Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session B56: Kagome and Triangular Lattice Magnetism |
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Sponsoring Units: DCMP GMAG Chair: Gyanendra Dhakal, Univ of Central Florida Room: Mile High Ballroom 2C |
Monday, March 2, 2020 11:15AM - 11:27AM |
B56.00001: Valence bond phases of herbertsmithite and related copper kagome materials Michael Norman, Nicholas Laurita, David Hsieh Recent evidence from magnetic torque, electron spin resonance, and second harmonic generation indicate that the prototypical quantum spin liquid candidate, herbertsmithite, has a symmetry lower than its x-ray refined trigonal space group. Here, we consider known and possible distortions of this mineral class, along with related copper kagome oxides and fluorides, relate these to possible valence bond patterns, and comment on their relevance to the physics of these interesting materials. |
Monday, March 2, 2020 11:27AM - 11:39AM |
B56.00002: Evidence for a parity broken monoclinic ground state in the S = 1/2 Kagome antiferromagnet Herbertsmithite Nicholas Laurita, Alon Ron, Jeong Woo Han, Jongseok Lee, Allen Scheie, John P Sheckelton, Rebecca Smaha, Wei He, Jiajia Wen, Young Sang Lee, Michael Norman, David Hsieh Nearest neighbor interacting S = ½ spins on the ideal Kagomé lattice have been predicted to form a variety of novel quantum entangled states, including quantum spin-liquid (SL) and valence bond solid phases. However, in real materials, the presence of additional perturbative spin interactions may greatly expand the variety of entangled states, which recent theoretical analyses have shown are identifiable through the spontaneous loss of particular discrete point group symmetries. Here we comprehensively resolve the ground state point group symmetries of the prototypical Kagomé SL candidate ZnCu3(OH)6Cl2 (Herbertsmithite) using a combination of optical ellipsometry and wavelength-dependent multi-harmonic optical polarimetry. We uncover a subtle parity breaking monoclinic structural distortion well above the nearest neighbor exchange energy scale. Surprisingly, the parity-breaking order parameter is dramatically enhanced upon cooling and closely tracks the build-up of nearest neighbor spin correlations, suggesting that it is energetically favored by the SL state. The refined low temperature symmetry group greatly restricts the number of viable ground states, and, in the perturbative limit, points toward the formation of a nematic SL ground state, a SL analogue of a liquid crystal. |
Monday, March 2, 2020 11:39AM - 11:51AM |
B56.00003: Magnetism on ideal triangular lattices in NaBaYb(BO3)2 Alireza Ghasemi, Shu Guo, Robert J. Cava, Collin Leslie Broholm In the search for compounds that have a stable spin liquid phase, the triangular lattice of Yb is of interest. Frustration combined with the quantum spin-orbital degree of freedom of Yb provides potentially favorable conditions for a quantum spin liquid. Here we study NaBaYb(BO3)2, which has two different triangular layers of Yb; one sandwiched between two layers of Na, the other between two layers of Ba. Here we discuss, the low-temperature heat capacity and susceptibility of this compound. Our high-field heat capacity data and susceptibility data indicate Yb3+ has a Kramers doublet ground state. From lower field data where the Schottky anomaly moves to lower T, we infer the energy scale of inter-site interactions is similar to that of magnetic dipole interactions. While we find a small anomaly in the zero-field heat capacity at 0.41(2)K, most of the Rln2 magnetic entropy is retained to the lowest T=0.1 K accessed here [1]. |
Monday, March 2, 2020 11:51AM - 12:03PM |
B56.00004: Magnetic and charge susceptibilities in the half-filled triangular lattice Hubbard model Shaozhi Li, Emanuel Gull The spin spectra of a triangular system have been studied extensively using the Heisenberg model. But for a correlated metallic triangular system, where the Heisenberg model is not valid, the spin properties are unknown. In this talk, I present momentum-dependent magnetic and charge susceptibilities of the half-filled two-dimensional triangular Hubbard model in the metallic, Mott insulating, and crossover regions of parameter space, using the dual fermion approach. In the insulating state, I find strong spin fluctuations at the K point at low energy corresponding to the 120o antiferromagnetic order. The spin spectra of the insulating state are consistent with the inelastic neutron spectra of the triangular compound Ba8CoNb6O24. I also find that the spin fluctuations at the K point persist into the metallic phase and move to higher energy. These results are in agreement with the studies of spin-lattice relaxation times in k-(ET)2Cu2(CN)3. Finally, I present charge susceptibilities in different areas of parameter space, which should correspond to momentum-resolved electron-loss spectroscopy measurements on triangular compounds. |
