Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session T51: Pyrochlore SystemsFocus Recordings Available
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Sponsoring Units: GMAG DMP Chair: Martin Mourigal, Georgia Tech Room: McCormick Place W-474B |
Thursday, March 17, 2022 11:30AM - 11:42AM |
T51.00001: Dynamical ground state in the XY pyrochlore Yb2GaSbO7 Adam A Aczel, Paul M Sarte, Christopher R Wiebe, Joseph Paddison, Minseong Lee, Matthew B Stone, Daniel M Pajerowski, Stuart Calder, Lucile Mangin-Thro, Yiming Qiu, Mitchell Bordelon, Brenden Ortiz, Dalmau Reig-i-Plessis, J. Paul Attfield, Stephen D Wilson, Chris Stock, Haidong Zhou, Alannah M Hallas The magnetic ground state of the pyrochlore Yb2GaSbO7 has remained an enigma for a long time. Here I will discuss the synthesis and characterisation of Yb2GaSbO7 that enabled us to revisit the nature of the magnetic ground state. Through DC and AC magnetic susceptibility, heat capacity, and neutron scattering experiments, we observe evidence for a dynamical ground state that makes Yb2GaSbO7 a promising quantum spin liquid candidate. This state is quite fragile, being tuned to a splayed ferromagnet in a modest magnetic field H = 1.5 T. |
Thursday, March 17, 2022 11:42AM - 11:54AM |
T51.00002: Investigation of the monopole magneto-chemical potential in spin ices using capacitive torque magnetometry Christianne Beekman, Naween Anand, Kevin T Barry, Jennifer N Neu, David E Graf, Haidong Zhou, Hitesh J Changlani, Theo Siegrist Geometrically frustrated systems have an inherent incompatibility between the lattice geometry and the magnetic interactions, resulting in macroscopically degenerate ground-state manifolds. The single-ion anisotropy and magnetic interactions in spin-ice systems give rise to unusual non-collinear spin textures, such as Pauling states and emergent quasiparticle excitations equivalent to magnetic monopoles. The effective spin correlation strength (Jeff) determines the relative energies of the different spin-ice states and the magneto-chemical potential (MCP) associated with monopole formation. Capacitive torque magnetometry is used as a unique tool to characterize the transitions between noncollinear spin textures in spin-ice samples, which allows extraction of Jeff and the MCP of monopole formation [1]. We build a magnetic phase diagram of Ho2Ti2O7, from which we determine that the field-induced magnetic phase transitions cannot be described by a single value of Jeff. Rather, the MCP depends on the spin sublattice (α or β), i.e., the Pauling state, involved in the transition. There is an enticing potential of using these monopoles for the development of new quantum information applications, which requires thin films. Thin films grown on yttria-stabilized zirconia substrates show modified spin ice physics [2] depending on the growth conditions. |
Thursday, March 17, 2022 11:54AM - 12:06PM |
T51.00003: Absence of moment fragmentation in the mixed B-site pyrochlore Nd2GaSbO7 Steven J Gomez Nd-based pyrochlore oxides of the form Nd2B2O7 have garnered a significant amount of interest owing to the moment fragmentation physics observed in Nd2Zr2O7 and speculated in Nd2Hf2O7. Notably this phenomenon is not ubiquitous in this family, as it is absent in Nd2Sn2O7, which features a smaller ionic radius on the B site. Here, we explore the necessary conditions for moment fragmentation in the Nd pyrochlore family through a detailed study of the mixed B-site pyrochlore Nd2GaSbO7. The B site of this system is characterized by significant disorder and an extremely small average ionic radius. Similarly to Nd2Sn2O7, we find no evidence for moment fragmentation through our bulk characterization and neutron scattering experiments, indicating that chemical pressure (and not necessarily the B-site disorder) plays a key role in the presence or absence of this phenomenon in this material family. Surprisingly, the presence of significant B-site disorder in Nd2GaSbO7 does not generate a spin-glass ground state and instead the same all-in-all-out magnetic order identified in other Nd pyrochlores is found here. |
Thursday, March 17, 2022 12:06PM - 12:18PM |
