Argonne National Laboratory

Argonne X-rays validate quantum magnetism model

May 20, 2015

Scientists at the U.S. Department of Energy’s Argonne National Laboratory and Max Planck Institute for Solid State Research in Stuttgart, Germany have validated a theorized model of quantum magnetism by observing it firsthand in a honeycomb lattice.

The research is featured in an article titled “Direct evidence for dominant bond-directional interactions in a honeycomb lattice iridate Na2IrO3” published May 11 online at Nature Physics.

Researchers used resonant techniques at two X-ray beamlines at the Advanced Photon Source, a U.S. Department of Energy user facility at Argonne National Laboratory, to directly observe the Kitaev interaction—a theoretical mechanism to describe the spin of atoms in a crystalline, honeycomb lattice. The interactions between the three-direction bond and six-direction spin of individual atoms give rise to a novel type of magnetic frustration that could advance the field of quantum computing, which shows enormous promise in cybersecurity, tackling large, complex calculations, and indexing.

Quantum computing by the creation of quasiparticles known as anyons has the potential to overcome the stability-decoherence problem, which produces errors in computation—inherent in standard quantum computers. The Kitaev honeycomb model is one of the simplest systems for creating anyons to target this roadblock to quantum computing.

“This is the first direct experimental evidence for this new type of magnetic frustration mechanism proposed by theoretical [Kitaev] model” said Jong-Woo Kim, a physicist in the APS’s X-ray Science Division.

While neutron scattering has traditionally been the gold standard for determining magnetic structure, the unique capabilities of the APS allowed the team to use a smaller sample—in this case a single crystal—to determine the spin directions and probe the novel nanoscale magnetic structure with a higher energy resolution. Whereas standard X-rays are plagued by background signals that make resolving the actual magnetic energy difficult, “the unparalleled resolution capability available at the APS significantly improves signal-to-background ratio,” said Sae Hwan Chun of Argonne’s Material Sciences Division. “Essentially, it makes the useful features easier to see.”

The finding falls short of the ultimate goal in quantum magnetism: the realization of a quantum spin liquid, a sought-after state of matter for both quantum computing and high-temperature superconductivity. However, it does bring the researchers one step closer. The X-rays revealed a new type of magnetic frustration that exists in nature and revealed the mechanism responsible for it. Due to perturbations inherent in the material, however, it doesn’t quite achieve the liquid state.

The next step is to quantify the energy scale of the phenomenon—work that is currently ongoing at the APS. Then researchers can begin to pursue a predictable model of the magnetic frustration with advanced techniques in concert with Hamiltonian and microscopic theories.

“This work will provide a pathway to manipulate the key parameters and eventually realize the quantum spin liquid state,” said Jungho Kim, a physicist in the APS's X-ray Science Division.

The research utilized data from previous experiments at the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. Work at the APS was carried out at beamlines 6-ID-B, 9-ID, 27-ID and 30-ID.

The team included: Sae Hwan Chun, H. Zheng, Constantinos C. Stoumpos, C. D. Malliakas, and J.F. Mitchell, all of Argonne’s Material Science Division; Jong-Woo Kim, Jungho Kim, Y. Choi, and T. Gog of the APS; Kavhita Mehlawat and Yogesh Singh of the Indian Institute of Science Education and Research (IISER); A. Al-Zein, M. Moretti Sala, and M. Krisch of ESRF; J. Chaloupka of the Central European Institute of Technology; and G. Khaliullin, B.J. Kim, and G. Jackeli (also of the Institute for Functional Matter and Quantum Technologies at the University of Stuttgart) of the Max Planck Institute for Solid State Research.

The use of the APS and the work was funded in part by the US Department of Energy, Office of Science, Basic Energy Sciences and a grant from India.

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