Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session Y18: Frustrated Magnetism: Pyrochlore and Kagome LatticesFocus
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Sponsoring Units: GMAG DMP Chair: Lucile Savary, MIT Room: 317 |
Friday, March 18, 2016 11:15AM - 11:51AM |
Y18.00001: Quantum spin ices and magnetic states from dipolar-octupolar doublets on the pyrochlore lattice Invited Speaker: Gang Chen We consider a class of electron systems in which dipolar-octupolar Kramers doublets arise on the pyrochlore lattice. In the localized limit, the Kramers doublets are described by the effective spin 1/2 pseudospins. The most general nearest-neighbor exchange model between these pseudospins is the XYZ model. In additional to dipolar ordered and octupolar ordered magnetic states, we show that this XYZ model exhibits two distinct quantum spin ice (QSI) phases, that we dub dipolar QSI and octupolar QSI. These two QSIs are distinct symmetry enriched U(1) quantum spin liquids, enriched by the lattice symmetry. Moreover, the XYZ model is absent from the notorious sign problem for a quantum Monte Carlo simulation in a large parameter space. We discuss the potential relevance to real material systems such as Dy2Ti2O7, Nd2Zr2O7, Nd2Hf2O7, Nd2Ir2O7, Nd2Sn2O7 and Ce2Sn2O7. [Preview Abstract] |
Friday, March 18, 2016 11:51AM - 12:03PM |
Y18.00002: Layered kagome spin ice James Hamp, Sian Dutton, Martin Mourigal, Paromita Mukherjee, Joseph Paddison, Harapan Ong, Claudio Castelnovo Spin ice materials provide a rare instance of emergent gauge symmetry and fractionalisation in three dimensions: the effective degrees of freedom of the system are emergent magnetic monopoles, and the extensively many `ice rule' ground states are those devoid of monopole excitations. Two-dimensional (kagome) analogues of spin ice have also been shown to display a similarly rich behaviour. In kagome ice however the ground-state `ice rule' condition implies the presence everywhere of magnetic charges. As temperature is lowered, an Ising transition occurs to a charge-ordered state, which can be mapped to a dimer covering of the dual honeycomb lattice. A second transition, of Kosterlitz-Thouless or three-state Potts type, occurs to a spin-ordered state at yet lower temperatures, due to small residual energy differences between charge-ordered states. Inspired by recent experimental capabilities in growing spin ice samples with selective (layered) substitution of non-magnetic ions, in this work we investigate the fate of the two ordering transitions when individual kagome layers are brought together to form a three-dimensional pyrochlore structure coupled by long range dipolar interactions. We also consider the response to substitutional disorder and applied magnetic fields. [Preview Abstract] |
Friday, March 18, 2016 12:03PM - 12:15PM |
Y18.00003: Theory of quantum kagome ice Yi-Ping Huang, Michael Hermele Some pyrochlore oxides realize novel dipolar-octupolar (DO) doublets on the sites of the pyrochlore lattice of corner-sharing tetrahedra. With magnetic field along the (111) direction, such systems can approximately be described as decoupled layers of a $S=\frac{1}{2}$ XYZ model on Kagome planes, with perpendicular magnetic field. A recent quantum Monte Carlo study found a zero temperature disordered phase in this model, dubbed quantum kagome ice, and proposed that it is a type of $Z_2$ quantum spin liquid (J. Carrasquilla, Z. Hao and R. G. Melko, \emph{Nat. Comm.}, \textbf{6}, 7421). We will describe an effective theory for this putative $Z_2$ spin liquid, and present results on its symmetry fractionalization and resulting properties that may be tested in future numerical simulations. [Preview Abstract] |
Friday, March 18, 2016 12:15PM - 12:27PM |
Y18.00004: Dynamics of quantum excitations in square ice Claudio Castelnovo, Stefanos Kourtis The study of emergent excitations in classical spin ice has culminated in the discovery of a condensed-matter realization of magnetic monopoles. In spin-ice materials where quantum fluctuations play an important role, excitations acquire quantum properties that promote them to more complicated and exciting objects. To understand these quantum excitations better in a relatively simple context, we construct a toy model of excited square ice and solve it both exactly by tuning it to a Rokhsar-Kivelson point and numerically for small clusters. We furthermore numerically evaluate the dynamic spin structure factor and compare it to effective free-particle theories. Our results offer a useful point of comparison for further theoretical and experimental work. [Preview Abstract] |
Friday, March 18, 2016 12:27PM - 12:39PM |
