Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session M4: Focus Session: Pyrochlore magnets: spin ice and spin liquid |
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Sponsoring Units: GMAG Chair: Oleg Tchernyshyov, Johns Hopkins University Room: 112/110 |
Wednesday, March 5, 2014 11:15AM - 11:27AM |
M4.00001: Dy$_{2}$Ti$_{2}$O$_{7}$ Spin Ice Thin-Films Laura Bovo, Xavier Moya, Dharmalingam Prabhakaran, Yeong-Ah Soh, Andrew T. Boothroyd, Neil D. Mathur, Gabriel Aeppli, Steve T. Bramwell Spin ice[1] illustrates much novel science, including unusual phases, degeneracies, quasiparticles and topology[1-4]. A characteristic feature of spin ice is its apparent violation of the Third Law of thermodynamics. This leads to a number of interesting properties including the emergence of an effective vacuum for `magnetic monopoles' and their currents - `magnetricity'. Here we add a new dimension to the experimental study of spin ice by fabricating thin epitaxial films of Dy$_{2}$Ti$_{2}$O$_{7}$ on an inert substrate. The films show the distinctive characteristics of spin ice at temperatures greater than $2$ ${\rm K}$, but at lower temperature we find evidence of a zero entropy state. This restoration of the third law in spin ice thin films is consistent with a predicted [5] strain-induced ordering. Our results illustrates how the fabrication and study of thin films opens up new possibilities for the control and manipulation of the unusual magnetic properties of spin ice materials and related frustrated magnets. \\[4pt] [1] Harris M.J. et al. PRL 79, 2554(1997) [2] Ramirez A.P. et al. Nature 399, 333(1999) [3] Ryzhkin I.A. J. Exp. and Theor. Phys. 101, 481(2005) [4] Castelnovo C. et al. Nature 451, 42(2008) [5] Jaubert L.D.C. PRL 105, 087201(2010) [Preview Abstract] |
Wednesday, March 5, 2014 11:27AM - 11:39AM |
M4.00002: Wien Effect on a Lattice Vojtech Kaiser, Steven Bramwell, Peter Holdsworth, Roderich Moessner The Second Wien Effect is an increase of conductivity of Coulomb gas in an external field, driven by enhanced dissociation of Coulombically bound pairs. The importance of the Wien effect for spin ice was suggested previously since spin ice maps to a Coulomb gas of magnetic monopoles. We present simulations of a lattice Coulomb gas and spin ice. The results confirm Onsager's theory of the Wien effect and reveal additional corrections, while allowing access to microscopic dynamics underlying the increase in the charge carrier density. Main extensions of the original theory involve the Debye screening, field dependent mobility and the character of the association constant. We discuss further corrections specific to spin ice due its emergent topological charge and Dirac string network. [Preview Abstract] |
Wednesday, March 5, 2014 11:39AM - 11:51AM |
M4.00003: Hubbard Model on the Pyrochlore Lattice: a 3D Quantum Spin Liquid B. Normand, Z. Nussinov We demonstrate that the insulating one-band Hubbard model on the pyrochlore lattice contains, for realistic parameters, an extended quantum spin-liquid phase. This is a three-dimensional spin liquid formed from a highly degenerate manifold of dimer-based states, which is a subset of the classical dimer coverings obeying the ice rules. It possesses spinon excitations, which are both massive and deconfined, and on doping it exhibits spin-charge separation. We discuss the realization of this state in effective $S = 1/2$ pyrochlore materials. [Preview Abstract] |
Wednesday, March 5, 2014 11:51AM - 12:03PM |
M4.00004: Time Domain Terahertz Spectroscopy Study of Composite Spin Excitations in a Quantum Spin Ice LiDong Pan, Christopher M. Morris, Se Kwon Kim, Kate A. Ross, Edwin Kermarrec, S.M. Koohpayeh, Oleg Tchernyshyov, Bruce D. Gaulin, N. Peter Armitage We report the terahertz transmission spectra of the quantum spin ice material $Yb_{2}Ti_{2}O_{7}$. Several branches of magnetic absorption are observed with applied magnetic field. We compare the experimental results with classical spin wave analysis, and identify signatures of composite spin excitations. [Preview Abstract] |
Wednesday, March 5, 2014 12:03PM - 12:15PM |
