A Spin Active Diamond Anvil for High-Pressure Technologies – SADAHPT

Applying a pressure in the magnitude order of 100 GPa on a material leads to structural rearrangements and chemical bonds modifications. New resulting properties and many amazing results illustrate this wealth: the xenon atomic crystal is transformed into a metal at high temperature and with a high melting temperature; dense oxygen becomes a solid formed from O8 molecules; nitrogen forms a solid polymer; dense solid hydrogen has a rich polymorphism due to nuclear quantum interactions and its metallic atomic phase is predicted with remarkable properties such as superconductivity at room temperature. The consequences of high pressure on pure elements can be observed on other compounds under more modest pressure conditions thanks to a chemical pre- compression effect. This was recently illustrated by the observation of a record superconductivity temperature of 203 K for hydrogen sulfide H2S when this very simple compound is compressed to 150 GPa. This analogy with metallic hydrogen opens the possibility to have a super-hydride that would be superconducting at room temperature with metastability under ambient conditions. This progress is largely due to the possibility of reaching pressures in the order of 100 GPa, i.e. one million of atmospheres, in the laboratory, using diamond anvil cells (DAC). Many diagnostics have also been developed, either in synchrotron or in laboratory. In addition, the community is now large enough to explore many types of materials. However, high-pressure magnetic measurements remain difficult and are only accessible to a few groups. SADAHPT will develop a magnetic diagnostic which would make it possible to characterize materials at high pressure and which could detect the predicted superconductivity of hydrogen-rich elements. Different approaches are used to characterize the magnetic properties of a sample within a DAC: use a very small cell to fit the sampling chamber of a SQUID; integrate electric coils inside the DAC to detect the magnetic susceptibility of the sample; use synchrotron X-ray radiation either by circular magnetic X-ray dichroism or nuclear resonance scattering. But these measurements remain difficult to implement and their use beyond 100 GPa becomes extremely delicate. SADAHPT proposes a radically different approach based on the magnetic field sensitivity of "artificial atoms" created in one of the two anvils of the DAC. These elementary quantum systems are point defects in the diamond crystal, called NV centers, that are formed by a nitrogen-N impurity replacing a carbon atom bounded to a vacancy-V on an adjacent site of the carbon lattice. The magnetic resonance spectrum of these defects, which can be optically detected, makes it possible to determine in situ the magnetic field applied to them. It is then possible to measure the magnetization of the compressed material in the DAC that creates this magnetic field and detect the appearance of a superconducting phase using the Meissner effect. With the complementary expertise of LAC on magnetometry with NV centers, CEA-DAM and Institut Néel on high-pressure experiments, LSPM on the CVD synthesis of ultrapure or nitrogen-doped layers of single-crystal diamond, and LSI for modeling the NV properties using ab initio calculations, SADAHPT aims to demonstrate the potential of this technique and especially its reproducibility. Thus, a major technological lock for the characterization of new materials under very high pressure and the characterization of new physical phenomena under these extreme conditions will be lifted.