Probing condensed matter order with nitrogen-vacancy center scanning magnetometry
The development of new scientific instruments often leads to the observation of new physical phenomena. In this thesis, I will describe a new imaging instrument aimed at uncovering and understanding electronic and magnetic phenomena arising on the nanometer length scale. In recent years, the nitrogen-vacancy center in diamond (NV) has emerged as a unique tool for sensing magnetic fields with high spatial resolution. We have extended these sensing capabilities to a wide temperature range and harnessed this temperature control to probe the emergence of nanoscale collective order in condensed matter systems. The NV center has properties that make it an ideal tool for this purpose: it is an atomic-sized defect with a spin that can be optically initialized and read-out, it is noninvasive, and it retains quantum coherence from cryogenic to room temperature. In this thesis, I will describe the design and construction of a variable temperature scanning microscope built around the sensing capabilities of single NV centers. I will also describe the design and fabrication of the NV sensors themselves. The scanning microscope consists of a confocal microscope for addressing NVs, an atomic force microscope housed in a closed-cycle optical cryostat, and single-crystal diamond probes containing NV centers. Scanning NV magnetometry excels in probing systems that require good magnetic field sensitivity and high spatial resolution. I will describe how we have applied this measurement technique to several condensed matter systems that display magnetic or electronic structure on the nanometer to micrometer scale. I will focus on our measurements of magnetic skyrmions, where we show that NV magnetometry can be used to simultaneously probe the structure and dynamics of chiral domain walls. And I will describe our measurements exploring different regimes of electron transport in graphene, where the NV is used as local probe of current density, in search for signatures of electron hydrodynamic behavior.