Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W39: Invited Session: Emergent Symmetries in Magnetic and Ferroelectric Systems |
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Sponsoring Units: DCMP DMP Room: Mile High Ballroom 2A-3A |
Thursday, March 6, 2014 2:30PM - 3:06PM |
W39.00001: Critical Properties of the Kitaev-Heisenberg Model Invited Speaker: Natalia B. Perkins |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W39.00002: Spin dynamics in layered honeycomb iridates: implications for Kitaev physics Invited Speaker: Sungkyun Choi We explore the spin dynamics in the frustrated honeycomb magnets Na$_{2}$IrO$_{3}$ [1] and Li$_{2}$IrO$_{3}$, candidates to display novel magnetic states stabilized by the strong spin-orbit coupling at the 5d Ir ions. Theory [2] predicts composite spin-orbital J$=$1/2 moments at the Ir ions coupled by strongly-anisotropic and bond-directional exchanges, the so-called Kitaev honeycomb model, which has in its phase diagram novel magnetically-ordered ordered phases and a quantum spin liquid with exotic excitations. To search for such physics the experimental technique of choice is inelastic neutron scattering to probe the spin dynamics, however this is technically very challenging due to the large absorption cross-section of neutrons by the Ir nuclei. Using an optimised setup to minimise neutron absorption we have been successful in observing strongly dispersive spin-wave excitations of the Ir moments in both compounds and results are compared with predictions for a Kitaev-Heisenberg model as well as a Heisenberg model with further neighbour couplings. \\[4pt] [1] S. K. Choi, R. Coldea, A. N. Kolmogorov, T. Lancaster, I. I. Mazin, S. J. Blundell, P. G. Radaelli, Yogesh Singh, P. Gegenwart, K. R. Choi, S.-W. Cheong, P. J. Baker, C. Stock, and J. Taylor, Phys. Rev. Lett. 108, 127204 (2012). \\[0pt] [2] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010). [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W39.00003: Emergent Criticality and Ricci Flow in a 2D Frustrated Heisenberg Model Invited Speaker: Peter P. Orth In most systems that exhibit order at low temperatures, the order occurs in the elementary degrees of freedom such as spin or charge. Prominent examples are magnetic or superconducting states of matter. In contrast, emergent order describes the phenomenon where composite objects exhibit longer range correlations. Such emergent order has been suspected to occur in a range of correlated materials. One specific example are spin systems with competing interactions, where long-range discrete order in the relative orientation of spins may occur. Interestingly, this order parameter may induce other phase transitions as is the case for the nematic transition in the iron pnictides. In this talk, we introduce and discuss a system with emergent $Z_6$ symmetry, a two-dimensional frustrated Heisenberg antiferromagnet on the windmill lattice consisting of interpenetrating honeycomb and triangular lattices. The multiple spin stiffnesses can be captured in terms of a four-dimensional metric tensor, and the renormalization group flow of the stiffnesses is described by the Ricci flow of the metric tensor. The key result is a decoupling of an emergent collective degree of freedom given by the relative phase of spins on different sublattices. In particular, our results reveal a sequence of two Berezinskii-Kosterlitz-Thouless phase transitions that bracket a critical phase. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W39.00004: Z$_{2}$ x Z$_{3}$ Symmetry of Multferroic Vortices Invited Speaker: Sang-Wook Cheong Hexagonal REMnO$_{3}$ (RE$=$ rare earths) with RE$=$Ho-Lu, Y, and Sc, is an improper ferroelectric where the size mismatch between RE and Mn induces a trimerization-type 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 h-REMnO$_{3}$ meet in cloverleaf arrangements that cycle through all six domain configurations, 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. These ferroelectric vortices/antivortices are found to be associated with intriguing collective magnetism at domain walls, reflecting the multiferroic nature of vortices. We have found that an intriguing, but seemingly irregular network of a zoo of multiferroic vortices and antivortices in h-REMnO$_{3}$ can be neatly analyzed in terms of graph theory, and this graph theoretical analysis reveals the emergence of Z$_{2}$ $\times$ Z$_{3}$ symmetry in the vortices/antivortices network. In addition, poling or self-poling due to a surface charge boundary condition induces global topological condensation of the network through breaking of the Z$_{2}$ part of the Z$_{2}$ $\times$ Z$_{3}$ symmetry. The opposite process of restoring the Z$_{2}$ symmetry can be considered as topological evaporation. It turns out that these Z$_{2}$xZ$_{3}$ vortices are, in fact, three-dimensional vortex loops, which result from the emergent continuous U(1) symmetry near the critical temperature. This spontaneous trapping of topological defects in the process of undergoing a continuous phase transition is important to understand numerous novel phenomena such as the early stage of universe after big bang. The so-called Kibble-Zurek mechanism was proposed for the trapping process of topological defects right after big bang. It appears that the Kibble-Zurek mechanism is also responsible for the network formation of multiferroic vortices, and thus, hexagonal REMnO$_{3}$ is a test bed for the birth of this cosmos. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W39.00005: Hidden Monopolar Order in Magnetoelectrics Invited Speaker: Nicola Spaldin I will discuss a recently proposed [Monopole-based formalism for the diagonal magnetoelectric response, N. A. Spaldin, M. Fechner, E. Bousquet, A. Balatsky and L. Nordstrom, Phys. Rev. B 88, 094429 (2013)] form of hidden order -- the magnetoelectric monopole -- and its relationship to a material's magnetoelectric response. Using density functional calculations for the Li transition metal phosphate series, LiMPO$_4$, with M = Mn, Fe, Co and Ni, I will show that materials with the same overall antiferromagnetic ordering can have distinct ferromonopolar or antiferromonopolar orderings, that lead to different, and in principle measureable, magnetoelectric responses. The current status and open questions in both the theoretical formalism and experimental verification will be outlined. [Preview Abstract] |
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