Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session R47: Criticality and Low Dimensionality in Quantum MagnetsFocus
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Sponsoring Units: GMAG DCMP DCOMP Chair: Paul Goddard, Univ of Warwick Room: 710/712 |
Thursday, March 5, 2020 8:00AM - 8:36AM |
R47.00001: Chemical control of magnetism in S = 1 quantum materials: new twists on a seemingly old theme Invited Speaker: Jamie Manson We aim to control the spatial exchange and single-ion anisotropies (D) in bespoke S = 1 Ni(II) quantum magnets. Early work led to [Ni(HF2)(pyz)2]SbF6 (pyz = pyrazine) which contains Ni-FHF-Ni chains (JFHF) cross-linked by Ni-pyz-Ni segments (Jpyz) to form a tetragonal network [1]. The material undergoes XY-AFM order below TN = 12.2 K with collinear Ni(II) moments confined to the [Ni(pyz)2]2+ plane. High-field M(H) and inelastic neutron scattering (INS) revealed D ≈ JFHF >> Jpyz. In the related square lattices [2,3], NiX2(pyz)2 (X = F, Cl, Br, I, NCS), the electronegativity of X dictates the Ni(II) spin direction with XY- and Ising-like ground-states observed for X = F, Cl and X = Br, I, NCS, respectively. We used INS to determine Jpyz and J⊥. Chemical substitution of pyz for pyrimidine (pym) affords unusual topologies and enhanced magnetic interactions along Ni-pym-Ni relative to Ni-pyz-Ni. Thus, we synthesized two new Q1D chains: staggered [Ni(pym)(H2O)4]SO4●2H2O and chiral [Ni(pym)(H2O)4]SiF6●H2O, each showing multiple field-induced phase transitions below 5 K. Lastly, we will describe a unique 3D chiral Ni(II) framework that may display spin-liquid behavior at low temperatures. Time permitting, the structural and magnetic properties of these materials will be discussed. |
Thursday, March 5, 2020 8:36AM - 8:48AM |
R47.00002: A near-ideal molecule-based Haldane spin chain Paul Goddard, Robert C Williams, William J. Blackmore, Sam Curley, Martin Lees, Serena M Birnbaum, John Singleton, Benjamin Huddart, Thomas Hicken, Tom Lancaster, Stephen Blundell, Fan Xiao, Andrew Ozarowski, Francis L. Pratt, David J Voneshen, Zurab Guguchia, Christopher Baines, J.A. Schlueter, Danielle Y. Villa, Jamie Manson We report on a new molecule-based magnet NiI2(3,5-lutidine)4 characterized using x-ray diffraction, electron-spin resonance, magnetometry, inelastic neutron scattering and muon-spin relaxation. The system is a near-ideal realization of the S = 1 Haldane chain with intrachain exchange J = 17.5 K, energy gaps of 5.3 K and 7.7 K split by easy-axis anisotropy D=−1.2 K, and with no sign of magnetic order down to 20 mK. The value D/J=−0.07 implies one of the most isotropic Haldane systems yet discovered, while the ratio Δ/J = 0.40(1) (where Δ is the average gap size) is close to the ideal theoretical value, suggesting a very high degree of magnetic isolation of the chains. The Haldane gap is closed by orientation-dependent critical fields 5.3 T and 4.3 T, which are readily accessible experimentally. The fully polarized state occurs above 46.0 T. The results are explicable within the so-called fermion model, which is in contrast to other reported easy-axis Haldane systems. The flexible nature of this molecule-based material readily permits tuning via applied pressure and introduction of disorder along the exchange pathway. |
Thursday, March 5, 2020 8:48AM - 9:00AM |
R47.00003: Magnetic ground-state of two new highly one-dimensional ferromagnetic chain compounds M(NCS)2(thiourea)2; M = Co, Ni. Sam Curley, Robert C Williams, Paul Goddard, Rebecca Scatena, Stephen Blundell, Piero Macchi, Thomas Hicken, Tom Lancaster, Fan Xiao, James C Eckert, Vivien Zapf, Jacqueline Villa, Jamie Manson Low-dimensional magnetic materials have garnered much attention from theorists and experimentalists alike for many years. Chains of low-spin, ferromagnetically (FM) or antiferromagnetically (AFM) coupled magnetic-ions can both harbour exotic magnetic ground-states: such as the gapped Haldane ground state in Heisenberg AFM chains, or the field-induced quantum paramagnetic state in Ising-like FM chains. In terms of physically realizing such systems, co-ordination chemistry has had great success in promoting crystal architectures that can lead to quasi-1D behaviours. Altering the bridging or non-bridging ligand species can modify both the Heisenberg exchange (J) and single-ion anisotropy (D). It is the interplay of these two parameters that ultimately determines the magnetic ground-state. |
Thursday, March 5, 2020 9:00AM - 9:12AM |
