Abstract
Since the discovery of high-temperature superconductivity in cuprates, Josephson junction based phase-sensitive experiments are believed and used to provide the most convincing evidence for determining the pairing symmetry. Regardless of different junction materials and geometries used, quantum tunneling involved in these experiments is essentially a nanoscale process, and thus, actual experimental results are extremely sensitive to atomic details of the junction structures. The situation has led to controversial results as to the nature of the pairing symmetry of cuprates: while in-plane junction experiments generally support -wave pairing symmetry, those based on out-of-plane (-axis) Josephson junctions between two rotated cuprate blocks favor -wave pairing. In this work, we revisit the -axis experiment by fabricating Josephson junctions with atomic-level control in their interface structure. We fabricate over 90 junctions of ultrathin (BSCCO) flakes by state-of-the-art exfoliation technique and obtain atomically flat junction interfaces in the whole junction regions as characterized by high-resolution transmission electron microscopy. Notably, the resultant uniform junctions at various twist angles all exhibit a single tunneling branch behavior, suggesting that only the first half of a unit cell on both sides of the twisted flakes is involved in the Josephson tunneling process. With such well-defined geometry and structure and the characteristic single tunneling branch, we repeatedly observe Josephson tunneling at a nominal twist angle of 45°, which is against the expectation from a purely -wave pairing scenario. Our results strongly favor the scenario of a persistent -wave order parameter in the junction.
- Received 13 March 2020
- Accepted 20 May 2021
DOI:https://doi.org/10.1103/PhysRevX.11.031011
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
The mechanism underlying high-temperature superconductivity has been the subject of much research since its discovery more than three decades ago. One aspect of this puzzle is understanding precisely how electrons pair up to form the Cooper pairs that give rise to unimpeded electrical current: In an -wave superconductor, the paired electrons have a net orbital angular momentum of zero; in a -wave superconductor, the net angular momentum is 2. Here, we present experimental evidence for -wave pairing in a copper-oxide-based, or cuprate, superconductor.
-wave and -wave pairings can be distinguished with the help of a Josephson junction, where two superconductors are weakly coupled together. In this setup, a current can flow without any applied voltage. If two -wave superconductors are stacked vertically and twisted by 45°, the current should vanish; in the same setup for two -wave superconductors, the current should flow.
We stack two ultrathin flakes of a typical cuprate superconductor on top of each other to form a Josephson junction. By isolating the Josephson current at the atomically flat interface, we observe the same current at twist angles of 0° and 45°. Our results are incompatible with trivial explanations such as disorder and strongly support the existence of an -wave pairing.
These results are essential to resolving the pairing mechanism of high-temperature superconductivity in cuprates. In addition, the twisted cuprate Josephson junction may constitute the building block for future superconducting circuitry such as superconducting quantum interference devices.
