Experiments with diamond anvil cells (DACs) that create extreme pressure conditions allow investigation of material behavior in environments like those deep inside the Earth and how new phases or superconductivity emerge. However, measuring pressure in a DAC can be complex. The pressure inside the tiny sample chamber can vary from one place to another, and the stress is typically biased in a particular direction (i.e., nonhydrostatic). As a result, spatial pressure distributions and stress anisotropy cannot be adequately evaluated from local measurements alone, making it difficult to assess intrinsic material changes.　Techniques that precisely measure physical quantities based on principles of quantum mechanics are referred to as quantum sensing. In this study, we used nanodiamonds containing nitrogen-vacancy (NV) centers as quantum sensors. We applied a spin-sensitive optical technique called optically detected magnetic resonance (ODMR), in which a microwave frequency is swept under optical illumination and the intensity of photoluminescence is measured as a function of frequency. The local pressure can be determined because the resonance frequency shifts with pressure. Furthermore, we can also obtain a two-dimensional pressure map from the ODMR spectrum at each camera pixel using wide-field microscopy.

This method provides information beyond “pressure” as a single scalar quantity. By fitting the spectral shape, we can estimate the effective pressure component along the loading axis and that perpendicular to it . The ratio of these two quantities is a simple indicator of a stress anisotropy. When , the condition is nearly hydrostatic, whereas a larger indicates stronger uniaxial stress.

We compared pressure environments for different numbers of solid NaCl pressure-medium layers in contact with nanodiamond quantum sensors. In the single-layer configuration, NaCl was placed only on one side of the nanodiamond layer, making it easier for the nanodiamonds to come into direct contact with the diamond anvils. Consequently, the ODMR spectrum was broadened and became asymmetrical and varied widely, and reached a value of approximately 1.6–1.8 near the center of the chamber. In the double-layer configuration, the spectrum was nearly symmetrical, the pressure distribution narrowed to 17–20 GPa, and decreased to approximately 1.2–1.5. These results indicate that the environment approached hydrostatic conditions.

Thus, we used nanodiamond-based pressure imaging to visualize the pressure distribution inside a DAC with micrometer-scale resolution. This approach enables the evaluation of how the arrangement of the pressure medium affects nonhydrostaticity. This method can be extended to other pressure media and serve as a platform for conducting materials research under extreme conditions with higher reliability when combined with other NV-based sensing capabilities.

(Written by Ryotaro Suda on behalf of all authors.)

Share this topic

Fields