Tunable Topological Transitions Probed by the Quantum Hall Effect in Twisted Double Bilayer Graphene

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Physics

Background:

• Moiré systems offer tunable platforms for exploring exotic quantum phases.
• The displacement field (D) is critical for tuning electronic structures and topological properties in twisted double bilayer graphene (TDBG).

Purpose of the Study:

• To investigate D-tunable topological transitions in the quantum Hall phases of TDBG valence bands.
• To understand the impact of the displacement field on Landau level sequences and Berry phase in TDBG.

Main Methods:

• Experimental observation of quantum Hall phases in TDBG under varying displacement fields.
• Theoretical calculations to analyze Landau level sequences and Berry phase alterations.

Main Results:

• Observed alternating emergence of quantum Hall regions (full-filling and half-filling) attributed to D-induced Lifshitz transitions.
• Documented a shift in Landau level sequences (from 8N + 4 to 8N) in remote valence bands upon application of D.
• Unveiled an alteration in Berry phase linked to the observed Landau level transitions.

Conclusions:

• TDBG serves as an exemplary system for studying D-tunable topological transitions.
• The findings provide insights into the fundamental mechanisms driving topological phase changes in graphene-based moiré systems.