Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session S38: Frustrated Square Lattice and Other LowDimensional StructuresFocus

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Sponsoring Units: GMAG DCMP DMP Chair: Oleg Tchernyshyov, Johns Hopkins University Room: BCEC 206B 
Thursday, March 7, 2019 11:15AM  11:27AM 
S38.00001: Quantum Spin Liquid with Broken Unidirectional Symmetry in a Square Lattice Spin1 Model Wenjun Hu, Shoushu Gong, HsinHua Lai, Haoyu Hu, Qimiao Si, Andriy Nevidomskyy Quantum spin liquids (QSL) are known to arise in strongly frustrated spin systems as a result of competing interactions. Here we consider a spin1 model on a square lattice, where despite the absence of geometric frustration, competition between the nearest neighbor Heisenberg (J_{1}) and biquadratic (K_{1}) interactions results in a QSL around the J_{1}=K_{1 }point, as evidenced by our density matrix renormalization group (DMRG) studies. At that point, the model has an emergent SU(3) symmetry and calculations based on N=3 flavorwave theory indicate the presence of large quantum fluctuations that destabilize the nearby antiferromagnetic and quadrupolar orders. What emerges is a QSL with no longrange order in spin or quadrupolar channels, which nevertheless has fluctuations peaked at the wavevector (π, 2π/3) and spontaneously breaks the C_{4 }rotational symmetry of the square lattice [1]. We demonstrate, by studying an anisotropic model, that this latticenematic spin liquid is distinct from the limit of weakly coupled Haldane chains. Instead, analysis of the spectral gaps and entanglement entropy is consistent with the QSL being either gapless or having a very small gap. 
Thursday, March 7, 2019 11:27AM  11:39AM 
S38.00002: Characterization of quantum spin liquids and their spinon band structures via functional renormalization Johannes Reuther, Max Hering, Jonas Sonnenschein, Yasir Iqbal We combine the pseudofermion functional renormalization group (PFFRG) method with a selfconsistent Focklike meanfield scheme to calculate lowenergy effective theories for emergent spinon excitations in spin1/2 quantum spin liquids. Using effective spin interactions from PFFRG as an input for the Fock equation and allowing for the most general types of free spinon ansatze as classified by the projective symmetry group (PSG) method, we are able to systematically determine spinon band structures for spinliquid candidate systems beyond meanfield theory. We apply this approach to the antiferromagnetic J1J2 Heisenberg model on the square lattice and to the antiferromagnetic nearestneighbor Heisenberg model on the kagome lattice. For the J1J2 model, we find that in the regime of maximal frustration a SU(2) piflux state with Dirac spinons yields the largest meanfield amplitudes. For the kagome model, we identify a gapless Z2 spin liquid with a small circular spinon Fermi surface and approximate Diraccones at low but finite energies. 
Thursday, March 7, 2019 11:39AM  11:51AM 
S38.00003: Critical Level Crossings and Gapless Spin Liquid in the SquareLattice Spin1/2 J1J2 Heisenberg Antiferromagnet Ling Wang, Anders W Sandvik We use the density matrix renormalization group method to calculate several energy eigenvalues of the frustrated S=1/2 squarelattice J1J2 Heisenberg model on 2L × L cylinders with L ≤ 10. We identify excitedlevel crossings versus the coupling ratio g=J2/J1 and study their drifts with the system size L. The lowest singlettriplet and singletquintuplet crossings converge rapidly (with corrections ∝ L^−2) to different g values, and we argue that these correspond to groundstate transitions between the Neel antiferromagnet and a gapless spin liquid, at gc1≈0.46, and between the spin liquid and a valencebond solid at gc2≈0.52. Previous studies of order parameters were not able to positively discriminate between an extended spin liquid phase and a critical point. We expect levelcrossing analysis to be a generically powerful tool in density matrix renormalization group studies of quantum phase transitions. 