M4.00005: Sm$_{2}$Ti$_{2}$O$_{7}$: An exchange spin ice candidate? Paul Sarte, Harlyn Silverstein, Arzoo Sharma, Alannah Hallas, Haidong Zhou, Bruce Gaulin, Christopher Wiebe A phase pure single crystal of Sm$_{2}$Ti$_{2}$O$_{7}$ was grown from phase pure powder synthesized by a standard solid state reaction. A Curie-Weiss fit yielded a Curie constant corresponding to a smaller $\mu_{eff}$ compared to $\mu_{eff,free}$ and a value for $\theta_{CW}$ corresponding to dominant AFM interactions. C$_{p}$ measurements were performed at 0 T and 9 T down to 0.35 K with both yielding a low T anomaly but with the latter being shifted to lower T with an increase in $\Delta$ from a high T expansion of the Schottky anomaly. While the Schottky fit was successful for 0 T, the fit proved unsuccessful for 9 T indicating possible ordering. With the reduced $\mu_{eff}$ and the lack of an LRO state down to 0.35 K, providing an ${f}>>$1, the system is frustrated with its ${\cal H}$ being $J$ dominated. Future work will consist of growing an isotopically pure crystal for neutron scattering, lower T DC $\chi$ measurements to reduce CF effects, AC $\chi$ to yield $\tau$ through degenerate configurations with the objective for providing a comparison with spin ices in literature. Furthermore, additional C$_{p}$ at multiple H$_{o}$ and at lower T will be performed to determine both $\Delta(H_{o}$) and if the 9 T anomaly is indeed a transition that is electronic in origin. [Preview Abstract] |
Wednesday, March 5, 2014 12:15PM - 12:27PM |
M4.00006: Far from equilibrium behaviour of spin ice materials Claudio Castelnovo, Sarah Mostame, Roderich Moessner, Shivaji Sondhi Non-equilibrium physics in spin ice is a novel setting which combines kinematic constraints, emergent topological defects, and magnetic long range Coulomb interactions. In spin ice, magnetic frustration leads to highly degenerate yet locally constrained ground states. Together, they form a highly unusual magnetic state -- a ``Coulomb phase'' -- whose excitations are pointlike defects -- magnetic monopoles -- in the absence of which effectively no dynamics is possible. At low temperatures, the monopoles are sparse and dynamics becomes very slow. These systems are therefore prone to falling out of equilibrium at low temperatures, for instance following comparatively rapid changes in temperature or applied magnetic field. In this regime, a wealth of dynamical phenomena occur, including reaction diffusion behaviour, slow dynamics due to kinematic constraints, as well as behaviour that mimic the deposition of interacting dimers on a lattice. The situation is further complicates by the presence of disorder that, even at small densities, appears to have a sizeable effect on the low-temperature dynamics of these systems. Here we investigate some of these phenomena and we propose how to effectively extend existing theories to to describe spin ice far from equilibrium. [Preview Abstract] |
Wednesday, March 5, 2014 12:27PM - 12:39PM |
M4.00007: Engineering entropy: novel phases on the pyrochlore lattice Owen Benton, Han Yan, Ludovic Jaubert, Nic Shannon Frustrated pyrochlores such as Yb$_2$Ti$_2$O$_7$ push our understanding of magnetism to its limits [1, 2]. Here we explore a highly general model for spins on the pyrochlore lattice. We establish a complete phase diagram for the model [3] and are able to identify several previously unstudied limits where classical order breaks down entirely. Here we focus on two limits of special interest: a classical spin liquid and a ``hidden order" spin nematic. These ideas are explored in the context of experiments on the pyrochlore stannates and titanates. [1] J. S. Gardner, M. J. P. Gingras, J. E. Greedan, Rev. Mod. Phys. {\bf 82}, 53, (2010). [2]~K.~A.~Ross, L. Savary, B. D. Gaulin and L. Balents, Phys. Rev. X {\bf 1}, 021002 (2011). [3]~H.~Yan, O.~Benton, L. Jaubert and N.~Shannon, arXiv:1311.3501. [Preview Abstract] |
Wednesday, March 5, 2014 12:39PM - 12:51PM |
M4.00008: Quantum effects in a realistic model of spin ice Nic Shannon, Olga Sikora, Frank Pollmann, Karlo Penc, Paul McClarty, Roderich Moessner The spin ice materials Ho2Ti2O7 and Dy2Ti2O7 offer a widely-studied example of a classical spin liquid, complete with magnetic monopole excitations. Here we use exact diagonalization and quantum Monte Carlo simulation and to explore how quantum tunnelling between different ice states changes the ground state of a realistic model of a spin ice. We find that the competition between long-range dipolar interactions and second-neighbour exchange interactions helps to stabilize a quantum spin liquid phase and, for large enough exchange, a ferromagnet ground state. We discuss the implications of these results for the spin ice Dy2Ti2O7. [Preview Abstract] |
Wednesday, March 5, 2014 12:51PM - 1:03PM |
