Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session R37: Pyrochlores II: Classical and Quantum Spin IceFocus
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Sponsoring Units: GMAG DMP Chair: David Tennant, Oak Ridge National Laboratory Room: BCEC 206A |
Thursday, March 7, 2019 8:00AM - 8:36AM |
R37.00001: Refined spin Hamiltonian for Yb2Ti2O7 and its two competing low field states Invited Speaker: Allen Scheie We present a neutron scattering study of pyrochlore Yb2Ti2O7 in a [111] magnetic field using high-quality stoichiometric single crystals. High magnetic fields (1.5 T) reveal sharp spin wave modes, which we use to further refine the magnetic Hamiltonian, offering corrections to previous Hamiltonian refinements. Low magnetic fields reveal diffuse features which suggest an unconventional ground state, and we discuss mechanisms explaining how such a spectrum can be caused by proximity to a ferromagnetic-antiferromagnetic phase boundary. We present a model which accounts for the observed spectrum, and resolves several long-standing questions concerning the ground state of Yb2Ti2O7 [1]. |
Thursday, March 7, 2019 8:36AM - 8:48AM |
R37.00002: Semiclassical simulation of quantum spin ice: seeing beyond the light Claudio Castelnovo, Attila Szabó We devise a semiclassical numerical method to investigate quantum spin ice, based on classical Monte Carlo and dynamical simulations of the effective ring exchange Hamiltonian in the large-$S$ limit. At low temperatures, this approach is asymptotically equivalent to a path integral field theory, and indeed we find excellent agreement with known results from the latter regarding the dispersion of the photonic modes of the system. We further argue that our method samples the finite-temperature path integral formulation of the quantum spin ice problem accurately, and thus allows us to go beyond the results attainable using the field theoretic description, and study also the isolated as well as collective behaviour of vison excitations. |
Thursday, March 7, 2019 8:48AM - 9:00AM |
R37.00003: Vison and photon electrodynamics in quantum spin ice Attila Szabó, Claudio Castelnovo We investigate quantum spin ice on the pyrochlore lattice using semiclassical Monte Carlo and dynamical simulations of the effective ring exchange Hamiltonian. The model exhibits normal modes equivalent to photons as well as gapped vortices of the emergent magnetic field analogous to visons. Contrary to the behaviour of spinons in classical spin ice, we find that visons form a weak electrolyte; that is, their strong interaction makes the energy cost of a bound pair lower than that of an isolated vison. This leads to a qualitatively different temperature dependence of the pinch points in the corresponding emergent magnetic field. Finally, we observe that the thermal bath of photons dresses the visons and drastically reduces their energy. This suggests that visons may hybridise with the photon spectrum in quantum spin ice, which has potential implications on detecting them experimentally. |
Thursday, March 7, 2019 9:00AM - 9:12AM |
R37.00004: Probing the magnetic excitations in the quantum magnet Yb2Ti2O7 Xinshu Zhang, Steffen Säubert, Seyed Koohpayeh, Peter Armitage The pyrochlore magnet Yb2Ti2O7 has shown exotic magnetic properties and has been considered to be a quantum spin ice although there is still considerable uncertainty about its Hamiltonian parameters. We study the evolution of its magnetic excitations as function of magnetic field and temperature using time-domain THz spectroscopy. We perform a thorough sum rule analysis of the low energy excitations by combining time domain terahertz spectroscopy and magnetization measurements. The distribution of spectral intensity indicates strong quantum fluctuations and short range spin correlations in Yb2Ti2O7 at low magnetic field. It also shows the very different manifestation of thermal fluctuations vs. quantum fluctuations in this compound. |
Thursday, March 7, 2019 9:12AM - 9:24AM |
R37.00005: Capacitive Torque Magnetometry: Study of field-induced magnetic transitions and transient states in the Spin-Ice Material Ho2Ti2O7 Kevin Barry, Naween Anand, Jennifer Neu, Haidong Zhou, David E Graf, Theo Siegrist, Christianne Beekman We used capacitive torque magnetometry to compile phase diagrams that show the field-induced magnetic transitions between specific spin textures of the spin ice state. High quality single crystals have been measured at 500 mK in applied fields up to 11 T applied along various crystallographic directions. Utilizing reported results from neutron scattering as our starting point we have developed a phenomenological model that fully characterizes the anisotropic magnetic phase diagram of the system. This model clearly shows the evolution of the spin textures between the previously identified Q = 0, Q = X, and 3-in/1-out states, and it shows the existence of transient states between them. These transient states are characterized by spin flip excitations taking the spin ice away from the well-defined spin textures. Activation energies associated with these excitations have been extracted. This study demonstrates the applicability of torque magnetometry in probing specific spin textures hosted by the spin ice state. |
