Neutron scattering studies of a rare earth metal oxide have identified fundamental pieces to the quantum spin liquid puzzle, revealing a better understanding of how and why the magnetic moments within these materials exhibit exotic behaviors such as failing to freeze into an ordered arrangement even near absolute zero temperatures.
In a paper published in Nature Physics, a team of researchers from the Georgia Institute of Technology, the University of Tennessee and the Department of Energy's Oak Ridge National Laboratory used neutrons to examine the origins of unusual magnetic behavior in a rare earth-based metal oxide, ytterbium-magnesium-gallium-tetraoxide (YbMgGaO4). The material, discovered in 2015, is known to have strange magnetic properties, putting it in a unique category of materials classified as quantum spin liquids.
Red arrows represent electron spin orientations in a portion of the YbMgGaO4 crystal structure, where antiferromagnetic interactions between groups of magnetic moments cause neighboring spins to align anti-parallel to one another. This mechanism is partially responsible for the quantum spin liquid behavior observed in the neutron scattering data, illustrated on the hexagonal tiles.
Image credit: ORNL/Jill Hemman
"A quantum spin liquid is an exotic state of matter characterized by the entanglement of particles over long distances across the atomic scale," said lead investigator Martin Mourigal, an assistant physics professor at the Georgia Institute of Technology.
Think of Schrödinger's cat, the thought experiment, he said: Many particles participate in a quantum superposition, where multiple quantum states combine to form a new quantum state, and cannot be characterized by the behavior of individual particles.
By definition, he said, "it's something we can't explain with classical physics."
In a series of experiments at ORNL's Spallation Neutron Source, the researchers revealed three key features underpinning the material's exotic properties:
antiferromagnetic interactions, where groups of electron spins have an antiparallel alignment with their respective neighbors; spin space anisotropy, meaning that individual magnetic moments strongly prefer aligning themselves alongside specific directions in the material; and chemical disorder between the material's magnetic layers that randomizes the interactions between electron spins.
Neutrons are well suited for studying magnetism because their lack of electric charge allows them to penetrate through materials, even when the neutrons' energy is low. The neutrons also have magnetic moments, allowing researchers to directly probe the behavior of spins within materials.
"Neutron scattering is the only technique that allows us to study the dynamics of quantum spin liquids at the lowest temperatures," Mourigal said.
However, quantum spin liquids present a challenge because their magnetic moments are constantly changing. In typical materials, researchers can lock the spins into certain symmetric patterns by lowering the temperature of the sample, but this approach doesn't work on spin liquids.
In the team's first neutron scattering measurements of an YbMgGaO4 single-crystal sample at the SNS's Cold Neutron Chopper Spectrometer, CNCS, the researchers observed that, even at a temperature of 0.06 kelvins (approximately negative 460 degrees Fahrenheit), magnetic excitations remained disordered or "fuzzy." This fluctuating magnetic behavior, known to occur to quantum spin liquids, runs counter to the laws of classical physics.
"The material screamed spin liquid when we put it in the beam," Mourigal said.
To overcome this fuzziness, the team used an 8 Tesla magnet to create a magnetic field that locked the spins into an ordered and partly frozen arrangement, allowing for better measurements.
"Once we applied the magnetic field, we were able to measure coherent magnetic excitations in the material that propagate sort of like sound waves," said CNCS instrument scientist Georg Ehlers. "When a neutron comes into the material, it flies by a magnetic moment and shakes it. The nearby magnetic moments see this happening, and they all begin to vibrate in unison. The frequency of these vibrations is determined by the energy between neighboring spins."
Those magnetic field measurements enabled the team to directly validate theoretical expectations and provided a physical understanding of the spin behavior and the system as a whole.
"A quantum spin liquid is an intrinsically collective state of matter," said Mourigal. "But if you want to understand the society, you need to understand the individuals as well."
The team then turned to another SNS instrument, the Fine-Resolution Fermi Chopper Spectrometer instrument, SEQUOIA, to understand the individual properties of the magnetic moments.
