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 Low-Dimensional StructuresFocus
|
Hide Abstracts |
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 Spin-1 Model Wenjun Hu, Shoushu Gong, Hsin-Hua 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 spin-1 model on a square lattice, where despite the absence of geometric frustration, competition between the nearest neighbor Heisenberg (J1) and biquadratic (K1) interactions results in a QSL around the J1=K1 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 flavor-wave theory indicate the presence of large quantum fluctuations that destabilize the nearby antiferromagnetic and quadrupolar orders. What emerges is a QSL with no long-range order in spin or quadrupolar channels, which nevertheless has fluctuations peaked at the wavevector (π, 2π/3) and spontaneously breaks the C4 rotational symmetry of the square lattice [1]. We demonstrate, by studying an anisotropic model, that this lattice-nematic 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 self-consistent Fock-like mean-field scheme to calculate low-energy effective theories for emergent spinon excitations in spin-1/2 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 spin-liquid candidate systems beyond mean-field theory. We apply this approach to the antiferromagnetic J1-J2 Heisenberg model on the square lattice and to the antiferromagnetic nearest-neighbor Heisenberg model on the kagome lattice. For the J1-J2 model, we find that in the regime of maximal frustration a SU(2) pi-flux state with Dirac spinons yields the largest mean-field amplitudes. For the kagome model, we identify a gapless Z2 spin liquid with a small circular spinon Fermi surface and approximate Dirac-cones at low but finite energies. |
Thursday, March 7, 2019 11:39AM - 11:51AM |
S38.00003: Critical Level Crossings and Gapless Spin Liquid in the Square-Lattice Spin-1/2 J1-J2 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 square-lattice J1-J2 Heisenberg model on 2L × L cylinders with L ≤ 10. We identify excited-level crossings versus the coupling ratio g=J2/J1 and study their drifts with the system size L. The lowest singlet-triplet and singlet-quintuplet crossings converge rapidly (with corrections ∝ L^−2) to different g values, and we argue that these correspond to ground-state transitions between the Neel antiferromagnet and a gapless spin liquid, at gc1≈0.46, and between the spin liquid and a valence-bond 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 level-crossing 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 low-dimensional 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 freedom---typified by the classical XY model---due 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 Kosterlitz-Thouless (KT) phase transition has been predicted in the quantum system by theory and simulation [2]. Here we demonstrate a large-scale quantum simulation of this phenomenon in a network of 1,800 in situ programmable superconducting flux qubits arranged in a fully-frustrated square-octagonal lattice [3]. Essential to the critical behavior, we observe the emergence of a complex order parameter with continuous rotational symmetry, and the onset of quasi-long-range order as the system approaches a critical temperature. We use a simple but previously undemonstrated approach to statistical estimation with an annealing-based 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 J1 - J2 quantum antiferromagnet BaCdVO(PO4)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 1-st and 2-nd 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 J1-J2 spin-1/2 model on a square lattice Juraj Hasik, Federico Becca The optimization of infinite projected entangled-pair 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 multi-variable functions in a plethora of different contexts. In the quest for an efficient representation of the ground state wavefunction of many-body 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 many-body Jastrow factors for the study of spin systems. In particular, we apply the RBM factor on top of a Gutzwiller-projected fermionic wavefunction [2], in order to construct a variational ansatz for the ground state of the spin-1/2 J1-J2 Heisenberg model in two dimensions. In the magnetically ordered phase (J2=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 Ba2CoTeO6 revealed by magnetostriction and magnetocaloric effect measurements Dagmar Weickert, Carolina Corvalan-Moya, Myron B Salamon, Chao Dong, Koichi Kindo, Yoshimitsu Kohama, Nobuyuki Kurita, Hidekazu Tanaka Ba2CoTeO6 is composed of two nearly