| Metadata | Details |
|---|
| Publication Date | 2020-04-01 |
| Journal | New Journal of Physics |
| Authors | K R Joshi, N. M. Nusran, M.A. Tanatar, K.Cho, S.L. Bud′ko |
| Institutions | Ames National Laboratory, Iowa State University |
| Citations | 23 |
| Analysis | Full AI Review Included |
- Core Discovery: A sharp, anomalous peak was observed in the zero-temperature London penetration depth ($\lambda$) within the superconducting (SC) dome of electron-doped Ba(Fe1-xCox)2As2 (Co-Ba122).
- QPT Coincidence: This peak occurs precisely at the optimal doping concentration ($x = 0.057$), which coincides with the extrapolated location of the Antiferromagnetic (AFM) Quantum Phase Transition (QPT) boundary (T = 0 K).
- Methodology: The measurement utilized highly sensitive, minimally-invasive optical magnetometry based on Nitrogen-Vacancy (NV) centers embedded in diamond, operating at a fixed temperature of 4.5 K.
- Universality Implication: This behavior is strikingly similar to that previously observed in the cleaner, isovalently-substituted, nodal superconductor BaFe2(As1-yPy)2 (P-Ba122), suggesting that the $\lambda$ peak is a universal manifestation of magnetic quantum fluctuations in iron-based superconductors, regardless of disorder level or gap structure.
- Theoretical Challenge: The existence of this sharp peak is surprising because theoretical models suggest that while critical fluctuations enhance $\lambda$, a QPT alone does not necessarily guarantee a distinct peak structure.
- Engineering Relevance: The NV magnetometry technique successfully probed fundamental SC parameters (Hc1 and $\lambda$) in small, complex materials, demonstrating its utility for characterizing quantum materials.
| Parameter | Value | Unit | Context |
|---|
| Measurement Temperature (Texp) | 4.5 | K | Fixed temperature for $\lambda$ and Hc1 measurements. |
| Optimal Doping Concentration (x) | 0.057 | Dimensionless | Location of the sharp peak in $\lambda(x)$ and extrapolated QPT. |
| Superconducting Tc Range (Co-Ba122) | 10 to 24 | K | Range across the studied doping concentrations. |
| London Penetration Depth ($\lambda$) Peak Value | ~300 | nm | Observed peak magnitude near optimal doping. |
| NV Center Sensor Material | Diamond | N/A | Electronic-grade single crystalline plate with [100] surface. |
| Diamond Plate Thickness | 40 | µm | Thickness of the NV-activated diamond sensor. |
| NV Center Activation Depth | ~20 | nm | Depth of the NV centers from the surface in contact with the sample. |
| Sample Geometry | Cuboid | N/A | Samples cleaved to ensure sharp edges and well-defined demagnetization factors. |
| Sample Thickness (Platelets) | Typically 50 | µm | Initial thickness of cleaved BaCo122 crystals. |
| P-Ba122 Scaling Factor (y/x) | 5.3 | Dimensionless | Factor used to match the phase diagrams of P-Ba122 and Co-Ba122. |
| Estimated $\lambda$ Change (T/Tc = 0.5) | < 10 | % | Relative change based on the two-fluid model, confirming fixed Texp is acceptable. |
- Crystal Synthesis: High-quality single crystals of Ba(Fe1-xCox)2As2 were grown using the self-flux solution growth technique.
- Compositional Analysis: Cobalt concentration ($x$) was precisely determined for each sample using Wavelength Dispersive Spectroscopy (WDS).
- Sample Preparation: Crystals were cleaved into thin platelets (typically 50 µm thick) and further shaped into cuboid samples with four sharp edges along the [100] and [010] tetragonal directions. Edge quality was verified via Scanning Electron Microscopy (SEM).
- NV Sensor Integration: A 40 µm thick diamond plate containing NV centers activated ~20 nm deep on one side was placed in direct contact with the (001) surface of the superconducting sample.
- Magnetic Induction Measurement: The local magnetic induction was measured using Optically Detected Magnetic Resonance (ODMR), monitoring the Zeeman splitting of the NV centers.
- Lower Critical Field (Hc1) Determination: The sample was cooled in zero magnetic field to 4.5 K. The applied magnetic field (H) was increased perpendicular to the sample face (along the c-axis). Hc1 was determined by detecting the onset of the first penetration of Abrikosov vortices (Hp) near the sample edge.
- Penetration Depth Calculation: The measured Hp was corrected using the effective demagnetization factor (N) for the cuboid geometry, and $\lambda$ was calculated using the formula relating Hc1 to the flux quantum ($\Phi$0) and coherence length ($\xi$): Hc1 = Hp / (1 - N).
- Quantum Sensing and Magnetometry: The NV center platform provides ultra-high spatial resolution and sensitivity for magnetic field mapping, enabling the development of advanced quantum sensors for non-invasive characterization of magnetic and superconducting materials at cryogenic temperatures.
- Superconducting Component Design: The precise measurement of the London penetration depth ($\lambda$) is critical for optimizing the performance of superconducting radio-frequency (RF) cavities and resonators, where surface impedance (directly related to $\lambda$) dictates energy loss.
- Cryogenic Electronics and Computing: Fundamental research into the interplay between quantum criticality and superconductivity informs the design of robust, high-Tc materials suitable for next-generation superconducting quantum interference devices (SQUIDs) and components in quantum computers.
- Advanced Materials Characterization: NV magnetometry offers a unique, minimally-invasive tool for probing local magnetic properties and homogeneity in complex, small-scale samples (e.g., thin films or micro-structures), which is essential for quality control in materials science R&D.
- Phase Transition Engineering: Understanding how disorder and quantum fluctuations affect the SC state allows engineers to tune material compositions to maximize critical parameters (like Tc or Hc1) near QPTs for practical applications.
View Original Abstract
Abstract Unconventional superconductivity often emerges in close proximity to a magnetic instability. Upon suppressing the magnetic transition down to zero temperature by tuning the carrier concentration, pressure, or disorder, the superconducting transition temperature T c acquires its maximum value. A major challenge is the elucidation of the relationship between the superconducting phase and the strong quantum fluctuations expected near a quantum phase transition (QPT) that is either second order (i.e. a quantum critical point) or weakly first order. While unusual normal state properties, such as non-Fermi liquid behavior of the resistivity, are commonly associated with strong quantum fluctuations, evidence for its presence inside the superconducting dome are much scarcer. In this paper, we use sensitive and minimally invasive optical magnetometry based on NV-centers in diamond to probe the doping evolution of the T = 0 penetration depth in the electron-doped iron-based superconductor Ba(Fe 1− x Co x ) 2 As 2 . A non-monotonic evolution with a pronounced peak in the vicinity of the putative magnetic QPT is found. This behavior is reminiscent to that previously seen in isovalently-substituted BaFe 2 (As 1− x P x ) 2 compounds, despite the notable differences between these two systems. Whereas the latter is a very clean system that displays nodal superconductivity and a single simultaneous first-order nematic-magnetic transition, the former is a charge-doped and significantly dirtier system with fully gapped superconductivity and split second-order nematic and magnetic transitions. Thus, our observation of a sharp peak in λ ( x ) near optimal doping, combined with the theoretical result that a QPT alone does not mandate the appearance of such peak, unveils a puzzling and seemingly universal manifestation of magnetic quantum fluctuations in iron-based superconductors and unusually robust quantum phase transition under the dome of superconductivity.