Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session A3: Recent Progress on 'Order by Disorder' PhenomenaInvited

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Sponsoring Units: DCMP GMAG Chair: Israel Klich, University of Virginia Room: Ballroom III 
Monday, March 14, 2016 8:00AM  8:36AM 
A3.00001: Order From disorder in Frustrated Spin Systems Invited Speaker: Piers Coleman This talk will review the phemomenon of "Order from disorder": the mechanism by which fluctuations remove a degeneracy within a frustrated spin system. An important consequence of orderfromdisorder, is the ability of frustrated Heisenberg spin systems to overcome the MerminWagner theorem, developing new forms of discrete order, even when the spins themselves remain disordered with a finite correlation length. The most wellknown example, is the twodimensional frustrated $J_1J_2$ Heisenberg model, which undergoes a finite temperature Ising phase transition into a stripy or "nematic" state, even though the spins do not order until absolute zero[1,2]. Nematic ordering of this kind is believed to occur in the ironbased superconductors, such as $BaFe_2As_2$. More recently, it has been possible to theoretically study the triangularhoneycomb versions of the $J_1J_2$ model, called a windmill model[34], in which orderfrom disorder drives the development of sixstate clock order. Remarkably, in this case, orderfromdisorder leads to an intermediate powerlaw spin phase, despite the underlying Heisenerg spins.\\ \hbox{[1]}C. L. Henley, Phys. Rev. Lett. 62, 2056 (1989).\\ \hbox{[2]}P. Chandra, P. Coleman, and A. I. Larkin, Phys. Rev. Lett. 64, 88 (1990).\\ \hbox{[3]}P. P. Orth, P. Chandra, P. Coleman, and J. Schmalian, Phys. Rev. Lett. 109, 237205 (2012).\\ \hbox{[4]}P. P. Orth, P. Chandra, P. Coleman, and J. Schmalian, Phys. Rev. B 89, 094417 (2014).\\ \hbox{[4]}B. Jeevanesan, P. Chandra, P. Coleman, P. P. Orth, Phys. Rev. Lett. 115, 177201 (2015). [Preview Abstract] 
Monday, March 14, 2016 8:36AM  9:12AM 
A3.00002: Love triangles, quantum fluctuations and spin jam Invited Speaker: SeungHun Lee When magnetic moments are interacting with each other in a situation resembling that of complex love triangles, called frustration, a large set of states that are energetically equivalent emerge. This leads to exotic spin states such as spin liquid and spin ice. Recently, we presented evidence for the existence of a topological glassy state, that we call spin jam, induced by quantum fluctuations.[1,2] The case in point is SrCr$_{9p}$Ga$_{129p}$O$_{19}$ (SCGO(p)), a highly frustrated magnet, in which the magnetic Cr ions form a quasitwodimensional triangular system of bipyramids. This system has been an archetype in search for exotic spin states. Understanding the nature of the state has been a great intellectual challenge. Our new experimental data and theoretical spin jam model provide for the first time a coherent understanding of the phenomenon. Furthermore, the findings strongly support the possible existence of purely topological glassy states. Reference: [1] \textit{Spin jam induced by quantum fluctuations in a frustrated magnet, }J. Yang \textit{et al}., Proc. Natl. Acad. Sci. of U.S.A. Vol. 127, 1151911523 (2015). [2] \textit{Glassiness and exotic entropy scaling induced by quantum fluctuations in a disorderfree frustrated magnet, }I. Kich, S.H. Lee, K. Iida, Nature Communications \textbf{5}, 3497 (2014). [Preview Abstract] 
Monday, March 14, 2016 9:12AM  9:48AM 
A3.00003: The many faces of orderbydisorder in rareearth pyrochlore magnets Invited Speaker: Michel J P Gingras Orderbydisorder (ObD) is a concept of central importance in the field of frustrated magnetism. Saddled with large accidental degeneracies, a subset of states, those that support the largest quantum and/or thermal fluctuations, may be selected to form true longrange order. More formally, one often begins describing a system in terms of some order parameter $m$ with the lowenergy description framed in terms of an effective action $\Gamma(m)$. In each ObD scenario, one starts from an artificial limit where there is an accidental degeneracy; that is the effective action at this point, $\Gamma_0(m)$, has an accidental symmetry. One may then view ObD phenomena as cases where the corrections to $\Gamma_0(m)$ arise through some form of fluctuation corrections, may they be thermal, quantum or virtual, towards an enlarged higher energy Hilbert space. In the rareearth pyrochlore oxides, of formula $R_2$$M_2$O$_7$, the trivalent magnetic rareearth ions $R^{3+}$ (e.g $R=$Gd, Er, Yb; $M=$Ti, Sn is nonmagnetic) reside on a threedimensional pyrochlore lattice of cornersharing tetrahedra. This architecture is prone to a high degree of magnetic frustration, with the $R_2M_2$O$_7$ pyrochlore materials having been found over the past twenty