New hybrid structures could pave the way for more stable quantum computers

RHEED patterns during MBE growth. (a) Graphene bilayer substrate terminated 6H-SiC(0001). (b) Monolayer NbSe₂ film developed on bilayer graphene. (vs) 5 QL Bi₂Se₃/monolayer NbSe₂ heterostructure developed on bilayer graphene. Credit: Natural materials (2022). DOI: 10.1038/s41563-022-01386-z

A new way of combining two materials with special electrical properties – a single-layer superconductor and a topological insulator – provides the best platform yet for exploring an unusual form of superconductivity called topological superconductivity. The combination could provide the basis for topological quantum computers that are more stable than their traditional counterparts.

Superconductors – used in powerful magnets, digital circuits and imaging devices – allow electric current to flow without resistance, while topological insulators are thin films only a few atoms thick that restrict the movement of electrons towards their edges, which can result in unique properties. A team led by Penn State researchers describes how they combined the two materials in an article published Oct. 27 in the journal Natural materials.

“The future of quantum computing depends on a type of material we call a topological superconductor, which can be formed by combining a topological insulator with a superconductor, but the actual process of combining these two materials is difficult,” said Cui-Zu Chang. , Henry W. Knerr Early Career Professor and Associate Professor of Physics at Penn State and leader of the research team.

“In this study, we used a technique called molecular beam epitaxy to synthesize both topological and superconducting insulating films and create a two-dimensional heterostructure that provides an excellent platform to explore the phenomenon of topological superconductivity.”

In previous experiments aimed at combining the two materials, superconductivity in thin films usually disappears once a layer of topological insulator is grown on top. Physicists were able to add a topological insulating film to a three-dimensional “bulk” superconductor and retain the properties of both materials.

However, applications of topological superconductors, such as low-power chips inside quantum computers or smartphones, are expected to be two-dimensional.

In this paper, the research team stacked a topological insulating film made of bismuth selenide (Bi₂Se₃) with different thicknesses on a single-layer niobium diselenide superconducting film (NbSe₂), resulting in a two-dimensional end product. By synthesizing the heterostructures at very low temperatures, the team was able to retain both topological and superconducting properties.

“In superconductors, electrons form ‘Cooper pairs’ and can flow with zero resistance, but a strong magnetic field can break these pairs,” said Hemian Yi, postdoctoral researcher at Penn State’s Chang Research Group and first author. of the item. .

“The single-layer superconducting film we used is known for its “Ising-type superconductivity”, which means that the Cooper pairs are very robust against in-plane magnetic fields. We would also expect the topological superconducting phase formed in our heterostructures be robust in this way.”

By subtly adjusting the thickness of the topological insulator, the researchers found that the heterostructure shifted from Ising-type superconductivity – where the electron spin is perpendicular to the film – to another type of superconductivity called “Rashba-type superconductivity”. – where the electron spin is parallel to the film.

This phenomenon is also observed in the theoretical calculations and simulations of the researchers.

This heterostructure could also be a good platform for exploring Majorana fermions, an elusive particle that would go a long way toward making a topological quantum computer more stable than its predecessors.

“This is an excellent platform for exploring topological superconductors, and we hope to find evidence for topological superconductivity in our continued work,” Chang said. “Once we have strong evidence for topological superconductivity and demonstrate Majorana physics, this type of system could be suitable for quantum computing and other applications.”