Thursday, March 7, 2019 11:51AM  12:27PM 
S38.00004: Observation of topological phenomena induced by frustration and quantum fluctuations in a programmable lattice of 1,800 qubits Invited Speaker: Isil Ozfidan The celebrated work of Berezinskii, Kosterlitz and Thouless in the 1970s [1] revealed exotic phases of matter governed by topological properties of lowdimensional materials such as thin films of superfluids and superconductors. Key to this phenomenon is the appearance and interaction of vortices and antivortices in an angular degree of freedomtypified by the classical XY modeldue to thermal fluctuations. In the 2D Ising model this angular degree of freedom is absent in the classical case, but with the addition of a transverse field it can emerge from the interplay between frustration and quantum fluctuations. Consequently a KosterlitzThouless (KT) phase transition has been predicted in the quantum system by theory and simulation [2]. Here we demonstrate a largescale quantum simulation of this phenomenon in a network of 1,800 in situ programmable superconducting flux qubits arranged in a fullyfrustrated squareoctagonal lattice [3]. Essential to the critical behavior, we observe the emergence of a complex order parameter with continuous rotational symmetry, and the onset of quasilongrange order as the system approaches a critical temperature. We use a simple but previously undemonstrated approach to statistical estimation with an annealingbased quantum processor, performing Monte Carlo sampling in a chain of reverse quantum annealing protocols. Observations are consistent with classical simulations across a range of Hamiltonian parameters. We anticipate that our approach of using a quantum processor as a programmable magnetic lattice will find widespread use in the simulation and development of exotic materials. 
Thursday, March 7, 2019 12:27PM  12:39PM 
S38.00005: Thermodynamics of a frustrated quantum magnet on a square lattice Vivek Bhartiya, Kirill Povarov, Zewu Yan, Andrey Zheludev We report the magnetic and calorimetric measurements in single crystal samples of the square lattice J_{1 } J_{2} quantum antiferromagnet BaCdVO(PO_{4})_{2} [1]. An investigation of the scaling of magnetization reveals a “dimensionality reduction” indicative of a strong degree of geometric frustration. Below a characteristic temperature less than 150 mK we observe the emergence of a new strongly fluctuating quantum phase close to full magnetic saturation. It is separated from the magnetically ordered state by 1st and 2nd order phase transitions, depending on the orientation of the applied magnetic field. We suggest that the new phase may indeed be the theoretically predicted spin nematic state. 
Thursday, March 7, 2019 12:39PM  12:51PM 
S38.00006: Improving the full update and probing frustrated J_{1}J_{2} spin1/2 model on a square lattice Juraj Hasik, Federico Becca The optimization of infinite projected entangledpair state (iPEPS) ansatz 
Thursday, March 7, 2019 12:51PM  1:03PM 
S38.00007: Accuracy of Restricted Boltzmann Machine wavefunctions for frustrated spin systems Francesco Ferrari, Juan Carrasquilla, Federico Becca Artificial neural networks (ANNs) constitute a powerful tool to approximate multivariable functions in a plethora of different contexts. In the quest for an efficient representation of the ground state wavefunction of manybody quantum systems, ANNs have recently emerged as a flexible framework to construct variational ansatze [1]. We investigate the accuracy of Restricted Boltzmann Machine (RBM) networks as manybody Jastrow factors for the study of spin systems. In particular, we apply the RBM factor on top of a Gutzwillerprojected fermionic wavefunction [2], in order to construct a variational ansatz for the ground state of the spin1/2 J_{1}J_{2} Heisenberg model in two dimensions. In the magnetically ordered phase (J_{2}=0), Monte Carlo results show that the RBM correlator yields a significant improvement of the variational energy [2]. On the other hand, when competing interactions are turned on and the system becomes highly frustrated, the energy gain due to the RBM becomes remarkably less pronounced. 