Monday, March 2, 2020 12:03PM - 12:15PM |
B56.00005: Topological flat bands in the 3d transition metal-based kagome lattices Min Gu Kang, Shiang Fang, Linda Ye, Hoi Chun Po, Jonathan Denlinger, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Efthimios Kaxiras, Joseph G Checkelsky, Riccardo Comin Electronic flat bands in momentum space, arising from strong localization of electrons in real space, are an ideal stage to realize strong correlation phenomena as highlighted by the recent example of twisted-bilayer graphene. In certain lattice systems, electronic flat bands with nontrivial topology may naturally arise from the combination of geometrical frustration, spin-orbit coupling, and reduced dimensionality, while their experimental realization has been elusive so far. Here, we report the observation of topological flat bands in series of 3d transition metal-based kagome compounds. Using angle-resolved photoemission spectroscopy, we directly show how the dispersion of the flat bands is strongly quenched along all three momentum directions. Spin-orbit coupling opens a large gap at the quadratic band touching point between the Dirac and flat bands, endowing a nonzero Z2 topological invariant to the flat band. Our observation opens a promising route to engineer novel emergent phases at the crossroad between strong correlated and topological materials. |
Monday, March 2, 2020 12:15PM - 12:27PM |
B56.00006: Localized states of the topological flat bands in the 3d transition metal kagome compound Shiang Fang, Min Gu Kang, Linda Ye, Hoi Chun Po, Jonathan Denlinger, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, Joseph G Checkelsky, Riccardo Comin, Efthimios Kaxiras The kagome lattice can host both Dirac electrons and the flat band in the spectrum. With spin-orbit coupling, the flat band will acquire non-trivial topology. On the other hand, when the kinetic energy scale is quenched as with a flat band, the electron interactions could facilitate the formation of correlated phases. Hence, the kagome lattice system has the potential to open a door to study the interplay between topology and electron correlations. In synthesized kagome compounds, additional degrees of freedom such as the orbitals further enrich the physics. To elucidate the nature of the electronic structure, we perform the ab initio density functional theory calculations. We focus on the 3d transition metal kagome compound with topological flat bands, observed experimentally by angle-resolved photoemission spectroscopy. The effective Hamiltonians of these topological flat bands are investigated with the derivations of Wannier states. The implications of topological obstructions and crystalline symmetries are discussed. These localized Wannier functions with intricate spin and orbital texture in the real space reveal their origin from the frustrated kagome structure. |
Monday, March 2, 2020 12:27PM - 12:39PM |
B56.00007: Colossal transverse response from non-centrosymmetric kagome ferromagnets Tomoya Asaba, Sean Thomas, Eric Bauer, Filip Ronning The interest of the anomalous Hall effect (AHE) and anomalous Nernst effect (ANE) has been renewed since the discoveries of large values of the AHE and ANE due to topologically nontrivial origins, and their potential application to spintronics and efficient energy conversion. Good examples are kagome magnets Mn3Sn and Co2MnGa. Among topological materials, in particular, uranium compounds are one of the best candidates to find large responses of AHE and ANE due to the correlated, flat f-band feature and large spin-orbit coupling that is the key ingredient for topology. However, the study of AHE and ANE for uranium compounds has been very sparse with only a few superconducting exceptions. Here, we will present the giant transverse response from uranium kagome ferromagnets. Our results shed light on the potential of uranium compounds as new spintronic and energy-conversion materials. |
Monday, March 2, 2020 12:39PM - 12:51PM |
