Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session L47: Kitaev Magnetism: Experiment |
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Sponsoring Units: GMAG DMP Chair: Kemp Plumb, Brown University Room: 710/712 |
Wednesday, March 4, 2020 8:00AM - 8:12AM |
L47.00001: Strain effects on bulk α-RuCl3 and α-RuCl3/graphene bilayer Sananda Biswas, Ying Li, David Kaib, Kira M. Riedl, Stephen Winter, Johannes Knolle, Roser Valentí α-RuCl3 has gained considerable attention recently, being the ‘proximate’ Kitaev spin liquid candidate. We discuss the effect of electronic proximity of α-RuCl3 to graphene which gives rise to an insulator to metal transition in α-RuCl3 with the Fermi energy located close to the bottom of the upper Hubbard band of the t2g manifold. These results suggest the possibility of realizing metallic and even exotic superconducting states. Moreover, we show that in the strained α-RuCl3 monolayer, the Kitaev interactions are enhanced compared to the unstrained bulk structure. Furthermore, we investigate the thermal expansion and magnetostriction coefficients of bulk α-RuCl3 via ab initio based methods. |
Wednesday, March 4, 2020 8:12AM - 8:24AM |
L47.00002: Magneto-oscillations in the Thermal Conductivity of Kitaev Magnet RuCl3 Peter Czajka, Tong Gao, Jingjing Lin, Max Hirschberger, Arnab Banerjee, Paula Lampen-Kelley, Jiaqiang Yan, David Mandrus, Stephen E Nagler, N. Phuan Ong RuCl3 has been the subject of much intrigue and controversy due to it’s status as a possible realization of Kitaev’s honeycomb model and it’s exotic physics. Much of this attention has focused on the half-integer quantized thermal hall conductivity reported by Kasahara et al [1]. Here, we report a striking new observation in the system’s thermal transport physics. When cooled below 4K, RuCl3 exhibits strong magneto-oscillations in its longitudinal thermal conductivity. Additionally, the thermal conductivity follows a butterfly-shaped hysteresis loop that also displays oscillations. We discuss possible explanations for this surprising effect and provide updates on our attempts to reproduce the half-integer quantized thermal hall conductivity reported by Kasahara et al. |
Wednesday, March 4, 2020 8:24AM - 8:36AM |
L47.00003: Kitaev-Γ-Heisenberg exchange in Ni3+ honeycomb magnet NaNi2BiO6-δ Allen Scheie, Collin Leslie Broholm, Kate Ross, Panagiotis Peter Stavropoulos, Elizabeth Seibel, Jose Rodriguez-Rivera, Joel Tang, Yi Li, Hae-Young Kee, Robert J. Cava We present experimental and theoretical evidence for bond-dependent Kitaev-Γ-Heisenberg exchange in honeycomb Ni3+ magnet NaNi2BiO6-δ. Heat capacity and neutron diffraction reveal a low-temperature ordered magnetic state for T < 4.8 K with (1/3, 1/3, 0.15(1)) order. In-plane magnetic correlations match the 120° ordered state of the Kitaev-Γ-Heisenberg exchange. Electron spin resonance and density functional theory reveal a uniform 3+ valence in Ni intermediate between a S=3/2 and J=1/2 state. DFT shows that covalent bonding between oxygen and Bi produces an enhanced spin orbit coupling in Ni3+, which allows for strong bond-dependent exchange. The ordering wave vector, in-plane magnetic correlations, missing entropy, spin state, and superexchange pathways are all consistent with bond-dependent Kitaev-Γ-Heisenberg exchange interactions in NaNi2BiO6-δ. |
Wednesday, March 4, 2020 8:36AM - 8:48AM |
L47.00004: Intermediate magnetic phase of the proximate Kitaev system in the in-plane magnetic field Beom Hyun Kim, Shigetoshi Sota, Tomonori Shirakawa, Young-Woo Son, Seiji Yunoki a-RuCl3 is the antiferromagnetic insulator with a zigzag order which is believed to be proximate to the Kitaev spin liquid. Its magnetic continuum excitation likely resulted from fractionalized Majorana fermions has been evidently observed in recent experiments. The zigzag long-range order can be suppressed and an intermediate phase, a possible quantum spin liquid, can emerge in between the zigzag phase and parallelly polarized phase when the magnetic field is applied. A few theories have attempted to explain the origin of intermediate magnetic phase in the presence of magnetic field. However, they have failed to manifest the intermediate phase in the in-plane external field observed by some experiments. |
Wednesday, March 4, 2020 8:48AM - 9:00AM |
L47.00005: Analysis of the high-field evolution of magnetic excitations in α-RuCl3 David Kaib, Stephen Winter, Roser Valenti The antiferromagnetic order of α-RuCl3 can be suppressed by an in-plane magnetic field of approximately 7.5 tesla. Above this critical field, a rather unconventional field-dependence of the magnetic excitations has been reported. More recent Raman and terahertz spectroscopy experiments went up to significantly higher fields [1, 2]. We theoretically analyze these and compare the response of realistic anisotropic models. To this end we discuss different scattering mechanisms for the Raman response. We find that the Fleury-Loudon mechanism (usually discussed as the "two-magnon mechanism") can explain the strikingly strong intensity of the single-magnon in the Raman spectra, as a consequence of strong Kitaev and anisotropic couplings. Our study firmly establishes the partially-polarized quantum disordered character of the high-field phase. |
