Kohn, Luttinger Superconductivity and Nematicity Interplay in Twisted Bilayer Graphene

The pursuit of superconductivity, the transmission of electricity with zero resistance, receives a boost from investigations into unconventional materials like graphene, a single layer of carbon atoms arranged in a hexagonal lattice. Researchers, including M. Yu. Kagan from National Research University Higher School of Economics, M. M. Korovushkin and V. A. Mitskan from the Kirensky Institute of Physics, and colleagues, explore how superconductivity arises not just in single layers, but also in stacked and twisted bilayer graphene structures. Their work demonstrates that a mechanism known as Kohn-Luttinger superconductivity, combined with other forms of ordering, can produce unusual properties in both superconducting and normal states, potentially paving the way for new materials with enhanced electrical conductivity and novel electronic behaviours. This detailed analysis of bilayer and twisted graphene reveals a complex interplay between superconductivity and other states, offering insights into designing future superconducting devices and understanding the fundamental physics of these materials.

It has been shown that the Kohn, Luttinger superconductivity mechanism, operating alongside other types of electronic ordering, can be realized in systems possessing a hexagonal lattice structure. Investigations into these systems in their normal, non-superconducting phase also yield valuable insights.

Superconductivity and Correlations in Twisted Graphene

This compilation details research into superconductivity and correlated electron behavior in graphene and other two-dimensional materials. The central theme is the exploration of unconventional superconductivity, particularly in graphene, twisted bilayer graphene, and related materials, including understanding the mechanisms driving superconductivity, the role of interactions between electrons, and how stacking order and twist angle influence material behavior. A significant portion of the research focuses on twisted bilayer graphene, specifically the “magic angle” where unique flat bands emerge and superconductivity is observed. The magic angle in twisted bilayer graphene is a key area of investigation, with researchers trying to understand why superconductivity emerges at this specific angle and the nature of the electron pairing. Flat bands, arising at the magic angle, are believed to enhance electron interactions and promote superconductivity. Evidence suggests that superconductivity in twisted bilayer graphene is unconventional, not following the standard BCS theory, with the pairing mechanism likely arising from interactions between electrons rather than vibrations within the material.

Some research indicates that twisted bilayer graphene can exhibit Mott insulating behavior at certain electron densities, demonstrating strong correlation effects, and the influence of strain and pressure on electronic structure and superconducting properties is also being investigated. Stacking different two-dimensional materials, such as graphene with hexagonal boron nitride, allows for tuning electronic properties and creating novel devices. The large number of references demonstrates the intense research activity in this rapidly evolving field with frequent new discoveries. The research involves a combination of theoretical physics, materials science, and experimental techniques, with twisted bilayer graphene becoming a central focus due to its unique properties and the emergence of superconductivity. This mechanism offers an alternative to conventional superconductivity, which relies on vibrations, and is particularly promising in systems where strong electron correlations are present. The team’s work builds upon previous investigations into superconductivity in graphene and bilayer graphene, extending the understanding of how electron behavior can lead to this remarkable state. The research reveals that the Kohn-Luttinger mechanism doesn’t simply appear in isolation, but interacts with other forms of electronic ordering within the material.

Specifically, the superconducting state can coexist and interplay with spin density waves, creating complex electronic phases. In twisted bilayer graphene, this interplay leads to the emergence of nematic superconductivity, a state where superconducting properties exhibit directional dependence, adding complexity and potential for novel applications. This discovery expands the range of materials where unconventional superconductivity might be achievable, with calculations showing that the critical temperature for superconductivity can be significantly enhanced under certain conditions. By manipulating the spin polarization of electrons or utilizing a two-band electronic structure, the critical temperature can reach 1 to 5 Kelvin, opening possibilities for practical applications, linked to the strengthening of the Kohn anomaly, which promotes attractive interactions between electrons.

Furthermore, the research demonstrates that the type of electron pairing depends on the material’s properties. In graphene, calculations predict the emergence of triplet pairing with specific symmetries, such as px + ipy and f-wave pairing, while other systems, resembling cuprate superconductors, favor d-wave pairing. This ability to tailor pairing symmetry through material design is a crucial step towards creating superconductors with optimized properties for specific applications, providing a theoretical framework for understanding and potentially engineering unconventional superconductivity in a range of materials.

Kohn-Luttinger Superconductivity in Graphene Structures

This research investigates the possibility of unconventional superconductivity in graphene, exploring mechanisms beyond traditional understandings of electron pairing. The team demonstrates that the Kohn-Luttinger mechanism, which transforms repulsive interactions into attraction, can contribute to superconductivity in various graphene configurations, including monolayer, bilayer, and twisted bilayer structures, significantly enhancing superconducting transition temperatures in idealized graphene systems, although its effectiveness is reduced when considering real-world complexities like interlayer repulsion and competing orderings. The study extends to examine the interplay between Kohn-Luttinger superconductivity and the formation of spin density waves, revealing that these two phenomena can coexist in twisted bilayer graphene, unlike in standard bilayer structures where they typically compete. However, the inclusion of realistic factors, such as imperfections and substrate interactions, diminishes the impact of the Kohn-Luttinger mechanism, potentially allowing other superconductivity pathways, like the electron-phonon interaction, to become dominant, with future work likely focusing on incorporating these complexities to gain a more complete understanding of superconductivity in graphene and to explore the potential for designing materials with enhanced superconducting properties.