Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session S45: Magnetic Frustration and Quantum Spin Liquids |
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Sponsoring Units: DCMP GMAG Chair: Travis Williams, Oak Ridge Natl. Lab. Room: LACC 505 |
Thursday, March 8, 2018 11:15AM - 11:27AM |
S45.00001: Dynamical and thermal properties of the Kitaev honeycomb-lattice magnet RuCl3 Takafumi Suzuki, Sei-ichiro Suga Honeycomb-lattice magnet RuCl3 is considered to be a potential candidate of realizing Kitaev spin liquid [1], although this material undergoes a phase transition to the zigzag ordered state at a low temperature. Recent inelastic neutron-scattering (INS) measurements on RuCl3 have unveiled characteristic dynamical properties [2]. In this study, we calculate dynamical spin structure factors of three ab-initio models [3-5] and an empirical model [6] for RuCl3 with an exact numerical diagonalization method. We also calculate the temperature dependences of the heat capacity, C(T) by using thermal pure quantum states. We find that the four models are difficult to explain experimental key features observed in C(T) and the INS simultaneously. This suggests that the Kitaev interactions estimated in the ab-initio calculations are small. We thus propose the model with larger Kitaev coupling that can reproduce the key features not only qualitatively but also quantitatively. [1]J.Chaloupka, et al., PRL 110, 097204 (2013). [2] A. Banerjee, et al., Science 356, 1055-1059 (2017). [3] H. S. Kim and H. Y. Kee, PRB 93, 155143 (2016). [4] S. M. Winter, et al., PRB 93, 214431 (2016). [5] R. Yadav, et al., Sci. Rep. 6, 37925 (2016). [6] S. M. Winter, et al., arXiv:1702.08466. |
Thursday, March 8, 2018 11:27AM - 11:39AM |
S45.00002: Unconventional spin dynamics in the honeycomb-lattice material α-RuCl3: high-field ESR studies Sergei Zvyagin, Alexei Ponomaryov, E. Schulze, Joachim Wosnitza, Paula Lampen-Kelley, Arnab Banerjee, Jiaqiang Yan, Craig Bridges, David Mandrus, Stephen Nagler, Alexei Kolezhuk We performed comprehensive multi-frequency high-field electron spin resonance (ESR) studies of α-RuCl3, an anisotropic spin system with a honeycomb structure, a top candidate for exhibiting Kitaev's spin-liquid physics. Two modes of antiferromagnetic resonance were detected in the zigzag-ordered phase. Magnetic field applied in the ab plane suppresses the zigzag order, resulting in a significant change of the excitation spectrum. Four ESR modes were observed in the field-induced quantum paramagnetic state above Hc ~ 7 T, suggesting an unconventioanl multiparticle nature of magnetic excitations in α-RuCl3. The obtained data are compared with results of recent numerical calculations, revealing good qualitative agreement. |
Thursday, March 8, 2018 11:39AM - 11:51AM |
S45.00003: Optical waveguide modes in α-RuCl3 Lin Xiong, Aaron Sternbach, Paige Kelley, Sai Sunku, Stephen Nagler, David Mandrus, Dimitri Basov The Kitaev quantum spin liquid (KQSL) is an exotic state of matter predicted to exhibit Majorana fermions and gauge flux excitations. This novel state may be realized in α-RuCl3. Here we report a nano-imaging study of optical waveguide modes in an α-RuCl3 thin film. The waveguide modes that we study are confined electromagnetic waves propagating within the host material. These modes couple to orbital/electronic transitions, magnetic resonances and potentially to itinerant Majorana fermions, thus offering a sensitive probe of different degrees of freedom in the system. We found that the waveguide mode propagation length varies with probe energy, due to absorption by d-d orbital transitions, and reaches a maximum of 12μm at photon energy of 830 meV. |
Thursday, March 8, 2018 11:51AM - 12:03PM |
S45.00004: Antiferromagnetic Resonance and Terahertz Continuum in α-RuCl3 Arielle Little, Liang Wu, Paula Lampen-Kelley, Arnab Banerjee, Shreyas Patankar, Dylan Rees, Craig Bridges, Jiaqiang Yan, David Mandrus, Stephen Nagler, Joseph Orenstein We report time-domain THz spectroscopy measurements of α-RuCl3 as a function of temperature and applied in-plane magnetic field, H, in the energy range 0.3 to 8.3 meV. Polarized measurements show that at zero field, 3-fold rotational symmetry is broken in the honeycomb plane from 2 K to 300 K. Below TN, and with B(t) parallel H, we observe an antiferromagnetic resonance (AFMR) peak at 2.56 meV atop a large broadband continuum which can attributed largely to electric dipole absorption. We track the evolution of the AMFR peak from H = 0 T to 7 T as it broadens and shifts to lower frequency. From spectral weight analysis of this peak, we place an upper bound on the contribution to the dc susceptibility from a magnetic excitation continuum. Ref. arxiv:1704.07357 |