Wednesday, March 4, 2020 9:00AM - 9:12AM |
L47.00006: Half-integer quantized thermal Hall conductance without out-of-plane magnetic field in a chiral spin liquid state of α-RuCl3 Yuichi Kasahara, Taichi Yokoi, Sixiao Ma, Shigeru Kasahara, Takasada Shibauchi, Nobuyuki Kurita, Hidekazu Tanaka, Joji Nasu, Yukitoshi Motome, Ciaran Hickey, Simon Trebst, Yuji Matsuda Kitaev quantum spin liquid (QSL) displays the fractionalization of quantum spins into Majorana fermions. In magnetic fields, the emergence of Majorana edge current is predicted to manifest itself in the form of half-integer quantized thermal Hall plateau, which has recently been reported for two-dimensional honeycomb material α-RuCl3 [1]. While the conventional electronic Hall effect requires a perpendicular magnetic field, we find that the thermal Hall plateau appears even for a magnetic field with no out-of-plane components. In addition, the topological Chern number determinied by the sign of the quantized thermal Hall conductance is consistent with that expected in the pure Kitaev spin liquid. These results demonstrate that the Kitaev interaction is primarily responsible and that the non-Abelian topological order persists even in the presence of non-Kitaev interactions in α-RuCl3. |
Wednesday, March 4, 2020 9:12AM - 9:24AM |
L47.00007: New Kitaev Magnet with Compelling Thermodynamic Evidence of Proximity to the Kitaev Spin-Liquid Faranak Bahrami, William Lafargue-Dit-Hauret, Oleg I. Lebedev, Roman Movshovich, Hung-Yu Yang, David Broido, Xavier Rocquefelte, Fazel Tafti Kitaev magnets are widely studied in search of quantum spin-liquid (QSL) ground states and Majorana excitations. Recent theoretical studies predict two potential signatures of a QSL at finite temperatures: a scaling behavior of thermodynamic quantities and a two-step release of the magnetic entropy. Here, we successfully synthesized a new Kitaev magnet, Ag3LiIr2O6 with both signatures in our AC magnetic susceptibility, magnetization and heat capacity measurements. In this new quaternary oxide, despite the existence of a large Weiss temperature, there is no evidence of long-range order in our DC magnetic susceptibility and heat capacity measurements. Based on our DFT calculations, a mixing between silver d and oxygen p orbitals enhances the spin-orbit coupling within the honeycomb layers suggesting a closer proximity to the QSL in Ag3LiIr2O6 compared to known Kitaev magnets, α-Li2IrO3 and Na2IrO3. |
Wednesday, March 4, 2020 9:24AM - 9:36AM |
L47.00008: Quasiparticle Relaxation Dynamics in the Kitaev Hyperhoneycomb β-Li2IrO3 Peter Kissin, Alejandro Ruiz, Peter Kim, Max Poore, Ananya Rai, Mayia A Vranas, James Analytis, Alex Frano, Richard Averitt We study quasiparticle relaxation dynamics in single crystals of the three dimensional Kitaev hyperhoneycomb β-Li2IrO3 (LIO) using time-resolved optical pump-probe spectroscopy (OPPS). OPPS is extremely sensitive to subtle changes in the low energy electronic structure and the emergence of new quasiparticles, which can be missed with other probes. For LIO, we show that the photoinduced change in reflectivity is sensitive to several distinct phases upon varying temperature, photoexcitation intensity, and magnetic field applied along the crystallographic b axis. We observe large differences in the relaxation dynamics in the incommensurate spiral phase at low applied fields as compared to the field-induced zig-zag phase at fields above 2.8 Tesla. In addition, OPPS is sensitive to the recently discovered bulk high temperature phase transition at approximately 100 Kelvin. By tracking the evolution of quasiparticle dynamics within this new phase as a function of temperature and field, our results shed light on the nature of this phase as well as its relationship to the known low temperature phases. |
Wednesday, March 4, 2020 9:36AM - 9:48AM |
L47.00009: A rare-earth Kitaev material candidate YbCl3 Eve Emmanouilidou, Jie Xing, Huibo Cao, Chaowei Hu, Jinyu Liu, David E Graf, Arthur Ramirez, Gang Chen, Ni Ni
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Wednesday, March 4, 2020 9:48AM - 10:00AM |
