Scientists at Argonne National Laboratory have developed a new nanoscale technique, surface-sensitive spintronic terahertz spectroscopy (SSTS), to probe the quantum behaviour of materials at interfaces.
This breakthrough offers unprecedented insights into surface phonons and interfacial superconductivity, paving the way for advancements in quantum material and technologies
Scientists are in a global race to develop the next generation of quantum technologies, from ultra-powerful computers to incredibly precise sensors. However, translating these promising technologies from the lab to practical applications requires a much deeper understanding of how quantum materials behave, particularly at their surfaces and interfaces.
Now, researchers at the U.S. Department of Energy’s Argonne National Laboratory have unveiled a groundbreaking new technique that promises to revolutionise the study of these materials, paving the way for advancements in quantum computing and sensing. This innovation, called surface-sensitive spintronic terahertz spectroscopy (SSTS), offers an unprecedented glimpse into the quantum world at the interfaces between materials, unlocking secrets that could unlock the full potential of quantum technology.
Unveiling the secrets of surface phonons
At the heart of this breakthrough lies the concept of phonons – collective vibrations of atoms within a crystal lattice. While scientists have a relatively good understanding of phonons within the bulk of a material, the behaviour of surface phonons – those residing within nanometres of an interface – has remained largely a mystery.
The Argonne team’s research reveals that these surface phonons behave significantly differently from their bulk counterparts. They exhibit unique quantum behaviours, such as interfacial superconductivity. This discovery opens exciting new avenues for research and potential applications.
“This technique allows us to study surface phonons – the collective vibrations of atoms at a material’s surface or interface between materials,” explains Zhaodong Chu, a postdoctoral researcher at Argonne and the lead author of the study. “Our findings reveal striking differences between surface phonons and those in the bulk material, opening new avenues for research and applications.”
The promise of interfacial superconductivity
One of the most intriguing aspects of this research is its connection to interfacial superconductivity. Superconductivity, the phenomenon where electrons flow without resistance, has numerous applications, including MRI machines and particle accelerators. Interfacial superconductivity, a special type that only occurs at the boundary between two materials, holds immense promise for the development of novel quantum technologies.
Argonne physicist Anand Bhattacharya explains the origin of this research: “The idea for this discovery began with the finding some years ago that interfaces between two crystalline materials can exhibit superconducting behaviour neither one shows on its own. It is only when the two materials are together that the superconductivity magic happens at the interface, which is different from the bulk.”
Overcoming the challenges of terahertz radiation
The team hypothesised that a specific type of vibration, known as the TO1 phonon, triggers this interfacial superconductivity. However, proving this theory presented two major challenges. First, the interface is buried within the sample and is only a few nanometres thick, making it difficult to study with traditional methods. Second, the team needed to work with terahertz radiation, a frequency range a thousand times higher than 5G networks. Many crucial quantum effects occur in this terahertz range, but capturing them with high resolution is notoriously difficult.
Argonne physicist Haidan Wen elaborates on these challenges: “There were two main challenges. First, the interface is buried in the sample and only a few nanometres thick, making it hard to study using conventional methods. Second, the team needed to work with terahertz radiation. This happens in a frequency range a thousand times higher than 5G phone networks. Many important quantum effects happen in this terahertz range, but capturing them with high resolution is difficult.”
SSTS: A new window into the quantum world
To overcome these hurdles, the researchers developed their innovative SSTS method. This technique involves shining ultrafast laser pulses through an oxide crystal onto a thin magnetic film. The interaction between the laser light and the material generates terahertz vibrations at the oxide interface.
Using this method, the team successfully detected the elusive TO1 phonon and demonstrated that its behaviour within 5 nanometres of the interface differed significantly from that in the bulk material. This is analogous to how waves behave differently in the shallow end of a lake compared to the deeper waters.
A broad range of applications and future directions
The implications of this breakthrough are far-reaching. “Our interface-sensitive technique can be applied to a broad range of materials for probing elusive quantum behaviour, including magnetism and superconductivity,” says Michael Norman, Argonne Distinguished Fellow and director of the Argonne Quantum Institute. “We now have a new window into quantum materials that can point the way to novel quantum devices for future technologies.”
Bhattacharya adds, “Terahertz light interacting with matter can not only probe quantum materials in new ways, as in our study, but also induce entirely new states of matter. This is an incredibly exciting avenue for future investigation.”
This research, published in Science Advances, represents a significant step forward in our understanding of quantum materials. The development of SSTS provides a powerful new tool for scientists to explore the quantum realm, paving the way for the creation of advanced quantum technologies that could revolutionise computing, sensing, and many other fields.
The ability to observe and potentially manipulate quantum behaviour at interfaces opens up a world of possibilities for future research and innovation.
