Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A24: 2D Frustrated Spin Systems: Shastry Sutherland and Bipartite LatticesFocus

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Sponsoring Units: GMAG DMP Chair: Vivien Zapf, Los Alamos National Laboratory Room: LACC 403A 
Monday, March 5, 2018 8:00AM  8:36AM 
A24.00001: Mapping the Phase Diagram of Frustrated Quantum Magnets Using Magnetic Field, Pressure and Chemical Doping Invited Speaker: Sara Haravifard The interactions that define how spins arrange themselves in a material play a fundamental role in a wide variety of physical phenomena. Frustrated quantum magnets are systems in which the exchange interactions governing the interacting spins cannot be simultaneously satisfied, leading to a highly degenerate ground state and exotic states of matter. In this talk I will discuss how chemical doping, hydrostatic pressure and application of magnetic field regulates the underlying electronic and magnetic interactions in twodimensional geometrically frustrated systems, ultimately driving the ground state across the phase diagram and leading to emergent quantum critical phenomena. I will present our results for magnetometry, xray and neutron scattering experiments performed under extreme sample environments on ShastrySutherland model systems and rareearth triangular antiferromagnets. 
Monday, March 5, 2018 8:36AM  8:48AM 
A24.00002: High Magnetic Field, High Pressure Studies of Emergent Bound States in Chemically Doped ShastrySutherland System Zhenzhong Shi, William Steinhardt, David Graf, Philippe Corboz, Marcelo Jaime, Yaohua Liu, Daniel Silevitch, Casey Marjerrison, Hanna Dabkowska, Thomas Rosenbaum, Frederic Mila, Sara Haravifard The orthogonaldimer antiferromagnet SrCu_{2}(BO3)_{2} (SCBO) is an experimental realization of the twodimensional ShastrySutherland (SSL) model, which represents one of the very few topologies where the ground state of a frustrated lattice could be exactly solved. The delicate competition between the intradimer and interdimer exchange interactions in SCBO enables tuning of its ground state by a variety of experimental parameters, such as magnetic field, pressure, and chemical doping. While pure SCBO has been well studied, less is known about the doping effect on the SSL system. In this talk we present our recent highpressure, highmagnetic field neutron scattering and megnetometry results performed on chemically doped SCBO single crystals, where magnetic Cu^{2+} has been substituted with nonmagnetic isoelectronic Mg^{2+}. We discuss the interplay between the induced disorder and external tuning parameters, and explain how such correspondence results in stabilizing the emergent new bound states in SSL systems. 
Monday, March 5, 2018 8:48AM  9:00AM 
A24.00003: Raman Scattering and Dilatometry of Frustrated Spin Dimer Compound SrCu_{2}(BO_{3})_{2} in High Magnetic Fields to 45T Marcelo Jaime, Komalavalli Thirunavukkuarasu, Dmitry Smirnov, Zhengguang Lu, Peter Christianen, Mariana Ballotin, Hanna Dabkowska, Guillaume Radtke, Andres Saul SrCu_{2}(BO_{3})_{2} is a quasitwo dimensional orthogonal spin dimer system with a spin singlet ground state. It is a realization of the Shastry Sutherland model, and exhibits a sequence of magnetization plateaux at magnetic fields in excess of 20 T, i.e. high enough to close the low temperature spin gap [1]. The unique behavior of this quantum spin liquid results from the interplay between geometrical frustration and strong quantum fluctuations. We will discuss the origin and characteristics of the strong spinlattice coupling in SrCu_{2}(BO_{3})_{2}, as learned from experimetal studies of Raman scattering, thermal expansion, and magnetostriction performed in high DC magnetic fields to 45T. The interpretation of experimental results is supported by a theoretical analysis and prediction of active Raman modes that include the socalled panthograph mode [2]. 
Monday, March 5, 2018 9:00AM  9:12AM 
A24.00004: Thermal Transport Measurements of a ShastrySutherland Magnet with Capacitive Thermometry Colin Tinsman, Ziji Xiang, Dmitri Mihaliov, Tomoya Asaba, Lu Chen, Sara Haravifard, Lu Li The ShastrySutherland magnet SrCu_{2}(BO_{3})_{2} has been predicted to host excitations known as triplons which give rise to a thermal Hall conductivity. Measurements of the thermal Hall effect are challenging to make since they require sensitive thermometry at low temperatures and in intense magnetic fields. Most standard methods of measuring temperature are either not sensitive enough or have significant magnetic field induced deviations. We have developed a technique using Oxygen18 doped Strontium Titanate to make capacitive thermometers which are sensitive at the required temperature range while also not being affected by magnetic fields. By combining this method with more traditional methods of thermometry, we have been able to observe fielddependent thermal properties in SrCu_{2}(BO_{3})_{2}, including a strongly varying thermal conductivity as well as our exploration of the predicted thermal Hall conductivity. 
