Antiferromagnetism on an hyperkagome lattice in Na4Ir3O8
One of the most promising new materials in which to observe quantum spin liquid physics is Na4Ir3O8, in which the Ir atoms form spin-1/2 spins on an "hyperkagome" lattice. This is a network of corner-sharing triangles in three dimensions, which can be obtaining by selectively removing one site from each tetrahedron of a pyrochlore structure. It is similar to the two dimensional kagome lattice, but sparser, and so in a sense is likely to be even more disordered. The experiments are quite intriguing, showing a very large Curie-Weiss temperature (-650K) and no order down to below 1K. The specific heat has a broad peak around 30K and a roughly T2 behavior below this, indicating gapless low energy excitations.
^ TOPOur work:
We have two papers on this subject. In the first, we consider the quantum chemistry and especially the role of spin-orbit interactions for the Ir moments. Spin orbit coupling is very large for Ir (of order 0.5eV) which can dramatically alter the physics. Indeed, we argue that the Ir spins are actually strongly spin-orbit mixed and best thought of as J (L+S) eigenstates. A consideration of Goodenough-Kanamori reasoning suggests that the exchange is dominantly direct between the Ir ions. We point out that spin orbit coupling is probably essential in understanding the apparently contradictory behavior of the susceptibility and heat capacity at low temperature.
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Our second paper is a collaboration with the group at University of Toronto (Michael Lawler, Arun Paramekanti, and Yong-Baek Kim). In it we consider the nature of quantum spin liquid ground states of the hyperkagome antiferromagnet, assuming an Heisenberg model description. This is justified as a first approximation by our previous paper. We find a set of candidate states with gapless fermionic spinons.
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