Diamond’s latest Nature breaks the controversy!

Recently, Professor Huang Xiaoli and others from the School of Physics of Jilin University collaborated with the team of Professor Norman Yao from the University of California, Berkeley, Lawrence Berkeley National Laboratory, and Harvard University, and achieved significant results in experimental research on the superconductivity of superconducting hydride-rich compounds under high pressure. A breakthrough, using diamond nitrogen-vacancy center (NV center) quantum sensing technology, successfully achieved the Meissner effect experimental measurement of hydrogen-rich compounds under ultra-high pressure. This major research result was titled "Imaging the Meissner effect in hydride superconductors using quantum sensors" and was published in Nature on February 28, 2024. In the same Nature issue, Professor Yang Sen from the Hong Kong University of Science and Technology and others made a highlight review titled "Quantum sensor settles debate about superconductivity in hydrides".

Since the discovery of superconductivity of elemental Hg, superconductivity, especially room temperature superconductors, has been a dream pursued by scientists. Hydrogen-rich compounds under high pressure are regarded as potential room temperature superconductors and have therefore become the focus of research in related fields. The team of Professor Huang Xiaoli of Jilin University discovered a variety of new hydrogen-rich compounds using various experimental techniques at high temperature and high pressure. In particular, we were the first to obtain superhydrides CeH9 and CeH10 with a superconducting transition temperature exceeding 100K and a metal-like hydrogen sublattice below one million atmospheres. However, the experimentally reported room-temperature superconductors C-S-H and Lu-N-H were successively withdrawn by Nature, making people question the authenticity of hydrogen-rich compound superconductors. Therefore, researchers urgently need to obtain a complete chain of evidence for the high-temperature superconductivity of superconducting hydride, especially to verify its superconducting Meissner effect, in order to resolve the controversy surrounding hydride-rich superconductors.

Traditional measurement methods, such as induction coils and superconducting quantum interference devices, have limitations in terms of ultimate pressure, magnetic field sensitivity and spatial resolution. Therefore, developing new magnetic detection methods and achieving accurate magnetic measurements under ultrahigh pressure are urgent issues to be solved not only for superconducting hydride research, but also for the entire high-voltage research field. NV color centers are point defects inside diamond, which have excellent magnetic field sensitivity and spatial resolution and are emerging quantum sensors. Using NV color center measurement technology, the research team was able to achieve local measurement of the Meissner effect and magnetic flux capture in trace amounts of hydride-rich samples prepared under high temperature and high pressure conditions. This makes it possible to measure the zero resistance and Meissner effect of superconductivity at the same pressure.

In 2019, research teams from the United States, France, and China independently published papers in Science magazine, kicking off the application of NV color center sensing technology in the field of high-voltage research. Among them, Professor Norman Yao's team (Science 2019, 366, 1349–1354) measured the stress distribution on the diamond anvil surface under high pressure and characterized the magnetic transition of metallic iron/gadolinium with pressure/temperature. However, how to apply NV color center measurement technology with excellent sensitivity and resolution to the study of hydrogen-rich compound superconductors under ultrahigh pressure is a new challenge. In order to overcome this challenge, Professor Huang Xiaoli's team collaborated with Professor Norman Yao's team and selected cerium hydrogen compounds that are stable to lower pressures and have excellent superconducting properties as the research object. They innovatively proposed an experimental plan for electrical and magnetic characterization of the same sample under the same pressure.

In the experiment, the team used diamond in the cutting direction to maintain the symmetry of the NV color center along the pressing direction to the greatest extent. This enabled the research team to measure the absolute strength of the magnetic field at pressures above one million atmospheres, with a sensitivity of 35 μT/√Hz and a spatial resolution of sub-micron levels. In continuous mode, the fluorescence contrast reaches 15%; in pulse mode, the spin coherence time reaches 2.04 μs, and the Rabi oscillation frequency reaches 25 MHz.

Based on this technology, the research team measured the diamagnetic properties of high-temperature superconductor CeH9 samples at different positions through field cooling (FC) and zero field cooling (ZFC), and obtained results consistent with electrical measurements, which effectively verified the superconducting cerium Meissner effect of hydrides. In addition, hysteresis caused by magnetic flux pinning was also found in the superconducting sample. The results show that the magnetic field is unevenly distributed after superconducting transformation, which is consistent with the characteristics of superconductor repulsion of magnetic fields and proves the Meissner effect of superconducting samples. The magnetic detection technology based on NV color centers has excellent spatial resolution. The research team linearly scanned the diamagnetic distribution on the surface of the sample after superconducting transformation at a specific temperature and a uniform external magnetic field with a step size of 1 micron, achieving millions of Magnetic imaging above atmospheric pressure obtained the local diamagnetic characteristics of the inhomogeneous sample, thereby determining the spatial distribution of the CeH9 superconducting sample.

This research successfully achieved high-sensitivity and spatial resolution magnetic detection under million-atmosphere pressure conditions, opening up a broad research space for magnetic measurements of trace samples under extreme conditions.