Quantum spin liquids are unusual states of matter that can exist in quantum magnetic systems. Even at very low temperatures, the magnetic moments of electrons, known as spins, do not order themselves into conventional magnetic patterns in these materials.
Phil Anderson, a physicist, hypothesized in 1973 that the quantum spin liquid, or QSL, state existed on some triangular lattices, but he lacked the tools to investigate further. Fifty years later, a team led by researchers from the Department of Energy’s Oak Ridge National Laboratory’s Quantum Science Center confirmed the presence of QSL behavior in a new material with this structure, KYbSe2.
QSLs excel at stabilizing quantum mechanical activity in KYbSe2 and other delafossites because they are an unusual state of matter controlled by interactions among entangled, or intrinsically linked, magnetic atoms called spins. These materials are valued for their layered triangular lattices and promising properties, which could aid in the development of high-quality superconductors and quantum computing components.
Researchers from ORNL, Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, SLAC National Accelerator Laboratory, the University of Tennessee, Knoxville, the University of Missouri, the University of Minnesota, Stanford University, and the Rosario Physics Institute contributed to the paper, which was published in Nature Physics.
Researchers have studied the triangular lattice of various materials in search of QSL behavior. One advantage of this one is that we can swap out atoms easily to modify the material’s properties without altering its structure, and this makes it pretty ideal from a scientific perspective.Allen Scheie
“Researchers have studied the triangular lattice of various materials in search of QSL behavior,” said QSC member and lead author Allen Scheie, a staff scientist at Los Alamos. “One advantage of this one is that we can swap out atoms easily to modify the material’s properties without altering its structure, and this makes it pretty ideal from a scientific perspective.”
The team observed multiple hallmarks of QSLs using a combination of theoretical, experimental, and computational techniques, including quantum entanglement, exotic quasiparticles, and the proper balance of exchange interactions, which control how a spin influences its neighbors. Although previous efforts to identify these features were hampered by physical experiment limitations, modern neutron scattering instruments can produce accurate measurements of complex materials at the atomic level.
By examining KYbSe2’s spin dynamics with the Cold Neutron Chopper Spectrometer at ORNL’s Spallation Neutron Source – a DOE Office of Science user facility – and comparing the results to trusted theoretical models, the researchers found evidence that the material was close to the quantum critical point at which QSL characteristics thrive. They then analyzed its single-ion magnetic state with SNS’s Wide-Angular-Range Chopper Spectrometer.
The witnesses in question are the one-tangle, two-tangle, and quantum Fisher information, which has played a key role in previous QSC research focused on examining a 1D spin chain, or a single line of spins within a material. KYbSe2 is a 2D system, a quality that made these endeavors more complex.
“We are taking a co-design approach, which is hardwired into the QSC,” said Alan Tennant, a professor of physics and materials science and engineering at UTK who leads a quantum magnets project for the QSC. “Theorists within the center are calculating things they haven’t been able to calculate before, and this overlap between theory and experiment enabled this breakthrough in QSL research.”
This study aligns with the QSC’s priorities, which include connecting fundamental research to quantum electronics, quantum magnets and other current and future quantum devices.
“Gaining a better understanding of QSLs is really significant for the development of next-generation technologies,” Tennant went on to say. “This field is still in the fundamental research state, but we can now identify which materials we can modify to potentially make small-scale devices from scratch.”
Although KYbSe2 is not a true QSL, the fact that approximately 85% of its magnetism fluctuates at low temperatures indicates that it has the potential to be one. The researchers believe that slight changes to its structure or exposure to external pressure could help it reach 100%.
Parallel studies and simulations on delafossite materials are planned by QSC experimentalists and computational scientists, but the researchers’ findings established an unprecedented protocol that can also be applied to other systems. They hope to speed up the search for genuine QSLs by streamlining evidence-based evaluations of QSL candidates.
“The important thing about this material is that we’ve found a way to orient ourselves on the map so to speak and show what we’ve gotten right,” he said. “We’re pretty sure there’s a full QSL somewhere within this chemical space, and now we know how to find it.”