years to display a gamut of exotic phenomena. In this talk, I will discuss three such phenomena: (i) the intermediate partiallyordered multiple$k$ state between $0.7$K and $1$K in the Gd$_2$Ti$_2$O$_7$ Heisenberg antiferromagnet~\footnote{$\,$PRL \textbf{114}, 130601 (2015)}, (ii) the ordered $\psi_2$ state selection in the $XY$ Er$_2$Ti$_2$O$_7$ antiferromagnet~\footnote{$\,$arXiv:1510.04292} and (iii) the puzzling high sample sensitivity of the Yb$_2$Ti$_2$O$_7$ ``quantum spin ice'' candidate~\footnote{$\,$arXiv:1505.05499}. I will argue that in all three cases, some form of fluctuation corrections to their simplest $\Gamma_0(m)$ description play a significant role in the state selection and experimentally observed behaviors. [Preview Abstract] 
Monday, March 14, 2016 9:48AM  10:24AM 
A3.00004: Order by Disorder in the XY Pyrochlore Antiferromagnet Er$_2$Ti$_2$O$_7$ Invited Speaker: Bruce D. Gaulin Crystal field effects associated with Er$^{3+}$ magnetic moments in Er$_2$Ti$_2$O$_7$ give rise to local XY anisotropy and effective quantum S=1/2 spins which are antiferromagnetically coupled on this materialâ€™s cubic pyrochlore lattice [1]. Er$_2$Ti$_2$O$_7$ orders into a noncollinear antiferromagnetic $\Psi_2$ state below $\sim$ 1.2 K, in zero magnetic field, but the mechanism for its ground state selection has been a puzzle for more than a decade. We have carried out inelastic neutron scattering measurements on single crystal samples of Er$_2$Ti$_2$O$_7$ at low temperatures and in the presence of a strong [110] magnetic field, allowing us to determine the underlying spin Hamiltonian for this quantum antiferromagnet [2, 3]. These results point to ground state selection via an orderbyquantumdisorder mechanism [3], and a concomitant orderbydisorder gap of $\sim$ 0.05 meV has also been observed [4], associated with the pseudoGoldstone modes in the low field ordered state. In addition, we have explored the sensitivity of the ground state selection to magnetic dilution by preparing and studying single crystals of Er$_{2x}$Y$_x$Ti$_2$O$_7$ [5]. These studies are particularly topical in light of two theoretical predictions [6,7] that the $\Psi_2$ ordered state may be unstable to formation of the related $\Psi_3$ phase at low temperatures, in the presence of quenched disorder. [1] J.D.M. Champion et al., Phys. Rev. B 68, 020401 (2003). [2] J.P.C. Ruff et al., Phys. Rev. Lett., 101, 147205 (2005). [3] L. Savary et al. Phys. Rev. Lett., 109, 167201 (2012). [4] K.A. Ross et al. Phys. Rev. Lett. 112, 057201 (2014). [5] J.F. Niven, Proc. R. Soc. A, 470: 20140387 (2014). [6] V. S. Maryasin and M. E. Zhitomirsky, Phys. Rev. B 90, 094412 (2014). [7] A. Andreanov and P. A. McClarty, Phys. Rev. B 91, 064401 (2015). [Preview Abstract] 
Monday, March 14, 2016 10:24AM  11:00AM 
A3.00005: Quantum orderbydisorder and excitations in anisotropic kagomelattice antiferromagnets Invited Speaker: Alexander Chernyshev Our recent works have advanced theoretical understanding of the quantum effects in kagomelattice antiferromagnets and have provided insights into the quantum orderbydisorder mechanism, important for a broad class of frustrated spin systems. In particular, we have challenged a general expectation that the quantum and thermal orderbydisorder mechanisms always select the same ground state. We have shown that the nonlinear terms in the quantum hamiltonian of the anisotropic kagomelattice antiferromagnets can yield a rare example of the ground state that is different from the one favored by thermal fluctuations. We have also demonstrated that the order selection is generated by topologically nontrivial tunneling processes, yielding a new energy scale in the system. \\ Related to the groundstate selection mechanism are the nonlinear effects in the spectra of the kagomelattice systems. Further progress has been made in understanding spectral properties of realistic kagomelattice antiferromagnets such as Fejarosite, for which we have demonstrated a remarkable wipeout effect for a significant portion of the spectrum. This phenomenon is related to an existence of the socalled "flat mode," a ubiquitous feature of the kagomelattice and other highlyfrustrated antiferromagnets, and is due to a resonantlike decay processes involving two of such modes.\\ References: [1] A. L. Chernyshev and M. E. Zhitomirsky, "Quantum Selection of Order in an XXZ Antiferromagnet on a Kagom'e Lattice", Phys. Rev. Lett. 113, 237202 (2014). [2] A. L. Chernyshev, "Strong quantum effects in an almost classical antiferromagnet on a kagome lattice", Phys. Rev. B 92, 094409 (2015). [3] A. L. Chernyshev and M. E. Zhitomirsky, "Order and excitations in largeS kagomelattice antiferromagnets", Phys. Rev. B 92, 144415 (2015). (Editors' Suggestion). [Preview Abstract] 
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