Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F37: Pyrochlores I: Moment Fragmentation, Quantum Spin Liquids, and BeyondFocus
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Sponsoring Units: GMAG DMP Chair: Bella Lake, Helmholtz-Zentrum Berlin Room: BCEC 206A |
Tuesday, March 5, 2019 11:15AM - 11:51AM |
F37.00001: Search for quantum phases in the rare earth zirconate and hafnate pyrochlore frustrated magnets Invited Speaker: Monica Ciomaga Hatnean Pyrochlores are interesting due to their versatile structure, frustrated magnetic lattice and their wide variety of exotic magnetic ground states (spin ices, spin frozen states, or long-range ordered states). The most studied pyrochlore oxides are the classical spin ices Dy2Ti2O7 and Ho2Ti2O7. One of the most interesting avenues of current research is into systems which exhibit novel magnetic ground states, such as quantum spin liquid (QSL) and quantum spin ice (QSI). Pr, Tb or Yb pyrochlores are good candidates for the realisation of quantum states. We have studied the zirconate and hafnate R2M2O7 (R= Rare Earth, M= Zr, Hf) oxides. Recent results show that they exhibit a variety of exciting magnetic behaviours and emergent properties, ranging from a disordered state with quadrupolar correlations and spin-ice-like excitations in Pr2Zr2O7, the fragmentation of magnetic moments into an ordered phase and a fluctuating state in Nd2Zr2O7, a disorder-induced QSL state in Tb2Hf2O7, to a potential QSI ground state with fractionalized excitations in Pr2Hf2O7. These investigations have been motivated by the availability of large high-quality single crystals of these materials. I will discuss the structural and magnetic characteristics of some of the zirconate and hafnate pyrochlores. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F37.00002: Neutron Scattering Study on the Spin Disordered Ground State in the Pyrochlore Antiferromagnet Ce2Zr2O7 Evan Smith, Jonathan Gaudet, Jérémi Dudemaine, Casey Marjerrison, Andrea Bianchi, Connor Buhariwalla, Michael Nicklas, Roman Movshovich, Nicholas Butch, Matthew Brandon Stone, Bruce Gaulin The pyrochlore antiferromagnet Ce2Zr2O7 is sensitive to an instability of the Ce3+ oxidation state, whereby the 4f1 electronic configuration of Ce3+ can be diluted via sample oxidation and the concomitant introduction of Ce4+ to the lattice.1 For that reason, reduction of Ce2Zr2O7 samples is required to enable a neutron scattering study of this material, the subject of this presentation. The spins in Ce2Zr2O7 possess a local Ising anisotropy and interact via an antiferromagnetic coupling, which typically results in all-in all-out antiferromagnetic order. However the crystal electric field (CEF) wavefunctions of the Ce3+ ground state can have a dipole-octupole nature which is an ingredient for moment fragmentation and can allow for a disordered U(1) quantum spin liquid ground state.2 High energy neutron spectroscopy confirms the dipole-octupole CEF ground state, and higher energy resolution inelastic neutron scattering shows a disordered ground state for Ce2Zr2O7. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F37.00003: Neutron scattering study of breathing pyrochlore lattice material LiGaCr4S8 Ganesh Pokharel, Hasitha Suriya Arachchige, Andrew May, Tao Hong, Stuart Calder, Gabriele Sala, Georg Ehlers, David George Mandrus, Andrew D Christianson LiGaCr4S8 is a breathing pyrochlore lattice where the ordering of the Li+ and Ga3+ cations on the A-site of the spinel structure leads to the periodic expansion and contraction of Cr4 tetrahedra. Strong magneto-elastic coupling drives negative thermal expansion in the temperature range 12-110 K. Neutron diffraction and inelastic neutron scattering experiments were carried out on polycrystalline samples of LiGaCr4S8 to investigate the spin configuration and spin-spin interactions. No long-range magnetic order is observed above 1.5 K. However, neutron diffraction shows the gradual appearance of a magnetic signal centered at |Q|~0.51 Å-1 on cooling below ~100 K and a second contribution at |Q|~0.64 Å-1 appears below 15 K. The inelastic neutron scattering data shows spin excitations with a zone boundary energy 12.6 meV at 4 K. The intensity of the inelastic signal weakens above 10 K though persists to nearly 100 K. At low energies the dynamic susceptibility can be described with a single temperature dependent relaxation. