Chemistry and physics are combining forces at Columbia, and it's leaving everyone frustrated—in a good way. New work, published today in Nature Physics , describes a new two-dimensional material capable of complex quantum behaviors that arise from its underlying chemistry, rather than its atomic structure.
"It's a classic Columbia story—multiple groups in physics and chemistry came together to work on this new material, and we found exciting new results about how electrons move," said Aravind Devarakonda , an applied physicist at Columbia Engineering.
The material, Pd5AlI2, exhibits what's known as frustration of electron motion. It's metallic, air-stable, and can be peeled into atom-thin layers, and it represents a simple new starting point in the search for flat bands. These are unique electronic structures that could one day lead to new quantum technologies like better superconductors, rare-earth-free room-temperature magnets, and more.
Many quantum phenomena, like superconductivity and unique forms of magnetism, arise when electrons behave in ways that contradict the laws of classical physics, which declare that these elementary particles repel each other. But there are circumstances in which electrons can be forced to pair up. One means is by introducing frustration.
Frustration occurs when the electrons in a material can't find a stable place to settle with respect to each other's energies. To date, this has been due to geometry: crystals made up of triangles or squares create a physical conflict between electrons and trap them together. Et voila, collective quantum behaviors can manifest…at least in theory; materials with frustrated geometries are rare.
Pd5AlI2 brings frustration into the mix by way of its chemistry, rather than its crystal structure alone. "We've found an entirely new way to think about frustration, one that combines how chemists think about chemical bonds with how physicists think about crystal lattices," said Columbia chemist Xavier Roy , whose lab made the new metal.
At a glance, Pd5AlI2's lattice looked pretty straightforward, said Devarakonda, who led the work as a Simons Fellow working with Roy and Columbia physicist Cory Dean . Two members of Roy's group, graduate student Christie Koay (now a postdoctoral researcher at Princeton Chemistry) and postdoctoral researcher Daniel Chica, created it for Dean, who was looking for an air-stable metal that could be peeled into atom-thin layers.
During preliminary measurements, Devarakonda recognized a curious electronic feature that is characteristic of a geometrically frustrated structure called a Lieb lattice. Lieb lattices are made up of squares—and their unusual behavior had yet to be studied outside of theoretical models.
Devarakonda showed the data to theoretical physicist Raquel Queiroz , who made the connection between his observation and Pd5AlI2's chemistry: its orbitals, a fundamental concept in chemistry that determines where an electron can roam around its originating atom, combine into a checkerboard pattern that mimics the geometry of the Lieb lattice, but now, in a real material.
"That was our Eureka moment," said Devarakonda. "The lattice may be simple, but it's because of the orbitals that it becomes so interesting."
The signal that Devarakonda observed was a coveted electronic flat band. Flat bands are electronic structures that force electrons to all share the same energy, an inherently unstable position that can give rise to unusual quantum behaviors, like superconductivity.
The team continues to probe and prod Pd5AlI2 and similar frustrated materials—Devarakonda, for example, is literally pulling on samples to introduce strain—in efforts to coax out and ultimately control these behaviors. They are excited by the prospects of this new source of frustration.
As a layered crystal, they successfully peeled it down to a single atomic layer in the current publication; this raises the possibility of combining it with other 2D materials to create entirely new kinds of physics, a focus of Dean's lab. The fact that Pd5AlI2 is metallic, one of the first to stably exist while so very thin, also means that he will be able to shrink the stacked structures he creates even further.
Devarakonda also points to potential applications, like creating new quantum sensors and high-temperature magnets. Because the electrons are held in place, it may be possible to record their properties, like the direction they are spinning, to sense changes in their environment. At a larger scale, most magnets found in, for example, electric motors or wind turbines, require rare-earth elements; insights from Pd5AlI2 could help reduce reliance on increasingly difficult and expensive materials.
However, Pd5AlI2isn't exactly cheap. So the team plans to incorporate AI techniques to more rapidly identify crystals that might have orbital frustration hiding within their chemical bonds.
"The possibility of frustrated hopping from orbitals was articulated theoretically, but now we have a concrete example. We're now trying to see what other combinations of elements can come together to frustrate electrons," said Devarakonda. "There are so many models that people have come up with over the decades, but now we can use our newfound insights about lattices and orbitals to chase them from a different angle."
