The discovery of a novel quantum phenomenon in a crystalline material known as “hybrid topology” by physicists has excellent prospects for next-generation quantum technologies. Princeton scientists examined a solid crystal composed of arsenic (As) atoms and made the discovery, which was published in the journal Nature on April 10.
They investigated and captured images of this distinct quantum state using photoemission spectroscopy and a scanning tunneling microscope (STM). This state presents a novel sort of topological quantum behavior never observed in the same material before, combining edge states with surface states. This is the first time these behaviors have been shown together, resulting in a unique state of matter, even though they had been seen independently in earlier tests.
In recent years, the study of topological states of matter has drawn significant attention from engineers and physicists worldwide. This area of study combines topology—a branch of mathematics that studies geometric qualities that can be changed without affecting their underlying nature—with quantum physics.
Scientists have been studying strange quantum phenomena in bulk solids using materials based on bismuth (Bi) for more than ten years. Usually, these materials are compounds, like bismuth and selenium (Se). But this latest experiment is the first to show topological consequences in crystals composed entirely of arsenic (As) atoms.
This discovery of new topological properties in a simple solid material is highly significant. It opens up exciting possibilities for fundamental research and practical applications in quantum science and engineering.
The researchers discovered these unexpected topological states in arsenic crystals using cutting-edge experimental methods created in their Princeton lab. Because of its simplicity in synthesis and cleanliness, bismuth has been the element that has been investigated most for its topological features. However, the discovery in arsenic points to a new approach for this type of research.
For the first time, researchers have demonstrated that distinct topological orders can also interact and give rise to new and intriguing quantum phenomena similar to correlated ones.
Topological materials are needed to unlock the mysteries of quantum topology. These materials’ interiors function as insulators, preventing electrons from moving freely and conducting electricity. Nevertheless, the material’s edges are conductive because electrons are free to move. The fascinating thing about these edge electrons is that their unique topological characteristics prevent them from being impacted by faults or deformations.
By examining the quantum electronic features of matter, this kind of device can further our understanding of matter and develop technology.
M. Zahid Hasan, the Eugene Higgins Professor of Physics at Princeton University, underlined how intriguing it is to use topological materials in real-world applications. Nonetheless, for this to occur, two significant developments are required. First, at higher temperatures, one must be able to see quantum topological phenomena. Second, researchers must find materials that can display these topological phenomena. These materials should be basic and elemental, like silicon in traditional electronics.
Building on the foundations of the quantum Hall effect—a notable topological phenomenon that was awarded the 1985 Nobel Prize in Physics—topological materials have been discovered. Since then, scientists have researched topological phases and produced several quantum materials with distinct electronic structures.
Notable scientists in this area include Princeton University‘s Daniel Tsui and F. Duncan Haldane, awarded Nobel Prizes for their findings regarding topological phase transitions and the quantum Hall effect.
Dr. Hasan and his colleagues at Princeton have been investigating various facets of topological insulators and hunting for novel states of matter, following in their footsteps. Finding the first examples of three-dimensional topological insulators in 2007 was a significant accomplishment. For the past ten years, they have searched for a new topological state that can function at room temperature.
To fully exploit the potential of topological insulators in real-world applications, Dr. Hasan stresses the significance of integrating theoretical computations, structural design, and atomic chemistry. This calls for a thorough grasp of materials and extensive experiments to find good candidates. They have explored a variety of bismuth-based materials along the way, which has produced several significant breakthroughs in the field of topological materials.
Materials based on bismuth can theoretically support a topological state of matter at very high temperatures. However, they necessitate intricate material preparation under extremely high vacuum settings, therefore the scientists chose to investigate several alternative methods. Md. Shafayat Hossain, a postdoctoral researcher, proposed an arsenic crystal because it can be generated in a cleaner form than many bismuth compounds.
When Hossain and Yuxiao Jiang, a graduate student in the Hasan lab, turned the STM on the arsenic sample, they made a striking discovery: grey arsenic, a metallic-looking kind of arsenic, concurrently hosts both topological surface states and edge states.
Postdoctoral researcher Md. Shafayat Hossain said, “We were surprised. Grey arsenic was supposed to have only surface states. But when we examined the atomic step edges, we also found beautiful conducting edge modes.”
Jiang, a co-first author of the work, continued, “A gapless edge mode should not exist in an isolated monolayer step edge.
Calculations made by Rajibul Islam, a postdoctoral researcher at the University of Alabama in Birmingham, Alabama, and Frank Schindler, a postdoctoral fellow and condensed matter theorist at Imperial College London in the United Kingdom, demonstrate this.
Schindler states, “The surface states hybridize with the gapped states on the edge and form a gapless state once an edge is placed on top of the bulk sample.”
“This is the first time we have seen such a hybridization.”
Physically, neither solid nor higher-order topological insulators alone are predicted to exhibit a gapless state on the step edge. Instead, it is only anticipated in hybrid materials that exhibit both kinds of quantum structure. This gapless state differs from hinge or surface states in higher-order and topological solid insulators. Consequently, the Princeton team’s experimental discovery unveiled a topological state that had never been observed before.
David Hsieh, the Caltech Physics Division Chair and an independent researcher, emphasized the research’s novel conclusions. He pointed out that some materials can simultaneously be in two different topological classes. These two topologies’ boundary states can potentially interact and create a more complicated new quantum state.
The researchers employed scanning tunneling microscope measurements and high-resolution angle-resolved photoemission spectroscopy to further validate their findings.
Some of the photoemission measurements were carried out by Zi-Jia Cheng, a graduate student in the Hasan group and one of the co-first authors of the work. He said, “The grey As sample is very clean, and we found clear signatures of a topological surface state.”
The researchers investigated the unique correlation between the bulk, surface, and edge associated with the hybrid topological state by integrating multiple experimental methodologies, which confirmed the experimental results.
The finding has two implications. First, observing the surface state and integrated topological edge mode opens the door to creating novel electron transport pathways. This may result in the creation of novel tools for quantum computing or quantum information science. The Princeton researchers showed that these topological edge modes are only present along particular geometrical configurations that coincide with the crystal’s symmetries, providing a method to construct different kinds of future spin-based electronics and nanodevices.
Society gains from discovering new materials and qualities when looking at things more broadly. The discovery of elemental solids as material platforms in the field of quantum materials—such as bismuth for higher-order topology or antimony for robust topology—has sparked the creation of innovative materials that have significantly improved the study of topological materials.
“We foresee arsenic, with its unique topology, serving as a new platform for developing novel topological materials and quantum devices that are not currently achievable through existing platforms,” noted Hasan. “This opens up an exciting new frontier in material science and novel physics!”
Journal Reference:
- Md Shafayat Hossain, Frank Schindler, Rajibul Islam, Zahir Muhammad et al. A hybrid topological quantum state in an elemental solid. Nature. DOI: 10.1038/s41586-024-07203-8