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F37.00004: Non-spin-ice quantum spin liquid (QSL) with Ce-based dipole-octupole doublets Ce2Zr2O7 Bin Gao, Tong Chen, Kalyan Sasmal, Manivannan Nallaiyan, David W Tam, Chien-Lung Huang, Gabriele Sala, Feng Ye, Matthew Brandon Stone, D. T. Adroja, Christopher Baines, Joel Barker, Huibo Cao, Songxue Chi, Stefano Spagna, Sang-Wook Cheong, Emilia Morosan, M Brian Maple, Gang Chen, Pengcheng Dai The quantum spin liquid (QSL) state, which has no conventional order parameter associated with a broken symmetry, is an exotic state where the spins are highly entangled with one another. Although recent claims on some QSL candidates are under debate due to disorder and spin freezing, we find a non-spin-ice QSL with Ce-based dipole-octupole doublets Ce2Zr2O7. High-quality single crystals of Ce2Zr2O7 are grown by a floating zone method. We confirmed the Kramer doublet ground state in Ce3+ ions from crystal electric field measurements. We revealed that the quasi-elastic structure factor and Pauling entropy is totally suppressed in this system by carrying out diffuse neutron scattering and heat capacity measurements. Our AC magnetic susceptibility and muon-spin relaxation measurements suggest no spin freezing down to 0.02 K. Inelastic neutron scattering experiments display broad excitation continua along Brillouin zone boundaries, indicating fractional spin excitation, which is the key feature of QSL. This, together with the absence of features of spin ice and spin glass, suggest the system is a non-spin-ice QSL state at low temperature, where spins are highly correlated but fluctuate strongly. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F37.00005: Pyrochlore antiferromagnet CdYb2Se4 studied by neutron scattering Guratinder Kaur, Oksana Zaharko, Vladimir Tsurkan, Tom Fennell, Christian Rueegg The geometrically frustrated pyrochlore lattice with exotic magnetic behavior is well established in titanates R2Ti2O7 (R= rare earth) [1]. Spinel compounds AR2X4 with R-ions also residing on the pyrochlore lattice similarly exhibit unconventional magnetism [2, 3]. The basic distinction between the two families is the local environment of the rare earth ions [4, 5]. Here, we present a neutron scattering study of the crystal electric field (CEF), magnetic ground state, and spin dynamics of CdYb2Se4. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F37.00006: Ultrasound investigation of Ce2Zr2O7 at low temperatures and in high magnetic fields. Andrea Bianchi, Jérémie Dudemain, Jonathan Gaudet, Evan Smith, Bruce Gaulin, Yulia Gritsenko, Sergei Zherlitsyn, Joachim Wosnitza Through careful controlling the atmosphere during the growth of Ce2Zr2O7 we have been able to stabilize Ce3+ and prevent the formation of non-magnetic Ce4+.1 This allows us to study the interesting case of doublet-octupole moment on a pyrochlore lattice. In the pyrochlore structure the Ce3+ J = 5/2 multiplet is split by crystal field interactions resulting in wave functions for the ground state Kramers doublet which correspond to a linear combination of mJ = ±3/2 states. A generic model for these unusual doublets supports two distinct symmetry enriched U(1) QSL ground states in the corresponding quantum spin ice regimes.2 We measured ultrasound down to 20 mK and in magnetic fields of up to 10 T with the field applied along the [111] direction for transversal and longitudinal modes. In zero field we observe a softening of the modes for temperatures below 0.8 K, which is an indication for quantum fluctuations. At 20 mK, an increase of the magnetic field leads to decrease of the sound velocity and the curves show hysteresis which is reminiscent of the behaviour observed in Dy2Ti2O7 and Ho2Ti2O7. