Quantum spin liquid (QSL) with a macroscopic degeneracy of spin configurations down to absolute zero temperature represents one of the most exotic state of matters with its intriguing connections to high temperature superconductivity and possible applications for quantum computation[1]. Finding a material platform for such physics turns out to be extremely challenging due to the general tendency of nature that finds a way to remove the degeneracy via coupling to lattice. Recently, a new type of QSL avoiding this materials complication is proposed in a bipartite honeycomb lattice (named as Kitaev spin liquid), which is driven by the magnetically frustrated situation among bond-directional anisotropic exchanges [2]. Iridate A2IrO3 (A = Na, Li) are the material candidates, because spin-orbit entangled Jeff = 1/2 state of iridium ions built in the particular bond geometries may result in dominant anisotropic interactions while the other interactions are suppressed [3]. However, their relevance to the Kitaev spin liquid has remained elusive as the residual interactions contribute to forming long-range ordered magnetic states [4]. In this talk, I discuss the direct evidence of magnetic frustration driven by the bond-directional magnetic interactions in Na2IrO3 from a study of diffuse magnetic x-ray scattering [5]. This work
establishes the importance of the bond-directional anisotropy and the proximity of Na2IrO3 to the Kitaev spin liquid, offering a novel route to create magnetically frustrated lattices with spin-orbit entangled states.

[1] L. Balents, Nature 464, 199 (2010);

[2] A. Kitaev, Ann. Phys. 321, 2 (2006);

[3] G. Jackeli and G.Khaliullin, Phys. Rev. Lett. 102, 017205 (2009);

[4] Y. Singh and P. Gegenwart, Phys. Rev. B 82, 064412  (2010);

[5] S. H. Chun et al., Nature Phys. 11, 462 (2015).

Dr. Chun is currently a post-doctoral researcher at Argonne National Laboratory. He completed one postdoc
at Seoul National University, where he completed his Ph.D.