Low-field all-optical detection of superconductivity using NV nanodiamonds

Technical Summary: Low-field all-optical detection of superconductivity using NV nanodiamonds

Problem Statement
While Nitrogen-Vacancy (NV) centers in diamond have emerged as powerful quantum sensors for studying superconductors, many established techniques rely on microwave-induced optically detected magnetic resonance (ODMR) or advanced pulsed methods (e.g., $T_1$ and $T_2$ measurements). These approaches present significant limitations: high-power microwaves can alter the local properties of sensitive samples, and the implementation of microwave control can be intrusive or technically challenging. Furthermore, existing methods often struggle with samples possessing rough surfaces or complex geometries. There is a need for a non-invasive, microwave-free sensing modality capable of probing superconducting properties, such as the Meissner effect and critical parameters, with high spatial resolution.

Methodology
The authors demonstrate a microwave-free, all-optical sensing platform utilizing near-zero-field cross-relaxation magnetometry. The experimental setup involves:

• Sensors: Commercially available 140 nm carboxylated fluorescent nanodiamonds (NDs) containing NV centers, deposited via drop-casting onto YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) thin films (200 nm thick) and control glass substrates.
• Detection Scheme: A widefield microscope setup using a 515 nm laser for excitation and a photodiode for collecting red fluorescence. The system employs a "salt-and-pepper" technique where NDs are sprinkled over the sample surface.
• Measurement Principle: The method relies on cross-relaxation between NV spins and nearby paramagnetic impurities. This resonant energy exchange enhances the NV spin-lattice relaxation rate, appearing as a dip in fluorescence when the energy splitting of NV sublevels matches that of the impurity system. This effect is most prominent near zero magnetic field.
• Experimental Protocol:
  • Temperature Characterization: The cross-relaxation feature was characterized on glass substrates from 5 K to 294 K to establish baseline linewidth and contrast dependencies.
  • Critical Temperature ($T_c$) Measurement: A 1 mT magnetic field was modulated at 3 Hz (square wave) while cooling the YBCO sample. The amplitude of the fluorescence modulation was tracked using a lock-in amplifier.
  • Field Penetration Studies: Magnetic fields were swept linearly from 0 to 6.5 mT (forward) and back (backward) at various temperatures. Measurements were taken at both the center and the edge of the superconductor to observe flux expulsion and penetration.

Key Contributions and Results

1. Microwave-Free $T_c$ Determination: The study successfully measured the critical transition temperature of the YBCO film without microwaves. By monitoring the fluorescence modulation amplitude, the authors observed a sharp drop in signal as the sample cooled below the superconducting transition, indicating the expulsion of the modulated magnetic field (Meissner effect). Derivative analysis of the signal identified two transition peaks at 87.5 K and 88.2 K, consistent with the manufacturer's specifications and attributed to oxygen inhomogeneity.
2. Observation of the Meissner Effect and Flux Penetration: The technique detected the transition from the Meissner state to the mixed (vortex) state.
   • Center Measurements: At temperatures below $T_c$, the superconductor expelled the field up to a specific threshold. For instance, at 83 K, flux penetration began at approximately 3.64 mT, marked by a distinct change in fluorescence. Hysteresis was observed between forward and backward sweeps due to vortex pinning.
   • Edge Measurements: The study highlighted significant field enhancement at the sample edges. At 83 K, flux penetration at the edge occurred at a much lower field (~0.5 mT) compared to the center (3.64 mT), demonstrating the focusing of magnetic flux at the edges of the thin film.
3. Non-Invasive Surface Compatibility: The method proved effective on the YBCO thin film, demonstrating that the "salt-and-pepper" deposition of nanodiamonds allows for probing superconductors with rough or textured surfaces without requiring intimate, flat contact or invasive sample preparation.

Significance and Claims
The paper claims that this work establishes a minimally invasive, microwave-free alternative for probing superconducting materials. By leveraging nanodiamond fluorescence modulation driven by magnetic field variations, the authors demonstrate the ability to:

• Quantitatively measure critical parameters ($T_c$ and penetration fields) without the risk of microwave-induced sample heating or perturbation.
• Operate effectively on samples with rough surfaces, facilitating the study of flux vortices and critical phenomena in complex geometries.
• Bridge macroscopic measurements and microscopic physics in superconductivity research.

The authors note that while the current widefield approach averages signals over a 120 $\mu$m $\times$ 120 $\mu$m area, the method offers a versatile platform for local magnetic measurements. They suggest that future iterations could utilize confocal microscopy for higher spatial resolution, but the current widefield technique already opens possibilities for imaging superconducting properties over a broader field of view. The study does not claim to have solved all limitations of NV sensing but positions this specific all-optical, low-field approach as a practical and less intrusive tool for the field.