Thursday, March 7, 2019 9:24AM - 9:36AM |
R37.00006: Investigations of scaling and dynamics in the kagome ice phase of Ho2Ti2O7 Alexandra Turrini, Sean Giblin, Edward Riordan, Peter C. W. Holdsworth, Patrik Henelius, Tom Fennell Applying a medium strength field along the [111] direction of the geometrically frustrated spin ices Ho2Ti2O7 and Dy2Ti2O7 results in a magnetization plateau called kagome ice. Competition between the ice rules and applied field separates the pyrochlore structure into alternating kagome and triangular planes, creating a quasi-two dimensional analog of spin ice with distinct spin correlations [1]. The concentration of spin flip excitations (recontextualized as magnetic monopole quasiparticles [2]) experiences various crossovers with increased field and temperature visible as critical inflections in the magnetization, specific heat [3] and susceptibility [4]. As the energy of spin flip excitations in the kagome plane changes with field, the mobility of monopole quasiparticles is constrained further from their spin ice counterparts. Using diffuse neutron scattering and high frequency susceptibility measurements on a Ho2Ti2O7 single crystal, alongside Monte Carlo simulations, we have observed and characterized signals of scaling and crossovers attributed to changing monopole density above the critical point. |
Thursday, March 7, 2019 9:36AM - 9:48AM |
R37.00007: Fitting crystal field excitations using point-charge model
-- application to rare earth pyrochlore, Kagome, and triangular magnets Zhiling Dun, Xiaojian Bai, Joseph Paddison, Matthew Brandon Stone, Haidong Zhou, Martin Mourigal Determining the single ion crystal field (CF) Hamiltonian is usually an indispensable step to understand the collective low-temperature magnetism in rare earth magnets. This sometimes becomes challenging in low-symmetry systems due to the limited experimental observables compared to a large number of fitted CF parameters. Here I describe a general method to fit neutron CF excitation spectrum using point charge models. By benchmarking with the existing inelastic neutron scattering measurements of pyrochlore oxides (R2X2O7), we achieve a simple and universal point-charge model. This model is then modified and applied to the newly discovered tripod Kagome magnets (R2Mg2Sb3O14), through which the principal axes and local g-tensor in the pseudo-spin basis can be determined and diagonalized. Finally, we apply the point-charge calculation to the triangular lattice magnet YbMgGaO4 and relates its mysterious extra CF levels to the local structure disorder. |
Thursday, March 7, 2019 9:48AM - 10:00AM |
R37.00008: Correlated Quantum Tunnelling of Monopoles in Spin Ice Bruno Tomasello, Claudio Castelnovo, Roderich Moessner, Jorge Quintanilla The spin ice materials Ho2Ti2O7 and Dy2Ti2O7 are by now perhaps the best-studied classical frustrated magnets. A crucial step towards the understanding of their low temperature behaviour – both regarding their unusual dynamical properties and the possibility of observing their quantum coherent time evolution – is a quantitative understanding of the spin-flip processes which underpin the hopping of magnetic monopoles. We attack this problem in the framework of a quantum treatment of a single ion subject to the crystal, exchange and dipolar fields from neighbouring ions. By studying the fundamental quantum mechanical mechanisms, we discover a bimodal distribution of hopping rates which depends on the local spin configuration, in broad agreement with rates extracted from experiment. Applying the same analysis to Pr2Sn2O7 and Pr2Zr2O7, we find an even more pronounced separation of time scales signalling the likelihood of coherent many-body dynamics |
Thursday, March 7, 2019 10:00AM - 10:12AM |
R37.00009: Spectroscopy of Spin Fluctuations in Spin Ices Ritika Dusad, Franziska Kirschner, Jesse Hoke, Benjamin Roberts, Anna Eyal, Felix Flicker, Graeme Luke, Stephen Blundell, James C Davis In the past decade, the theoretical prediction of emergent magnetic charges in several lanthanide-pyrochlore magnetic insulators1 has spawned renewed experimental interest in the study of these elusive particles, albeit in a condensed matter system. Thermodynamic and transport studies of of the spin ice of Dy2Ti2O7 and Ho2Ti2O7 are the focus of research. We report new studies of the spin fluctuation spectrum in these materials and analyze the data in the context of theories for thermally generated magnetic monopole pairs. |
