Perfect and Controllable Nesting in Minimally Twisted Bilayer Graphene

Area of Science:

• Condensed Matter Physics
• Materials Science

Background:

• Parallel regions of a Fermi surface (FS) are known to drive electron fluid instabilities.
• In higher dimensions, only a fraction of the FS typically consists of parallel sheets, limiting nesting phenomena.

Purpose of the Study:

• To investigate the possibility of a completely nested Fermi surface in two-dimensional materials.
• To explore the role of interlayer bias in controlling Fermi surface nesting in twist bilayer graphene (TBLG).

Main Methods:

• Theoretical investigation of the electronic structure of twist bilayer graphene.
• Analysis of Fermi surface geometry and nesting properties under varying interlayer bias.
• Comparison with experimental scanning tunneling microscopy (STM) data.

Main Results:

• Demonstrated a phase in tiny-angle TBLG where the Fermi surface is entirely composed of nestable Fermi lines.
• Confirmed the existence of this completely nested FS phase in both ideal and relaxed TBLG structures.
• Established excellent agreement between theoretical predictions and recent STM observations of topological states.

Conclusions:

• Twist bilayer graphene offers the first example of a completely nested Fermi surface in a two-dimensional material.
• The geometry of the Fermi lines network is tunable via interlayer bias, enabling control over FS nesting physics.
• TBLG provides a novel platform for studying electron fluid instabilities driven by Fermi surface nesting.