Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P39: Low Dimensional Magnets in the Quantum LimitFocus Live
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Sponsoring Units: GMAG DMP Chair: Kemp Plumb, Brown University |
Wednesday, March 17, 2021 3:00PM - 3:12PM Live |
P39.00001: Evidence for Vector Chiral Magnetic Order in κ-(BETS)2Mn[N(CN)2]3 Stephen Winter, Kira Riedl, Steffi Hartmann, David Zielke, Natalia Kushch, Elena Gati, Werner Biberacher, Roser Valenti, Michael Lang, Mark Kartsovnik, Eduard Yagubskii Organic CT salts have long provided ideal experimental playgrounds for studying the interplay between magnetic and charge degrees of freedom. This has culminated in the experimental discoveries of spin-liquid states in the vicinity of the Mott transition, for example in k-(ET)2Cu2(CN)3 (κ-CN). Such states are thought to be stabilized by large multi-spin ring-exchange interactions. We discuss the properties of the recently synthesized κ-(BETS)2Mn[N(CN)2]3 (κ-Mn). Based on analysis of specific heat, magnetic torque, and NMR measurements combined with ab-initio calculations, we will address: (i) The nature and role of π-d interactions, (ii) the magnetic interactions within the BETS layers including SOC and ring-exchange, (iii) the origin of anomalous angle dependence and therefore (iv) the character of the magnetic ground state at low pressure. In particular, we argue that all experiments and calculations support an exotic four-sublattice vector chiral order, which is selected by the combination of large ring exchange and DM interactions. Ring exchange in κ-Mn plays an opposite role as in κ-CN – namely, the ring-exchange helps to stabilize an unconventional ordered state rather than a spin-liquid. |
Wednesday, March 17, 2021 3:12PM - 3:24PM Live |
P39.00002: Realization of the orbital-selective Mott state at the molecular level in Ba3LaRu2O9 Adam Aczel, Qiang Chen, Aime Verrier, Djamel Ziat, Amanda J Clune, Redha Rouane, Xavier Bazier-Matte, Jie Ma, Stuart Calder, Keith Taddei, Clarina Dela Cruz, Alexander Kolesnikov, Jinguang Cheng, Z. Liu, Jeffrey A Quilliam, Janice Musfeldt, Haidong Zhou Molecular magnets based on heavy transition metals have recently attracted significant interest in the quest for novel magnetic properties. For systems with an odd number of valence electrons per molecule, high or low molecular spin states are typically expected in the double exchange or quasi-molecular orbital limits respectively. In this work, we use bulk characterization, muon spin relaxation, neutron diffraction, and inelastic neutron scattering to identify a rare intermediate spin-3/2 per dimer state that cannot be understood in a double exchange or quasi-molecular orbital picture and instead arises from orbital-selective Mott insulating behavior at the molecular level. Our measurements are also indicative of stripe magnetic order below TN = 25 K for these molecular spin-3/2 degrees-of-freedom, which is consistent with expectations for an ideal triangular lattice with significant next nearest neighbor in-plane exchange. Finally, we present neutron diffraction and Raman spectroscopy data under applied pressure that reveal a coincident crystal symmetry lowering and spin state transition from S = 3/2 to S = 1/2 at a modest pressure P ~ 1 GPa, which highlights the delicate balance between competing energy scales in this system. |
Wednesday, March 17, 2021 3:24PM - 4:00PM Live |
P39.00003: Spin-Lattice Coupling in the Spin-1 Kagome Compound Na2Ti3Cl8 Invited Speaker: Turan Birol The coupling between the lattice vibrations or static distortions and the magnetic Hamiltonians (the spin-phonon and spin-lattice couplings) are ubiquitous in ionic transition metal compounds. These effects give rise to phenomena such as spin-splitting of phonon modes, or breaking of the degeneracy of elastic constants below magnetic ordering temperatures. Historically, first principles computational approaches have been extremely successful in predicting and explaining the coupling between the crystal structure, its excitations, and the magnetic properties. In this talk, after giving a brief introduction to the idea of spin-lattice coupling, I am going to discuss a case study from a first principles theory point of view: Na2Ti3Cl8 is a 2 dimensional frustrated antiferromagnet, which hosts spin-1 Ti cations on a Kagome lattice. The crystal structure of Na2Ti3Cl8 undergoes a phase transition at 200 K, below which the system develops a polarization, which is in principle switchable, and the magnetic moments are suppressed. Our combination of first principles density functional theory and exact diagonalization calculations show that this compound not only has a magnetic Hamiltonian with unexpected higher order `ring exchange’ terms, but also that the coincident structural and magnetic phase transition is driven by the strong spin-lattice coupling. I will conclude by a discussion on how the lower-T electronic structure of this compound is better explained from a molecular point of view, rather than the usual approach of well defined atomic moments in a solid. |