Thursday, March 8, 2018 12:03PM - 12:15PM |
S45.00005: Spin liquid and quantum phase diagram on a spin-orbit coupled triangular lattice Mac Lee, Donna Sheng, Say-Peng Lim Spin-orbit coupling breaks SU(2) spin symmetry, which leads to exotic physics that people are starting to take note of in recent years. Among the many unusual phases and behaviors that materials exhibit when SU(2) symmetry is absent, our paper focuses on the search of a spin-liquid phase on an anisotropic triangular lattice. We use DMRG and exact diagonalization to study ground state physics of this particular model and look for tell-tale signs of a spin-liquid phase. In the process, we explore the relationship of different ordered and disordered states with spin-orbit coupling based on finite-size study of the energy spectrum, entanglement spectrum, spin correlations, and excitation gaps. Through comparison with the theoretical work of J. Iaconis et al. (arXiv:1708.07856v2 [cond-mat.str-el] 15 Sep 2017), we also determine the interesting regimes for different spin liquid candidates. |
Thursday, March 8, 2018 12:15PM - 12:27PM |
S45.00006: Quenching the Kitaev honeycomb model Louk Rademaker I studied the non-equilibrium response of an initial Néel state under time evolution with the Kitaev honeycomb model. This time evolution can be computed using a random sampling over all relevant flux configurations. With isotropic interactions the system quickly equilibrates into a steady state valence bond solid. Anisotropy induces an exponentially long prethermal regime whose dynamics are governed by an effective toric code. Signatures of topology are absent, however, due to the high energy density nature of the initial state. |
Thursday, March 8, 2018 12:27PM - 12:39PM |
S45.00007: Low-lying excitations of the Kitaev-Heisenberg model on a honeycomb lattice Sei-ichiro Suga Some transition-metal compounds on honeycomb lattices such as RuCl3 are regarded as promising candidates for realizing the Kitaev spin liquid (KSL), although these materials undergo phase transitions to the zigzag magnetic states [1]. Since their ground states are considered to be located close to the KSL state, low-lying excitations are difficult to calculate using analytical methods. The S = 1/2 Kitaev-Heisenberg (KH) model on a honeycomb lattice is a minimal model for these materials [2]. We investigate ground state energies and low-lying excitations of the KH model on a honeycomb lattice by using series expansions. We find that dimer series expansions can approach the close vicinity of the Kitaev limit in the zigzag phase, where the Heisenberg interaction is absent, in the lower order expansion than Ising series expansions. When the system approaches the Kitaev limit, the low-lying mode becomes flatter except for the Bragg wave numbers. We discuss the results in relation to the inelastic neutron scattering experiments [3]. [1] K. W. Plumb, et al., Phys. Rev. B 90, 041112(R) (2014). [2] J. Chaloupka, et al., Phys. Rev. Lett. 110, 097204 (2013). [3] A. Banerjee, et al., Nat. Mater. 15, 733 (2016). |
Thursday, March 8, 2018 12:39PM - 12:51PM |
