Superconductors allow the electric current to pass without resistance, while topological insulators are thin films only a few atoms thick that restrict the movement of electrons to their edges, resulting in unique properties. A research team at Penn State has found a new way to combine two materials with special electrical properties. Their method offers the basis for topological quantum computers that are more stable than their traditional counterparts.
Researchers in this study used a molecular beam epitaxy technique to synthesize a topological insulator and superconductor films. They then created 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. A topological insulator sheet was added to a three-dimensional “bulk” superconductor by physicists, preserving both materials’ characteristics. However, applications for topological superconductors, such as chips with low power consumption inside quantum computers or smartphones, would need to be two-dimensional.
In this study, researchers stacked a topological insulator film made of bismuth selenide (Bi2Se3) with different thicknesses on a superconductor film made of monolayer niobium diselenide (NbSe2), resulting in a two-dimensional end-product. By synthesizing the heterostructures at a very lower temperature, the team retained the topological and superconducting properties.
Hemian Yi, a postdoctoral scholar in the Chang Research Group at Penn State and the paper’s first author, said, “In superconductors, electrons form ‘Cooper pairs’ and can flow with zero resistance, but a strong magnetic field can break those pairs.”
“The monolayer superconductor film we used is known for its ‘Ising-type superconductivity,’ which means that the Cooper pairs are robust against the in-plane magnetic fields. We expect the topological superconducting phase formed in our heterostructures to be robust 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. This phenomenon is also observed in the researchers’ theoretical calculations and simulations.
This heterostructure could also be a good platform for exploring Majorana fermions. This elusive particle would significantly make a topological quantum computer more stable than its predecessors.
Cui-Zu Chang, Henry W. Knerr Early Career Professor and Associate Professor of Physics at Penn State, said, “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. Once we have solid evidence of topological superconductivity and demonstrate Majorana physics, this system could be adapted for quantum computing and other applications.”