T51.00004: Rapid microwave-assisted synthesis of mixed B-site pyrochlores: stabilization and discovery of Ln2InSbO7 family Brenden Ortiz, Paul M Sarte, Michael Garcia, Marcos Marmolejo, Ganesh Pokharel, Stephen D Wilson In the field of condensed matter physics, the pyrochlore oxides are one of the most intensely studied classes of materials. The combination of a magnetic rare-earth ion within a frustrated tetrahedral motif imparts a wide variety of exotic emergent phenomena, ranging from candidate quantum spin-liquids to spin-glasses to moment fragmentation. Here we present the synthesis of the entire family of mixed B-site pyrochlores Ln2InSbO7 (Ln: La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) using rapid microwave-assisted synthesis. Our methods produce phase pure, crystalline powders in short (<10 min. heating) time scales. Additionally, our methods enable the stabilization of the mixed B-site pyrochlores from Tb-Lu, which are classically outside of the stability-field for typical pyrochlore oxides. Magnetic characterization on the entire class of compounds draws striking parallels to the stannite analogs, suggesting several of the compounds are prime candidates for future avenues of investigation. |
Thursday, March 17, 2022 12:18PM - 12:30PM |
T51.00005: Quantum Monte Carlo Simulation of an SO(N) Singlet Projector Model on the Pyrochlore Lattice Jared R Sutton, Matthew S Block Over the last decade, the search for deconfined quantum critical points on a variety of models and lattices has revealed possible opportunities for identifying and characterizing phase transitions involving exotic quantum phases, such as valence bond solids and quantum spin liquids. Due to the lattice’s geometric frustration, quantum Monte Carlo (QMC) simulations deployed on the pyrochlore lattice have potential to exhibit such transitions at sufficiently high values of the number of spin colors, N. Thus, we deployed an SO(N)-symmetric singlet-projector model, using a stochastic series expansion QMC simulation, to explore these possibilities. Our primary focus involved using high-performance computing resources to identify the critical value Nc such that, for N>Nc, magnetic order is destroyed. Moreover, we added to our singlet-projection model (J1 coupling) a second J2 term, which favors magnetic order, to allow for controlled study of the phase transition as approached from a large value of N. Our characterization of the phase transition (as either first-order or continuous) based on data obtained from large system sizes near the transition point will be presented. Finally, based on these results, potential future directions for this project are briefly discussed. |
Thursday, March 17, 2022 12:30PM - 12:42PM |
T51.00006: Understanding reentrance in frustrated magnets: the case of the Er2Sn2O7 pyrochlore Danielle R Yahne, Darren Pereira, Ludovic D Jaubert, Duminda Sanjeewa, Joseph Kolis, Matthew J Enjalran, Michel J P Gingras, Kate A Ross In frustrated magnets, most of the theoretical focus has been devoted to the physics near zero temperature. However, a number of frustrated magnetic materials develop long-range order at a nonzero critical temperature (Tc), therefore, it is important to ask if the physics near Tc can inform our understanding of the ground state physics. We investigate this question through a detailed single crystal study of Er2Sn2O7, a frustrated pyrochlore antiferromagnet known to reside near the boundary of two classical phases. We propose an original mechanism for the phenomenon of reentrance, where an ordered phase is surrounded on both sides by the same less ordered phase as an external parameter is tuned. While reentrance has been found in a variety of systems, the microscopic mechanisms behind it are not often studied in detail. We show Er2Sn2O7 has multiple instances of reentrance in its experimental field vs temperature phase diagram, and through extensive theoretical calculations, we propose that the origins of this reentrance are linked to soft modes arising from T=0 phase competition. The ground state phase competition enhances thermal fluctuations that entropically stabilize the ordered phase, leading to increased transition temperatures for specific field values, and thus reentrance. |
Thursday, March 17, 2022 12:42PM - 1:18PM |
