UC Berkeley

Tuning pressure using diamond anvil cells (DACs) provides an interesting playground for exploring novel condensed phases and understanding important geophysical processes. However, the extension of conventional probes for materials characterization to extreme pressure environments is often constrained by large stress gradients and limited signal strengths. Here, I outline a novel platform for high pressure metrology in DACs based on the Nitrogen Vacancy (NV) color center in diamond. By integrating the NV center, a robust and versatile quantum sensor, directly into the diamond culet, we demonstrate in situ vector magnetometry and tensorial stress imaging with high sensitivity and diffraction-limited sub-micron spatial resolution. Proof of principle measurements demonstrate the platform’s versatility by imaging the dipole strength of Iron (Fe) across the α ↔ ε structural transition, mapping the full stress tensor across the culet surface, and exploring the pressure-temperature phase diagram of gadolinium (Gd). Applying this technique in synergy with piezoelectric displacement sensing and X-ray diffraction (XRD) to study the amorphization of chrysotile Serpentine, we observe phase transformational nanoseismicity driven by shear stress - a possible mechanism underpinning deep earthquakes. Furthermore, we explore surprising qualitative changes in the behavior of the NV center up to megabar pressures, motivating further studies to understand the spin-orbit, and spin-spin coupling in the excited electronic state and the inter-system crossing mechanism. We demonstrate the viability of using the NV center as a robust pressure calibrant capable of easily measuring spatial gradients in multiple stress components. By playing with different crystal cuts of diamond anvils, we extend continuous wave and pulsed NV sensing to pressures nearing ∼ 150 GPa achieving excellent signal contrast and sensitivity. Finally, we use simultaneous transport measurements and NV magnetometry to probe the dual hallmarks superconductivity in a superhydride material at megabar pressures. Our work constitutes the first spatially resolved measurement of the Meissner effect and provides clear evidence of flux trapping in these novel compounds.