Abstract
We construct high-quality graphene-based van der Waals devices with narrow superconducting niobium nitride (NbN) electrodes, in which superconductivity and a robust fractional quantum Hall (FQH) state coexist. We find a possible signature for crossed Andreev reflection (CAR) across the superconductor separating two FQH edges. Our observed CAR probabilities in the particlelike fractional fillings are markedly higher than those in the integer and hole-conjugate fractional fillings and depend strongly on temperature and magnetic field unlike the other fillings. Further, we find a filling-independent CAR probability in integer fillings, which we attribute to spin-orbit coupling in NbN allowing for Andreev reflection between spin-polarized edges. These results provide a route to realize novel topological superconducting phases in FQH-superconductor hybrid devices based on graphene and NbN.
- Received 20 September 2021
- Accepted 4 May 2022
DOI:https://doi.org/10.1103/PhysRevX.12.021057
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Superconductivity and the fractional quantum Hall effect are two unique physical phenomena, each representing an extreme state of matter, that result from interactions of electrons. In superconductivity, these interactions induce electron pairings that produce perfect conductance; in the fractional quantum Hall effect, strong interactions break electrons into fractionalized charges. Combining these states can produce a new type of topological device that can be used in fault-tolerant quantum computing. However, superconductivity is typically destroyed by a magnetic field whereas the fractional quantum Hall effect requires it. Here, we present high-mobility nanodevices that combine both these extreme states of matter for the first time.
Our devices are based on graphene and a niobium nitride superconductor-electrical contact. A large magnetic field induces the fractional quantum Hall effect while the superconductivity in the electrode is preserved. Our devices show the conversion of interacting fractionalized charge carriers in a quantum Hall graphene to electron pairs in a conventional superconductor. An external electric field tunes the carrier density, revealing certain densities with increased conversion probability, one that also strongly depends on temperature and magnetic field. This observation suggests the presence of spin-orbit coupling in the superconductor, explaining the unexpected conversion of charges in a quantum Hall graphene to Cooper pairs in a conventional superconductor.
While topological devices are not yet able to perform fault-tolerant algorithms, our demonstration of charge fractionalization removes a long-standing barrier to topological quantum computing.
