title: Two-Dimensional Disorder in Black Phosphorus and Monochalcogenide Monolayers.
abstract: Ridged, orthorhombic two-dimensional atomic crystals with a bulk Pnma structure such as black phosphorus and monochalcogenide monolayers are an exciting and novel material platform for a host of applications. Key to their crystallinity, monolayers of these materials have a 4-fold degenerate structural ground state, and a single energy scale EC (representing the elastic energy required to switch the longer lattice vector along the x- or y-direction) determines how disordered these monolayers are at finite temperature. Disorder arises when nearest neighboring atoms become gently reassigned as the system is thermally excited beyond a critical temperature Tc that is proportional to EC/kB. EC is tunable by chemical composition and it leads to a classification of these materials into two categories: (i) Those for which EC ≥ kBTm, and (ii) those having kBTm > EC ≥ 0, where Tm is a given material's melting temperature. Black phosphorus and SiS monolayers belong to category (i): these materials do not display an intermediate order-disorder transition and melt directly. All other monochalcogenide monolayers with EC > 0 belonging to class (ii) will undergo a two-dimensional transition prior to melting. EC/kB is slightly larger than room temperature for GeS and GeSe, and smaller than 300 K for SnS and SnSe monolayers, so that these materials transition near room temperature. The onset of this generic atomistic phenomena is captured by a planar Potts model up to the order-disorder transition. The order-disorder phase transition in two dimensions described here is at the origin of the Cmcm phase being discussed within the context of bulk layered SnSe.
authors: Department of Physics, University of Arkansas , Fayetteville, Arkansas 72701, United States.
authors: Mehrshad
authors: M
authors: Mehboudi
authors: Department of Physics, University of Arkansas , Fayetteville, Arkansas 72701, United States.
authors: Alex M
authors: AM
authors: Dorio
authors: Department of Electrical and Computer Engineering, University of Illinois , Urbana, Illinois 61820, United States.
authors: Wenjuan
authors: W
authors: Zhu
authors: Department of Mechanical Science and Engineering, University of Illinois , Urbana, Illinois 61820, United States.
authors: Arend
authors: A
authors: van der Zande
authors: Department of Physics, University of Arkansas , Fayetteville, Arkansas 72701, United States.
authors: Hugh O H
authors: HO
authors: Churchill
authors: Departamento de Ingeniería Mecánica, Universidad del Norte , Barranquilla, Colombia.
authors: Alejandro A
authors: AA
authors: Pacheco-Sanjuan
authors: Department of Mathematical Sciences, University of Arkansas , Fayetteville, Arkansas 72701, United States.
authors: Edmund O
authors: EO
authors: Harriss
authors: Department of Physics, University of Arkansas , Fayetteville, Arkansas 72701, United States.
authors: Pradeep
authors: P
authors: Kumar
authors: Department of Physics, University of Arkansas , Fayetteville, Arkansas 72701, United States.
authors: Salvador
authors: S
authors: Barraza-Lopez
mesh_terms: D018011:Chalcogens / Q000737:chemistry*; D056004:Molecular Dynamics Simulation; D049329:Nanostructures / Q000737:chemistry*; D044367:Phase Transition; D010758:Phosphorus / Q000737:chemistry*; D013696:Temperature; D013816:Thermodynamics
chemical_list: D018011:Chalcogens; D010758:Phosphorus
keywords: 2D atomic materials; black phosphorus; layered monochalcogenides; molecular dynamics; phase transitions; structural degeneracies