Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session V37: Spin Ice: Kagome, Artificial, and TheoryFocus

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Sponsoring Units: GMAG DCMP DMP Chair: Franziska Weickert, Florida State University Room: BCEC 206A 
Thursday, March 7, 2019 2:30PM  3:06PM 
V37.00001: Dynamic kagome ice state 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. 
Thursday, March 7, 2019 3:06PM  3:18PM 
V37.00002: Quantum Spin Fragmentation in Kagome Ice Ho3Mg2Sb3O14 Zhiling Dun, Xiaojian Bai, Joseph Paddison, Emily Hollingworth, Franz Demmel, Nicholas Butch, Clarina Dela Cruz, Matthew Brandon Stone, Tao Hong, Martin Mourigal, Haidong Zhou A promising route to realize entangled magnetic states combines geometrical frustration with quantum tunneling effects. Spinice materials are canonical examples of frustration, and Ising spins in a transverse magnetic field are the simplest manybody model of quantum tunneling. Here, we show that the tripod kagome lattice material Ho3Mg2Sb3O14 unites an icelike magnetic degeneracy with quantumtunneling terms generated by an intrinsic splitting of the Ho3+ groundstate doublet, realizing a frustrated transverse Ising model. Using neutron scattering and thermodynamic experiments, we observe a symmetrybreaking transition at 0.32 K to a remarkable quantum spinfragmented states with strongly reduced ordered/local moment. Using exact diagonalization and meanfield calculation, we demonstrate that whereas the transverse field tends to drive the system into a spinliquid state with zero onsite moment, the hyperfine interaction helps to stabilize the fragmented local moments. Our results establish that Ho3Mg2Sb3O14 realizes a quantum spinfragmented state on the kagome lattice. 
Thursday, March 7, 2019 3:18PM  3:30PM 
V37.00003: Exact diagonalization studies of an effective model for quantum kagome ice KaiHsin Wu, YiPing Huang, YingJer Kao We study the spin1/2 kagome Heisenberg XYZh model in the socalled quantum kagome ice regime[1]. From our recent topological entanglement entropy and thermal entropy studies, we find that the system does not show a Z_{2} topological order down to β=48, while the thermal entropy down to β=200 is consistent with the residual entropy of a classical kagome ice in a magnetic field [2]. Using degenerate perturbation theory (DPT) out of the classical ice manifold, we derive an effective model which shows an intricate competition between the ringexchange and diagonal processes. Here, we perform exact diagonalization on the effective Hamiltonian. By tuning the weight of the diagonal term, we find that the competition can lead to a quasidegenerate energy spectrum, consistent with the Quantum Monte Carlo simulation results. 
Thursday, March 7, 2019 3:30PM  3:42PM 
V37.00004: MFM Study of Magnetic Charge Order in FibonacciDistorted, Honeycomb Artificial Spin Ice Justin Woods, Barry W Farmer, Yonglei Wang, WaiKwong Kwok, Jeffrey T Hastings, Lance De Long We study the ordering of magnetic charges in honeycomb artificial spin ice (ASI) having distortions created by applying an aperiodic Fibonacci sequence of binary digits mapped onto short (d_{1}) and long (d_{2}) 2D lattice parameters. Patterned Permalloy (Ni_{0.80}Fe_{0.20}) thin films were deposited on Si_{3}N_{4} using standard electron beam lithography (EBL) to create samples with a range of distortion ratios d_{2}/d_{1} = 1.00, 1.15, 1.30, 1.45 and 1.62. 

