Two-Dimensional Disorder in Black Phosphorus and Monochalcogenide Monolayers

Area of Science:

• Materials Science
• Condensed Matter Physics
• Crystallography

Background:

• Two-dimensional atomic crystals with Pnma structure, like black phosphorus and monochalcogenide monolayers, are novel materials.
• Their crystallinity is characterized by a 4-fold degenerate structural ground state.
• A single energy scale, EC, dictates their disorder at finite temperatures.

Purpose of the Study:

• To classify two-dimensional atomic crystals based on their thermal disorder and phase transition behavior.
• To investigate the relationship between elastic energy (EC), melting temperature (Tm), and the occurrence of order-disorder transitions.
• To explain the origin of the Cmcm phase in bulk layered SnSe.

Main Methods:

• Theoretical classification of materials into two categories based on EC and Tm.
• Utilizing a planar Potts model to capture the order-disorder transition phenomena.
• Analyzing the impact of chemical composition on EC and transition temperatures.

Main Results:

• Materials are categorized into those that melt directly (EC ≥ kBTm) and those with an intermediate order-disorder transition (kBTm > EC ≥ 0).
• Monochalcogenide monolayers with EC > 0 undergo a two-dimensional transition before melting.
• GeS, GeSe, SnS, and SnSe monolayers transition near room temperature due to their EC/kB values.

Conclusions:

• The order-disorder phase transition in two dimensions is a generic phenomenon in these materials.
• This transition is responsible for the Cmcm phase observed in bulk layered SnSe.
• Tunable EC offers a pathway to control the properties and applications of these novel atomic crystals.