Thursday, March 7, 2019 1:03PM  1:15PM 
S38.00008: Magnetic superstructures inside the ⅓ and ½ magnetization plateaux in Ba_{2}CoTeO_{6} revealed by magnetostriction and magnetocaloric effect measurements Dagmar Weickert, Carolina CorvalanMoya, Myron B Salamon, Chao Dong, Koichi Kindo, Yoshimitsu Kohama, Nobuyuki Kurita, Hidekazu Tanaka Ba_{2}CoTeO_{6} is composed of two nearly decoupled magnetic spinsystems stacked as alternating 2D layers along the crystallographic cdirection in the trigonal crystal structure. Subsystem A is a S = ½ triangular Heisenberg antiferromagnet that orders at T_{A} = 3K. System B is a J_{1}J_{2} honeycomb Ising antiferromagnet with ordering temperature T_{B} = 12K. Both underlying spinmodels are interesting due to geometrical frustration (A) and bond frustration (B) causing magnetic plateaux upon the application of magnetic fields H. We present a combination of magnetostriction and magnetocaloric effect (MCE) measurements in high pulsed and static magnetic fields to map out the phase diagram for H ll and perp. to the A and B layers. We observe distinct anomalies in the sample length ΔL/L as a function of field and temperature indicating phase transitions as well as signatures in the MCE measurement indicating a change of entropy. These anomalies are accompanied by significant hysteresis in decreasing compared to increasing magnetic fields. 
Thursday, March 7, 2019 1:15PM  1:27PM 
S38.00009: Spin structure and dynamics of Pb_{2}VO(PO_{4})_{2} Simon Bettler, Florian Landolt, Ömer M. Aksoy, Zewu Yan, Eric Ressouche, Sergei Zvyagin, Alexey Ponomaryov, Ketty Beauvois, Yiming Qiu, Stephane Raymond, Severian Gvasaliya, Andrey Zheludev Pb_{2}VO(PO_{4})_{2} is one of few quantum magnets proposed to realize the frustrated spin1/2 square lattice with ferromagnetic nearestneighbor exchange. We have determined the magnetic structure and excitation spectrum of Pb_{2}VO(PO_{4})_{2} by means of single crystal neutron diffraction and neutron spectroscopy going beyond previous limited powder sample studies. The spin dynamics show qualitative differences – which were independently confirmed by ESR – from what is expected for an ideal square lattice. These are attributed to having as many as three nearest neighbor and two nextnearest neighbor. We discuss the implications for closely related materials proposed to realize a frustrated square lattice. 
Thursday, March 7, 2019 1:27PM  1:39PM 
S38.00010: Magnetisation process in the rare earth tetraborides, NdB_{4} and HoB_{4} Oleg Petrenko, Daniel Brunt, Navid Qureshi, Geetha Balakrishnan, Daniel Mayoh, Martin Lees We report a study into the fieldinduced magnetisation of the two frustrated rare earth tetraborides, HoB_{4 }and NdB_{4}. NdB_{4} shows a fractional magnetisation plateau occurring at M/M_{sat} ≈ 1/5 before saturating in a field of 33 kOe. On cooling down to 0.5 K the temperature dependent susceptibility of NdB_{4} shows an unconventional transition where the system returns to the zero field antiferromagnetic state from a highertemperature ferrimagnetic state. We are able to reconstruct the magnetic phase diagram of NdB_{4} from the magnetisation, susceptibility and resistivity measurements for both H//c and H⊥c. For HoB_{4}, the most interesting behaviour is found at the lowest temperature of 0.5 K, where the field dependent magnetisation demonstrates a new fractional 1/2magnetisation plateau. Further insight into the relations between the exchange interactions and single ion effects is gained through highfield magnetisation measurements in both HoB_{4} and NdB_{4}. 
