To develop new, high-performance optical and electronic materials, scientists need to measure how material properties such as magnetism and strength change under extreme conditions. Diamond anvil cells have made it possible for scientists to safely re-create these conditions in the lab, but most conventional sensors cannot be used to measure the experimental results, because they cannot withstand the crushing forces inside a diamond anvil cell. A team of scientists led by the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and University of California, Berkeley (UC Berkeley) took advantage of the intrinsic sensing properties found in nitrogen-vacancy (NV) centers — atomic defects found in a diamond’s crystal structure — to develop a tool that can be used to perform experiments that are inaccessible to conventional sensors. The researchers engineered a thin layer of NV centers directly into a diamond anvil cell in order to image the phenomena taking place within the high-pressure chamber of the cell. First, they generated a layer of NV center sensors a few hundred atoms thick inside 0.1-carat diamonds. They then tested the NV sensors’ ability to measure the diamond anvil cell’s high-pressure chamber. The sensors glowed red when excited with laser light. By probing the brightness of this fluorescence, the researchers could determine how the sensors responded to small changes in their environment. What they found surprised them: The NV sensors indicated that the once-flat surface of the diamond anvil began to curve in the center under pressure. Professor Raymond Jeanloz and his team identified this phenomenon as “cupping” — a concentration of pressure toward the center of the anvil tips. At left, natural diamonds glow under ultraviolet light owing to their various NV centers. At right, a schematic depicting the diamond anvils in action, with NV centers in the bottom anvil. The NV sensors glow a brilliant shade of red when excited with laser light. By probing the brightness of this fluorescence, the researchers were able to see how the sensors responded to small changes in their environment. Courtesy of Norman Yao/Berkeley Lab and Ella Marushchenko. According to professor Norman Yao, scientists “had known about this effect for decades but were accustomed to seeing it at 20 times the pressure, where you can see the curvature by eye. Remarkably, our diamond anvil sensor was able to detect this tiny curvature at even the lowest pressures.” In another experiment, when a methanol/ethanol mixture underwent a transition from a liquid to a solid, the diamond surface turned from a smooth bowl to a jagged, textured surface. Mechanical simulations performed by professor Valery Levitas of Iowa State University and Ames Laboratory confirmed the result. “This is a fundamentally new way to measure phase transitions in materials at high pressure, and we hope this can complement conventional methods that utilize powerful x-ray radiation from a synchrotron source,” researcher Satcher Hsieh said. The researchers also used their NV sensors to capture magnetic “snapshots” of iron and gadolinium. In the case of iron, the researchers directly imaged the pressure-induced phase transition from magnetic phase to nonmagnetic phase by measuring the depletion of the magnetic field generated by a micron-size bead of iron inside the high-pressure chamber. With gadolinium, the researchers took a different approach. The researchers noted that the NV center sensors could flip into different magnetic quantum states in the presence of magnetic fluctuations. The electrons inside gadolinium move in random directions, generating a fluctuating magnetic field that the NV sensor can measure. The researchers postulated that by timing how long it took for the NV centers to flip from one magnetic state to another, they could characterize the gadolinium’s magnetic phase by measuring the magnetic noise emanating from the motion of the gadolinium. They found that when gadolinium is in a nonmagnetic phase, its electrons are subdued and its magnetic field fluctuations are weak. The NV sensors stayed in a single magnetic quantum state for nearly 100 microseconds. Conversely, when the gadolinium sample changed to a magnetic phase, the electrons moved around rapidly, causing the nearby NV sensor to swiftly flip to another magnetic quantum state and indicating that the gadolinium had entered a different magnetic phase. The researchers’ technique allowed them to pinpoint magnetic properties across the sample with submicron precision. The team hopes that its “noise spectroscopy” technique will provide scientists with a new tool for exploring phases of magnetic matter that can be used as the foundation for smaller, faster, and cheaper ways of storing and processing data. Now that they’ve demonstrated how to engineer NV centers into diamond anvil cells, the researchers plan to use their device to explore the magnetic behavior of superconducting hybrid materials, whose use could transform how energy is stored and transferred. They would also like to explore the application of their device outside of physics. “What’s most exciting to me is that this tool can help so many different scientific communities,” Hsieh said. “It’s sprung up collaborations with groups ranging from high-pressure chemists to Martian paleomagnetists to quantum materials scientists.” The research was published in Science (www.dx.doi.org/10.1126/science.aaw4352). For more information on this research, see Researchers Create Nanoscale Sensors to Better See How High Pressure Affects Materials, Iowa State University News Service, Dec. 31, 2019.
