An innovative technique for combining two materials with unique electrical capabilities The best tool to date for studying a peculiar type of superconductivity termed topological superconductivity is a monolayer superconductor and a topological insulator. The resulting system might serve as the foundation for topological quantum computers that are more reliable than conventional ones.
Topological insulators are thin films only a few atoms thick that restrict the movement of electrons to their edges, which can result in unusual properties. Superconductors are thick films that allow the electric current to flow without resistance and are used in powerful magnets, digital circuits, and imaging devices. A team led by researchers at Penn State describe how they have paired the two materials in a paper appearing on Oct. 27, 2022, in the journal Nature Materials.
“The future of quantum computing depends on a kind of material that 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 challenging,” 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 insulator and superconductor films and create a two-dimensional heterostructure that is an excellent platform to explore the phenomenon of topological superconductivity.”
The superconductivity in thin films in earlier studies to mix the two materials typically vanishes once a topological insulator layer is developed on top. The properties of both materials might be preserved by adding a topological insulator sheet on a three-dimensional “bulk” superconductor, according to physicists. Topological superconductors would need to be used in two-dimensional systems for devices like cell phones or low-power circuits within quantum computers.
The future of quantum computing depends on a kind of material that 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 challenging.
Professor Cui-Zu Chang
In this study, the research group created a two-dimensional final product by stacking topological insulator films composed of bismuth selenide (Bi2Se3) with varying thicknesses over superconductor films built of monolayer niobium diselenide (NbSe2). The team was able to maintain the heterostructures’ topological and superconducting characteristics by synthesizing them at very low temperatures.
“In superconductors, electrons form ‘Cooper pairs’ and can flow with zero resistance, but a strong magnetic field can break those pairs,” said Hemian Yi, a postdoctoral scholar in the Chang Research Group at Penn State and the first author of the paper.
“The monolayer superconductor film we used is known for its ‘Ising-type superconductivity,’ which means that the Cooper pairs are very robust against the in-plane magnetic fields. We would also expect the topological superconducting phase formed in our heterostructures to be robust in this way.”
The researchers discovered that the heterostructure changed from Ising-type superconductivity, where the electron spin is perpendicular to the film, to “Rashba-type superconductivity,” where the electron spin is parallel to the film, by subtly changing the thickness of the topological insulator. In the researchers’ theoretical calculations and simulations, this behavior is also seen.
The study of Majorana fermions, an illusive particle that would significantly increase the stability of a topological quantum computer compared to earlier models, could also be facilitated by this heterostructure.
“This is an excellent platform for the exploration of topological superconductors, and we are hopeful that we will find evidence of topological superconductivity in our continuing work,” said Chang.
“Once we have solid evidence of topological superconductivity and demonstrate Majorana physics, then this type of system could be adapted for quantum computing and other applications.”
In addition to Chang and Yi, the research team at Penn State includes Lun-Hui Hu, Yuanxi Wang, Run Xiao, Danielle Reifsnyder Hickey, Chengye Dong, Yi-Fan Zhao, Ling-Jie Zhou, Ruoxi Zhang, Antony Richardella, Nasim Alem, Joshua Robinson, Moses Chan, Nitin Samarth, and Chao-Xing Liu. The team also includes Jiaqi Cai and Xiaodong Xu at the University of Washington.
This work was primarily supported by the Penn State MRSEC for Nanoscale Science and also partially supported by the National Science Foundation, the Department of Energy, the University of North Texas, and the Gordon and Betty Moore Foundation.
