In conjunction with our colleagues in the Ultrafast Nano-Optics Group, we use shallow NV centers to probe exciton spin relaxation and coherence dynamics in van der Waals heterostructures. Recent measurements on spin relaxation and coherence times using optical Kerr spectroscopy have demonstrated extremely long electron spin lifetimes.  Our setup consists of back-gated monolayer transition metal dichalcogenides exfoliated on a diamond substrate with shallow NVs implanted within a few nanometers of the diamond surface.

Since the NV electronic spin couples directly to both DC and AC magnetic fields, the energy splitting between the NV's Zeeman sub-levels behave as a sensitive probe of any spin polarization of the TMD excitons. In particular, we plan to use wide-field imaging to directly map out the vector magnetic field generated by circularly polarized excitation of the TMD. This is enabled by the fact that excitons in the K and K' valleys selectively couple to left and right circularly polarized light. In combination with strong spin-orbit coupling in the valence band, this enables one to excite both spin and valley polarized excitons by using a combination of polarization and frequency selection. The ultimate sensitivity of our approach is determined by a combination of spin coherence time and spin projection noise. We will utilize Ramsey spectroscopy, which maps the magnetic field generated by the excitons into a relative phase difference accumulated by the NV's electronic spin states. By alternating the TMD excitation between left and right circular polarization, we can perform spin-echo spectroscopy, which significantly enhances the NV sensitivity.

Since their introduction, diamond anvil cells (DACs) have become the most versatile approach to generating sustained high pressures inside the laboratory. By compressing a thin sample between two opposing diamonds, pressures over hundreds of gigapascal can be achieved. These systems provide a useful platform for realizing pressure-driven quantum phase transitions that can elucidate our understanding of quantum materials. However, owing to the sharp pressure gradient between the micron-scale sample chamber and its ambient surroundings in the laboratory, localized measurements of stress gradients, magnetic fields, or temperature are challenging. We propose the development of NV centers as a tool for pressure calibration and nanoscale sensing in DAC systems.

By preparing a diamond anvil with an ensemble of NV centers, one can study spatially resolved tensorial stress gradients inside the anvil over a millimeter scale. While these gradients have been simulated extensively, a complete experimental measurement has remained elusive. The systematic study of these stress gradients may inform the design of novel anvil geometries that are capable of sustaining new pressure regimes more reliably.

Using alternative but complementary sensing modalities, one can perform magnetometry and thermometry using a thin layer of NV centers near a diamond anvil cell sample chamber to study pressure-induced magnetic phase transitions. We are using this tool to construct spatially resolved pressure-temperature phase diagrams of skrymion materials, for which pressure has already proved to be an interesting order parameter for extending the thermodynamic stability region of the skyrmion phase.  The combination of nanoscale spatial resolution and simultaneous sensing of stress, magnetism, and temperature afforded by the NV center provides a rich and unexplored phase space for the study of quantum phases of matter.