Abstract
Vortices are topological defects of type-II superconductors in an external magnetic field. In a similar fashion to a quantum anomalous Hall insulator, quantum anomalous vortices (QAV) spontaneously nucleate due to orbital-and-spin exchange interaction between supercurrent and magnetic impurity moment without an external magnetic field. Here, we used scanning superconducting quantum interference device microscopy (sSQUID) to search for its signatures in iron-chalcogenide superconductor Fe(Se,Te). Under zero magnetic field, we found a stochastic distribution of isolated anomalous vortices and antivortices with flux quanta . By applying a small local magnetic field under the coil of the nano-SQUID device, we observed hysteretic flipping of the vortices reminiscent of the switching of ferromagnetic domains, suggesting locally broken time-reversal symmetry. We further observed vectorial rotation of a flux line linking a vortex-antivortex pair by manipulating the local field. These unique properties of the anomalous vortices satisfy the defining criteria of QAV. Our observation suggests an emergent quantum phase with spontaneously nucleated vortex-antivortex matter in an iron-based superconductor with nontrivial topological band structure.
- Received 29 August 2022
- Revised 8 November 2022
- Accepted 9 January 2023
DOI:https://doi.org/10.1103/PhysRevX.13.011046
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
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Superconducting Vortices Made Without Magnetic Fields
Published 27 March 2023
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Popular Summary
Vortices are whirlpools of dissipationless electrical current in a superconductor. Conventional vortices appear only in the presence of a magnetic field, with the sense of circulation determined by the direction of the field. However, quantum anomalous vortices (QAVs) appear spontaneously due to the quantum-mechanical interactions between the superconducting electrons and magnetic impurities in the superconductor. As no magnetic field is present, both senses of circulation may occur, leading to vortices and antivortices. QAVs represent a new phase of vortex matter that could host emergent particles. However, because observation requires extraordinarily sensitive instrumentation in a zero magnetic field environment, they have not been detected—until now. Here, we present the first direct observations of QAVs.
To image a QAV, we take advantage of the direct flux sensitivity of scanning superconducting quantum interference device microscopy (sSQUID), a type of highly sensitive magnetometry that relies on superconducting circuits. In a well-calibrated, zero-magnetic-field environment, we image magnetic flux from a sample of Fe(Se,Te), a superconductor with quantized energy levels in its vortices suitable for hosting QAVs. We directly observe QAVs and antivortices appearing spontaneously and randomly after repeated coolings. A closely situated QAV and antivortex show connected flux lines and bond into a pair. By applying a local field from the sSQUID probe, we tilt the pair and eventually switch their relative circulation. These are unique vortex behaviors due to the underlying quantum-mechanical interaction.
Our findings not only establish a new form of vortex matter but also suggest interesting venues for future investigation. Since vortices interact with each other, QAVs may exhibit collective ferromagnetism when their concentration is high enough, which will be useful as a low-power cryogenic memory. Furthermore, the efficient manipulation of QAVs may potentially enable their application in quantum information technology.