M4.00009: Effective electromagnetism in rare-earth pyrochlore oxides Han Yan, Ludovic Jaubert, Owen Benton, Nic Shannon Rare-earth pyrochlore oxides show a fabulously diverse range of different forms of magnetism, including both classical and quantum spin-liquid phases. Here we develop a unified picture of ordered and disordered states in a pyrochlore magnet with anisotropic exchange interactions. We find that the great majority of these phases can be understood in terms of a simple classical field theory, reminiscent of electromagnetism. [Preview Abstract] |
Wednesday, March 5, 2014 1:03PM - 1:39PM |
M4.00010: Magnetoelastic spin liquid in Tb2Ti2O7? Invited Speaker: Tom Fennell In the rare earth pyrochlore Tb$_2$Ti$_2$O$_7$, a three-fold puzzle exists - the mechanism by which Tb$_2$Ti$_2$O$_7$ escapes both magnetic order and/or a structural distortion, and furthermore, the nature of the spin liquid which exists instead, are long standing questions in the field of frustrated magnetism. Recent theories propose that classical spin order is suppressed by virtual crystal field excitations which renormalize the antiferromagnetic exchange, making Tb$_2$Ti$_2$O$_7$ into a type of quantum spin ice [1]; or that an undetected structural distortion leads to a spin-liquid state built of singlets [2]. Using polarized neutron scattering, we have recently shown that, at low temperature, Tb$_2$Ti$_2$O$_7$ has power-law spion correlations, manifested by pinch point scattering, somewhat similar to a spin ice [3]. We have also discovered that an acoustic phonon is coupled to an excited crystal field state, producing a sharp, dispersive mode with both magnetic and phononic character [4]. I will show that the overall structure of the low temperature state of Tb$_2$Ti$_2$O$_7$ should therefore be viewed as a Coulomb phase with propagating spin excitations [4,5]. \\[4pt] [1] Molavian et al., PRL 98, 157204 (2007);\\[0pt] [2] Petit et al., PRB 86, 174403 (2012);\\[0pt] [3] Fennell et al., PRL 109, 017201 (2012);\\[0pt] [4] Fennell et al., arXiv:1305.5405;\\[0pt] [5] Guitteney et al., PRL 111, 087201 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 1:39PM - 1:51PM |
M4.00011: Spin freezing in geometrically frustrated magnets Jorge Rehn, Arnab Sen, Alexei Andreanov, Antonello Scardicchio, Kedar Damle, Roderick Moessner Materials which are believed to be faithfully represented by classical frustrated magnets with macroscopically degenerate groundstates, often exhibit spin-freezing. The latter is a transition to a spin-glass phase. Explaining the mechanism of such freezing is not always a simple task, since conventional ingredients, like randomness of the interactions, is not always present in the systems under study. We present a model, where dilution alone generates frustrating interaction between certain spins in the systems and leads to their freezing. The effective model deals with antiferromagnetically coupled Heisenberg spins in 2D. Both the long-range nature of the interaction and its dependence on the distance are crucial for the existence of the glass phase. We confirm our predictions by performing Monte-Carlo simulation of the effective model. [Preview Abstract] |
Wednesday, March 5, 2014 1:51PM - 2:03PM |
M4.00012: Stability of Gapless Quantum Spin-Liquids Tarun Grover Strong correlations can lead to novel quantum phases with striking features, such as, emergent fermions and photons in a bosonic system, or even phases which lack any sharply defined quasiparticle. Given their scarcity, a fundamental question is: when are such `fractionalized' phases stable? In this talk, I will employ the recently discovered results which relate quantum entanglement and the renormalization group, to determine the stability of several gapless quantum spin-liquids. I will also provide a general argument which shows that the phase transitions out of a topological phase necessarily lie beyond Landau-Ginzburg paradigm. [Preview Abstract] |
Wednesday, March 5, 2014 2:03PM - 2:15PM |
M4.00013: Spectroscopic signatures of crystal momentum fractionalization Andrew Essin, Michael Hermele In spin liquids with fractional excitations, the low-energy edge $\Omega(q)$ of the two-spinon continuum carries information about the single-spinon physics. This physics is accessible experimentally in inelastic neutron scattering, for example, in the dynamical spin structure factor. We show that some types of quantum-number fractionalization in gapped, $Z_2$ spin liquids lead to dramatic signatures in $\Omega(q)$. Notably, it may need to repeat within the first Brillouin zone, which is a direct signature of fractional crystal momentum, remarkable in the absence of symmetry-breaking spatial order. [Preview Abstract] |
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