Frustrating a quantum magnet

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Science  06 Nov 2015:
Vol. 350, Issue 6261, pp. 631-632
DOI: 10.1126/science.aad3556

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Being able to determine the ground state of a frustrated quantum magnet has been a long-sought goal in condensed matter physics. In general, frustration results from the inability to achieve a desired outcome. In solid-state systems, frustration can arise in relation to electron spin, one of an electron's degrees of freedom. The antiferromagnetic (AFM) interaction between electron spins means that the spins minimize their energy by orienting opposite to their neighbors. For example, if spin A was “up,” spin B would be “down,” antiparallel to spin A. However, introducing the third spin, as in an antiferromagnetically coupled equilateral triangle of electron spins, presents frustration (see the figure, panel A). Here, spin C cannot simultaneously satisfy the up-down interactions with spins A and B. Because spin C cannot achieve its “desired outcome” of being oppositely aligned with both its nearest neighbors, it is “frustrated.” On page 655 of this issue, Fu et al. (1) investigate the ground state of such a system by using nuclear magnetic resonance (NMR) to measure the intrinsic low-energy spin excitation of the ideal frustrated quantum magnet ZnCu3(OH)6Cl2.