Silicon has been the backbone of modern technology for decades, serving as the foundation for everything from computer chips to image sensors in cameras. The rapid miniaturization of silicon-based components has driven remarkable progress in computing power and efficiency. However, as these components shrink to the atomic scale, fundamental physical limitations arise, making it increasingly difficult to continue this trend. Engineers and scientists are now looking beyond silicon, turning to a new class of materials that operate on entirely different principles—quantum materials.
These materials derive their unique properties from the fundamental rules of quantum mechanics, which govern the behavior of particles at the smallest scales. Unlike traditional semiconductors, quantum materials exhibit behaviors such as superconductivity, magnetism, and exotic electronic states that could revolutionize the future of computing, sensing, and communication. One of the most promising avenues for exploring these materials is through a powerful technique known as resonant inelastic X-ray scattering (RIXS).
Unlocking the Potential of Quantum Materials
Quantum materials differ from conventional ones because their electrons do not behave as they do in silicon or other traditional semiconductors. Instead, these electrons exhibit strong interactions, meaning that the behavior of one electron can significantly influence the others. This interaction leads to complex quantum states that could enable ultra-fast computing, advanced data storage, and highly sensitive sensors. However, before these materials can be integrated into real-world applications, scientists must first understand how they function at a fundamental level.
Mark Dean, a physicist at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory, is one of the researchers at the forefront of this effort. As the leader of the Dynamics and Control Group in Brookhaven’s Condensed Matter Physics and Materials Science Department, Dean and his colleagues are working to decode the behavior of quantum materials. Their tool of choice, RIXS, allows them to observe and manipulate these materials with unprecedented precision.
The Science Behind RIXS
Resonant inelastic X-ray scattering is a technique that provides deep insights into the inner workings of quantum materials. By bombarding a sample with ultrabright X-rays, scientists can induce tiny disturbances, or “excitations,” in the material. These disturbances alter the energy and momentum of the X-rays, and by measuring these changes, researchers can reconstruct how electrons within the material interact.
Steven Johnston, a professor of physics and astronomy at the University of Tennessee, explains the concept using a simple analogy: “It’s like a guitar string. When a musician presses down on different frets, they change the length of the string, altering the sound. By analyzing the frequency of the sound, we can determine the properties of the string. In the same way, RIXS allows us to ‘listen’ to the behavior of electrons and uncover hidden details about their interactions.”
Unlike traditional X-ray techniques that provide only partial glimpses of electron behavior, RIXS offers a complete picture of microscopic electron excitations. This ability to directly probe quantum effects makes RIXS an invaluable tool for studying materials that could form the foundation of future quantum computers, superconductors, and next-generation electronic devices.
State-of-the-Art RIXS Facilities
One of the world’s leading RIXS research facilities is the Soft Inelastic X-ray Scattering (SIX) beamline at the National Synchrotron Light Source II (NSLS-II), a DOE Office of Science user facility at Brookhaven Lab. The SIX beamline is specifically designed to capture extremely small energy changes at high resolution, making it one of the most powerful instruments for studying quantum materials.
Dean and his collaborators, including Matteo Mitrano from Harvard University, Steven Johnston, and Young-June Kim from the University of Toronto, have used this beamline to conduct groundbreaking research. Their work has led to the discovery of previously unknown behaviors in quantum materials, such as the formation of excitons—microscopic particle-like objects that play a crucial role in energy transfer within materials.
Beyond NSLS-II, other advanced X-ray sources like the Linac Coherent Light Source (LCLS) at DOE’s SLAC National Accelerator Laboratory are expanding the capabilities of RIXS even further. These facilities allow scientists to perform ultrafast measurements, capturing quantum states that last for mere trillionths of a second. This ability to study fleeting quantum phenomena is essential for developing next-generation technologies that operate at unprecedented speeds.
Probing Quantum Entanglement with RIXS
One of the most exciting frontiers in quantum research is quantum entanglement, a phenomenon where two or more particles become inseparably linked, even if separated by vast distances. Entanglement is the foundation of quantum computing and secure quantum communication, but directly observing it in materials has remained a challenge.
RIXS may soon change that. Researchers believe that this technique can provide direct proof of entanglement within quantum materials, allowing them to unlock new ways to harness this effect for practical applications. “Quantum has become a buzzword in recent years,” says Mitrano, “but we need to define precisely what makes a material ‘quantum.’ That’s why we are embarking on this journey to detect entanglement and determine how to control it.”
Manipulating Quantum States with Light
In addition to observing entanglement, RIXS is also being used to actively control quantum materials. Scientists can use laser pulses to manipulate a material’s quantum state and then observe these changes in real-time with RIXS. This approach enables researchers to explore entirely new states of matter that exist only for a fraction of a second but could revolutionize electronic and optical technologies.
“RIXS is giving us a new playground to understand how light can be used to manipulate materials at the quantum level,” explains Kim. These insights could lead to breakthroughs in quantum switches, advanced memory storage, and ultra-efficient energy transfer.
The Future of RIXS and Quantum Materials
The future of RIXS research looks incredibly promising. As X-ray sources become more powerful and detection methods improve, scientists will be able to probe even smaller energy shifts and observe quantum interactions in greater detail than ever before. The combination of RIXS with other techniques, such as neutron scattering and ultrafast laser spectroscopy, will provide a comprehensive understanding of quantum materials, accelerating their development for real-world applications.
Beyond computing, these materials could lead to ultra-sensitive sensors that detect minute changes in temperature, pressure, or magnetic fields. They could also pave the way for high-efficiency superconductors that transmit electricity without energy loss, revolutionizing power grids and electronic devices.
Despite its immense potential, the field of quantum materials research is still in its early stages. “The way X-rays interact with materials is incredibly complex,” notes Johnston, emphasizing the need for continued collaboration between experimentalists and theorists. But with rapid advancements in both experimental techniques and computational modeling, researchers are confident that they are on the brink of transforming the landscape of modern technology.
More information: M. Mitrano et al, Exploring Quantum Materials with Resonant Inelastic X-Ray Scattering, Physical Review X (2024). DOI: 10.1103/PhysRevX.14.040501