Bilayer Superconductor Junctions Exhibit Double Andreev Reflection and Normal Reflection Between Two Cones

Andreev reflection, a phenomenon where electrons transform into holes at a superconductor interface, typically behaves predictably, but new research demonstrates this process becomes surprisingly nuanced in a specific material: AA-stacked bilayer graphene. Wei-Tao Lu and Qing-Feng Sun, from Peking University, investigate how this reflection changes depending on the unique ‘cone’ structure within the graphene, revealing a dependence previously unobserved. Their work shows that by manipulating an interlayer potential, researchers can control whether Andreev reflection occurs as a ‘specular’ or ‘retro’ process, effectively directing electrons based on their cone characteristics. This control opens possibilities for spatially separating different types of charge carriers within the material and could lead to novel electronic devices that exploit these cone-dependent properties, offering a new avenue for manipulating and understanding superconductivity in graphene.

Cone-dependent retro and specular Andreev reflections in AA-stacked bilayer graphene Wei-Tao Lu and Qing-Feng Sun AA-stacked bilayer graphene, with its unique electronic band structure, presents exciting opportunities for exploring novel phenomena at interfaces with superconductors. This work theoretically investigates Andreev reflection, a process where an electron transforms into a hole upon encountering an interface, in a junction between AA-stacked bilayer graphene and a superconductor. Understanding this reflection in this system is crucial for developing new superconducting devices and exploring unusual quantum phenomena arising from bilayer graphene’s band structure. This research focuses on how the cone-like shape of the energy bands in bilayer graphene influences Andreev reflection, specifically examining both retro- and specularly reflected holes.

The research identifies that the anisotropic reflectivity strongly depends on the ‘cone’ characteristics of the material’s electronic structure. By adjusting the interlayer potential, the researchers demonstrate that reflection can be specular in one cone and retro in the other. Introducing an interlayer potential difference enables scattering between cones, allowing for double reflection between the two cones. Specular reflection between cones is possible across a broad range of potential values.

Graphene and Bilayer Graphene Literature Review

This document presents a comprehensive collection of references related to graphene, bilayer graphene, and related phenomena, serving as a detailed bibliography. It covers a vast range of topics within the field, providing a broad overview of current research and establishing the current state of knowledge. The extensive number of references indicates a thorough investigation of existing literature. The review covers fundamental properties of graphene and bilayer graphene, including their electronic, optical, and transport characteristics. It also focuses on the impact of stacking order, particularly the unique properties of AA-stacked bilayer graphene, AB-stacked bilayer graphene, and twisted bilayer graphene.

The review also explores graphene heterostructures and devices, including graphene/superconductor junctions and graphene-based transistors. Finally, it covers theoretical and computational studies, such as density functional theory, and specific phenomena like Andreev reflection and the quantum Hall effect. The review demonstrates a strong emphasis on AA-stacked bilayer graphene and graphene/superconductor interfaces, suggesting these areas are particularly active and important. The broad coverage and inclusion of recent research indicate a comprehensive understanding of the field and its rapid development.

Controlling Andreev Reflection via Cone Indexing

This research investigates Andreev reflection, the conversion of an electron into a hole, in a specific bilayer material combined with a superconductor. The study demonstrates that the behaviour of this reflection is strongly linked to the ‘cone’ characteristics within the material’s electronic structure. Specifically, the team found that by adjusting an interlayer potential, they could control whether the Andreev reflection is ‘specular’ or ‘retro’, and even induce different types of reflection for electrons originating from different cones. This control arises because different cones exhibit different reflection characteristics under specific potential conditions.

The team modelled Andreev conductance near critical potential values, confirming the potential for separating electrons based on their cone characteristics. The authors acknowledge that their calculations are based on a theoretical model and do not account for potential imperfections or disorder in real materials, which could influence the observed behaviour. Future work could explore the impact of these factors and investigate the potential for utilizing this cone-dependent Andreev reflection in novel electronic devices.