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F37.00007: Extended Coulomb Liquid of Paired Hardcore Boson Model on a Pyrochlore Lattice Chun-Jiong Huang, Changle Liu, Zi Yang Meng, Yue Yu, Youjin Deng, Gang Chen There is a growing interest in the U(1) Coulomb liquid in both quantum materials in pyrochlore ice and cluster Mott insulators and cold atom systems. We explore a paired hardcore boson model on a pyrochlore lattice. This model is equivalent to the XYZ spin model that was proposed for rare-earth pyrochlores with ''dipole-octupole'' doublets. Since this model has no sign problem for quantum Monte Carlo(QMC) simulations in a large parameter regime, we carry out both analytical and QMC calculations. We find that the U(1) Coulomb liquid is quite stable and spans a rather large portion of the phase diagram with boson pairing. Moreover, we numerically find thermodynamic evidence that the boson pairing could induce a possible Z2 liquid in the vicinity of the phase boundary between Coulomb liquid and Z2 symmetry-broken phase. Besides the materials' relevance with quantum spin ice, we point to quantum simulation with cold atoms on optical lattices. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F37.00008: Magnetic phase diagram of dipolar-octupolar pyrochlores Owen Benton Recent experiments have revealed intriguing magnetic properties in a variety of rare-earth pyrochlores R2M2O7 with magnetic ions R=Nd, Sm, Ce. Common to these systems is a low energy crystal field doublet with so-called "dipolar-octupolar" character. This is associated with several interesting phenomena including magnetic moment fragmentation and possible quantum spin liquid ground states. Additionally, the application of a magnetic field in the (1,1,1) or (1,1,0) directions allows tuning from a fully 3-dimensional state in zero field to 2-dimensional or 1-dimensional states in high field. This rich set of phenomena calls for a unified description with which we can quantitatively understand and predict the properties of dipolar-octupolar pyrochlores. In this talk, we will explore the magnetic phase diagram of these materials in applied magnetic fields using a combination of analytical and numerical methods. Particular attention will be paid to the emergent low dimensional states induced by external fields along the (1,1,1) and (1,1,0) directions. This work will enable us to explain the behaviour of diverse dipolar-octupolar pyrochlores and the emergence of new quantum states in these systems. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F37.00009: Dynamical structure factor of the three-dimensional quantum spin liquid candidate NaCaNi2F7 Hitesh Changlani, Shu Zhang, Kemp Plumb, Oleg Tchernyshyov, Roderich Moessner We study the spin-1 pyrochlore material NaCaNi2F7 [1] with a combination of molecular dynamics simulations, stochastic dynamical theory and linear spin wave theory. The dynamical structure factor from inelastic neutron scattering is well described by a near-ideal Heisenberg Hamiltonian incorporating small anisotropic terms and weak second-neighbor interactions [2]. We find that all three approaches reproduce remarkably well the momentum dependence of the scattering intensity and its energy dependence with the exception of the lowest energies. We find (i) a complete lack of sharp quasiparticle excitations in momentum space over much, if not all, of the energy range; (ii) linear spin-wave theory appears to apply in a regime where it would be expected to fail for a number of reasons. We elucidate what underpins these surprises, and note that basic questions about the nature of quantum spin liquidity in such systems pose themselves as a result [3]. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F37.00010: Intermultiplet transitions and long-range order in Sm-based pyrochlores Viviane Pecanha-Antonio, Erxi Feng, D. T. Adroja, Fabio Orlandi, Thomas Brückel, Yixi Su We present bulk and neutron scattering measurements performed on the isotopically enriched 154Sm2Ti2O7 (titanate) and 154Sm2Sn2O7 (stannate) samples. Both compounds display sharp heat capacity anomalies, at 350 mK and 440 mK, respectively. Inelastic neutron scattering measurements are employed to solve the crystalline electric field (CEF) excitations scheme, which includes transitions between the ground and first excited J multiplets