T51.00007: Spin Dynamics and Unconventional Coulomb Phase in Nd2Zr2O7 Invited Speaker: Elsa Lhotel Coulomb phases form a novel exotic state of matter which, because of frustration, lacks long range order, yet is described by a local organizing principle. Spin ice is the emblematic example of this physics: in a lattice where magnetic ions occupy the vertices of corner sharing tetrahedra (pyrochlore lattice), the combination of strong Ising anisotropy along local <111> axes with ferromagnetic interactions leads to the so called ice rule (2 spins point in and 2 spins point out in each tetrahedron). In the presence of quantum fluctuations, which can be introduced through transverse couplings (as opposed to Ising coupling terms at play in classical Coulomb phases), quantum spin ice and more generally U(1) quantum spin liquids can be stabilized. However, if too large, the transverse terms are expected to stabilize ordered phases. In this context, the question whether classical ordered phases may be stabilized out of Coulomb phases via a Higgs mechanism, has become an important issue. The Nd2Zr2O7 pyrochlore magnet is an excellent candidate to explore this physics: its ground state is known to be antiferromagnetically ordered in the so-called all-in all-out (AIAO) state, but its paramagnetic phase above the ordering temperature could be a novel example of Coulomb phase. Recently, it was proposed that the transition from this possible Coulomb phase towards the AIAO phase could be driven by a Higgs mechanism. Through a careful neutron scattering study of the dynamics and correlations below and above the transition temperature TN, we confirm the coulombic nature of the phase above TN, yet its organizing principle is different from canonical spin ice. In addition, in this Coulomb phase, the spin dynamics contains features typical of the low temperature AIAO phase. Our observations suggest that the transition arises in the thermal regime of the Coulomb phase and is likely not associated to a Higgs mechanism. |
Thursday, March 17, 2022 1:18PM - 1:30PM |
T51.00008: Observing Magnetic Monopoles in Spin Ice via Electron Holography Ankur Dhar, Ludovic D Jaubert, Tsumoru Shintake, Nic Shannon While the magnetic monopole proposed by Pierre Curie and Paul Dirac have remained elusive, emergent monopoles of the H field have been shown to exist in spin ice [1]. Signatures of these monopoles have been indirect so far, leaving their direct observation an open challenge since their discovery [2]. One such technique that could realize this direct observation is electron holography, due to the electron's sensitivity to magnetic fields via the Aharonov-Bohm effect [3]. We explore the possibility of imaging monopoles via electron holography through experimental measurements of monopole and spin ice analogs, and computational simulation of how monopoles would appear in a pyrochlore spin ice thin film [4]. These results suggest that imaging the dynamics of monopoles in thin films of spin ice, including the emergence of electric fields associated with fluctuations of closed loops of spins, through electron holography is a realistic possibility. |
Thursday, March 17, 2022 1:30PM - 1:42PM |
T51.00009: Probing Flat Band Physics in Spin Ice Systems via Polarized Neutron Scattering Kristian T Chung, Jeremy G Kang, Aritro Mukherjee, Wen Jin, Daniel Lozano-Gomez, Michel J P Gingras In this talk we summarize and expand on recent results elucidating how polarized neutron scattering can be used to isolate the correlations of spin modes forming flat bands in pyrochlore magnets. Starting with the nearest-neighbor model of spin ice, we explain why the spin-flip (SF) scattering channel produces pinch point features while the non-spin-flip (NSF) channel produces a completely dispersionless response in the (hhl) scattering plane, a consequence of the fact that the NSF in this plane probes only the modes forming the flat bands. By consider an extended spin ice (ESI) model with further-neighbor interactions, we demonstrate that the NSF is highly sensitive to perturbations lifting the flat band degeneracy, and we use these results to explain the non-flat NSF measured experimentally in the dipolar spin ice compound Ho2Ti2O7. |
Thursday, March 17, 2022 1:42PM - 1:54PM |
T51.00010: The suppression of the spin ice state in the mixed B-site pyrochlore Dy2ScNbO7. Megan R Rutherford, Cole D Mauws, Adam A Aczel, Yixi Su, James W Beare, Haidong Zhou, Casey Marjerrison, Sara Haravifard, Graeme M Luke, Christopher R Wiebe The pyrochlore oxides (with the chemical formula A2B2O7) have been central to the study of geometric frustration for the past several decades. Pyrochlores in which the A-site is occupied by dysprosium exhibit a spin ice state, a phenomenon that has been well studied in Dy2Ti2O7, Dy2Sn2O7, and Dy2Ge2O7. To explore the robustness of the spin ice state in the face of chemical perturbation, we present an investigation of Dy2ScNbO7, where the non-magnetic B-site is occupied by a stoichiometric mixture of Sc3+ and Nb5+ cations. Using magnetometry, heat capacity, muon spin relaxation, and inelastic neutron scattering measurements, we show that Dy2ScNbO7 does not appear to adopt the spin ice ground state. Experimental results that provide evidence that the B-site disorder appears to destroy the expected Ising anisotropy of the system, which is a crucial ingredient for the spin ice state, will be presented. |