Y18.00005: Magnetic monopoles in quantum spin ice Olga Petrova, Roderich Moessner, Shivaji Sondhi Typical spin ice materials can be modeled using classical Ising spins. The geometric frustration of the pyrochlore lattice causes the spins to satisfy ice rules, whereas a violation of the ice constraint constitutes an excitation. Flipping adjacent spins fractionalizes the excitation into two monopoles. Long range dipolar spin couplings result in Coulombic interactions between charges, while the leading effect of quantum fluctuations is to provide the monopoles with kinetic energy. We study the effect of adding quantum dynamics to spin ice, a well-known classical spin liquid, with a particular view of how to best detect its presence in experiment. For the weakly diluted quantum spin ice, we find a particularly crisp phenomenon, namely, the emergence of hydrogenic excited states in which a magnetic monopole is bound to a vacancy at various distances [1]. \\ [4pt] [1] O. Petrova, R. Moessner, S. L. Sondhi, Phys. Rev. B \textbf{92}, 100401 (2015) [Preview Abstract] |
Friday, March 18, 2016 12:39PM - 12:51PM |
Y18.00006: Spinon walk in quantum spin ice Yuan Wan, Juan Carrasquilla, Roger Melko Quantum spin ice is a novel family of spin ice magnets that possess substantial quantum fluctuations. The fractional excitations are spinons, which are quantum analog of the monopoles in classical spin ice. The spinon propagates in quantum spin ice via quantum tunnelling. As opposed to a conventional quantum particle, the spinon moves in a background of disordered spins. The orientation of background spins controls the spinon motion, whereas the spinon motion in turn alters the spin background. One may naturally ask what a suitable framework for understanding the dynamics of spinon is in quantum spin ice, and furthermore, whether the spinon propagation is coherent. In this talk, we address these issues by investigating a minimal model that captures the essential features of single spinon dynamics in quantum spin ice. We demonstrate that the spinon motion can be thought of as a quantum walk with entropy-induced memory. Our numerical simulation shows that the simple quasi-particle behaviour emerges out of the intricate interplay between the spinon and the background spins . [Preview Abstract] |
Friday, March 18, 2016 12:51PM - 1:03PM |
Y18.00007: Neutron scattering in $Er_{2-x}Y_{x}Ti_2O_7$ Jonathan Gaudet, Alannah Hallas, Dalini Maharaj, Edwin Kermarrec, Nicholas P. Butch, Hanna Dabowska, Bruce Gaulin $Er_2Ti_2O_7$(ETO) is a strong candidate for ground state selection via the order by disorder mechanism. A $\psi_2$ magnetic ground state appears below T$_N$=1.2 K, where $\psi_2$ and $\psi_3$ are the two basis states of the irreducible representation $\Gamma_5$. No sample dependance has been observed in the thermodynamics properties of ETO at low temperature, and in particular on its phase transition to long range magnetic order. ETO's ordered Neel state has been shown to be robust even to a relatively high level of magnetic dilution, as occurs with non-magnetic $Y^{3+}$ substitution of $Er^{3+}$.However, recently two theoretical studies have predicted that ETO's $\psi_2$ ground state should be unstable to formation of the $\psi_3$ state, in the presence of such disorder. To explore this possibility,we grew single crystals of $Er_{2-x}Y_{x}Ti_2O_7$(EYTO) with x = 0,0.2 and 0.4 and performed a systematic inelastic neutron scattering studies using the Disk Chopper time-of-flight spectrometer (DCS) at the National Institute of Standards and Technology (NIST). We will show elastic and inelastic neutron scattering at low temperatures and as a a function of applied magnetic field for all three samples and discuss the role of such quenched disorder on the spin dynamics of EYTO. [Preview Abstract] |
Friday, March 18, 2016 1:03PM - 1:15PM |
Y18.00008: Semi-classical Theory of Quantum Spin Ice Michal Kwasigroch, Claudio Castelnovo The low temperature properties of quantum spin ice compounds Yb$_2$Ti$_2$O$_7$ and Pr$_2$Zr$_2$O$_7$ are described by spin-1/2 degrees of freedom associated with magnetic Yb or Pr atoms residing on vertices of corner-sharing tetrahedra. Strong Ising exchange enforces the well-known 2-in-2-out rules for each tetrahedron at low temperatures. These describe the macroscopically degenerate spin ice configurations. Recently, it has been shown [Phys. Rev. B 69, 064404 (2004)] that the addition of weak easy-plane exchange can lead to hybridisation of the classically allowed spin-ice configurations and the emergence of a gapless quantum spin liquid. We show that a semi-classical treatment of this U(1) liquid phase captures the QED-like physics and we derive quantitative estimates of the low-energy dispersion and the dynamic structure factor. These compare well with the existing Monte Carlo simulations. [Preview Abstract] |
Friday, March 18, 2016 1:15PM - 1:27PM |
Y18.00009: From pinch points to pinch lines: a new spin liquid on the pyrochlore lattice Owen Benton, Ludovic Jaubert, Han Yan, Nic Shannon One of the most fascinating disoveries in the study of spin liquids has been the existence of emergent gauge fields arising out of a disordered magnetic ground state. The best known example is provided by the spin ice pyrochlores Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$, whose underlying gauge structure is revealed by the presence of pinch-point singularities in the neutron scattering response. Here we report the discovery of a new spin liquid on the pyrochlore lattice, the low temperature fluctuations of which are naturally described by the fluctuations of a tensor field with a continuous gauge freedom. This gauge structure underpins a novel form of spin correlations, giving rise to ``pinch-line'' singularities- line-like analogues of the pinch-point singularity extending along the $\langle 111 \rangle$ directions of reciprocal space. Remarkably, our theory reproduces several otherwise unaccounted for features of neutron scattering experiments on Tb$_2$Ti$_2$O$_7$. [Preview Abstract] |