Monday, March 5, 2018 9:12AM  9:24AM 
A24.00005: Visualizing Magnetic Domains at Fractional Magnetization Plateaus in a Metallic ShastrySutherland Isingtype Rare Earth Tetraboride Paul Sass, Weida Wu, Linda Ye, Joseph Checkelsky Recently, unconventional magnetic and electronic effects born out of lattice geometric frustration have been observed in metallic frustrated systems. The rare earth tertraboride TmB_{4} is one such system with both lattice frustration and itinerant electronic behavior exhibiting complex magnetic phenomena. The Tm ions form a sublattice topologically equivalent to the ShastrySutherland lattice in the abplane which enters an antiferromagnetic ground state below 11.8 K. Interestingly, the system enters a fieldinduced ferrimagnetic state at high field, with strong caxis anisotropy, eventually reaching a fieldinduced paramagnetic state, exhibiting hysteretic fractional magnetization plateaus along the way, induced by frustration and complex spin flip processes.^{2} Magnetic hysteresis indicates domain formation and the coexistence of fractional magnetization phases. To this end, we report our variable temperature magnetic force microscopy studies on refined floating zone grown TmB_{4} single crystals. Various multidomain patterns were observed at different saturation magnetization plateaus below T_{N}. The evolution of these magnetic domains at various temperatures and magnetic fields will be presented. 
Monday, March 5, 2018 9:24AM  9:36AM 
A24.00006: Frustrated Ground State in the metallic Ising 2D Antiferromagnet Nd_{2}Ni_{2}In Gabriele Sala, Silvie Maskova, Matthew Stone

Monday, March 5, 2018 9:36AM  9:48AM 
A24.00007: Dynamical responses and instabilities of the ShastrySutherland model Zhentao Wang, Cristian Batista The ShastrySutherland model (SSM) is famous for holding an exact ground state solution for interdimer exchange J'/J≤0.7. In this exact ground state, spins form singlets on the dimers. Single triplon excitations have a rather flat dispersion that renders perturbative treatments of the triplontriplon interactions simply inadequate. In addition, the multiple competing length scales of this model also limits the applicability of finite size numerical simulations. 
Monday, March 5, 2018 9:48AM  10:00AM 
A24.00008: Entanglement Studies of Resonating Valence Bonds on the Frustrated Square Lattice Alexander Seidel, Julia Wildeboer We study a shortrange resonating valence bond (RVB) wave function with diagonal links on the square lattice that permits signproblem free wave function MonteCarlo studies. Special attention is given to entanglement properties, in particular, the study of minimum entropy states (MES) according to the method of Zhang et. al. We provide evidence that the MES associated with the RVB wave functions can be lifted from an associated quantum dimer picture of these wave functions, where MES states are certain linear combinations of eigenstates of a 't Hooft ``magnetic loop''type operator. From this identification, we calculate a value consistent with $\ln(2)$ for the topological entanglement entropy directly for the RVB states via wave function MonteCarlo. This corroborates the $\mathbb{Z}_{2}$ nature of the RVB states. We furthermore extract information about the modular $\mathcal{S}$ and $\mathcal{U}$matrices. Aspects of a related classical dimer partition function will also be discussed. 
Monday, March 5, 2018 10:00AM  10:12AM 
A24.00009: The phase diagram of spin1/2 square J_{1}J_{2} Heisenberg model: U(1)symmetric infinite PEPS study Reza Haghshenas The tensornetwork states have shown to be quite promising in studying the frustrated systems. Specifically, infinite projected entangledpair state (iPEPS) anstaz, a generalization of powerful DMRG, could efficiently address the lowlying excited states of such systems and provide quite competitive variational energy. In this talk, we aim to introduce an "improved" U(1)symmetric iPEPS anstaz and study phase diagram of the canonical spin1/2 square J_{1}J_{2} Heisenberg model. We discuss a simple strategy to selects automatically relevant symmetric sectors without losing accuracy and also discuss an improved optimization method. We show variational groundstate energies of the model set better upper bound on the true energy, improving previous tensornetworkbased results. By studying magnetization parameter and correlation functions, we find a direct NéeltoVBS transition at J_{2}=0.53. The estimated critical anomalous exponents η_{s}=0.65(5) and η_{d}=1.9(1) show strong deviation from those of the wellknown JQ models, suggesting possibly different universality class. We compare our results with earlier DMRG and PEPS studies and suggest future directions for resolving remaining issues. 