B56.00008: Wigner solid of charge transfer excitons in Yin-Yang Kagome lattice Gurjyot Sethi, Congjun Wu, Feng Liu We demonstrate that photo absorption between two enantiomorphic topological flat bands, as hosted in a Yin-Yang Kagome lattice, provides a perfect platform for the realization of excitonic Wigner crystallization. By solving the Bethe Salpeter equation in Tamm Dancoff approximation within a tight-binding framework, we study the flat-band excitons in terms of their binding energy and dispersion. We show that they are charge-transfer excitons, characterized by a constant binding energy (dispersion-less) in momentum space and highly localized electron-hole pair distribution function in real space. By virtue of electronic and excitonic flat bands our results are indicative of excitonic Wigner crystallization. This would be the first-time realization of Wigner solid of charge-transfer excitons in a lattice model. Our findings open the door to further investigation of photo excitation and many-body physics of correlated electrons in multiple flat bands. |
Monday, March 2, 2020 12:51PM - 1:03PM |
B56.00009: Three-colored quantum scars in the kagome quantum antiferromagnet Hitesh Changlani, Arijeet Pal Non-equilibrium properties of quantum materials present many intriguing properties, among them athermal behavior, which violates the eigenstate thermalization hypothesis. Such behavior has primarily been observed in disordered systems. More recently, experimental and theoretical evidence for athermal eigenstates, known as "quantum scars", has emerged in non-integrable disorder-free models in one dimension with constrained dynamics. In this work we investigate quantum scar eigenstates and the associated dynamical properties for the two dimensional kagome antiferromagnet. The model we study was previously shown to harbor a special exactly solvable point with three-color ground states, which involves only nearest neighbor XXZ interactions [H. J. Changlani et al. Phys. Rev. Lett. 120, 117202 (2018)]. In particular, we demonstrate the occurrence of low entanglement states spread throughout the middle of the spectrum and show quantum revivals for appropriately chosen initial states. We also present natural generalizations of our ideas to higher dimensions. |
Monday, March 2, 2020 1:03PM - 1:15PM |
B56.00010: A new quantum spin liquid candidate with Co-based triangular lattice Ruidan Zhong, Shu Guo, Guangyong Xu, Zhijun Xu, Robert J. Cava Currently under active study in condensed matter physics, both theoretically and experimentally, are quantum spin liquid (QSL) states. However, the existing QSL candidates all have their intrinsic disadvantages, and solid evidence for quantum fluctuations is scarce. Here, we report a previously unreported compound, Na2BaCo(PO4)2, a geometrically frustrated system with effective spin-1/2 local moments for Co2+ ions on an isotropic two-dimensional (2D) triangular lattice. Magnetic susceptibility and neutron scattering experiments show no magnetic ordering down to 0.05 K. Thermodynamic measurements show that there is a tremendous amount of magnetic entropy present below 1 K in zero magnetic field. The presence of localized low-energy spin fluctuations is revealed by inelastic neutron measurements. At low applied fields, these spin excitations are confined to low energy and contribute to the anomalously large specific heat. In larger applied fields, the system reverts to normal behavior as evident by both neutron and thermodynamic results. Our experimental characterization thus reveals that this material is an excellent candidate for the experimental realization of a QSL state. |
Monday, March 2, 2020 1:15PM - 1:27PM |
B56.00011: SU(4) antiferromagnetism on the triangular lattice Anna Keselman, Lucile Savary, Leon Balents In systems with many local degrees of freedom, high-symmetry points in the phase diagram can provide an important starting point for the investigation of their properties throughout the phase diagram. In systems with both spin and orbital (or valley) degrees of freedom, such as twisted multilayer graphene or transition metal dichalcogenides, such a starting point gives rise to SU(4)- symmetric models. Here we consider SU(4)-symmetric “spin” models, corresponding to Mott phases at half-filling, i.e. the six-dimensional representation of SU(4). In particular, we study the SU(4) antiferromagnetic “Heisenberg” model on the triangular lattice, both in the classical limit and in the quantum regime. Carrying out a numerical study using the density matrix renormalization group (DMRG), we argue that the ground state is non-magnetic. We then carry out a dimer expansion of the SU(4) spin model. An exact diagonalization study of the effective dimer model suggests that the ground state breaks translation invariance, forming a valence bond solid (VBS) with a 12-site unit cell. Finally, we consider the effect of SU(4)-symmetry breaking interactions due to Hund’s coupling, and argue for a possible phase transition between a VBS and a magnetically ordered state. |
