This article was updated on the 11th September 2019.
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Researchers from Boston College’s Departments of Physics and Chemistry, in collaboration with Harvard University’s School of Engineering and Applied Sciences and Center for Nanoscale Systems, have developed a new magnetically frustrated honeycomb iridate. It is much closer to the Kitaev spin liquid than its predecessors.
Understanding the ‘Frustation’ of Magnetism
Frustrated magnetic moments can occur from large degeneracy of a system’s ground state when competing spin exchange interactions are not all satisfied at the same time. Unlike other ferromagnetic spin states, a quantum spin liquid is a state achieved with interacting quantum spins which are disordered like liquid water until very low temperatures. Quantum spin liquids are particularly interesting in condensed matter physics due to their high temperature superconductivity.
This geometric frustration is known to suppress magnetic order in triangular, pyrochlore and kagome lattices. Quantum spin liquids result when there is a complete suppression of magnetic order in an S=1/2 system. Sodium iridate (Na2IrO3) and lithium iridate (Li2IrO3) are the most well-known materials that show the properties of quantum spin liquids to some extent. In both the Na2IrO3 and Li2IrO3, the iridate ions (Ir4+) are arranged in the octahedral crystal field with effective S=1/2 placed onto the honeycomb lattice. Although these two materials satisfy the basic requirements of the Kitaev model, they both failed to be true spin liquids because of the antiferromagnetic (AFM) order they show at 15K.
Obtaining the Remarkable Honeycomb Structure
This change in magnetism from a Kitaev limit (spin liquid) to a Heisenberg limit (AFM order) is attributed to the magnitude of competing interactions on the honeycomb lattices. Several research teams have tried to produce other honeycomb iridates using other alkali elements such as potassium (K), Rubidium (Rb) and Cesium (Cs) but none were successful because of the large ionic radii of these elements. Unlike the previous attempts with transition metals, the Boston College and Harvard University team, led by Assistant Professor of Physics Fazel Tafti, were able to describe a new Iridite with a transition metal copper for the first time.
The Successful Production of Copper Iridate
The work carried out by Fazel Tafti’s team was published recently in the Journal of American Chemical Society. The initial fabrication of this material involved heating the two parent compounds of Na2IrO3 and copper chloride (CuCl) to 350° C for 16 hours, which is a relatively mild condition for chemical experiments. During this topotactic reaction, Na2IrO3 (a solid-state chemical) and CuCl produced a unique structure of the product’s crystals that are ultimately determined by the structure of the parent compound.
While the parent compound Na2IrO3 has been shown to be incapable of achieving an ideal spin liquid on its own, the combination of these two materials successfully developed the first of its kind copper iridate metal oxide (Cu2IrO3).
Under these conditions, the replacement of copper within the honeycomb lattice structure of the Na2IrO3 parent compound produced the highly desirable and rarely achievable honeycomb structure of the final Cu2IrO3 product. The immobilization of the electrons within the Cu2IrO3 structure were found to be highly immobilized, thereby creating an ideal insulation material and weak short magnetism – both of which are ideal conditions for the maintenance of the honeycomb structure.
Despite the limitations of its parent compound, the Cu2IrO3 material exhibited a 2.7 K electron disorder with far fewer distortions present within the honeycomb structure as compared to its predecessors. Additionally, the Cu2IrO3 material achieved bond angles that measured much closer to the ideal 120° measurements as compared to Na2IrO3.
Future Research
The successful production of the Cu2IrO3 will only further propagate the growing trend of various industries to turn to quantum computing for their specific needs. Quantum computers, which are capable of performing calculations at an exponentially faster rate as compared to some of the fastest computers currently available on the market, have found useful applications in industries ranging from finance to life sciences and manufacturing. The large scale production of the Cu2IrO3 material could greatly enhance these computers for unimaginable properties in the future.
References:
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Abramchuk, M., Ozsoy-Keskinbora, C., Krizan, J.W., Metz, K.R., Bell, D.C. and Tafti, F. (2017). Cu2IrO3: A New Magnetically Frustrated Honeycomb Iridate. Journal of the American Chemical Society, 139(43), pp.15371–15376.
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