The three-pronged approach discerns the qualities of quantum spin liquids

In 1973, physicist Phil Anderson hypothesized that the quantum spin liquid state, or QSL, existed in some triangular lattices, but he didn’t have the tools to dig deeper. Fifty years later, a team led by researchers associated with the Quantum Science Center, based at the Department of Energy’s Oak Ridge National Laboratory, confirmed the presence of QSL behavior in a new material with this structure, KYbSe._two.

QSLs – an unusual state of matter controlled by interactions between entangled or intrinsically linked magnetic atoms called spins – excel at stabilizing quantum mechanical activity in KYbSe_two and other delafosites. These materials are valued for their layered triangular networks and promising properties that could contribute to the construction of high-quality superconductors and quantum computing components.

The article, published in Nature Physics, has 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 Rosário Physics Institute.

“Researchers studied the triangular lattice of various materials in search of QSL behavior,” said QSC member and lead author Allen Scheie, Los Alamos staff scientist. “One advantage of this is that we can easily swap atoms to modify the properties of the material without changing its structure, and this makes it quite ideal from a scientific point of view.”

Using a combination of theoretical, experimental and computational techniques, the team observed multiple characteristics of QSLs: quantum entanglement, exotic quasiparticles and the right balance of exchange interactions, which control how one spin influences its neighbors. Although efforts to identify these characteristics have historically been hampered by the limitations of physical experiments, modern neutron scattering instruments can produce precise measurements of complex materials at the atomic level.

When examining KYbSe_twoof 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 with reliable theoretical models, the researchers found evidence that the material was close to the quantum critical point in which QSL features thrive. They then analyzed its single-ion magnetic state with SNS’s wide-angle Chopper spectrometer.

The witnesses in question are the one-entangled, two-entangled, and quantum Fisher information, which played a key role in previous QSC research focused on examining a 1D spin chain, or a single line of spins within a material. KYbSe_two it is a 2D system, a quality that has made these endeavors more complex.

“We are taking a co-design approach, which is integrated into the QSC,” said Alan Tennant, a professor of physics and materials science and engineering at UTK who is leading a quantum magnet project for the QSC. “Theorists at the center are calculating things they couldn’t calculate before, and this overlap between theory and experiment has enabled this advancement in QSL research.”

This study aligns with QSC 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 developing next-generation technologies,” said Tennant. “This field is still in a state of fundamental research, but we can now identify which materials we can modify to potentially manufacture small-scale devices from scratch.”

Although KYbSe_two is not a true QSL, the fact that about 85% of magnetism fluctuates at low temperatures means it has the potential to become one. Researchers anticipate that small changes to its structure or exposure to external pressure could potentially help it reach 100%.

QSC experimenters and computational scientists are planning parallel studies and simulations focused on delafossite materials, but the researchers’ findings establish an unprecedented protocol that can also be applied to study other systems. By simplifying evidence-based assessments of QSL candidates, they aim to accelerate the search for genuine QSLs.

“The important thing about this material is that we found a way to orient ourselves on the map, so to speak, and show what we got right,” Scheie said. “We are sure that a complete QSL exists somewhere in this chemical space, and now we know how to find it.”