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The standard point of view is that at low energies Mott insulators exhibit only magnetic properties, while charge degrees of freedom are frozen out, because electrons are localized. We demonstrate [1] that in general this is not true : for certain spin textures there exist quite nontrivial effects in the ground and lowest excited states, connected with charge degrees of freedom. We show that in some cases there may exist spontaneous circular currents in the ground state of insulators, proportional to the scalar chirality; this clarifies the meaning of the latter and opens the ways to directly experimentally access it. For other spin structures there may exist spontaneous charge redistribution, so that average charge at a site may be different from 1. This can lead to the appearance of dipole moments and possibly of the net spontaneous polarization. This is a novel, purely electronic mechanism of multiferroic behavior. I will discuss also some dynamic consequences of the effects discovered, such as dipole-active "ESR" transitions, rotation of electric polarization by magnetic field, and possibility to get negative refraction. [1] L.N. Bulaevskii, C.D. Batista, M.V. Mostovoy and D.I. Khomskii, "Electronic Orbital Currents and Polarization in Mott Insulators", Phys.Rev.B 78, 028402 (2008) |
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