of the Sm3+ ion. In order to further validate those results, the single-ion magnetic susceptibility of the compounds is calculated and compared with the experimental static susceptibility measured in low applied magnetic fields. We show that the inclusion of intermultiplet transitions in the crystal field analysis is fundamental to the understanding of the intermediate and, more importantly, low temperature magnetic behaviour of the Sm-based pyrochlores. Finally, the heat capacity anomally is shown to correspond to the onset of an all-in-all-out long-range order in the stannate sample, while in the titanate a dipolar long-range order can be only indirectly infered. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F37.00011: The magnetic structure of the antiferromagnetic pyrochlore Gd2Ti2O7 Joseph Paddison, Georg Ehlers, Jason Gardner, Ross Stewart Canonical examples of highly frustrated antiferromagnets are the gadolimium pyrochlores Gd2Ti2O7 (GTO) and Gd2Sn2O7 (GSO) both of which order at ~1 K with different structures. GSO forms the Palmer-Chalker structure with a k = 0 propagation vector while GTO is reported to form a more complicated - and still controversial - partially ordered state with a propagation vector of k=(1/2, 1/2, 1/2), splitting the crystallographically equivalent Gd sites into non-equivalent magnetic sites [1]. Standard neutron diffraction techniques don't allow unambiguous determination of the k-state, (i.e. 1 or a combination of the 4 possible propagation vectors). However, using a combination of single crystal diffraction, diffuse and inelastic neutron scattering on both powder and single crystal samples of GTO, we have determined that the partially ordered magnetic structure is double-k with orthorhombic space group symmetry. This is in agreement with recent theoretical predictions of a double-k structure based on mean-field-theory with thermal fluctuations [2] |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F37.00012: Tuning Epitaxial Strain in XY Pyrochlore Er2Ti2O7 Connor Buhariwalla, Evan Smith, Jonathan Gaudet, Adam Aczel, Patrick Fournier, Bruce Gaulin Tuning parameters in frustrated magnetism allow us to explore the complex landscape of magnetically fragile ground states. In epitaxial thin film samples, we can study the statespace of eptiaxial strain (caused by the lattice mismatch between substrate and film) and the reduced dimensionailty inherent to a thin film. Thin film works on frustrated titanate pyrochlores have focused on spin ice compounds (Ho2Ti2O7 and Dy2Ti2O7)[1-3] and the unconventional magnetic ground state of Tb2Ti2O7[4]. This study examines the ordered magnetic ground state of Er2Ti2O7. The ground state selection between the Ψ2 and Ψ3 basis states of the Γ5 manifold and the relative stability of Γ5 to Palmer-chalker configurations have been shown to be sensitive to a number of tuning parameters such as magnetic dilution and isovalent non-magnetic substitution. By fabricating films of different thicknesses using pulsed-laser deposition, we tune the epitaxial strain on the magnetic lattice, and examine the subsequent magnetic groundstate through bulk characterization and neutron scattering. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F37.00013: Pyrochlore titanates RETi2O7 (RE = Ho & Er): Stoichiometry and Crystal Growth Alireza Ghasemi, Seyed Koohpayeh The pyrochlore cubic structure, with a corner sharing tetrahedra network, is the host for candidates of frustrated magnetic materials such as spin ices, spin liquids, and compounds with non-colinear XY orders [1–3]. Pyrochlore titanates, as a subgroup of the pyrochlore family, have been extensively studied; however, the reported physical properties have been shown to significantly vary from one sample to another. Here, we report a systematic study of synthesis and crystal growth by the float-zoning techniques. Structural quality, lattice parameters, stuffing defects and oxygen vacancies were found to depend on synthesis and growth conditions. A method to grow stoichiometric and high-quality single crystals of titanates is also reported [4]. |
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