Complex charge density waves in simple electronic systems of two-dimensional III₂-VI₃ materials

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid-State Chemistry

Background:

• Charge density wave (CDW) is a quantum electronic state characterized by periodic modulation of electron density and lattice distortion.
• CDW formation is typically linked to Fermi surface nesting or electron-phonon coupling, with the CDW vector (Q_CDW) related to electronic susceptibility or phonon frequencies.

Purpose of the Study:

• To propose and investigate a new family of multiple charge density wave (CDW) orders in two-dimensional (2D) III₂-VI₃ van der Waals materials.
• To explore the unique mechanisms driving diverse CDW phases in these materials.

Main Methods:

• First-principles calculations were employed to investigate the electronic and vibrational properties of 2D III₂-VI₃ materials.
• Analysis focused on identifying large and flat imaginary frequencies in optical phonon branches and their role in CDW formation.

Main Results:

• A new family of multiple CDW orders, including chiral Star-of-David configurations, was identified in nine 2D III₂-VI₃ van der Waals materials.
• The materials exhibit large and flat imaginary phonon frequencies, facilitating diverse CDW phase formation.
• CDW transitions induce metal-to-insulator and insulator-to-insulator transitions, significantly increasing bandgaps due to enhanced electronic localization.

Conclusions:

• The study reveals a novel class of 2D materials exhibiting multiple CDW orders.
• The findings provide deeper insights into the origins and mechanisms of charge density wave phenomena.
• The unique electronic and vibrational properties of 2D III₂-VI₃ materials make them promising for future research in condensed matter physics.