Superconductivity in twisted graphene multilayers

Superconductivity, the phenomenon where electrical current flows without resistance, has long captivated scientists and engineers. This unique state of matter holds promise for revolutionizing technologies, from power grids to magnetic levitation. A recent study ¹ delves into the intriguing world of graphene-based superconductors, shedding light on how twisting multiple layers of graphene can influence their superconducting properties.

Graphene and its superconducting potential

Graphene, a single layer of carbon atoms arranged in a hexagonal lattice, is renowned for its exceptional electrical, mechanical, and thermal properties. Beyond its strength and conductivity, graphene has exhibited superconducting behaviour under specific conditions. Notably, in 2018, researchers discovered that when two layers of graphene are stacked with a slight rotational misalignment—referred to as “twisted bilayer graphene”—the material can transition into a superconducting state at low temperatures. This groundbreaking discovery opened new avenues for exploring superconductivity in graphene-based systems.

The role of twisting

The angle at which graphene layers are twisted plays a pivotal role in determining their electronic properties. When layers are misaligned by a “magic angle” of approximately 1.1 degrees, the electronic interactions between the layers create a unique energy landscape. This configuration leads to the formation of “moiré patterns,” large-scale interference patterns that significantly alter the material’s electronic behaviour. In twisted bilayer graphene, these moiré patterns have been linked to the emergence of superconductivity and other correlated electronic phases.

Why not more than two?

Building upon the insights from twisted bilayer graphene, researchers have extended their investigations to systems involving more than two layers. The study in question focuses on “twisted double bilayer graphene” and “helical trilayer graphene.” In twisted double bilayer graphene, two bilayer graphene sheets are stacked with a twist, while helical trilayer graphene consists of three layers twisted in a specific sequence. These configurations introduce new complexities and opportunities in understanding superconductivity.

The researchers employed advanced theoretical models to analyze how long-range charge fluctuations—variations in the distribution of electric charge over distances—affect superconductivity in these multilayer graphene systems. Their findings reveal that the critical temperature and the order parameter differ significantly between twisted double bilayers and helical trilayers on one hand, and twisted bilayer graphene on the other.

Notable differences

The temperature at which a material becomes superconducting, known as the critical temperature, differs significantly between the systems studied. Twisted double bilayer and helical trilayer graphene exhibit distinct critical temperatures compared to twisted bilayer graphene, suggesting that adding more layers and varying twist angles can tune the superconducting properties.

The order parameter is a measure that describes the state of the superconducting phase. Variations in this parameter among the different graphene configurations indicate that the nature of the superconducting state changes with the number of layers and their arrangement.

The study highlights the role of moiré Umklapp processes, interactions where electrons scatter in a manner influenced by the moiré pattern. These processes differ between the systems, contributing to the observed variations in superconducting behaviour.

Future directions

Understanding how twisting and layering graphene affects its superconducting properties is more than an academic pursuit; it has practical implications for developing advanced materials and technologies. By manipulating the twist angles and stacking sequences, scientists can engineer graphene-based materials with tailored superconducting characteristics, potentially leading to more efficient energy transmission, novel electronic devices, and quantum computing components.

The exploration of superconductivity in twisted graphene multilayers exemplifies the dynamic interplay between fundamental research and technological advancement. As scientists continue to unravel the complexities of these materials, the potential for groundbreaking applications becomes increasingly tangible. This study not only deepens our understanding of superconductivity in graphene-based systems but also paves the way for future innovations in material science and electronic engineering.

Author: César Tomé López is a science writer and the editor of Mapping Ignorance

Disclaimer: Parts of this article may have been copied verbatim or almost verbatim from the referenced research paper/s.