The Impact of Ionic Frustration on Electronic Order

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Science  04 May 2012:
Vol. 336, Issue 6081, pp. 547-548
DOI: 10.1126/science.1221364

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For some metal oxides, the simplest applications of band theory predict that they should be good electrical conductors, but in fact they are Mott insulators—that is, the electrons that should be mobile are localized to metal atoms. These electrons retain internal degrees of freedom; they may occupy more than one type of orbital of the metal (they have an orbital degree of freedom) and also may adopt different spin states. These degrees of freedom, along with accompanying structural changes, can create materials with exotic electronic ordering that have potential applications such as magnetoelectric devices and quantum computing. If the spins and orbital degrees of freedom can both adopt more than one quantum state, they may entangle—that is, a measurement of spin determines the orbital state, and vice versa (1). On page 559 of this issue, Nakatsuji et al. (2) introduce a new ingredient, an ionic degree of freedom that creates a lattice of dipoles that is “frustrated”—incapable of completely ordering locally. Ordering is seen on nanometer dimensions, and it strongly affects the spin and orbital character of the electronic state.