Monday, March 2, 2020 1:27PM - 1:39PM |
B56.00012: Stability of chiral spin liquid phase of the triangular lattice Hubbard model with hopping anisotropy Aaron Szasz, Johannes Motruk In our previous work (arXiv: 1808.00463), we showed using density matrix renormalization group (DMRG) simulations on infinite cylinders that the triangular lattice Hubbard model hosts a chiral spin liquid phase at intermediate interaction strength. However, the model we studied featured isotropic hopping, whereas real triangular lattice spin liquid candidate materials, such as κ-(BEDT-TTF)2Cu2(CN)3, are not precisely isotropic. To better connect to experimental results in such spin liquid candidates, in this work we investigate the stability of the chiral spin liquid against anisotropy in the hopping and show that the phase survives, providing further support for the applicability of our findings to experiments. |
Monday, March 2, 2020 1:39PM - 1:51PM |
B56.00013: Dynamical DMRG study of spin excitation dynamics in triangular lattice spin-1/2 antiferromagnet Shigetoshi Sota, Tomonori Shirakawa, Seiji Yunoki, Takami Tohyama Recently, the theoretical understanding of the ground state of frustrated two-dimensional spin-1/2 antiferromagnetic Heisenberg models has been dramatically progressed. In contrast, much less has been understood for the spin excitation dynamics. For example, it is well established by now that 120 degree Néel order appears in the ground state of the triangular lattice spin-1/2 antiferromagnetic Heisenberg model. However, the spin excitation dynamics is still under active debate. In fact, the spin excitations observed by recent neutron scattering for Ba3CoSb2O9, modeled by the triangular lattice spin-1/2 antiferromagnetic Heisenberg model, cannot be explained by the linear spin wave theory. By using the dynamical DMRG method, here we investigate the spin excitation dynamics of the triangular lattice spin-1/2 antiferromagnetic Heisenberg model. We find that our results are in good agreement with the experimental results for Ba3CoSb2O9, implying that the quantum fluctuations beyond the linear spin wave theory play an important role on the spin excitations dynamics of Ba3CoSb2O9. |
Monday, March 2, 2020 1:51PM - 2:03PM |
B56.00014: Spin properties of triangular-lattice rare-earth compounds K2CsYb(PO4)2 and K2CsEr(PO4)2 Luwei Ge, Qing Huang, Zhiling Dun, Haidong Zhou, Martin Mourigal On the way to discover frustrated magnetic materials that realize unconventional phases of matter such as spin liquids, recent attention has been devoted to triangular-lattice materials comprising lanthanoid atoms. Here we report our detailed study of two such rare-earth-based compounds K2CsYb(PO4)2 and K2CsEr(PO4)2, where the magnetic ions form equilateral triangular lattices and where site mixing/distortion effects are absent. In spite of the limited size of the single crystals that we could obtain, we conducted detailed thermo-magnetic and inelastic neutron scattering (INS) studies unravelling the spin dynamics and crystal electric field effects in these systems. Our results indicate that exchange interactions are weak and magnetic excitations essentially accounted for by weakly coupled paramagnetic anisotropic effective spin degrees of freedom. We hope our research will benefit future efforts in this exciting line of research. |
Monday, March 2, 2020 2:03PM - 2:15PM |
B56.00015: Imaging antiferromagnetic domains of PdCrO2 Changmin Lee, Arielle Little, Ritika Dusad, Elina Zhakina, Philippa McGuinness, Veronika Sunko, Seunghyun Khim, Clifford W. Hicks, Andrew Mackenzie, Joseph Orenstein The layered antiferromagnet PdCrO2 is an exceptionally pure system, providing an ideal platform for studying how magnetic frustration is relieved in a triangular lattice. Here we spatially resolve the antiferromagnetic domains of PdCrO2 using a novel temperature-modulated birefringence imaging setup. The material undergoes a weakly first-order transition at the Néel temperature (38 K), forming three distinct types of antiferromagnetic domains that are oriented at 120 degrees from one other. The domain structure is observed to reorient after each cooling cycle (upon heating to sufficiently high temperatures), indicating that the domain walls are highly mobile under external perturbations. The effect of strain on the domain population and the phase transition will also be discussed. |
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