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

TL;DR

This work tackles the challenge of probing superconductivity without microwave radiation, which can perturb delicate states. It introduces a microwave-free, all-optical cross-relaxation magnetometry scheme using NV centers in nanodiamonds placed on YBCO thin films and reads out fluorescence under near-zero magnetic-field modulation. The approach yields a $T_c$ near 87.5–88.2 K and maps the penetration field with Meissner screening and vortex-penetration signatures, including edge-enhanced fields and pinning-induced hysteresis. The method is minimally invasive and robust to surface roughness, with widefield readout enabling imaging over a sizable area; future work could push toward confocal localization and applications to vortex dynamics, heterostructures, and topological superconductors.

Abstract

Nitrogen-vacancy centers in nanodiamond offer a microwave-free, noninvasive platform for probing superconductors via near zero-field cross-relaxation magnetometry. We demonstrate this by depositing nanodiamonds on YBCO thin films to measure critical parameters: transition temperature and penetration field. This method leverages nanodiamond fluorescence modulation as a result of magnetic field variation with 1mT amplitude to observe the Meissner effect and field scans to measure the penetration field. The approach is minimally invasive and can be applied to superconducting samples with rough surfaces, facilitating the study of flux vortices and critical phenomena in complex geometries.

Figures (5)

• Figure 1: Schematic of the low-temperature widefield magnetic imaging setup and three different phases of high T$_c$ superconductor. (A) NDs of 140 nm size are deposited on top of YBCO, which is placed on the sample stage of the cryostat. The green laser light is directed onto the NDs and red fluorescence is collected on a photodiode, selected with a flip mirror. A signal generator is used to produce a square-wave signal which is fed into the power supply (not shown) of the magnetic coil as well as to the lock-in amplifier as a reference. (B) Schematic representation of the three characteristic states of a superconductor. In the normal state, magnetic field lines penetrate uniformly through the material. In the Meissner state (below the critical temperature and field), the superconductor exhibits perfect diamagnetism by expelling magnetic flux. In the vortex (mixed) state of a type‑II superconductor, magnetic flux penetrates the material as quantized vortices, each comprising a normal core encircled by superconducting currents.
• Figure 2: Cross-relaxation feature characterization for NDs. (A) The feature recorded at different temperatures; (B) contrast, linewidth, and the linewidth-to-contrast ratio of the cross-relaxation feature as a function of temperature. The gray area indicates the temperature region of interest for studying the superconductor. The temperature dependence of FWHM is fitted with linear dependence, The error bars are the fitting errors, which are not purely statistical.
• Figure 3: Lock-in amplifier output of the amplitude modulated fluorescence signal as a function of temperature. The magnetic field is modulated between 0 and 1 mT with 3 Hz and the resulting $R$ output from the lock-in amplifier is continuously acquired. The amplitude level is nearly zero below the superconducting transition temperature, and the level rises when the temperature reaches the transition temperature. Figure (B) shows the derivative of the acquired signal, which is fitted with a double Gaussian function. The peak position obtained from the fit is taken as a transition temperature.
• Figure 4: Plots for the local transition from the Meissner state to intermediate state at the center of the superconductor. The magnetic field is scanned in both forward (blue) and backward (red) direction at various temperatures including above and below the transition temperature.
• Figure 5: The amplification of the magnetic field at the superconductor edge. The external field was scanned in both forward (blue) and backward (red) directions across a range of temperatures, spanning regimes both above and below the superconducting transition temperature.