decoupled magnetic spin-systems stacked as alternating 2D layers along the crystallographic c-direction in the trigonal crystal structure. Subsystem A is a S = ½ triangular Heisenberg antiferromagnet that orders at TA = 3K. System B is a J1-J2 honeycomb Ising antiferromagnet with ordering temperature TB = 12K. Both underlying spin-models 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 Pb2VO(PO4)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 Pb2VO(PO4)2 is one of few quantum magnets proposed to realize the frustrated spin-1/2 square lattice with ferromagnetic nearest-neighbor exchange. We have determined the magnetic structure and excitation spectrum of Pb2VO(PO4)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 next-nearest 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, NdB4 and HoB4 Oleg Petrenko, Daniel Brunt, Navid Qureshi, Geetha Balakrishnan, Daniel Mayoh, Martin Lees We report a study into the field-induced magnetisation of the two frustrated rare earth tetraborides, HoB4 and NdB4. NdB4 shows a fractional magnetisation plateau occurring at M/Msat ≈ 1/5 before saturating in a field of 33 kOe. On cooling down to 0.5 K the temperature dependent susceptibility of NdB4 shows an unconventional transition where the system returns to the zero field antiferromagnetic state from a higher-temperature ferrimagnetic state. We are able to reconstruct the magnetic phase diagram of NdB4 from the magnetisation, susceptibility and resistivity measurements for both H//c and H⊥c. For HoB4, the most interesting behaviour is found at the lowest temperature of 0.5 K, where the field dependent magnetisation demonstrates a new fractional 1/2-magnetisation plateau. Further insight into the relations between the exchange interactions and single ion effects is gained through high-field magnetisation measurements in both HoB4 and NdB4. |
Thursday, March 7, 2019 1:39PM - 1:51PM |
S38.00011: Evolution of the spin-flop antiferromagnetic transition and the emergence of frustration in
saw-tooth lattice Mn2SiS4-xSex (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 A2BX4 structure takes Pnma symmetry where the A site forms the triangle-based one dimensional saw-tooth chain structure and are known for optoelectronics, thermoelectric and magentic applications. We studied the olivines, Mn2SiS4-xSex (0 < x < 4), using magnetization, specific heat, thermal conductivity and ab initio density functional theory (DFT) calculations and tracked the evolution of spin-flop transition and frustration. It is seen that the antiferromagnetic transition temperature TN shows a linear decrease from 86 K for Mn2SiS4 towards 66 K for Mn2SiSe4 as x varied; whereas the critical field for spin-flop varied non-linearly. The magnetic phase transitions characterized using specific heat reveal very low magnetic entropy evolved at the TN, suggesting the underlying frustrated nature of the Mn2+ ions in the saw-tooth like triangular arrangement. A semiconducting-like 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 saw-tooth Mn lattice. |
Thursday, March 7, 2019 1:51PM - 2:03PM |
S38.00012: Quantum disordered state in the Jeff =1/2 triangular lattice antiferromagnet NaYbO2 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 symmetry-breaking structural or magnetic processes or innate chemical disorder that lifts ground state degeneracies. NaYbO2 (R-3m) contains geometrically-frustrated triangular layers of Jeff =1/2 Yb3+ ions with ideal chemical order, a robust lattice geometry to 330 mK and lacks conventional signs of magnetic ordering under zero-field conditions to 50 mK. Under a moderate applied field, NaYbO2 enters an antiferromagnetic phase below 1 K. This material stands as an appealing candidate to realize a dynamically-disordered quantum ground state driven by highly-anisotropic, spin-orbit entangled Yb ions whose transition into a nearby field-driven magnetically-ordered 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 SrCo2As2 Bing Li, Benjamin Ueland, Wageesha T Jayasekara, Douglas L Abernathy, Sangeetha N. S., David C Johnston, Qing-Ping 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 SrCo2As2 stripe-type antiferromagnetic (AF) spin fluctuations, though no long-range 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 local-moment J1-J2 model suggests that itinerancy enhances frustration. We conclude that SrCo2As2 is a candidate for frustrated itinerant magnetism and harbors unique frustrated magnetism on the boundary between itinerant ferromagnetic and stripe AF instabilities. |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700