In the realm of energy and materials science, a team of researchers from various institutions, including Penn State University, National Institute for Materials Science in Japan, and Université Grenoble Alpes in France, have made a significant stride in understanding multiterminal Josephson junctions. These junctions, which are a type of superconducting device, offer unique opportunities for exploring exotic superconducting and topological phenomena that are not possible with conventional two-terminal devices.
The researchers, led by Asmaul Smitha Rashid and Morteza Kayyalha, have directly observed Cooper quartet resonances in a graphene three-terminal Josephson junction (3TJJ). Cooper pairs are pairs of electrons that are responsible for superconductivity, and a quartet is a group of four such pairs. The observation of these resonances is a signature of correlated tunneling of two Cooper pairs across the device. Using tunneling spectroscopy, the team visualized how Andreev bound states (ABS), which are localized states that form at the interface between a superconductor and a normal material, evolve across a two-dimensional superconducting phase space. This space is controlled by the two independent phase differences in the 3TJJ.
The measurements revealed sharp local minima in the differential conductance spectra locked in a specific phase condition of superconducting phase variables. The resulting quantized trajectories around the compact torus of the superconducting phase variables reveal an underlying topological winding in the multipair transport. To interpret their results, the researchers developed a theoretical model that connects the observed quartet resonances to the coherent hybridization of multiple ABS branches. This hybridization is a hallmark of the rich pairing process enabled by multiterminal geometries.
The practical applications of this research for the energy sector are still in the early stages of exploration. However, the ability to engineer superconducting states and design topological band structures based on phase-controlled, higher-order superconducting transport could potentially lead to more efficient and robust superconducting devices. These devices could be used in a variety of applications, from energy transmission and storage to quantum computing and sensing. The research was published in the journal Nature Nanotechnology.
This article is based on research available at arXiv.