Recently, superconductivity under high pressure in hydrogen-rich compounds, such as H3S (Tc=203K at 150 GPa) and LaH10 (Tc=253K at 200 GPa), has attracted extensive research interest due to their high superconducting transition temperatures (Tc). However, magnetic measurements of these materials are significantly challenging in terms of sensitivity and spatial resolution. Consequently, magnetometry under high pressure using nitrogen-vacancy (NV) centers has recently begun to be implemented. The NV center is a promising sensor and is expected to overcome this problem because of its properties, including its size and measurement principles. In this article, we conducted wide-field magnetic imaging using NV centers at low temperatures and pressures of several GPa. The details of the developed measurement system and its application to a cuprate superconductor are demonstrated. By employing our method, the visualization of the Meissner effect in hydride superconductors is anticipated.
Recently, superconductivity under high pressure in hydrogen-rich compounds, such as H3S (Tc=203K at 150 GPa) and LaH10 (Tc=253K at 200 GPa), has attracted extensive research interest due to their high superconducting transition temperatures (Tc). However, magnetic measurements of these materials are significantly challenging in terms of sensitivity and spatial resolution. Consequently, magnetometry under high pressure using nitrogen-vacancy (NV) centers has recently begun to be implemented. The NV center is a promising sensor and is expected to overcome this problem because of its properties, including its size and measurement principles. In this article, we conducted wide-field magnetic imaging using NV centers at low temperatures and pressures of several GPa. The details of the developed measurement system and its application to a cuprate superconductor are demonstrated. By employing our method, the visualization of the Meissner effect in hydride superconductors is anticipated.
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.
The spatial distribution of a magnetic field can be determined with micrometer resolution using widefield nitrogen vacancy (NV) center magnetic imaging. Nevertheless, reconstructing the magnetic field from the raw data can be challenging due to the degeneracy of the four possible NV axes and the tremendous amount of data. While a qualitative approach is sufficient for most analyses, a quantitative analysis offers deeper insight into the physical system. Here, we apply NV widefield magnetic imaging to a HgBa2Ca2Cu3O8+δ (Hg-1223) superconducting microcrystal at a pressure of 4 GPa. We fit the results with solutions from the Hamiltonian describing the NV center ground state and take into account the relative intensities of the resonances to determine the local magnetic field magnitude and angle. Thus, we reconstruct the temperature-dependent expulsion of the magnetic field due to the Meissner effect around the superconductor. By comparing the resulting parameters to Brandt's model, which describes the magnetic behavior of a type-II superconductor, we extract the critical current density jc. Overall, this work showcases the first widefield quantitative reconstruction of the Meissner effect under pressure and an optical method to study critical current density. Thus, it provides new insights into the application of NV magnetometry to superconductivity research at high pressures.
We demonstrate wide-field optically detected magnetic resonance (ODMR) under simultaneous high-pressure and high-temperature conditions using nitrogen-vacancy (NV) centers. Although NV-center magnetometry has been widely used for spatially resolved magnetic-field imaging, its application to extreme environments combining pressure and temperature remains challenging. In this work, we show that ODMR can be observed at 5 GPa and 500 K, demonstrating the feasibility of NV spin readout under such combined extreme conditions. We further perform wide-field ODMR of iron at 7 GPa and 500 K, where the stray magnetic field from the sample is spatially visualized through the pressure cell. These results establish NV-center magnetometry as a promising platform for imaging magnetic phenomena in materials under high-pressure and high-temperature environments.
The diamond anvil cell-based high-pressure technique is a unique tool for creating new states of matter and for understanding the physics underlying some exotic phenomena. In situ sensing of spin and charge properties under high pressure is crucially important but remains technically challenging. While the nitrogen-vacancy (NV) center in diamond is a promising quantum sensor under extreme conditions, its spin dynamics and the quantum control of its spin states under high pressure remain elusive. In this study, we demonstrate coherent control, spin relaxation, and spin dephasing measurements for ensemble NV centers up to 32.8 GPa. With this in situ quantum sensor, we investigate the pressure-induced magnetic phase transition of a micron-size permanent magnet Nd 2 Fe 14B sample in a diamond anvil cell, with a spatial resolution of ∼2 μm, and sensitivity of ∼20 μT/Hz 1/2. This scheme could be generalized to measure other parameters such as temperature, pressure and their gradients under extreme conditions. This will be beneficial for frontier research of condensed matter physics and geophysics.
Pressure has been established as a powerful way of tuning material properties and studying various exotic quantum phases. Nonetheless, measurements under pressure are no trivial matter. To ensure a stable pressure environment, several experimental restrictions must be imposed including the limited size of a sample chamber. These have created difficulties in assembling high-pressure devices and conducting measurements. Hence, novel sensing methods that are robust and compatible with high-pressure devices under pressure are highly in demand. In this review, we discuss the nitrogen-vacancy (NV) center in diamond as a versatile quantum sensor under pressure. The excellent sensitivity and superior resolution of the NV center enable exciting developments in recent years. The NV center has great potential in sensing under pressure, especially beneficial to magnetic-related measurements.
In modern-day quantum metrology, quantum sensors are widely employed to detect weak magnetic fields or nanoscale signals. Quantum devices, exploiting quantum coherence, are inevitably connected to physical constants and can achieve accuracy, repeatability, and precision approaching fundamental limits. As a result, these sensors have shown utility in a wide range of research domains spanning both science and technology. A rapidly emerging quantum sensing platform employs atomic-scale defects in crystals. In particular, magnetometry using nitrogen-vacancy (NV) color centers in diamond has garnered increasing interest. NV systems possess a combination of remarkable properties, optical addressability, long coherence times, and biocompatibility. Sensors based on NV centers excel in spatial resolution and magnetic sensitivity. These diamond-based sensors promise comparable combination of high spatial resolution and magnetic sensitivity without cryogenic operation. The above properties of NV magnetometers promise increasingly integrated quantum measurement technology, as a result, they have been extensively developed with various protocols and find use in numerous applications spanning materials characterization, nuclear magnetic resonance (NMR), condensed matter physics, paleomagnetism, neuroscience and living systems biology, and industrial vector magnetometry. In this chapter, NV centers are explored for magnetic sensing in a number of contexts. In general, we introduce novel regimes for magnetic-field probes with NV ensembles. Specifically, NV centers are developed for sensitive magnetometers for applications where microwaves (MWs) are prohibitively invasive and operations need to be carried out under zero ambient magnetic field. The primary goal of our discussion is to improve the utility of these NV center-based magnetometers.