S45.00008: Tunable Bi-frustrated Electron Spin and Charge States in a Cu-Hexaaminobenzene Framework Wei Jiang, Zheng Liu, Jia-Wei Mei, Bin Cui, Feng Liu A geometrically frustrated lattice may host frustrated electron spin or charge states that spawn exotic quantum phases. We show that a newly synthesized metal-organic framework of Cu-Hexaaminobenzene [Cu3(HAB)2] exhibits a multi spectra of unusual quantum phases long sought after in condensed matter physics. On one hand, the Cu2+ ions form an ideal S-1/2 antiferromagnetic kagome lattice. On the other hand, the conjugated-electrons from the organic ligands give rise to completely dispersionless energy bands around the Fermi level, reproducing a frustrated πx-πy hopping model on a honeycomb lattice. We propose to characterize the coexistence of frustrated local spins and conjugated electrons through scanning tunneling microscopy simulations. Most remarkably, their close energy proximity enables one to tune the system between the two frustrated states by doping up to one hole per HAB unit. Thus, Cu3(HAB)2 provides a unique exciting platform to investigate the interplay of frustrated spins and electrons in one single lattice, e.g. by gating experiments, which will undoubtedly raise interesting theoretical questions leading to possible new condensed matter phases. |
Thursday, March 8, 2018 12:51PM - 1:03PM |
S45.00009: Quantum Spin Liquids Unveil the Genuine Mott State Andrej Pustogow, Mathias Bories, Anja Löhle, Roland Rösslhuber, Elena Zhukova, Boris Gorshunov, Silvia Tomic, John Schlueter, Ralph Hübner, Takaaki Hiramatsu, Yukihiro Yoshida, Gunzi Saito, Reizo Kato, Tsung-Han Lee, Vladimir Dobrosavljevic, Simone Fratini, Martin Dressel The Widom line identifies the supercritical crossover from a gas-like to a more liquid-like behavior. A similar transition exists in correlated electron liquids, where the interplay of Coulomb repulsion, bandwidth and temperature triggers between the Mott insulating state and an incoherent conduction regime. Here we explore the electrodynamic response of three organic quantum spin liquids, where the absence of magnetic order enables unique insight into the nature of the genuine Mott state. Combining optical spectroscopy with pressure-dependent dc transport and theoretical calculations, we succeeded to construct a phase diagram valid for all Mott insulators on a quantitative scale. Our findings reveal the Pomeranchuk-like anomaly of the Mott transition and metallic fluctuations within the Mott gap, previously predicted but never observed. |
Thursday, March 8, 2018 1:03PM - 1:15PM |
S45.00010: Valence Bonds in Random Quantum Magnets: Theory and Application to YbMgGaO4 Itamar Kimchi, Adam Nahum, Senthil Todadri We will discuss the role of quenched disorder in spin-1/2 quantum magnets where magnetic frustration promotes the formation of local singlets. We find that the destruction of a valence-bond solid phase by weak quenched disorder leads inevitably to the nucleation of topological defects carrying spin-1/2 moments. This renormalizes the lattice into a strongly random network of defect spins, which yield interesting low-energy excitations. A similar conclusion is reached in a regime of stronger disorder where short-ranged valence bonds would otherwise freeze without local crystalline order. Motivated by these results, we conjecture Lieb-Schultz-Mattis-like restrictions on ground states for disordered magnets with spin-1/2 per unit cell. We apply insights from this study to propose an alternative interpretation of the phenomenology of YbMgGaO4, which was suggested to be a quantum spin liquid. Experimental signatures, including unusual specific heat, thermal conductivity, and dynamical structure factor, and their behavior in a magnetic field, are predicted from the theory, and compare favorably with existing measurements on YbMgGaO4 and YbZnGaO4. |
Thursday, March 8, 2018 1:15PM - 1:27PM |
S45.00011: Synthesis, Crystal Growth and Characterization of Ho2Ti2O7 Alireza Ghasemi Holmium titanate (as a candidate for the spin ice behavior [1]), is a member of the rare-earth titanate family with the A2B2O7 chemical formula and Pyrochlore structure. The ability to show a wide variety of behaviors in the Pyrochlores is due to structure adaptation with different cations' radius ratio. This flexibility can also ease the structural distortion which causes defect formation inside the crystals [2,3]. The wide range of the lattice parameters that are reported for Ho2Ti2O7 in the literature raises doubts as to the quality of previously grown crystals. We grew single crystals of Ho2Ti2O7 (HTO) by floating zone and traveling solvent floating zone methods from high-quality powder to investigate and illustrate the effect of synthesis and growth of the single crystal at the melting point of the Holmium titanate and below their melting point on the compositional and structural properties. We found that the crystal grown by typical floating zone method shows a continuous increase in its lattice parameter from top to bottom, but the crystal is grown by traveling solvent method shows a consistency of the lattice parameter along the crystal, similar to what has been observed for Ytterbium titanate [4]. |
