[Paper Review] Emergence of charge density waves and a pseudogap in single-layer TiTe2

Peng Chen, Woei Wu Pai+9 more|May 14, 2018

2D Materials and Applications3 citations

TL;DR

This study investigates single-layer TiTe₂ using angle-resolved photoemission spectroscopy and scanning tunneling microscopy, revealing a (2×2) charge density wave order with a transition temperature of 92±3 K and a 28 meV pseudogap at the Fermi level at 4.2 K. Surprisingly, no such transitions occur in bilayer or multilayer phases, challenging conventional understanding of charge density wave formation in low-dimensional systems.

ABSTRACT

Two-dimensional materials constitute a promising platform for developing nanoscale devices and systems. Their physical properties can be very different from those of the corresponding three-dimensional materials because of extreme quantum confinement and dimensional reduction. Here we report a study of TiTe$_2$ from the single-layer to the bulk limit. Using angle-resolved photoemission spectroscopy and scanning tunneling microscopy and spectroscopy, we observed the emergence of a (2 x 2) charge density wave order in single-layer TiTe$_2$ with a transition temperature of 92 $\pm$ 3 K. Also observed was a pseudogap of about 28 meV at the Fermi level at 4.2 K. Surprisingly, no charge density wave transitions were observed in 2- and multi-layer TiTe$_2$, despite the quasi-two-dimensional nature of the material in the bulk. The unique charge density wave phenomenon in the single layer raises intriguing questions that challenge the prevailing thinking about the mechanisms of charge density wave formation.

Motivation & Objective

• To investigate the electronic properties of single-layer TiTe₂ and compare them with multilayer and bulk phases.
• To understand the origin and emergence of charge density wave (CDW) order in two-dimensional transition metal dichalcogenides.
• To explore the presence and nature of a pseudogap in single-layer TiTe₂ at low temperatures.
• To resolve the discrepancy between the CDW behavior in single-layer versus multilayer TiTe₂, which defies expectations from dimensional reduction.

Proposed method

• Angle-resolved photoemission spectroscopy (ARPES) was used to probe the electronic band structure and Fermi surface of single-layer TiTe₂.
• Scanning tunneling microscopy and spectroscopy (STM/STS) were employed to image the real-space charge density wave order and measure the local density of states.
• Temperature-dependent measurements were conducted to identify the critical transition temperature for CDW formation.
• Comparative studies were performed across single-layer, bilayer, and multilayer TiTe₂ to isolate the role of layer count in CDW and pseudogap emergence.
• Theoretical analysis using density functional theory (DFT) and tight-binding modeling supported the interpretation of experimental data.

Experimental results

Research questions

• RQ1 What is the nature of the electronic order in single-layer TiTe₂, and does it differ from its multilayer counterparts?
• RQ2 At what temperature does the (2×2) charge density wave transition occur in single-layer TiTe₂?
• RQ3 Is a pseudogap present at the Fermi level in single-layer TiTe₂, and how does it evolve with temperature?
• RQ4 Why does the charge density wave order emerge only in the single-layer limit despite the quasi-2D nature of bulk TiTe₂?
• RQ5 What mechanisms govern the formation of charge density waves in atomically thin transition metal dichalcogenides?

Key findings

• A (2×2) charge density wave order emerges in single-layer TiTe₂ with a transition temperature of 92±3 K, as confirmed by ARPES and STM.
• A pseudogap of approximately 28 meV is observed at the Fermi level in single-layer TiTe₂ at 4.2 K, indicating a partial gap in the electronic density of states.
• No charge density wave transitions are detected in bilayer or multilayer TiTe₂, despite their quasi-two-dimensional character.
• The CDW state in single-layer TiTe₂ is highly sensitive to dimensionality, suggesting strong electron correlation or lattice coupling effects unique to the monolayer.
• The absence of CDW in thicker phases challenges conventional models of CDW formation based on nesting or dimensionality, pointing to novel many-body effects in 2D systems.