Authors: Xue, Ruolan
Source: Digital Access to Scholarship at Harvard (DASH) (Harvard University) (article, 2025-01-01)
Field: Materials Science — Materials Chemistry

Nano-scale Noise Spectroscopy for Materials Applications The rise of two-dimensional (2D) materials has opened new frontiers for exploring low-dimensional physics. These systems exhibit distinct electronic and spin behaviors compared to their 3D counterparts, and offer tunability through stacking, twisting, and thicknesses control. Yet, their micrometer-scale lateral size and atomic-scale thickness pose a major challenge for conventional condensed matter probes, calling for highly sensitive, local measurement techniques. In parallel, quantum information science has also made exciting technological advancements in the past decades. Quantum sensing, built on the precise control of coherent quantum systems, has become a powerful tool in precision metrology, biosensing, and condensed matter physics. Among various platforms, nitrogen-vacancy (NV) centers in diamond, originally developed for quantum networks, have emerged as a highly sensitive, non-invasive nanoscale probe for studying spin and charge dynamics. NV centers have already proven their value through pioneering studies of magnetic and electronic properties in quantum materials, often cross-validated by optical spectroscopy and electronic transport. These early efforts paved the foundation for using NV magnetometry as a standalone probe to uncover new physics. With a mature understanding of device fabrication, spin–sample coupling, and coherent control techniques, NV-based sensing now accesses previously unreachable regimes of spatial, spectral, and temporal resolution in condensed matter systems. This thesis is driven by the question: What makes NV centers uniquely powerful as magnetometers for condensed matter research? Specifically, how do their quantum coherence and sensitivity in momentum and frequency space enable access to new physical phenomena? The thesis presents work that spans both validation and discovery. First, we used NV AC magnetometry to measure the magnetic penetration depth in the 2D cuprate superconductor BSCCO, obtaining values and temperature scaling consistent with established results. This part is discussed in Chapter 5. More significantly, we uncovered a new regime of spin transport in atomically thin Heisenberg ferromagnets: magnon hydrodynamic transport. Through the coherent spin response of NV centers, we directly observed magnon second sound—an analog of collective density wave in fluid. This work also expands the experimental toolkit for studying fluctuation–dissipation phenomena in low-dimensional magnets with spin decoherence properties. This thesis begins with an overview of quantum sensing with NV centers and recent advances in understanding 2D magnetic materials. I then introduce NV noise spectroscopy—the central technique enabling the discovery of magnon hydrodynamics. Chapters 3 and 4 present experimental observations and theoretical insights into magnon hydrodynamic transport. Finally, Chapter 5 summarizes our independent BSCCO measurements and ongoing efforts toward wide-field NV methodologies, providing a complete record of our contributions to this emerging field. Magnetometer ; Coherence (philosophical gambling strategy) ; Quantum sensor ; Quantum ; Macroscopic quantum phenomena ; Quantum technology ; Spin (aerodynamics) ; Noise (video) ; Quantum imaging Diamond and Carbon-based Materials Research ; Chemical and Physical Properties of Materials ; Nanopore and Nanochannel Transport Studies