"In rare earth magnets, rich physics, like what was observed at the CNCS instrument, can emerge from the fact that the individual spins can prefer to point along certain directions in a crystal," said SEQUOIA instrument scientist Matthew Stone. "SEQUOIA examined the localized higher energy states to confirm the individual pieces of the model used to describe the CNCS data were correct."
Mourigal says the information gleaned from the experiments will enable researchers to develop better theoretical models to further study these quantum phenomena.
"While the exact nature of the quantum state hosted by this material has not been fully established yet, we've discovered that chemical disorder and other effects are important here," said Mourigal. "With these experiments, we've really been able to nail down what ingredients need to be taken into the recipe for a quantum spin liquid in this material."
The paper's authors are Joseph A. M. Paddison, Marcus Daum, Zhiling Dun, Georg Ehlers, Yaohua Liu, Matthew B. Stone, Haidong Zhou and Martin Mourigal.
The YbMgGaO4 sample was synthesized at the University of Tennessee. Supplementary measurements of the YbMgGaO4 crystal structure were made at the SNS CORELLI instrument.
The research received support from the Georgia Institute of Technology, the National Science Foundation, and DOE's Office of Science. SNS is a DOE Office of Science User Facility.
UT-Battelle manages ORNL for the DOE's Office of Science. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, please visit http://science.
Jeremy Rumsey | EurekAlert!
Moon's crust underwent resurfacing after forming from magma ocean
22.11.2017 | University of Texas at Austin
NASA's James Webb Space Telescope completes final cryogenic testing
21.11.2017 | NASA/Goddard Space Flight Center
The WHO reports an estimated 429,000 malaria deaths each year. The disease mostly affects tropical and subtropical regions and in particular the African continent. The Fraunhofer Institute for Silicate Research ISC teamed up with the Fraunhofer Institute for Molecular Biology and Applied Ecology IME and the Institute of Tropical Medicine at the University of Tübingen for a new test method to detect malaria parasites in blood. The idea of the research project “NanoFRET” is to develop a highly sensitive and reliable rapid diagnostic test so that patient treatment can begin as early as possible.
Malaria is caused by parasites transmitted by mosquito bite. The most dangerous form of malaria is malaria tropica. Left untreated, it is fatal in most cases....
The formation of stars in distant galaxies is still largely unexplored. For the first time, astron-omers at the University of Geneva have now been able to closely observe a star system six billion light-years away. In doing so, they are confirming earlier simulations made by the University of Zurich. One special effect is made possible by the multiple reflections of images that run through the cosmos like a snake.
Today, astronomers have a pretty accurate idea of how stars were formed in the recent cosmic past. But do these laws also apply to older galaxies? For around a...
Just because someone is smart and well-motivated doesn't mean he or she can learn the visual skills needed to excel at tasks like matching fingerprints, interpreting medical X-rays, keeping track of aircraft on radar displays or forensic face matching.
That is the implication of a new study which shows for the first time that there is a broad range of differences in people's visual ability and that these...
Computer Tomography (CT) is a standard procedure in hospitals, but so far, the technology has not been suitable for imaging extremely small objects. In PNAS, a team from the Technical University of Munich (TUM) describes a Nano-CT device that creates three-dimensional x-ray images at resolutions up to 100 nanometers. The first test application: Together with colleagues from the University of Kassel and Helmholtz-Zentrum Geesthacht the researchers analyzed the locomotory system of a velvet worm.
During a CT analysis, the object under investigation is x-rayed and a detector measures the respective amount of radiation absorbed from various angles....
The quantum world is fragile; error correction codes are needed to protect the information stored in a quantum object from the deteriorating effects of noise. Quantum physicists in Innsbruck have developed a protocol to pass quantum information between differently encoded building blocks of a future quantum computer, such as processors and memories. Scientists may use this protocol in the future to build a data bus for quantum computers. The researchers have published their work in the journal Nature Communications.
Future quantum computers will be able to solve problems where conventional computers fail today. We are still far away from any large-scale implementation,...
15.11.2017 | Event News
15.11.2017 | Event News
30.10.2017 | Event News
22.11.2017 | Medical Engineering
22.11.2017 | Materials Sciences
22.11.2017 | Health and Medicine