V37.00005: ABSTRACT WITHDRAWN

Thursday, March 7, 2019 3:54PM  4:06PM 
V37.00006: Magnetotransport of Vertex Frustrated Artificial Spin Ice Structures Elysia Sharma, Daan M Arroo, Nirat Ray, Lesley Cohen, Will Branford Artificial spin ice (ASI) systems are 2D analogues of pyrochlore spin ice, which exhibit frustrated magnetism and magnetic monopolelike defects [1]. ASI is commonly studied in honeycomb or square lattices, but other geometries which exhibit frustration are of growing interest. Previous magnetotransport studies have observed an asymmetric Hall signal and postulated this is related to the phase transition to the chirally ordered state in the kagome lattice [2]. Due to the possible effect of exchange bias [3] and the breakdown of the macrospin approximation for cobalt ASI systems below 50K [4] this has continued to be a topic of much interest and debate. The comparison of the response of different geometries could provide insight which could clarify the origin of this effect. Here we present an investigation of the Longitudinal and Hall resistivity, in the range 2K<T<290K, for nonconventional ASI geometries. Samples consist of permalloy nanowires in 5 geometries: kagome, square, Shakti, brickwork and tetris. 
Thursday, March 7, 2019 4:06PM  4:18PM 
V37.00007: Imaging the stray field of chiral artificial spin ice with scanning nanoSQUIDontip Marcus Wyss, Denis Vasyukov, Sebastian Gliga, Lorenzo Ceccarelli, Giulio Romagnoli, Robert L. Stamps, Martino Poggio We use a scanning nanometerscale superconducting quantum interference device (nanoSQUID) [1] to image the stray field of an artificial spin ice, which displays structural chirality. Experiments are carried out in a series of magnetic fields at 4.2 K. The "chiral ice" [2] is a 2D arrangement of lithographically patterned permalloy nanomagnets. Each stadiumshaped nanomagnet is much thinner than its inplane dimensions, producing a strong shape anisotropy that favors a singledomain magnetization configuration [3]. Neverthelesss, scanning nanoSQUID measurements, backed by micromagnetic simulations, show that the magnetization in the nanomagnets is not uniform, displaying a bending at the edges of the nanostructures. The results show that the number of degrees of freedom in artificial spin ice can be much larger than typically captured in dipolar models. These additional degrees of freedom contribute to the fieldinduced dynamics and may be used to create reprogrammable magnonic crystals [4]. 
Thursday, March 7, 2019 4:18PM  4:30PM 
V37.00008: Simulation of electron holography of pyrochlore spin ice Ankur Dhar, Ludovic Jaubert, Nicholas Shannon, Tsumoru Shintake A major topic in frustrated magnetism is emergent magnetic monopoles in spin ice [1]. These lowenergy excitations are effective classical analogues of Dirac monopoles and have only been seen indirectly thus far [2]. Electron holography could allow for direct observations due to its high sensitivity to magnetic fields. Electron holography has achieved 3D spatial imaging of spin phenomena with nanometer resolution [3]. This makes the technique uniquely suited for directly imaging spin ice monopoles. 
Thursday, March 7, 2019 4:30PM  4:42PM 
V37.00009: Zone center physics in magnetic diffuse neutron scattering Mikael Twengström, Laura Bovo, Patrik Henelius, Steven T. Bramwell Probing physical quantities using neutron scattering is crucial in many areas of physics. Magnetic neutron scattering measures a generalized magnetic susceptibility, but which susceptibility is actually measured as the scattering vector Q→0? We consider the case of spin ice, which lends itself well to such a study as it obeys both the static approximation of neutron scattering, the classical fluctuationdissipation theorem and it also has an extremely large paramagnetic susceptibility. By comparing a direct magnetic measurement using a SQUID magnetometer and a neutron scattering measurement near a Brillouin zone center, we show that the external susceptibility is recovered rather than the intrinsic one. Theoretically we investigate this result by Monte Carlo simulations of the spin ice Hamiltonian with different boundary conditions. Our conclusion is that when measuring the paramagnetic structure factor for a Brillouin zone center point in Fourier space, a demagnetizing transformation may be needed in order to recover the intrinsic susceptibility of a magnetic material. 
Thursday, March 7, 2019 4:42PM  4:54PM 
V37.00010: Spectroscopy of spinons in quantum spin ice Siddhardh Morampudi, Christopher Laumann, Frank Wilczek We calculate the effect of the emergent photon on spectroscopic crosssections of spinons in quantum spin ice which realizes an emergent phase of QED at low energies. We show that the photon drastically modifies threshold crosssections of spinons from a naive density of states analysis resulting in effects such as an analogue of the Sommerfeld enhancement. We point out signatures in neutron scattering and Raman spectroscopy and show that this explains some recent numerical and experimental results. 
Thursday, March 7, 2019 4:54PM  5:06PM 
V37.00011: Hopfions in lattice dimer model Grigory Bednik In this talk I consider hopfions in 3D lattice dimer model, i.e. topological defects, which can be characterized by 3D topological Hopf invariant. More specifically, I consider 3D bipartite lattice dimer model, define its configurations as equivalent if they can be transformed into each other by a set of local flips, and derive, that they preserve Hopf number. In this way, Hopf invariants answer the question of ergodicity in bipartite lattice dimer model. Furthermore, I consider the case of nonbipartite lattice dimer model, and by using neural networks, demonstrate that its topological phases are characterized by Z2 topological invariant. Since the lattice dimer model is known to describe classical spin ice, my work can be viewed as a proposal to search for hopfions in spin ice materials. 
Thursday, March 7, 2019 5:06PM  5:18PM 
V37.00012: Frustrated Spin Ice Nanomagnets Probed by Superconducting Vortex Dynamics. Jose Vicent, Victor Rollano, Alvaro MunozNoval, Alicia Gomez, Fernando ValdesBango, Maria Velez, Jose Martin, Manuel Osorio, Daniel Granados, Elvira Gonzalez We have studied the superconducting vortex dynamics on hybrid superconducting/magnetic samples. The samples are connected Co nanomagnets with honeycomb structure which are embedded in Nb superconducting film. The samples are patterned in a crossshaped bridge for magnetotransport measurements. Superconducting vortex lattice motion detects distinct magnetic states of the honeycomb array. These states are governed by the topology of the Co honeycomb which frustrates inplane magnetic configurations which must follow spin ice rules. Furthermore, in the honeycomb array, each vertex consists of two charged Néel walls. This hybrid system can be set easily in magnetic configuration corresponding to Ice II or Ice I states. We will show that the superconducting vortex flow is sensitive to subtle changes in the magnetic state of the spin ice and remarkably the asgrown state is not fully demagnetized; that is, this spin ice presents remanent magnetization in all the possible magnetic configurations. 
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