Thursday, March 7, 2019 10:12AM - 10:24AM |
R37.00010: Monopole equilibration in spin ice David Tennant, Anjana Samarakoon, Santiago Grigera, Qiang Zhang, Bastian Klemke, Alexander Kirste, Claudio Castelnovo, Roderich Moessner Spin ice shows remarkably rich behavior due to its monopole quasiparticles and coulombic interactions. In actual materials quantum tunneling provides millisecond spin flip times which bring out-of-equilibrium states into reach for experimental and computational study. Recently there has been interest in using neutrons and noise as probes of such behaviors. We present computational and experimental studies of the magnetic states in Dy2Ti2O7 under out-of-equilibrium conditions. These cover the feasibility of noise and neutron measurements to assess the states in the material under thermal and field quenching. |
Thursday, March 7, 2019 10:24AM - 10:36AM |
R37.00011: Anomalous magnetic ground state behaviour of the mixed B-site pyrochlore Dy2ScNbO7 Megan Rutherford, Cole D Mauws, Casey Marjerrison, Sara Haravifard, James Beare, Graeme Luke, Haidong Zhou, Christopher R Wiebe The spin ice state, which is of great interest due to observed residual pauling entropy, has been observed in rare-earth pyrochlores such as Dy2Ti2O7, and Dy2Sn2O7. In an effort to explore the robustness of the spin ice state in dysprosium based pyrochlores, a new species Dy2ScNbO7 has been synthesized and single crystals have successfully been grown. Early physical characterization has shown unique behaviour arising from the disordered b-site, with faster spin dynamics and an anomalous low ordering temperature. In an effort to further explore the unique behaviour of Dy2ScNbO7, heat capacity measurements have been performed with an applied magnetic field along the [111] kagomé ice plane and the [110] direction of the cubic unit cell. The phase diagrams constructed from these measurements and other recent physical characterization results show an unexpected divergence from spin ice behaviour, which is the focus of this presentation. |
Thursday, March 7, 2019 10:36AM - 10:48AM |
R37.00012: The magnetism of novel pyrochlores with a known muon site studied using muon-spin rotation Stephen Blundell, Franz Lang, Tom Lancaster, Dharmalingam Prabhakaran Pyrochlore oxides with the formula A2B2O7 display a rich variety of ground states and novel magnetic phenomena, encompassing cooperative paramagnetism, all-in all-out (AIAO) states and spin-ice behaviour. The magnetic ions on the A or B sites occupy sublattices of corner sharing tetrahedra. In muon experiments on pyrochlores, it is important to consider the extent to which an implated muon may perturb its local environment and whether it is capable of altering the intrinsic properties of the material under investigation, a significant effect for Pr-based pyrochlores [1]. We have determined the muon site in the pyrochlore structure and use this to understand experimental data obtained on Tm2Ti2O7 (singlet ground state), Y2V2O7 and Lu2V2O7 (candidate topological magnon insulator), and Lu2Ir2O7 (insulator, with AIAO structure). [1] F. R. Foronda, F. Lang, J. S. Möller, T. Lancaster, A. T. Boothroyd, F. L. Pratt, S. R. Giblin, D. Prabhakaran and S. J. Blundell, Phys. Rev. Lett. 114, 017602 (2015) |
Thursday, March 7, 2019 10:48AM - 11:00AM |
R37.00013: Antisymmetric transport coefficients of magnetic monopoles in quantum spin ice under an electric field Shigeki Onoda Unambiguously detecting magnetic monopoles in classical or quantum spin ice has been an intriguing issue. Of our interest is the bosonic U(1) quantum spin liquid phase of quantum spin ice, which accomodates emergent magnetic monopoles as fractionalized quasiparticles and hosts deconfined emergent U(1) gauge fields. It has recently been recognized that the emergent electric gauge flux in this quantum spin liquid can be generated by a real electric field and electric polarization [1,2]. Here, solving the Harper equation [3] for a prototypical quantum spin ice, we show that magnetic monopoles can form Landau levels under an electric field, bending the orbital motion of magnetic monopoles and thus producing antisymmetric transport coeffiients in response to the temperature gradient and the magnetic field gradient [4]. Possible experimental setups are also discussed. |
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