Physics Department Colloquium
Multiferroic Vortices and Graph Theory
Time: 4:00 PM
Place: 2241 Chamberlin Hall (coffee at 330 pm)
Speaker: SangWook Cheong, Rutgers Center for Emergent Materials
Abstract: The fascinating concept of topological defects permeates ubiquitously our understanding of the earlystage universe, hurricanes, quantum matters such as superfluids and superconductors, and also technological materials such as liquid crystals and magnets. Largescale spatial configurations of these topological defects have been investigated only in a limited degree. Exceptions include the cases of supercurrent vortices or liquid crystals, but they tend to exhibit either trivial or ratherirregular configurations.Hexagonal REMnO3 (RE= rare earths) with RE=HoLu, Y, and Sc, is an improper ferroelectric where the size mismatch between RE and Mn induces a trimerizationtype structural phase transition, and this structural transition leads to three structural domains, each of which can support two directions of ferroelectric polarization. We reported that domains in hREMnO3 meet in cloverleaf arrangements that cycle through all six domain configurations [1], Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the cycle of domain configurations is reversed. Vortices and antivortices are topological defects even in a strong electric field they won't annihilate. Recently we have found intriguing, but seemingly irregular configurations of a zoo of topological vortices and antivortices in hREMnO3 [2]. These configurations can be neatly analyzed in terms of graph theory and this graph theoretical analysis reflects the nature of selforganized criticality in complexity phenomena as well as the condensation and eventual annihilation processes of topological vortexantivortex pairs.
[1] Insulating Interlocked Ferroelectric and Structural Antiphase Domain Walls in Multiferroic YMnO3, T. Choi, Y. Horibe, H. T. Yi, Y. J. Choi, Weida. Wu, and SW. Cheong, Nature Materials 9, 253258 (2010).
[2] SelfOrganization, Condensation and Annihilation of Topological Vortices and Antivortices in a Multiferroic, S. C. Chae, Y. Horibe, D. Y. Jeong, S. Rodan, N. Lee, and S.W. Cheong, PNAS, in print.
Host: Perkins
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