Measuring magnetism under very high-pressure conditions

M. Lesik, L. Toraille, L. Rondin, J.-F. Roch, (ENS Paris-Saclay, LAC/LUMIN), F. Occelli, P. Loubeyre, T. Plisson (CEA), J. Renaud, O. Salord, A Delobbe (Orsay Physics), T. Debuisschert (Thales Research and Technology)

The study of matter subject to very high pressures makes it possible to describe conditions inside a planet, and also to highlight remarkable properties that we would like to stabilize at ambient pressure. For example, the solubility of hydrogen in metals increases sharply with pressure until it forms compounds that are very rich in hydrogen: super hydrides. These compounds lose their electrical resistivity and become superconducting at record critical temperatures. The compound LaH10, for example, becomes superconducting at -25°C at 1.8 million atmospheres. The synthesis of these new high pressure materials takes place in diamond anvils, whose pointed shape (see figure) enable such high pressures. In addition, the transparency of diamond provides a window for optical measurements for visualization and spectroscopy, as well as for X-ray measurements using synchrotron radiation. Numerous techniques have been developed to characterize samples inside these cells. However, until now, the measurement of magnetic properties has remained very difficult.

In a project partially funded by PALM (the AIMHIP project), physicists from the Laboratoire Aimé Cotton (ENS Paris-Saclay/CNRS/University Paris-Saclay – now called LUMIN), in collaboration with CEA and Thales R&T, have overcome this difficulty by observing the response of point defects in the diamond crystal created on the head of one of the two anvils of the cell. These defects were realized by implanting nitrogen ions with a focused ion beam microscope designed with the company Orsay Physics. In the presence of a magnetic field, their luminescence varies, which can be observed by optical microscopy through the anvil. The technique has been validated by the observation of superconductivity in MgB2 by optically detecting the Meissner effect, which reflects the expulsion of the magnetic field from a superconducting material.The results of this study provide a much better understanding of the relationship between electronic, magnetic and structural properties as a function of pressure. In particular, it will allow the thorough study of numerous metal hydride compounds. The experimental data will enable the identification of materials whose superconductivity would persist under conditions close to ambient pressure. Finally, this magnetic detection could reveal quantum effects in metallic hydrogen at high pressure, in particular its room temperature superconductivity. The scientific and industrial implications of such a discovery are obviously considerable.

Diamond anvil cell used to study the behaviour of matter under very high pressure. One of the anvils incorporates crystal defects (red) on the surface in the vicinity of the sample (blue). The magnetic field applied to the cell (arrows) is expelled from the sample when it becomes superconducting. The change in the magnetic field lines is then detected in situ by the light emitted by the defects when excited by a green laser © Margarita Lesik (ENS Paris-Saclay)

M. Lesik, T. Plisson, L. Toraille, J. Renaud, F. Occelli, M. Schmidt, O. Salord, A. Delobbe, T. Debuisschert, L. Rondin, P. Loubeyre et J.-F. Roch. Magnetic measurements on micron-size samples under high pressure using designed NV centers, Science (2019)

Résultats obtenus dans le cadre du projet AIMHIP financé par le thème 1 du LabEx PALM et porté par Jean-François Roch (Thales)