Thursday, March 17, 2022 1:54PM - 2:06PM |
T51.00011: Quantum aspects of fragmentation in spin ices Flavien Museur, Elsa Lhotel, Peter Holdsworth The apparent fragmentation of magnetic moments in spin ice systems into coexisting longitudinal, transverse and harmonic parts is a direct consequence of emergent electromagnetism in these systems. At the microscopic level, moments of fixed length act as elements of an emergent field from which magnetic monopoles can be excited. By construction, the leftover is divergence-free. In quantum spin ice systems the emergence is even more striking, as small quantum fluctuations give to the transverse part the dynamics of a compact, U(1) gauge theory. An example of fragmentation is the monopole crystal phase in classical spin ice, in which the longitudinal field has AF long-range order and the transverse part remains disordered. In this talk we identify new fragmented phases and discuss their stability with regard to quantum fluctuations, external fields, and other interactions. In particular we focus on the partially ordered topological phase of kagome spin ice. We show that the onset of the predicted long-range dimer order is explicitly reflected in the transverse part of the emergent field, accompanied by enhanced quantum fluctuations. As a consequence,the dimer ordering can be directly observed by neutron scattering. The same approach is applied to fragmented pyrochlore spin ice |
Thursday, March 17, 2022 2:06PM - 2:18PM |
T51.00012: High-field magnetostriction of spin ices Nan Tang, Masaki Gen, Mingxuan Fu, Akihiko Ikeda, Huiyuan Man, Yasuhiro Matsuda, Kazuyuki Matsuhira, Satoru Nakatsuji In frustrated magnets, geometrical frustration prevents long-range magnetic order and leads to exotic states of matter like spin liquids [1]. Spin ice is such a state whose ground state does not show long-range order, but a short-range “2-in, 2-out” correlation instead [2]. Local Ising anisotropy induced by the competition between crystal electric field (CEF) effect and magnetic interactions are crucial in order to form such spin-ice correlations. Generally, frustrated magnets show characteristic magnetostrictive responses [3] and here, we used magnetostriction as a probe to explore the regime beyond Ising limit in classical spin ice Ho2Ti2O7 and quantum spin ice Pr2Zr2O7 under high magnetic fields up to ~ 50 T. We observed that the longitudinal magnetostriction Δ??/?? of Ho2Ti2O7 under the [111] magnetic field exhibits nonmonotonic change, while that of Pr2Zr2O7 shrinks monotonically. Based on these observations, we discuss the role of CEF effect and exchange-striction effect in driving the behavior of Δ??/??. |
Thursday, March 17, 2022 2:18PM - 2:30PM |
T51.00013: Higher Dimensional Spin Liquid on the Centred Pyrochlore Lattice Rajah P Nutakki, Lode C Pollet, Ludovic D Jaubert, Dirk Volkmer, Richard Roess-Ohlenroth, Alexander Tsirlin, Anton Jesche, Phillip Gegenwart, Hans-Albrecht Krug von Nidda Magnetic systems on frustrated lattices have proven a fertile source for the discovery of exotic states of matter, from quantum spin liquids to spin ice. Typically, these frustrated lattices are constructed from triangular or tetrahedral clusters of three or four spins respectively. The metal-azolate framework, [Mn(ta)2], realizes a centred pyrochlore lattice, where the basic cluster is a centred tetrahedron of five magnetic ions. We study a relevant minimal Heisenberg model, finding a spin liquid in a large region of the phase diagram. Comparison to measurements of the magnetic properties of [Mn(ta)2] indicates that the material is proximate to such a state. We propose an effective description of the spin liquid as a three-dimensional slab of a Coulomb phase on a four-dimensional lattice, which rationalizes the low temperature correlations of the model for a large range of parameters. This introduces a novel lattice geometry within which to explore the rich physics of frustrated magnetism. |
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