Prof. Daniel Khomskii
University of Koeln
|TITLE:||"Spontaneous currents, dipoles and monopoles in frustrated Mott insulators"|
|DATE:||Thursday, October 27, 2011|
|PLACE:||Building 212 / A-157|
Refreshments will be served at 10:45 a.m.
ABSTRACT: 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. I demonstrate that in general this is not true [1, 2]: for certain spin textures there exist quite nontrivial effects in the ground and lowest excited states, connected with charge degrees of freedom. In particular this may happen in frustrated systems, e.g. containing triangles or tetrahedra as building blocks. I will 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 behaviour. In particular I show  that such electric dipoles should exist in spin ice materials at every tetrahedra with three-in/one-out or one-in /three-out spin configurations, which are equivalent to magnetic monopoles . Thus there should be an electric dipole attached to each magnetic monopole in spin ice. This leads to electric activity of magnetic monopoles, and opens the possibility to control magnetic monopoles by electric field. The possibility to use chirality as qubits will be also discussed.
 L.N.Bulaevskii, C.D.Batista, M.V.Mostovoy and D.I.Khomskii, Phys.Rev.B 78, 028402 (2008)
 D.I.Khomskii, JPCM 22, 164209 (2010)
 D.I.Khomskii, to be publ.
 C.Castelnovo, R.Moessner and S.L.Sondhi, Nature 451, 42 (2008)