Emergence of charge density waves and a pseudogap in single-layer TiTe₂

Area of Science:

• Condensed Matter Physics
• Materials Science
• Nanotechnology

Background:

• Two-dimensional (2D) materials offer distinct properties compared to their 3D counterparts due to quantum confinement.
• Understanding the behavior of layered materials like titanium telluride (TiTe2) is crucial for nanoscale device development.

Purpose of the Study:

• To investigate the electronic properties and phase transitions of TiTe2 across different layer thicknesses, from single-layer to bulk.
• To explore the emergence and characteristics of charge density wave (CDW) order in monolayer TiTe2.

Main Methods:

• Angle-resolved photoemission spectroscopy (ARPES) to probe electronic band structure.
• Scanning tunneling microscopy and spectroscopy (STM/STS) to investigate surface properties and electronic states.

Main Results:

• Observation of a (2×2) charge density wave (CDW) order in single-layer TiTe2 with a transition temperature of 92 ± 3 K.
• Detection of a 28 meV pseudogap at the Fermi level in monolayer TiTe2 at 4.2 K.
• Absence of CDW transitions in two-layer and multi-layer TiTe2, despite its quasi-2D nature in bulk.

Conclusions:

• The unique CDW phenomenon in monolayer TiTe2 challenges existing models of CDW formation.
• Reduced dimensionality significantly impacts the electronic properties and phase transitions of TiTe2.
• Further research is needed to elucidate the mechanisms behind the observed CDW behavior in 2D materials.