R47.00004: Cluster-Based Haldane States in Spin Cluster Chains Takanori Sugimoto, Katsuhiro Morita, Takami Tohyama The S=1 Haldane state has attracted much attention due to its topological characters. This state emerges not only in the S=1 spin chain, but also in an S=1/2 two-leg spin ladder with a ferromagnetic rung interaction. Additionally, our recent study on low-temperature magnetism in a naturally occurring mineral Fedotovite, has shown the Haldane state consisting of cluster-based S=1 spins, in collaboration with experimental groups. In this compound, edge-shared tetrahedral spin clusters which include six spins in a cluster, are linked by a nearest-neighbor antiferromagnetic inter-cluster interaction in one dimension. The low-energy state in a cluster is a triplet and the inter-cluster interaction is perturbative as compared with the intra-cluster excitation energy, so that we can explain the low-temperature physics in an effective S=1 spin model obtained by projection into cluster's triplet states. Furthermore, we extend the cluster-based Haldane state to spin-cluster systems which have odd-number S=1/2 spins in a cluster, and find cluster-based Haldane state with breaking time-reversal symmetry. Thus, we will present several varieties of cluster-based Haldane states obatined in possible spin lattices. |
Thursday, March 5, 2020 9:12AM - 9:48AM |
R47.00005: A Novel Strongly Spin-Orbit Coupled Quantum Dimer Magnet: Yb2Si2O7 Invited Speaker: Kate Ross The quantum dimer magnet (QDM) is the canonical example of 'quantum magnetism'. This state consists of entangled nearest-neighbor spin dimers and often exhibits a field-induced 'triplon' Bose-Einstein condensate (BEC) phase. I will discuss a new QDM in the strongly spin-orbit coupled, distorted honeycomb-lattice material Yb2Si2O7. Single crystal neutron scattering, specific heat, and ultrasound velocity measurements reveal a gapped singlet zero field ground state with sharp, dispersive excitations. We find a field-induced magnetically ordered phase reminiscent of a BEC phase, with exceptionally low critical fields of Hc1 ~0.4 T and Hc2 ~1.4 T. Using inelastic neutron scattering we observe a Goldstone mode that persists throughout the entire field-induced magnetically ordered phase, suggestive of the spontaneous breaking of U(1) symmetry expected for a triplon BEC. However, in contrast to other well- known cases of this phase, the high-field (H > 1.2T) part of the phase diagram in Yb2Si2O7 is interrupted by an unusual regime signaled by a change inthe field dependence of the ultrasound velocity and net magnetization, as well as the disappearance of a sharp anomaly in the specific heat. These measurements raise the question of how anisotropy in strongly spin-orbit coupled materials modifies the field induced phases of QDMs. |
Thursday, March 5, 2020 9:48AM - 10:00AM |
R47.00006: Dimensionality of the BEC state in structurally healed Sr-doped BaCuSi2O6 Dagmar Weickert, Marcelo Jaime, Pascal Puphal, Shusaku Imajo, Yoshimitsu Kohama, Joosep Link, Raivo Stern In the last 20 years, more than a dozen quantum paramagnets have been identified to show properties consistent with the formalism of Bose-Einstein condensation (BEC) when a magnetic field is used to induce XY-type AFM order at cryogenic temperatures. Among them, BaCuSi2O6 orders between critical fields Hc1 = 23.5T and Hc2 = 49T, below Tc,max = 4K. However, BaCuSi2O6 additionally undergoes a structural phase transition at 90K into a monoclinic lattice at lower temperature causing a spatial modulation of the spin gap along the c-axis. This spin gap modulation is suspected to be the reason for a dimensional crossover in the magnetic excitations spectrum observed in the critical exponent of the phase transition close to Hc1. We discuss experimental results on newly synthesized single crystals BaCuSi2O6 doped with 10% Sr, structurally healed, which do not exhibit a phase transition at 90K and allow the investigation of the BEC close to Hc1 in the absence of spin gap modulation. We establish the H-T phase diagram based on magnetization, magnetic torque, specific heat and magnetocaloric effect (MCE) measurements in DC and pulsed magnetic fields that address the dimensionality of excitations in the BEC critical exponents. |
Thursday, March 5, 2020 10:00AM - 10:12AM |
R47.00007: Unconventional critical behavior in the quasi-one-dimensional S = 1 chain NiTe2O5 Jun Han Lee, Marie Kratochvílová, Huibo Cao, Zahra-Sadat Yamani, Jungsoo Kim, Je-Guen Park, Gregory Randall Stewart, Yoon Seok Oh The physical quantities develop with critical behavior near the (quantum) critical regime of correlated systems, such as superfluid density of two dimensional superconductor, surface critical behavior in topological phase, Higgs mode in quantum antiferromagnet, quantum criticality in multiferroics, quantum phase transition in two-dimensional electron system, etc. Despite the variety of complex interactions and correlations, the critical behavior only depends on global properties such as the spatial dimension, the symmetry of the order parameter, and the range of interaction. Recently we found a new quasi-one-dimensional chain compound NiTe2O5, in which spin-1 of Ni2+ ions form a one-dimensional chain structure through NiO6 octahedra’s edge-sharing. Although it undergoes a long-range antiferromagnetic (AFM) order with an archetypical anisotropic AFM anomaly at TN = 30.5 K, the AFM order parameter develops with intriguing unconventional critical exponents. In this talk, we present magnetic properties and structure of NiTe2O5 and discuss its unconventional critical behavior. |
