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
|
Hide Abstracts |
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 order-from-disorder, is the ability of frustrated Heisenberg spin systems to overcome the Mermin-Wagner theorem, developing new forms of discrete order, even when the spins themselves remain disordered with a finite correlation length. The most well-known example, is the two-dimensional frustrated $J_1-J_2$ 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 iron-based superconductors, such as $BaFe_2As_2$. More recently, it has been possible to theoretically study the triangular-honeycomb versions of the $J_1-J_2$ model, called a windmill model[3-4], in which order-from disorder drives the development of six-state clock order. Remarkably, in this case, order-from-disorder leads to an intermediate power-law 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: Seung-Hun 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$_{12-9p}$O$_{19}$ (SCGO(p)), a highly frustrated magnet, in which the magnetic Cr ions form a quasi-two-dimensional triangular system of bi-pyramids. 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, 11519-11523 (2015). [2] \textit{Glassiness and exotic entropy scaling induced by quantum fluctuations in a disorder-free 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 order-by-disorder in rare-earth pyrochlore magnets Invited Speaker: Michel J P Gingras Order-by-disorder (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 long-range order. More formally, one often begins describing a system in terms of some order parameter $m$ with the low-energy 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 rare-earth pyrochlore oxides, of formula $R_2$$M_2$O$_7$, the trivalent magnetic rare-earth ions $R^{3+}$ (e.g $R=$Gd, Er, Yb; $M=$Ti, Sn is non-magnetic) reside on a three-dimensional pyrochlore lattice of corner-sharing 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 partially-ordered 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 material’s cubic pyrochlore lattice [1]. Er$_2$Ti$_2$O$_7$ orders into a non-collinear 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 order-by-quantum-disorder mechanism [3], and a concomitant order-by-disorder gap of $\sim$ 0.05 meV has also been observed [4], associated with the pseudo-Goldstone 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$_{2-x}$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 order-by-disorder and excitations in anisotropic kagome-lattice antiferromagnets Invited Speaker: Alexander Chernyshev Our recent works have advanced theoretical understanding of the quantum effects in kagome-lattice antiferromagnets and have provided insights into the quantum order-by-disorder mechanism, important for a broad class of frustrated spin systems. In particular, we have challenged a general expectation that the quantum and thermal order-by-disorder mechanisms always select the same ground state. We have shown that the non-linear terms in the quantum hamiltonian of the anisotropic kagome-lattice 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 non-trivial tunneling processes, yielding a new energy scale in the system. \\ Related to the ground-state selection mechanism are the non-linear effects in the spectra of the kagome-lattice systems. Further progress has been made in understanding spectral properties of realistic kagome-lattice antiferromagnets such as Fe-jarosite, for which we have demonstrated a remarkable wipe-out effect for a significant portion of the spectrum. This phenomenon is related to an existence of the so-called "flat mode," a ubiquitous feature of the kagome-lattice and other highly-frustrated antiferromagnets, and is due to a resonant-like 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 large-S kagome-lattice antiferromagnets", Phys. Rev. B 92, 144415 (2015). (Editors' Suggestion). [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700