Capteurs quantiques pour la recherche de supraconducteurs à haute pression

Abstract

When high static pressure is applied to a material, the arrangement of its atoms and electrons can dramatically change to allow higher density, thus leading to the creation of new phases or modifying electronic and magnetic properties of solids. Pressures of tens of kilobars (gigapascals) or even megabars can be achieved in laboratories using Diamond Anvil Cells (DACs), at the expense of working with samples of minute size. Recent studies have claimed to have uncovered materials, super-hydrides, retaining superconductivity close to room temperature under extreme pressure conditions. However, performing reliable measurements of the hallmarks of superconductivity on these samples synthesized at high-pressure is remarkably challenging, sometimes leading to controversial results. In this thesis, we chose to pursue the development of a new high-pressure optical magnetic measurement method based on the adaptation of a mature quantum technology to the DAC: Nitrogen-Vacancy (NV) centers of diamond. We fabricate ensembles of these color centers inside the tip of a diamond anvil, using them as local quantum magnetic probes based on their spin-dependent photoluminescence properties. We show that NV centers can be used to spatially map the Meissner effect in the presence of a cuprate superconductor at high pressure and record the sample's critical temperature evolution with pressure up to 31 GPa. We then systematically study the NV centers' magnetic sensing capabilities at high pressure, identifying non-hydrostatic anvil stress as a limitation above 40 to 50 GPa. Finally, we propose an anvil micro-structuring strategy lifting this constraint and enabling efficient NV magnetometry at least up to 130 GPa, paving the way toward a future settlement of the super-hydride dispute by this new diagnosis method.