiliation: Centre for Soft Condensed Matter Physics and Interdisciplinary Research, Soochow University, Suzhou 215006, China.|Kavli Institute for Theoretical Physics China, CAS, Beijing 100190, China.
authors.forename: Zexin
authors.initials: Z
authors.lastname: Zhang