Quantitative nanoscale vortex-imaging using a cryogenic quantum magnetometer (1511.02873v1)

Abstract: Microscopic studies of superconductors and their vortices play a pivotal role in our understanding of the mechanisms underlying superconductivity. Local measurements of penetration depths or magnetic stray-fields enable access to fundamental aspects of superconductors such as nanoscale variations of superfluid densities or the symmetry of their order parameter. However, experimental tools, which offer quantitative, nanoscale magnetometry and operate over the large range of temperature and magnetic fields relevant to address many outstanding questions in superconductivity, are still missing. Here, we demonstrate quantitative, nanoscale magnetic imaging of Pearl vortices in the cuprate superconductor YBCO, using a scanning quantum sensor in form of a single Nitrogen-Vacancy (NV) electronic spin in diamond. The sensor-to-sample distance of ~10nm we achieve allows us to observe striking deviations from the prevalent monopole approximation in our vortex stray-field images, while we find excellent quantitative agreement with Pearl's analytic model. Our experiments yield a non-invasive and unambiguous determination of the system's local London penetration depth, and are readily extended to higher temperatures and magnetic fields. These results demonstrate the potential of quantitative quantum sensors in benchmarking microscopic models of complex electronic systems and open the door for further exploration of strongly correlated electron physics using scanning NV magnetometry.

Quantitative Nanoscale Vortex-Imaging Using a Cryogenic Quantum Magnetometer

The paper presented in the paper integrates the cutting-edge approach of using a cryogenic quantum magnetometer with single Nitrogen-Vacancy (NV) electronic spins in diamond to image superconducting vortices at the nanoscale. It provides quantitative magnetometry while maintaining a highly non-invasive design under conditions of varied temperatures and magnetic fields. This research addresses the limitations of earlier nanoscale imaging techniques that often suffered from restrictions regarding temperature, spatial resolution, or invasiveness.

Key Findings and Methodology

The researchers successfully used NV magnetometry to analyze vortices in YBa₂Cu₃O₇-δ (YBCO), demonstrating deviations from the prevalent monopole approximation and validating Pearl's analytic model for vortex stray fields. The NV sensor achieves a sensor-to-sample distance of approximately ten nanometers, providing high-resolution imaging of magnetic perturbations induced by superconducting vortices. This technique, due to its cryogenic functionality, offers novel opportunities for studying complex electron systems under conditions typical for type-II superconductors.

Implications and Analysis

The NV magnetometry used in this paper uniquely achieves quantitative measurements at the nanoscale. It allowed for precise determination of the local London penetration depth, yielding values consistent with previous reports through Pearl model fitting. The ability to distinguish between competing models for vortex stray fields offers potential utility in verifying theoretical predictions about superconducting conditions and behavior. This approach could lead to breakthroughs in understanding the microscopic mechanism driving phenomena such as the pseudogap phase of high-Tc superconductors or inhomogeneous superconducting properties on a nanoscale.

Future Directions

Future extensions of this NV magnetometry include investigating properties central to strongly correlated electron systems and potentially probing the pseudogap phase. With the advent of quantum sensing techniques leveraging spin-relaxometry and dynamical decoupling, the dynamic range of NV center-based magnetometers could be expanded. This would enable researchers to dive deeper into temporal aspects of vortex behavior, further illuminating the complex dynamics that occur within superconducting thin films.

Technical Insights

The paper surmounts formidable challenges concerning cryogenic operations, offering sensitivity improvements to the nanotesla per Hz-range through coherent spin manipulation and demonstrating a cryogenic setup that minimizes vibrational disturbances. The experimental configuration includes a combination of confocal and atomic force microscopy in a Helium bath cryostat, providing stability and precision in magnetic field imaging.

Overall, this research highlights significant advancements in quantum magnetometry and sets a path for further exploration in the realms of superconductivity and condensed matter physics. The implications for advancing technological applications, particularly in refining superconductor materials and deploying precise, low-scale sensors, are profound. This illumination of vortex behavior and the ability to define superconducting parameters with unprecedented accuracy underscores the paper's contribution to the field.