Monday, March 5, 2018 10:12AM  10:24AM 
A24.00010: Fermionic spinon theory of square lattice spin liquids near the Néel state Alex Thomson, Subir Sachdev Quantum fluctuations of the Néel state of the square lattice antiferromagnet are usually described by a CP^{1} theory of bosonic spinons coupled to a U(1) gauge field with a global SU(2) spin rotation symmetry. Such a theory also has a confining phase with valence bond solid (VBS) order and deconfined phases with Z_{2} topological order. We present dual theories of these phases starting from a meanfield theory of fermionic spinons with a πflux in each plaquette. Fluctuations about this πflux state are described by 2+1 dimensional quantum chromodynamics (QCD_{3}) with a SU(2) gauge group and N_{f}= 2 flavors of massless Dirac fermions. It has recently been argued by Wang et al. (arXiv:1703.02426) that this theory describes the NéelVBS phase transition. We introduce adjoint Higgs fields in QCD_{3}, and obtain fermionic descriptions of both the VBS phase and the topologically ordered phases obtained earlier using the bosonic theory. The global phase diagram of these phases contains multicritical points, and our results imply new bosonfermion dualities. 
Monday, March 5, 2018 10:24AM  10:36AM 
A24.00011: Longitudinal (Higgs) Modes of Spin Fluctuations in Ironbased Superconductors Qiang Zhang, Jiangping Hu Ironbased superconductors can exhibit different magnetic orders and are in a critical magnetic region where frustrated magnetic interactions strongly compete with each other. The order parameters can fluctuate transversely as gapless Goldstone modes and longitudinally as gapped Higgs modes. Here we investigate the Higgs modes of spin fluctuations in the unified effective J_{1}J_{2}J_{3}K model for ironbased superconductors. In this work, the Higgs modes are viewed as twomagnon resonance from the linear spin wave approach for the local spin exchange model. We focus on the collinear antiferromagnetic order and calculate the longitudinal modes when different phase boundaries are approached. We compare it with the observed longitudinal gap in CaFe_{2}As_{2}, which was looked as the support for the itinerant picture. This is a systematic method to explore the longitudinal excitations in the local spin exchange model and we can easily extend it in three dimensions. 
Monday, March 5, 2018 10:36AM  10:48AM 
A24.00012: Entanglement Negativity and Sudden Death in the Toric Code at Finite Temperature Oliver Hart, Claudio Castelnovo We study the fate of quantum correlations at finite temperature in the two dimensional toric code using the logarithmic entanglement negativity. We are able to obtain exact results that give us insight into how thermal excitations affect quantum entanglement. We show that an $O(1)$ density of the lower energy defect is required to degrade the zerotemperature entanglement between two subsystems in contact with one another. However, one type of excitation alone is not sufficient to kill all quantum correlations, and an $O(1)$ density of the higher energy defect is required to cause the socalled sudden death of the negativity. Interestingly, if the energy cost of one of the excitations is taken to infinity, quantum correlations survive up to arbitrarily high temperatures, a feature that is likely shared with other quantum spin liquids and frustrated systems in general, when projected down to their low energy states. We further observe that the negativity per boundary degree of freedom at a given temperature increases (parametrically) with the size of the boundary, and that quantum correlations between subsystems with extended boundaries are more robust to thermal fluctuations. 
Monday, March 5, 2018 10:48AM  11:00AM 
A24.00013: Z_{2} Topological Quantum Paramagnet on a honeycomb bilayer Darshan Joshi, Andreas Schnyder Topological quantum paramagnets are exotic states of matter with trivial paramagnetic ground states hosting topological excitations. Here we show that a simple model of quantum spins on a honeycomb bilayer hosts a Z_{2} topological quantum paramagnet in the presence of spinorbit coupling. The Z_{2} invariant is the same as that in the case of the fermionic quantum spin Hall state. We further show that upon making one of the Heisenberg couplings stronger the system undergoes a topological quantum phase transition, where the Z_{2} invariant vanishes, to a different topological quantum paramagnet. In this case the edge states are disconnected from the bulk excitations. This phase is characterized by a different topological invariant. This physics is amenable to experiments, where an anisotropic coupling can be induced under pressure. 
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