Wednesday, March 17, 2021 4:00PM - 4:12PM Live |
P39.00004: Frustration-driven dimerization in two-leg Heisenberg ladder with trimer rungs Andreas Weichselbaum, Weiguo Yin, Alexei Tsvelik We study the antiferromagnetic spin-half Heisenberg ladder in the presence of an additional frustrating rung spin. The model is also relevant for the description of real two-dimensional materials such as the two- dimensional trimer magnet Ba4Ir3O10. We study the zero-temperature phase diagram, where we combine numerical and analytical methods into an overall consistent description. All numerical simulations are also accompanied by studies of the dynamical spin structure factor obtained via the density matrix renormalization group. In the regime of strong rung coupling we find a gapped dimerized phase related to competing symmetry sectors in Hilbert space. In the weak rung-coupling regime the system remains critical while it also exhibits a sharp coherent branch in the low-energy regime of the dynamical structure factor. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P39.00005: Two- and three dimensional quantum-spin-1/2 XXZ magnets in zero magnetic field: global phase diagrams, thermodynamics and entanglement Ozan Sariyer Global phase diagrams and critical properties of the field-free quantum-spin-1/2 XXZ model in two- and three-dimensions are obtained using a numerical renormalization group theory method. Quantum phase transitions at zero-temperature are also identified. The nearest-neighbour spin-spin correlations and entanglement measures, as well as internal energy and specific heat are calculated globally at all temperatures for the whole range of exchange interaction anisotropy, from XY limit to Ising limits, for both antiferromagnetic and ferromagnetic cases. It is argued that there exists long-range (quasi-long-range) order at low-temperatures, and that the low-lying excitations are gapped (gapless) in the Ising-like easy-axis (XY-like easy-plane) regime. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P39.00006: Pressure-evolution of a symmetry-broken S = ½ dimer magnet Sam Curley, Matthew J Coak, Robert C Williams, Saman Ghannadzadeh, Dmytro Kamenskyi, Andreas Schneider, Susumo Okubo, Takahiro Sakurai, Hitoshi Ohta, Benjamin Michael Huddart, Tom Lancaster, David E Graf, Jere Tidy, Mark Senn, Jamie L Manson, Paul Goddard A lattice of weakly interacting, antiferromagnetically coupled spin-half dimers can be well described within a Bose-Einstein condensate (BEC) of magnons model, provided the rotational symmetry of the spin Hamiltonian is preserved. The material CuVOF4(H2O)6H2O, appears to exhibit behaviour typical of a dimer system, with electron-spin resonance and magnetometry data confirming the field-induced closing of an energy-gap between the excited triplet-state and singlet ground-state energy levels. However, as the dimer-unit is composed of two unlike spin-half ions, Cu(II) and V(IV), we are presented with the interesting situation where the rotational symmetry of the system is broken by the spins in the dimer-units themselves. Here we discuss the unusual hydrogen-bond-mediated exchange pathways in this material and the resultant phase diagram. In addition, we show that the application of hydrostatic pressure serves to tune the magnitude of the intradimer exchange interaction, as well as push the system through a structural phase-transition at pressures beyond 20 kbar. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P39.00007: Decay of Higgs amplitude mode in quasi two-dimensional antiferromagnet Ba2FeSi2O7 Seunghwan Do, Hao Zhang, Travis Williams, Tao Hong, Vasile Garlea, Tae-Hwan Jang, Jae-Hoon Park, Cristian Batista, Andrew Christianson Spontaneous symmetry breaking produces longitudinal amplitude (Higgs) mode as a well-defined collective excitation [1]. We investigate the emergence of the longitudinal mode in two-dimensional square-lattice antiferromagnet Ba2FeSi2O7 –easy-plane antiferromagnet with effective S=1 near a continuous quantum phase transition into a quantum paramagnet [2,3]. Our inelastic neutron scattering measurements and extended spin wave theory analysis demonstrate its proximity to the quantum critical point (QCP), and reveal the presence of the longitudinal mode in the observed spectra. The longitudinal mode is visible thoughout the Brillouin zone. Noticeably, this mode is highly renormalized and broadened near the magnetic zone center, which though decay channels to the transverse mode. Finally, we discuss the necessary theoretical corrections to a generalized linear spin wave theory to accurately describe the decay of the longitudinal magnon in Ba2FeSi2O7. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P39.00008: Thermal conductivity of the quantum spin liquid candidate EtMe3Sb[Pd(dmit)2]2: No evidence of mobile gapless excitations Nicolas Doiron-Leyraud, Patrick Bourgeois-Hope, Francis Laliberte, Etienne Lefrancois, Gael Grissonnanche, Samuel René de Cotret, Louis Taillefer, Hengbo Cui, Reizo Kato, Shunsuke Kitou, Hiroshi Sawa The search for highly mobile gapless spin excitations in quantum spin liquids (QSL) is currently attracting considerable attention. We examined this specific issue via thermal conductivity measurements on one of the most promising QSL candidates, the organic insulator EtMe3Sb[Pd(dmit)2]2 (dmit-131). We performed heat conduction experiments down to 0.07 K on 8 high-quality single crystals of dmit-131 [1]. In agreement with a parallel study [2] and contrary to other reports [3], our body of data consistently and reproducibly shows the absence of a T-linear contribution to the thermal conductivity of dmit-131 at low temperatures, showing that no spin excitation contribute to the heat transport. Consequently, the spin excitations in dmit-131 are localized and not mobile. Our data further reveal a strongly suppressed phonon conductivity, evidence that the phonons are heavily scattered by localized spin excitations. Comparison with published data on numerous spin-liquid materials shows, in fact, that this is a widespread phenomenon and that it should be considered in any future theory of QSLs. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P39.00009: Exactly solvable Dimer Liquid and Ising Gauge Theory of a nonplanar Quantum Dimer Model on the Checkerboard Lattice Julia Wildeboer, Zohar Nussinov, Alexander Seidel We present analytic results for a special dimer model on the nonbipartite and nonplanar checkerboard lattice that does not allow for parallel dimers surrounding diagonal links. This fully integrable model realizes a fully gapped and disordered dimer liquid phase with topological degeneracy and deconfined fractional excitations. Results concerning the entanglement behavior and the identification of ground states with minimum entanglement entropy, which reflect the quasiparticle excitations of the topological phase, are obtained. Additionally we elaborate on an exact mapping to Z2 lattice gauge theory previously only investigated in the context of planar dimer models. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P39.00010: Quantum-to-classical correspondence in two-dimensional Heisenberg models Tao Wang, Xiansheng Cai, Kun Chen, Nikolai Prokof'ev, Boris Svistunov The quantum-to-classical correspondence (QCC) in spin models is a puzzling phenomenon |
Wednesday, March 17, 2021 5:24PM - 6:00PM Live |
P39.00011: Quantum Magnetism in the Honeycomb Lattice Material YbCl3 Invited Speaker: Andy Christianson In quantum magnetism, simple model systems exhibit rich behavior that enables the testing of fundamental ideas which in turn serve as the basis for understanding and identifying more complex behavior. This talk will focus on the physics of the honeycomb lattice Heisenberg model as probed by inelastic neutron scattering. The honeycomb lattice Heisenberg model is simple: only nearest neighbor interactions are considered; there is no frustration, and the ground state at T=0 is the Néel state. The model material discussed here is the rare earth halide YbCl3. YbCl3 exhibits a broad peak in the heat capacity at 1.8 K and very weak but sharper transition at 0.6 K corresponding to the onset of magnetic order. We have determined the crystal field Hamiltonian through simultaneous refinements of inelastic neutron scattering and magnetization data. The ground state crystal field doublet is well isolated and results in an effective spin-1/2 system. The low energy excitation spectrum consists of conventional spin waves and an unusually sharp feature within a broad continuum. By including both transverse and longitudinal channels of the neutron response, linear spin wave theory with a single Heisenberg interaction (J ~ 0.42 meV) on the honeycomb lattice reproduces all of the key features in the spectrum. In particular, the broad continuum corresponds to a two-magnon contribution from the longitudinal channel, while the sharp feature within this continuum is identified as a Van Hove singularity in the joint density of states. The experimental demonstration of a Van Hove singularity in a two-magnon continuum is important as a confirmation of basic notions of continua in quantum magnetism and additionally because analogous features in two-spinon continua could potentially be used to distinguish quantum spin liquids from merely disordered systems. |
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