Thursday, March 5, 2020 10:12AM - 10:24AM |
R47.00008: Magnetic phase diagram and anisotropy in the one-dimensional quantum spin system Rb2Cu2Mo3O12 Shohei Hayashida, Dominic Blosser, Kirill Povarov, Zewu Yan, Severian Gvasaliya, Alexey Ponomaryov, Sergei Zvyagin, Andrey Zheludev We report low temperature thermodynamic, magnetic and electron spin resonance (ESR) measurements on single crystal samples of a quantum spin chain material Rb2Cu2Mo3O12. In heat capacity measurement, phase transitions to magnetically ordered state are clearly observed, and the entire phase diagram is mapped out in detail. It is found that the criticalities of the transitions at lower and upper fields are dominated by three-dimensional order and one-dimensionality, respectively. The measured phase boundaries and ESR data prove that anisotropy heavily influences the magnetic state of Rb2Cu2Mo3O12 [1]. |
Thursday, March 5, 2020 10:24AM - 10:36AM |
R47.00009: Comparison of spin dynamics in green and black dioptase: from classical chains to quantum fluctuations Andrey Podlesnyak, Stanislav Nikitin, Oleksandr Prokhnenko, Alexander Kolesnikov, Timothy Prisk, Lawrence {Larry} M. Anovitz The magnetic excitations of a gem crystal green dioptase Cu6[Si6O18].6H2O and fully dehydrated black dioptase Cu6Si6O18 have been studied by inelastic neutron scattering. The intrachain Jc and interchain Jab interactions as well as the anisotropy of the Jc have been determined. The observed spectrum of green dioptase contains two sharp magnetic modes and a prominent spin gap that is consistent with the ordered ground state of Cu moments coupled antiferromagnetically in spiral chains along the c axis and ferromagnetically in ab planes on the hexagonal cell. We found that dehydration has a dual effect on the magnetic interactions: it significantly increases the intrachain exchange interaction Jc, and simultaneously decouples the chains by suppressing Jab. The spin dynamics of green dioptase are dominated by conventional magnon excitations, whereas the continuum of excitations observed in black dioptase indicates the presence of fractionalized spinons. We suggest that black dioptase behaves like a nearly ideal S=1/2 antiferromagnetic Heisenberg spiral chain with enhanced quantum fluctuations and weak spinon confinement Jab/Jc~0.02. |
Thursday, March 5, 2020 10:36AM - 10:48AM |
R47.00010: Using muons to probe magnetic phases in quantum disordered systems through muon-induced distortions Benjamin Huddart, Tom Lancaster, Francis L. Pratt In muon-spin spectroscopy (µ+SR) experiments, concerns are often raised about the influence of the implanted muon on its local environment. In this talk, we will present examples where the distortions induced by the muon provide sensitivity to the magnetic state of the system. Transverse-field µ+SR spectra for the strong-rung molecular spin ladder system (Hpip)2CuBr4 exhibit characteristic behaviour in each of the regions of the phase diagram [1]. Analysis of the muon stopping sites in this compound, obtained from density functional theory calculations, suggests that the magnetic field shift at the muon site in the quantum disordered phase results from a local muon-induced antiferromagnetic state. We propose that a similar mechanism enables the detection of field-induced transitions in the quantum spin liquids (QSLs) κ-(BEDT-TTF)2Cu2(CN)3 and κ-(BEDT-TTF)2Ag2(CN)3. In these cases, the field distributions seen by the muon correspond to those expected from adjacent ordered phases in the 2D triangular lattice QSL phase diagram. |
Thursday, March 5, 2020 10:48AM - 11:00AM |
R47.00011: Kaleidoscope of Quantum Phases in a Frustrated Spin-1 Chain with Single-ion Anisotropy Qiang Luo, Shijie Hu, Jize Zhao, Xiaoqun Wang We study the phases and phase transitions of a spin-1 chain with next-nearest neighbor (NNN) interaction J2 and uniaxial single-ion anisotropy D by density-matrix renormalization group method. We reconfirm the topological first-order transition of the J1-J2 chain by the bond reversal method [Q. Luo, et al., Phys. Rev. B 100, 121111(R) (2019)], and we show accurately that the transition occurs at J2,t = 0.7607(2). When considering a negative D, there is a spontaneously dimerized phase so as to avoiding the direct transition between the Neel phase and the NNN-Neel phase. The Neel--Dimer transition is of Gaussian type with central charge c = 1, while the NNN-Neel--Dimer transition belongs to the 2D Ising universality class. For the positive D case, there is a gapless chiral phase with a broken parity symmetry. It separates the NNN-Haldane phase and the large-D phase before the direct transition between the two. We also determine the topological critical points between the Haldane phase and the large-D phase by the level spectroscopy technique supplemented by the Richardson method. The critical point is obtained with spectacular accuracy, Dc = 0.96847256(2), for the frustration-free case. |
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