Thursday, March 8, 2018 1:27PM - 1:39PM |
S45.00012: Variational Monte-Carlo study of lightly doped kagome spin liquid Fan Yang, Yifan Jiang, Hong Yao We study the t-J model on the Kagome lattice with small but non-zero doping by the large-scale variational Monte-Carlo simulation. We propose a new variational state which is equivalent to the U(1) quantum spin-liquid state proposed by Ying Ran at zero doping (Y. Ran, et al, Phys. Rev. Lett. 98, 117205 (2007)). Instead of a superconducting state, we show that the lowest energy state is a chiral Fermi liquid state with enlarged unit cell. By comparing this state with various types of projected spin-liquid states and valence bond crystal states, we find that our state has lower energy at small doping. The relation between this state and the holon Wigner crystal proposed by density-matrix renormalization group is also discussed. |
Thursday, March 8, 2018 1:39PM - 1:51PM |
S45.00013: Bosons in flat band lattices: Kagome and beyond Saurabh Maiti, Tigran Sedrakyan We provide a scheme to investigate and identify systems with flat bands, like the Kagome and Lieb lattices, and address the sensitivity of the flat bands to the hopping parameters. A unique feature of the energy spectrum of a flat-band system, such as the Kagome lattice, is the parabolic band touching of the flat band and a dispersing one at the Γ-point. When the lattice sites are populated by hard core spins, it is known that the ground-state energy is determined solely by the occupancy of the flat-band up to a filling fraction of ν=1/9. Beyond this point, the parabolic band at higher energies begins to populate, leading to the physics of Bose-Einstein condensation. Upon fermionizing such a system, we can treat the bosonic excitations as fermions interacting with a Chern-Simons(CS) gauge field. These fermions have an interesting property that they occupy the flat-band up to ν=1/3. We investigate the precise role played by CS-gauge field which connects the fermionic ν=1/3 state to the bosonic ν=1/9 state. In the process we also address the formation of `moats’ in the electronic structure of such systems that prevents the formation of a condensate (which leads to a spin-liquid behavior). |
Thursday, March 8, 2018 1:51PM - 2:03PM |
S45.00014: Observation of Massive Dirac Fermions in Ferromagnetic Kagome metal Fe3Sn2 Min Gu Kang, Linda Ye, Junwei Liu, Felix Cube, Christina Wicker, Takehito Suzuki, Chris Jozwiak, Aaron Bostwick, Eli Rotenberg, David Bell, Liang Fu, Joseph Checkelsky, Riccardo Comin Recent theoretical developments suggest that the combination of unusual magnetism, spin-orbit coupling, and geometric frustration in Kagome metal possibly lead to a wide range of novel topological physics, such as fractional quantum Hall effect and intrinsic anomalous Hall effect. In these phenomena, a major role is played by the topologically nontrivial flat bands and massive Dirac cones, both of which are predicted to exist from the unique geometrical hopping pathways of Kagome lattice. Despite these predictions, the experimental band structure of Kagome compounds has long remained unreported. In this talk, we report the experimental band structure of bilayer Kagome compound Fe3Sn2, measured by high-resolution angle-resolved photoemission spectroscopy. We clearly observe a pair of quasi-2D Dirac cones near the Fermi level, with a 30 meV spin-orbit gap that serves as a singular source of Berry curvature. Combined with ferromagnetism in the Kagome plane, this fermiology supports the emergence of intrinsic anomalous Hall conductivity, unambiguously observed by transport measurement over a wide temperature range (0.6 K ~ 400 K). Thus, our experiments establish the first direct link between the electronic structure and emergent topological transport in a correlated Kagome metal. |
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