Quantum mechanics may give us powerful computers, but that’s not the whole story. Researchers working in the nanoscale world of atoms and molecules are also exploring a variety of materials that show promising, if odd, behaviors because of their quantum mechanical interactions.
With names like spin ice, quantum spin liquid and topological insulator, quantum materials have been around for decades, yet we don’t have a universally agreed-upon definition of what the term means. One accepted definition revolves around the behavior of electrons.
“Quantum materials represent a class of materials that exhibit emergent physical properties due to the quantum mechanical interaction of electrons,” explained Ho Nyung Lee, ORNL program manager for the Materials Sciences and Engineering Program of DOE’s Basic Energy Sciences.
“The recent rise of interest in quantum materials focuses on uncovering the role of properties that can ultimately provide the avenue to quantum information science and computing. ORNL’s materials science research is well positioned to take advantage of synthesis, theory, computation, imaging and neutron scattering.”
A spin ice, for example, is not ice as such. In this material, the electrons’ magnets, or spins, form a series of connected three-sided pyramids that reminded researchers of actual ice. By the same token, quantum spin liquids are not really liquids. Rather, the structure of their electrons—which are entangled but not ordered—reminds researchers of water.
Topological insulators, on the other hand, are materials that can be distorted without losing their shape. While electrons within the material cannot move, electrons at its edge can.
Exploiting quantum materials
Quantum materials have a variety of potential uses—in novel sensors or powerful quantum computers, for instance— but they no doubt have uses that we haven’t even considered yet. According to Steve Nagler, a corporate fellow in ORNL’s Neutron Scattering Division, this research is far too early in its development for us to have a clear idea of where it will lead.
“We are at the point of trying to achieve an understanding, and that understanding has moved forward and forward during the intervening years,” Nagler said. “Some of these systems could be useful for quantum information systems, and they could be useful for other things. You learn along the way.”
He pointed to the evolution of transistors dating back to the 1930s and ’40s to illustrate the point.
“That was the same stage as this; it was just basic research. And they worked out the quantum mechanical solution for electrons in solids, came up with this concept of energy bands, understood how they could use that to infer what the property of the material would be, and they understood why you had metals and semiconductors and insulators.
“And once this was understood, somebody realized, hey, we can use this knowledge to make a piece of solid material that will do the same thing as a radio tube.”
So the job ahead of modern researchers is to understand these materials better. Fortunately, ORNL has powerful tools—specifically neutron scattering facilities and high-powered microscopes—to advance that understanding.
The value of neutrons
Scattering at ORNL’s Spallation Neutron Source and High Flux Isotope Reactor is an especially effective way to study quantum materials, because of neutrons’ wavelengths, magnetic properties and energy.
“The key component of these quantum states is the magnetism in the material,” explained ORNL neutron scattering scientist Clarina dela Cruz, “and neutrons are the most powerful tool you can have in probing magnetism in any material. This is because neutrons are chargeless; unlike electrons they have some mass, but it’s small; and they have an effective magnetic moment.”
According to Nagler, the neutrons used in scattering experiments have wavelengths comparable to the spaces between atoms, allowing researchers to use them effectively to study the microscopic structure of materials.
The same neutrons are also advantageous for studying the motions of atoms and their associated magnetic materials, he said, because the energies of the neutrons and the atomic vibrations are very similar. This can be contrasted to X-ray photons, which are a billion times more energetic than neutrons at the wavelengths needed to study these materials.
Making materials with an electron microscope
Not only are scanning electron transmission microscopes effective at studying quantum materials, noted ORNL materials scientist Stephen Jesse, but they are also good at creating and manipulating these materials.
In particular, he said, these microscopes are especially good at creating and studying materials in which an atom of one element is placed in a lattice made up of another element. While the new atom is typically known as a defect, that is not necessarily bad. In fact, Jesse explained, such materials may be promising for use in quantum information systems.
“Defects are sometimes really, really good,” he said. “These optical systems may have, say, a single defect in a material that has special quantum properties, and it emits light in a special way that can be used in a quantum sensor.”
The microscope itself has two functions, he said. First, of course, it is used to image the materials, looking at atoms and the atomic structure. But, second, it can also help create the materials.
As an example, he said, researchers can use the microscope to punch a hole in graphene, which is made up of a single layer of carbon atoms. If there’s carbon present around the sample, it will typically fill in the hole, but if there’s a different element present, it, too, can fill in the hole.
“We are figuring out new ways to use this platform to not just image materials, but to actually transform them and add dopants and defects while we’re looking at them,” Jesse explained. “Our goal is to add defects and then study them and see if we can find new arrangements that give us the optical properties we want, so we’re building things atom by atom.”
On top of that, he said, researchers are able to use the microscope’s electron beam to move specific atoms around the material.
“A lot of what I’ve done in the past is make microscopes do things they’re not traditionally supposed to do,” he said.