Twisted Bilayer Graphene Demonstrates Second Fermi Liquid Phase, Enabling Superconductivity with Small Fermi Surface at 0

Superconductivity in twisted bilayer graphene remains a significant puzzle in modern physics, and scientists continue to investigate the origins of this remarkable phenomenon. Jing-Yu Zhao and Ya-Hui Zhang, both from Johns Hopkins University, alongside their colleagues, now propose a new understanding of how superconductivity arises within this material. Their work identifies a unique state of matter, termed a ‘second Fermi liquid’, which exists before superconductivity emerges, and challenges conventional theories about electron behaviour. This research establishes, for the first time, a unified theoretical framework that explains both the unusual metallic state observed at higher temperatures and the emergence of two-gap nematic superconductivity at lower temperatures, potentially paving the way for the design of novel superconducting materials.

A metallic state with small pockets of available energy exhibits behaviour consistent with a second Fermi liquid (sFL) phase. At a specific electron density, the system displays a picture where two electrons primarily localise on certain sites and form a paired state, driven by interactions mediated by vibrations within the material. This sFL phase represents a strongly interacting state, demonstrating unique electronic properties under specific conditions.

Spin-Fermion Liquid Theory of Superconductivity

This research investigates a theoretical model for understanding the emergence of superconductivity in strongly correlated electron systems, where interactions between electrons are crucial. The authors explore a “slave-particle” approach, specifically a spin-fermion liquid (SFL) phase, as a possible route to superconductivity. This involves breaking down electrons into simpler particles to simplify the problem, aiming to explain how materials can conduct electricity with zero resistance at relatively high temperatures. The research utilizes the concept of “slave particles”, where an electron is conceptually divided into a spinon, carrying the electron’s spin, and a holon, carrying its charge.

The resulting state is a spin-fermion liquid (SFL), a highly correlated state differing from the standard model for metals. The study also employs the Hubbard model, a simplified representation of interacting electrons, and gauge theory, which describes forces arising from symmetries. The authors propose that the transition from the SFL phase to the superconducting phase is driven by the condensation of a pair of slave bosons, effectively creating Cooper pairs responsible for superconductivity. They describe how the interactions between the slave particles transform into the interactions between the superconducting Cooper pairs, suggesting this transition is a type of phase transition driven by the formation of topological defects. This research is significant because it provides a theoretical framework for understanding the emergence of superconductivity in strongly correlated electron systems. The authors’ model could help to explain the behaviour of high-temperature superconductors and could lead to the development of new materials with even higher superconducting transition temperatures, pushing the boundaries of our understanding of these systems.

Twisted Graphene’s Unusual Metallic State Explained

This research presents a new theoretical understanding of superconductivity in twisted bilayer graphene, focusing on the unusual metallic state that exists before the material becomes superconducting. Scientists have proposed a “second Fermi liquid” (sFL) phase, which differs from conventional metallic states by exhibiting small pockets of available energy rather than a large collection. This sFL phase is characterized by pre-existing pairing of electrons localized at specific sites, a pairing driven by interactions mediated by vibrations within the material. The team demonstrates that superconductivity emerges from this sFL phase as the pre-formed pairing transfers to mobile electrons, creating a smaller superconducting gap.

This framework successfully explains experimental observations of both a pseudogap, a suppression of electron density, and a two-gap structure in the superconducting state of twisted bilayer graphene, offering a unified explanation for these previously distinct phenomena. The sFL represents a strongly correlated metallic phase that goes beyond the standard theoretical framework used to describe metals, suggesting a novel state of matter with unique properties. The authors acknowledge a limitation in their model, noting it currently does not fully account for symmetry-breaking effects observed in some experiments. Future work will focus on incorporating these effects to refine the theory, particularly at lower temperatures. However, the current findings provide a significant step forward in understanding the complex interplay of electron correlations and superconductivity in this fascinating material.