The (r3xr3)R30{\deg} honeycomb of silicene monolayer on Ag(111) was found to undergo a phase transition to two types of mirror-symmetric boundary-separated rhombic phases at temperatures below 40 K by scanning tunneling microscopy. The first-principles calculations reveal that weak interactions between silicene and Ag(111) drive the spontaneous ultra buckling in the monolayer silicene, forming two energy-degenerate and mirror-symmetric (r3xr3)R30{\deg} rhombic phases, in which the linear band dispersion near Dirac point (DP) and a significant gap opening (150 meV) at DP were induced. The low transition barrier between these two phases enables them interchangeable through dynamic flip-flop motion, resulting in the (r3xr3)R30{\deg} honeycomb structure observed at high temperature.
@article{arxiv.1212.3679,
title = {Spontaneous Symmetry Breaking and Dynamic Phase Transition in Monolayer Silicene},
author = {Lan Chen and Hui Li and Baojie Feng and Zijing Ding and Jinglan Qiu and Peng Cheng and Kehui Wu and Sheng Meng},
journal= {arXiv preprint arXiv:1212.3679},
year = {2015}
}