Friday, March 18, 2016 1:27PM - 1:39PM |
Y18.00010: Numerical evidence for a chiral spin liquid in the XXZ model on the kagome lattice in a magnetic field Hitesh Changlani, Krishna Kumar, Bryan Clark, Eduardo Fradkin Frustrated spin systems in two dimensions provide a fertile ground for discovering exotic states of matter, often with topologically non-trivial properties. In this work, we investigate the possible existence of a chiral spin liquid state in the spin 1/2 XXZ model on the frustrated kagome lattice in the presence of a magnetic field. This model is equivalent to a hard-core bosonic one with density-density interactions at finite filling fraction. Motivated by previous field theoretic predictions utilizing a Chern-Simons theory adapted for this lattice [1,2], we focus our attention to understanding the XY limit for the 2/3 magnetization plateau (equivalent to a system of hard-core bosons at 1/6 filling with weak nearest-neighbor repulsive interactions). Performing exact or accurate numerical computations, and based on energetics and construction of minimally entangled states and associated modular matrices, we provide evidence for such a spin liquid. We study the nature of this phase and examine its stability to additional interactions. [1] K. Kumar, K. Sun, and E. Fradkin, Phys. Rev. B 90, 174409 (2014) [2] K. Kumar, K. Sun, and E. Fradkin, Phys. Rev. B 92, 094433 (2015) [Preview Abstract] |
Friday, March 18, 2016 1:39PM - 1:51PM |
Y18.00011: Spin liquids on an anisotropic kagome lattice Robert Schaffer, Kyusung Hwang, Yejin Huh, Yong Baek Kim Much recent theoretical and experimental effort has been devoted to the search for quantum spin liquids, which arise in the presence of strong frustration of magnetic interactions. Motivated by recent experiments on the vanadium oxyfluoride material DQVOF, we examine possible spin liquid phases on an anisotropic kagome lattice of $S=1/2$ spins, in which the $C_6$ symmetry is broken to $C_3$. Using the projective symmetry group analysis, we determine the possible phases for both bosonic and fermionic $Z_2$ spin liquids on this lattice. Using VMC, we study the Heisenberg model on this lattice, and show that a $Z_2$ spin liquid emerges as the ground state in the presence of this anisotropy. [Preview Abstract] |
Friday, March 18, 2016 1:51PM - 2:03PM |
Y18.00012: Classification of $Z_2$ spin liquids in a hyperkagome lattice by projective symmetry groups Biao Huang, Yong Baek Kim, Yuan-Ming Lu Being a rare candidate material supporting 3D spin liquid states, Na$_4$Ir$_3$O$_8$ has attracted much theoretical and experimental interest in the past decade. Propelled by such developments, we give a complete classification of $Z_2$ spin liquid states in the hyperkagome lattice formed by Ir$^{4+}$ ions in the projective symmetry group approach. A list of mean field states with different fractional quasi-particle excitations are correspondingly given, and their excitation gaps are analyzed. The effects of spin-orbit coupling due to the $5d$ electrons in Ir are also discussed. This work paves the way for further variational Monte-Carlo study of the spin liquid physics in hyperkagome lattices. [Preview Abstract] |
Friday, March 18, 2016 2:03PM - 2:15PM |
Y18.00013: Z$_{\mathrm{2}}$ gauge theory for valence bond solids on the kagome lattice Kyusung Hwang, Yejin Huh, Yong Baek Kim We present an effective Z$_{\mathrm{2}}$ gauge theory that captures various competing phases in spin-1/2 kagome lattice antiferromagnets: the topological Z$_{\mathrm{2}}$ spin liquid (SL) phase, and the 12-site and 36- site valence bond solid (VBS) phases. Our effective theory is a generalization of the recent Z$_{\mathrm{2}}$ gauge theory proposed for SL phases by Wan and Tchernyshyov. In particular, we investigate possible VBS phases that arise from vison condensations in the SL. In addition to the 12-site and 36-site VBS phases, there exists 6-site VBS that is closely related to the symmetry-breaking valence bond modulation patterns observed in the recent density matrix renormalization group simulations. We find that our results have remarkable consistency with a previous study using a different Z$_{\mathrm{2}}$ gauge theory. Motivated by the lattice geometry in the recently reported vanadium oxyfluoride kagome antiferromagnet, our gauge theory is extended to incorporate lowered symmetry by inequivalent up- and down-triangles. We investigate effects of this anisotropy on the 12-site, 36-site, and 6-site VBS phases. Particularly, interesting dimer melting effects are found in the 36-site VBS. We discuss the implications of our findings and also compare the results with a different type of Z$_{\mathrm{2}}$ gauge theory used in previous studies. [Preview Abstract] |
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