Thursday, March 7, 2019 1:39PM  1:51PM 
S38.00011: Evolution of the spinflop antiferromagnetic transition and the emergence of frustration in
sawtooth lattice Mn_{2}SiS_{4x}Se_{x} (x = 0  4) chalcogenides Hariharan Nhalil, Raju Baral, Adrian Cosio, Bethuel O Khamala, Srinivasa Rao Singamaneni, Magdalena Fitta, Rajendra Zope, Tunna Baruah, Daniel Antonio, Krzysztof Gofryk, Bayrammurad Saparov, Harikrishnan Nair The olivine A_{2}BX_{4} structure takes Pnma symmetry where the A site forms the trianglebased one dimensional sawtooth chain structure and are known for optoelectronics, thermoelectric and magentic applications. We studied the olivines, Mn_{2}SiS_{4x}Se_{x} (0 < x < 4), using magnetization, specific heat, thermal conductivity and ab initio density functional theory (DFT) calculations and tracked the evolution of spinflop transition and frustration. It is seen that the antiferromagnetic transition temperature T_{N} shows a linear decrease from 86 K for Mn_{2}SiS_{4} towards 66 K for Mn_{2}SiSe_{4} as x varied; whereas the critical field for spinflop varied nonlinearly. The magnetic phase transitions characterized using specific heat reveal very low magnetic entropy evolved at the T_{N}, suggesting the underlying frustrated nature of the Mn^{2+} ions in the sawtooth like triangular arrangement. A semiconductinglike thermal conductivity is revealed by the olivines where the DFT calculations show a small energy gap (~ 0.5 eV) between the valence and conduction bands. Our results are explained based on the emergence of frustration in the triangular units of the sawtooth Mn lattice. 
Thursday, March 7, 2019 1:51PM  2:03PM 
S38.00012: Quantum disordered state in the J_{eff }=1/2 triangular lattice antiferromagnet NaYbO_{2} Mitchell Bordelon, Eric Kenney, Lorenzo Posthuma, Marzieh Kavand, Yuanqi Lyu, Mark Stephen Sherwin, Craig Brown, Michael John Graf, Leon Balents, Stephen Wilson There has been a great deal of interest in realizing unconventional quantum disordered magnetic ground states in frustrated antiferromagnets driven by strong fluctuations in the small spin limit. Experimentally, real materials often preclude this limit due to symmetrybreaking structural or magnetic processes or innate chemical disorder that lifts ground state degeneracies. NaYbO_{2} (R3m) contains geometricallyfrustrated triangular layers of J_{eff }=1/2 Yb^{3+} ions with ideal chemical order, a robust lattice geometry to 330 mK and lacks conventional signs of magnetic ordering under zerofield conditions to 50 mK. Under a moderate applied field, NaYbO_{2} enters an antiferromagnetic phase below 1 K. This material stands as an appealing candidate to realize a dynamicallydisordered quantum ground state driven by highlyanisotropic, spinorbit entangled Yb ions whose transition into a nearby fielddriven magneticallyordered phase can be probed experimentally and theoretically. 
Thursday, March 7, 2019 2:03PM  2:15PM 
S38.00013: Competing paramagnetic phases and itinerant magnetic frustration in SrCo_{2}As_{2} Bing Li, Benjamin Ueland, Wageesha T Jayasekara, Douglas L Abernathy, Sangeetha N. S., David C Johnston, QingPing Ding, Yuji Furukawa, Peter P. Orth, Andreas Kreyssig, Alan Ira Goldman, Robert McQueeney Cobalt arsenides with a layered square lattice tend to be itinerant ferromagnets. However, using inelastic neutron scattering we find in SrCo_{2}As_{2} stripetype antiferromagnetic (AF) spin fluctuations, though no longrange AF order is observed down to 50 mK. The stripe phase competes with the itinerant ferromagnetism and wins below 115 K, resulting in the development of AF spin fluctuations concomitant with a suppression of the uniform magnetic susceptibility. Comparison of our experimental data to exact diagonalization and classical Monte Carlo simulations of a frustrated localmoment J_{1}J_{2} model suggests that itinerancy enhances frustration. We conclude that SrCo_{2}As_{2} is a candidate for frustrated itinerant magnetism and harbors unique frustrated magnetism on the boundary between itinerant ferromagnetic and stripe AF instabilities. 
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