Negative electronic compressibility in charge islands in twisted bilayer graphene

New report from the Stampfer group in cooperation with the Kennes group on negative electronic compressibility in twisted bilayer graphene

In a recent paper by the Stampfer and Kennes groups, the scientists report a phenomenon called “negative electronic compressibility,” which happens under specific conditions and offers new insights into the behavior of electrons in this material.

The main finding: The researchers observed that the compressibility of the electron gas in twisted bilayer graphene can become negative near certain insulating states. Compressibility, in simple terms, measures how the density of electrons changes in response to applied pressure. A negative compressibility means that as pressure increases, the electron density actually decreases, which is quite unusual.

The method: The key to this discovery was the use of naturally occurring twist-angle domains in the graphene samples. These domains create small regions, or “charge islands,” with unique properties. By measuring how these islands responded to changes in voltage, the researchers could determine their capacitance, a measure related to compressibility.

The relevance: The researchers found negative compressibility not only near states where the material is normally insulating (not conducting electricity) but also at a special “band insulating” state where the graphene’s bands are fully occupied. This unusual behavior is significant because it provides new insights into the interactions between electrons in graphene.

The study suggests that the observed negative compressibility might be due to a phenomenon called “Wigner crystallization,” where electrons form a regular, lattice-like structure because of their mutual repulsion. This finding opens up new possibilities for understanding and manipulating the electronic properties of graphene, which could lead to advancements in various technologies.

Publication: Negative electronic compressibility in charge islands in twisted bilayer graphene. Robin J. Dolleman, Alexander Rothstein, Ammon Fischer, Lennart Klebl, Lutz Waldecker, Kenji Watanabe, Takashi Taniguchi, Dante M. Kennes, and Florian Libisch et al. Phys. Rev. B 109, 155430 – Published 23 April, 2024
DOI: https://doi.org/10.1103/PhysRevB.109.155430