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 J39: Spin Liquid IIFocus Live
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Sponsoring Units: GMAG DMP Chair: Martin Mourigal, Georgia Inst of Tech |
Tuesday, March 16, 2021 3:00PM - 3:36PM Live |
J39.00001: Theory of Competing Orders in Two Dimensional Quantum Magnets Invited Speaker: Xueyang Song Quantum magnets provide the simplest example of strongly interacting quantum matter, yet they continue to resist a comprehensive understanding above one spatial dimension. We explore a promising framework in 2D lattice , the Dirac spin liquid (DSL), a version of Quantum Electrodynamics (QED3) with four flavors of Dirac fermions coupled to photons. Importantly, its excitations include magnetic monopoles that drive confinement, and the symmetry actions on monopoles on square, honeycomb, triangular and kagome lattices contain crucial information about the DSL states. The underlying band topology of spinon insulators, e.g., wannier insulator protected by rotation, determines the elusive Berry phase of monopole under rotations. The stability of the DSL is enhanced on triangular and kagome lattices compared to bipartite (square and honeycomb) lattices. We obtain the universal signatures of the DSL on triangular and kagome lattices, including those of monopole excitations, as a guide to numerics and experiments on existing materials. Even when unstable, the DSL helps unify and organize the plethora of competing orders in correlated two-dimensional materials. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J39.00002: Magnetic phase diagram of the frustrated quantum ferro-antiferromagnet Cs2Cu2Mo3O12 Daniel Flavian Blasco, Shohei Hayashida, Lysander Huberich, Dominic Blosser, Kirill Povarov, Zewu Yan, Severian Gvasaliya, Andrey Zheludev A single-crystal sample of the frustrated quantum magnet Cs2Cu2Mo3O12 is investigated by means of magnetic and thermodynamic measurements. A combination of specific heat and magnetic torque measurements maps out the entire H-T phase diagram for three magnetic field orientations. Remarkably, a new phase emerges below the saturation field, irrespective of the crystal orientation. It is suggested that the presaturation phase represents spin-nematic order or other multimagnon condensate. The phase diagrams within the long-range ordered dome are qualitatively different for each geometry. In particular, multiple transitions are identified at low temperature in a field along the (010) direction. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J39.00003: Valence-Bond Order in a Honeycomb Antiferromagnet Coupled to Quantum Phonons Manuel Weber We use exact quantum Monte Carlo simulations to demonstrate that the Neel ground state of an antiferromagnetic SU(2) spin-1/2 Heisenberg model on the honeycomb lattice can be destroyed by a coupling to quantum phonons. We find a clear first-order transition to a valence-bond-solid state with Kekule order instead of a deconfined quantum critical point. However, quantum lattice fluctuations can drive the transition towards weakly first-order, revealing a tunability of the transition by the retardation of the interaction. In contrast to the one-dimensional case, our phase diagram in the adiabatic regime is qualitatively different from the frustrated J1-J2 model. Our results suggest that a coupling to bond phonons can induce Kekule order in Dirac systems. |
Tuesday, March 16, 2021 4:00PM - 4:36PM Live |
J39.00004: Band topology of magnetic excitations in anisotropic quantum magnets Invited Speaker: Judit Romhanyi Excitations of ordered magnets offer a natural path to realize magnetic analogs of topologically nontrivial Z2 and Chern bands. Here we explore the topology of multiplet excitations arising due to the local entanglement of spin and/or orbital degrees of freedom. In particular, we study the fate of the fragile Z2 topological bands realized by non-Kramers magnetic excitations in bilayer as well as spin-orbital systems. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J39.00005: Spin-Excitations of 1T-TaS2 and Related Magnetoresistance in the 4Hb Phase Itai Silber, Itamar Kimchi, David E Graf, Amit Kanigel, Yoram Dagan Spin-spin interactions can lead to exotic ground states with emergent excitations in frustrated quantum magnets. Such a system is the transition metal dichalcogenide TaS2. While the 1T phase of the material is electrically insulating and exhibits no magnetic ordering down to millikelvin temperatures, the specific heat has a linear fermionic-type contribution that suggests a large bandwidth. The magnetic torque is strongly anisotropic and has a singular response at low magnetic fields which we attribute to a continuous disassociation of singlet pairs. This picture is also supported by the perfect collapse of the specific heat data at various temperatures and magnetic fields onto a universal curve predicted for singlets with a random distribution of antiferromagnetic coupling strengths. We interpret our data in the framework of a quantum spin liquid ground state having a large bandwidth with minority spins forming an array of singlet pairs with a continuum of coupling constants. Moreover, by intercalating layers of the proposed spin liquid 1T phase with superconducting layers of 2H phase we observe surprising anisotropic magnetoresistance, possibly reminiscent of the spin interactions in the 1T phase. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J39.00006: Featureless quantum paramagnet with frustrated criticality and competing spiral magnetism on spin-1 honeycomb lattice magnet Jianqiao Liu, Fei-Ye Li, Gang Chen, Ziqiang Wang We study the spin-1 honeycomb lattice magnets with frustrated exchange interactions. The proposed microscopic spin model contains first and second neighbor Heisenberg interactions as well as the singleion anisotropy. We establish a rich phase diagram that includes a featureless quantum paramagnet(QP) and various spin spiral states induced by the mechanism of order by quantum disorder. Although the QP is dubbed featureless, it is shown that the magnetic excitations develop a contour degeneracy in the reciprocal space at the band minima. These degenerate excitations are responsible for the frustrated criticality from the quantum paramagnet to ordered phases. This work illustrates the effects of magnetic frustration on both magnetic orderings and the magnetic excitations. We discuss the experimental relevance to various Ni-based honeycomb lattice magnets. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J39.00007: Exact tensor network representations for a frustrated spin system at large finite S Po-Wei Lo, Michael Lawler The tensor network approach has been shown to be a powerful tool to study entanglement of frustrated spin systems. Nevertheless, it usually requires high computational capability even for spin-half case, e.g. the kagome lattice. Counterintuitively, we expect that in the semiclassical regime (large spin S case), the representation of a wavefunction could be found without high computational requirement. We study the phase diagram of the frustrated spin ladder model and find a highly frustrated point in the classical limit at which all the eigenstates at finite spin S that approach the ground state as S goes to infinity are identified. At spin S these states are the rung singlet states and the set of Haldane states of spin 1 to spin 2S. All these states can be represented by tensor networks. Studies in the semiclassical regime, in general, give a clue of the phase in the quantum regime, and thus point a direction to search for novel quantum phases in future numerical works. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J39.00008: Investigating the Magnetism of the Honeycomb Arsenate KCo2(AsO4)(HAsO4) Justin Felder Materials with triangular and honeycomb lattices are of interest as potential quantum materials due to the possibility of magnetic geometric frustration. Combining the triangular lattice of known structure types with ions with desirable electronic and magnetic configurations could induce exotic ground states, such as a quantum spin liquid. We have identified the distorted honeycomb compound KCo2(AsO4)(HAsO4) as a potential quantum spin liquid candidate. In this talk I will discuss our ongoing investigations into characterizing the properties of KCo2(AsO4)(HAsO4), including the hydrothermal synthesis of polycrystalline samples, X-ray diffraction measurements, magnetic susceptibility measurements, neutron diffraction measurements, and DFT calculations. Magnetic susceptibility measurements suggest a transition to a magnetically ordered state below 16 K. The ferromagnetic-like character in the magnetic susceptibility below the transition suggests that the material is not a simple antiferromagnet. DFT calculations suggest a zig-zag antiferromagnetic order within the [ab] -plane. Neutron diffraction measurements at POWGEN reveal a magnetic ordering below ~20 K, and refinement of the magnetic structure is ongoing. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J39.00009: Mode-Entangled Neutron Beams: A possible new tool for exploring Quantum Materials Stephen Kuhn, Samuel McKay, David Verge Baxter, Collin Leslie Broholm, Eric B Dees, Fankang Li, Roger Pynn, Abu Ashik Md. Irfan, Gerardo Ortiz, Jiazhou Shen, William Michael Snow, Vincent Vangelista An entangled neutron beam is an intriguing potential probe of the properties of quantum materials. We have generated bipartite (spin-path) and tripartite (spin-path-energy) mode-entangled neutrons at both pulsed and continuous neutron sources. The subsystems of individual neutrons are entangled as proven by the violation of a contextuality inequality (similar to the Bell inequality). The entanglement length (i.e. path separation) was varied between 85 nm and 1600 nm, much smaller than the separations available to traditional neutron interferometry. The entanglement is reproducible across different beamlines, different beam collimations, different neutron wavelengths and with different mode entangling devices, with the primary limitation arising from the finite polarization of the neutron beam. A recent theoretical investigation has shown that neutron scattering by entangled electron spins in a dimer show unique scattering signatures for particular parameter values, encouraging us to believe that entangled neutron scattering may be a promising novel technique to probe highly-correlated, frustrated magnetic systems. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J39.00010: Numerical Study of Bond-Nematic Order in Spin S=1/2 Frustrated Ferromagnets on the Square Lattice Matthias Gohlke, Tokuro Shimokawa, Nicholas Shannon In the absence of conventional magnetic order of spin-dipolar moments, ordering with higher-order moments like spin-quadrupoles may occur. Magnets with spin-1/2 degrees of freedom, however, can only exhibit spin-quadrupoles---or spin-nematic states---if two spin-1/2 are combined into an effective spin-1. Such bond-nematic phase has theoretically been observed in frustrated ferromagnets on the square lattice [Shannon2006]. Motivated by recent experiments suggesting the existence of spin-nematic order in volborthite in applied magnetic field [Kohama2019], we revisit the square-lattice frustrated ferromagnets using a combination of modern numerical techniques, namely matrix-product states based methods and high-field exact diagonalization. We confirm the existence of the bond-nematic phase sandwiched between the fully-polarized phase at high fields and a long-range magnetically ordered phase at low fields, and characterise its microscopic properties. These methods enable us to study its dynamical fingerprint---a characteristic Goldstone mode associated with the director of the spin-quadrupole---taking into account the strongly interacting many-body nature in a controlled way. |
Tuesday, March 16, 2021 5:48PM - 6:00PM On Demand |
J39.00011: Spiral Spin Liquid Regardless of Spin Component on a Honeycomb Lattice Chun-Jiong Huang, Jianqiao Liu, Gang Chen Spiral spin liquid is a special family of classical spin liquids and its degenerate manifolds of spin spirals are spiral surfaces or spiral lines in momentum space. In this work, we study the antiferromagnetic Ising, XY, and Heisenberg models with an additional second-neighbor antiferromagnetic exchange on a honeycomb lattice. We find they all develop spiral spin liquid in a certain parameter region where degenerate manifolds form closed contours in momentum space. We also propose several related materials which are expected to hold this exotic state. |
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