Strong Correlation Between Superconductivity and Ferromagnetism in an Fe-Chalcogenide Superconductor
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2021-08-20 |
| Journal | Nano Letters |
| Authors | Nathan J. McLaughlin, Hailong Wang, Mengqi Huang, Eric Lee-Wong, LunâHui Hu |
| Institutions | Brookhaven National Laboratory, Westlake University |
| Citations | 45 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research reports the direct observation and nanoscale imaging of superconductivity-induced ferromagnetism (SC-FM) in an iron-chalcogenide topological superconductor, FeTexSe1-x (FTS).
- Core Achievement: Clear evidence of the coexistence of superconductivity (SC) and ferromagnetism (FM) in exfoliated FeTe0.7Se0.3 (FTS) flakes (Tc = 14.5 K).
- Methodology: Utilized Nitrogen Vacancy (NV) centers in diamond as nanoscale quantum sensors for wide-field magnetic imaging and relaxometry.
- Key Finding (SC-FM): The induced magnetization (4ÏM) averages approximately 4 G below Tc, confirming the presence of an unconventional pairing mechanism and Time-Reversal Symmetry Breaking (TRSB).
- Imaging Capabilities: Successfully mapped the spatial distribution of supercurrents (Meissner screening effect) and reconstructed 2D maps of the London penetration depth (λ) and magnetization (M).
- Technique Validation: Demonstrated the NV ODMR techniqueâs ability to quantitatively separate the magnetic fields generated by supercurrents (BSU) and ferromagnetism (BFM) based on distinct spatial geometries.
- Material Significance: FTS is confirmed as an attractive platform for exploring the complex interplay among topology, SC, and magnetism in emergent quantum materials.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Superconducting Transition Temperature (Tc) | 14.5 | K | Characteristic Tc for the exfoliated FeTe0.7Se0.3 (FTS) flake. |
| FTS Flake Thickness | 140 | nm | Thickness of the van der Waals material flake used in the experiment. |
| NV Center Depth | ~5 | nm | Distance of NV centers below the diamond surface. |
| NV Center Density | ~1500 | /”m2 | Density used for ensemble NV wide-field imaging. |
| Maximum Supercurrent Density (Jsu) | ~1 | mA/”m2 | Reconstructed maximum density of circular supercurrents in FTS. |
| Average Induced Magnetization (4ÏM) | ~4 | G | Average value of spatially dependent magnetization measured at T < Tc. |
| Theoretical Maximum Magnetization (4ÏMmax) | 15 | Oe | Calculated upper limit assuming full charge carrier polarization. |
| External Magnetic Field (Bext) | ~60 | G | DC magnetic field applied to tilt the magnetization (39° relative to normal). |
| Measured Spin Susceptibility | ~0.1 | N/A | Measured at 12 K, approximately 200 times larger than the paramagnetic state value. |
| Rabi Oscillation Frequency (fRabi) | 4.0 to 6.0 | MHz | Range used to map the local microwave magnetic field (BAC). |
Key Methodologies
Section titled âKey Methodologiesâ- Sample Preparation and Integration: Exfoliated FTS flakes (~140 nm thick) were transferred onto a single-crystal diamond substrate containing near-surface NV centers (~5 nm deep). An on-chip Au stripline was fabricated to apply microwave (MW) currents for NV spin control.
- Supercurrent Imaging via NV Rabi Oscillations: NV wide-field microscopy was used to measure the Rabi oscillation frequency (fRabi). Changes in fRabi across Tc were used to map the internal microwave magnetic field (BSU) generated by circular AC supercurrents (Meissner screening effect).
- London Penetration Depth (λ) Reconstruction: The spatial distribution of BSU was analyzed using a machine learning model to reconstruct 2D maps of the London penetration depth (λ), revealing superconducting domain variations.
- Ferromagnetism Detection via Pulsed NV ODMR: The Optical Detection of Magnetic Resonance (ODMR) technique was employed to detect the static out-of-plane component (Bz) of the internal DC stray magnetic field.
- Separation of Magnetic Effects: Machine-learning assisted simulations were used to quantitatively separate the total measured field (Bz) into contributions from supercurrents (BSU) and ferromagnetism (BFM), confirming the characteristic âsign reversalâ feature of tilted magnetization (BFM).
- Confirmation of Unconventional Pairing: The observation of SC-FM provides experimental evidence for Time-Reversal Symmetry Breaking (TRSB) and the presence of spin triplet states or unconventional pairing symmetries in FTS.
- Magnon Detection (Relaxometry): NV relaxometry measurements were performed to detect fluctuating magnetic fields generated by thermal magnons, further corroborating the existence of superconductivity-induced ferromagnetism.
Commercial Applications
Section titled âCommercial Applicationsâ- Quantum Information Technologies: Developing hybrid quantum architectures by exploiting the spatially tunable coupling between NV centers and superconducting materials for next-generation, solid-state quantum devices.
- Topological Quantum Computing: Advancing the understanding of topological superconductors (like FTS) that are candidates for hosting Majorana fermions, essential for fault-tolerant quantum computation.
- Nanoscale Quantum Sensing: Commercialization of accessible, table-top NV quantum sensing and imaging platforms for high-resolution characterization of local electrical and magnetic behaviors in 2D van der Waals materials.
- Spintronics and Quantum Transport: Designing novel quantum materials and devices that leverage the coexistence of superconductivity and ferromagnetism to explore exotic spin and charge transport phenomena.
- Materials Characterization: Providing a versatile local probe for investigating complex material properties, such as domain structure, inhomogeneity, and unconventional pairing mechanisms in strongly correlated electron systems.
View Original Abstract
The interplay among topology, superconductivity, and magnetism promises to bring a plethora of exotic and unintuitive behaviors in emergent quantum materials. The family of Fe-chalcogenide superconductors FeTe<sub><i>x</i></sub>Se<sub>1-<i>x</i></sub> are directly relevant in this context due to their intrinsic topological band structure, high-temperature superconductivity, and unconventional pairing symmetry. Despite enormous promise and expectation, the local magnetic properties of FeTe<sub><i>x</i></sub>Se<sub>1-<i>x</i></sub> remain largely unexplored, which prevents a comprehensive understanding of their underlying material properties. Exploiting nitrogen vacancy (NV) centers in diamond, here we report nanoscale quantum sensing and imaging of magnetic flux generated by exfoliated FeTe<sub><i>x</i></sub>Se<sub>1-<i>x</i></sub> flakes, demonstrating strong correlation between superconductivity and ferromagnetism in FeTe<sub><i>x</i></sub>Se<sub>1-<i>x</i></sub>. The coexistence of superconductivity and ferromagnetism in an established topological superconductor opens up new opportunities for exploring exotic spin and charge transport phenomena in quantum materials. The demonstrated coupling between NV centers and FeTe<sub><i>x</i></sub>Se<sub>1-<i>x</i></sub> may also find applications in developing hybrid architectures for next-generation, solid-state-based quantum information technologies.