Researchers at the Department of Energy's Oak Ridge National Laboratory are pioneering the design and synthesis of quantum materials, which are central to discovery science involving synergies with quantum computation. These innovative materials, including magnetic compounds with honeycomb-patterned lattices, have the potential to host states of matter with exotic behavior.
Using theory, experimentation and computation, scientists synthesized a magnetic honeycomb of potassium cobalt arsenate and conducted the most detailed characterization of the material to date. They discovered that its honeycomb structure is slightly distorted, causing magnetic spins of charged cobalt atoms to strongly couple and align. Tuning these interactions, such as through chemically modifying the material or applying a large magnetic field, may enable the formation of a state of matter known as a quantum spin liquid. Unlike permanent magnets, in which spins align fixedly, quantum spins do not freeze in one magnetic state. Collective magnetic excitations could emerge from such quantum materials. Called Majorana fermions, these collective excitations could be developed and controlled to become building blocks for next-generation quantum technologies that may transform diverse sectors from energy to national security.
That possibility has scientists buzzing with excitement. They hope refinements will make their honeycomb material a suitable platform for developing the collective excitations, testing theories at the forefront of condensed matter, and opening pathways to innovative computational technologies.
A 2006 model by Caltech physicist Alexei Kitaev suggested collective excitations would form at the edge of a honeycomb crystal. Seeking this quarry, scientists worldwide have conducted experiments on candidate "Kitaev materials," including barium cobalt arsenate, which is similar to ORNL's potassium cobalt arsenate, as well as related honeycomb materials such as ruthenium chloride and sodium iridate. Kitaev materials have electrically insulating bulks but highly conductive edges. The ideal material is resistant to the loss of quantum properties when it interacts with its environment.
"This field is really tough, and quite young," said ORNL's Craig Bridges, who led the potassium cobalt arsenate study published in Inorganic Chemistry . "Scientists are just beginning to understand how to observe and possibly manipulate Majorana fermions. We can't claim seeing them yet in our honeycomb material, though we hope to eventually achieve that by modifying the material. We're doing very basic research. It's frontier work to try to find a material that will fit the predicted physics."
America's national labs aim to lead advances in quantum materials, sensors, algorithms
Studying powders and crystals of the honeycomb material, the ORNL scientists are trying to understand fundamental properties that could release its potential.
Quantum materials are an important part of DOE's Quantum Science Center , launched at ORNL in 2020, renewed in 2025 and led by QSC Director Travis Humble. The center is an interdisciplinary partnership of 21 institutions from industry, academia and government that brings together theory, experiment and computation to advance quantum technologies. It is one of five DOE National Quantum Information Science Research Centers. Since its 2020 founding, the QSC has pioneered the synthesis and characterization of quantum spin systems to probe their unique properties using state-of-the-art measurements such as neutron scattering. With its 2025 renewal, the QSC began new efforts to simulate these same material properties using quantum computing methods that are tailored to their unique geometries and compositions. These advances bolster QSC efforts to train the next-generation workforce to accelerate the development of quantum science and technology.
Bridges tried to make the honeycomb material for many years. His recent efforts bloomed with the help of Justin Felder, formerly at ORNL and now at DOE's Los Alamos National Laboratory, and Lucas Pressley of ORNL. They heated a solution containing a compound of potassium, arsenic, oxygen and cobalt at a low temperature to avoid decomposing the material. Then they crystallized the material out of the solution. With Parans Paranthaman and Saurabh Pethe, both of ORNL, they tested and analyzed the material. To verify its chemical composition, the team used inductively coupled plasma mass spectrometry and scanning electron microscopy with energy dispersive spectroscopy.
To help determine the crystal structure, Ray Unocic, now at North Carolina State University, and Zheng Gai of ORNL performed electron diffraction using transmission electron microscopy.
ORNL's Michael McGuire performed heat capacity and magnetic measurements that revealed a transition at which magnetic spins freeze into an ordered state, indicating the material may be near, but not quite in, a quantum spin liquid state. This type of magnetism emerges when unpaired electrons from ions interact collectively to produce a lower-energy state with fixed moment directions and little fluctuation.
To further explore the cobalt honeycomb's structure and magnetism, Colin Sarkis and Clarina Dela Cruz, both of ORNL, characterized powder samples using neutron scattering. Alan Tennant of the University of Tennessee, Knoxville, and Stephen Nagler, emeritus of ORNL, lent additional expertise. Because neutrons strongly interact with magnetic spins, they hold a unique place in studies of magnetic materials. These neutron studies at ORNL proved that the new synthesis method exclusively produced crystals with a honeycomb lattice.
Computational studies were performed by theoreticians John Villanova at Middle Tennessee State University and ORNL's Satoshi Okamoto and Tom Berlijn. Using the crystal structure information, they calculated how the cobalt spins are expected to interact with one another. This calculation showed that the Kitaev part of the interaction was relatively weak compared to other, more common interactions, explaining why the material forms a magnetically ordered state instead of a spin liquid. They suggested that this balance may be tipped by changing the chemical composition of the material or compressing it under high pressure. Challenges remain as scientists try to optimize their new material.
"It's unlikely that this material will provide an exact fit to the model," Bridges said. "However, it could be a platform in which we could change the composition slightly to tune the material's magnetism and achieve the physics that we need to realize a true quantum spin liquid."
The research was primarily supported by the DOE Office of Science through the Quantum Science Center. Preliminary investigations of the cobalt-containing honeycomb crystal were supported by ORNL's Laboratory Directed Research and Development program. Portions of the research used resources at the Center for Nanophase Materials Sciences (electron microscopy data collection) and the High Flux Isotope Reactor (neutron diffraction of powder samples), which are DOE Office of Science user facilities at ORNL.
UT-Battelle manages ORNL for DOE's Office of Science, the single largest supporter of basic research in the physical sciences in the United States. The Office of Science is working to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science . - Dawn Levy