L47.00010: Field-angle-resolved specific heat measurements in the Kitaev material α-RuCl3 Ohei Tanaka, Ryuhei Harasawa, Yuta Mizukami, Marcin Konczykowski, Nobuyuki Kurita, Hidekazu Tanaka, Eun-Gook Moon, Yuichi Kasahara, Yuji Matsuda, Kenichiro Hashimoto, Takasada Shibauchi Recently, 4d5 or 5d5 honeycomb materials with strong spin-orbit coupling have attracted great attention as a candidate for realizing a Kitaev quantum spin liquid (QSL). However, most of the candidate materials show magnetic orders at low temperatures. Indeed, α-RuCl3, one of the most promising candidates, also exhibits a zigzag antiferromagnetic order below TN (~ 7 K). However, the magnetic order of this material can be suppressed by applying a critical magnetic field H* of 7.5 T parallel to the honeycomb plane. Recent thermal transport measurements, as well as spectroscopic studies such as inelastic neutron scattering, suggest the emergence of spin fractionalization into Majorana fermions in the field-induced QSL phase. Here, we report specific heat measurements under in-plane magnetic fields up to 7 T on electron-irradiated α-RuCl3 single crystals, in which a systematic suppression of TN and H* was observed. We find a remarkable six-fold oscillation in the field-angle dependence of the specific heat at low temperatures above H*. This result is consistent with the field-angle dependence of the Majorana gap expected in the Kitaev QSL. |
Wednesday, March 4, 2020 10:00AM - 10:12AM |
L47.00011: Ruthenium Substitution in Beta-Lithium Iridate Mayia Vranas, Alejandro Ruiz, Benjamin Frandsen, Arani Acharya, Vikram Nagarajan, Gilbert Lopez, Nityan Nair, Nicholas Breznay, Zahirul Islam, Alex Frano, James Analytis In the Kitaev model, orthogonal bond-directional exchange interactions have been proposed to give rise to quantum spin liquid ground state with fractional Majorana excitations. However, the candidate honeycomb materials described by this model order at low temperatures. Nonetheless, external perturbations, such as applied magnetic field, pressure, and chemical substitution, have been shown to change the strengths of the magnetic interactions and alter their ground state. Additionally, recent work in beta-lithium iridate has shown a hidden magnetic anomaly at T~100K which may be connected to the Kitaev-like behavior in the material. In this work, we present thermodynamic, resonant X-ray diffraction, and muon spin relaxation studies on ruthenium-doped beta-lithium iridate (β-Li2RuxIr1-xO3). We describe how the addition of magnetic impurities on the iridium sites suppresses the incommensurate phase and gives rise to an anisotropic spin glass. Additionally, we discuss the impact that these impurities have on the high-temperature phase. |
Wednesday, March 4, 2020 10:12AM - 10:24AM |
L47.00012: Confinement of fractional excitations in the 3D Kitaev magnet β-Li2IrO3 Yiping Wang, Alejandro Ruiz, James Analytis, Kenneth Burch Kitaev quantum spin liquid (QSL) is a highly frustrated state in topological magnets with excitations from fractional particles. In candidate materials like RuCl3 and A2IrO3(A = Na, Li), there is always an ordered state below Tc. However, the transition from spin liquid like fractional excitations to the magnons of the ordered state is still unclear. Here, we report the Raman scattering result on three-dimensional magnet β-Li2IrO3. We identified the polarization and temperature dependence of the magnons and fractional excitations in Raman. This provides first direct evidence for confinement of fractional excitations into magnons in a kitaev system, which so far has only been observed in quasi-1d systems. |
Wednesday, March 4, 2020 10:24AM - 10:36AM |
L47.00013: Evolution of magnetic phases in beta-Li2IrO3 under high magnetic fields Vikram Nagarajan, Kimberly Modic, John Singleton, Mun K. Chan, Kevin Wang, James Analytis The Kitaev model describes a honeycomb net of Ising spins where each bond direction exchange-couples an orthogonal component of spin. Remarkably, this model is exactly solvable and predicts a quantum spin liquid ground state. Within the past decade, several materials have been theorized to realize the Kitaev model in their magnetic interactions. One of these materials is beta-Li2IrO3, which crystallizes in a 3D "hyperhoneycomb" structure. While this material does not possess a quantum spin liquid ground state, it contains a variety of interesting magnetic phases that coexist at intermediate temperature. In this talk, we describe how these phases evolve under strong magnetic fields using the state-of-the art technique of resonant torsion magnetometry. |
Wednesday, March 4, 2020 10:36AM - 10:48AM |
L47.00014: High-resolution RIXS as a probe of Kitaev Magnetic Excitations in β-Li2IrO3 Alejandro Ruiz, Anthony Allen, Vikram Nagarajan, Jake Korelek, Mary Upton, Jongho Kim, Nicholas Breznay, James Analytis, Diego M Casa, Alex Frano Spin-orbit entangled Mott insulators with strong bond-directional interactions have been proposed as a possible venue for the realization of the Kitaev honeycomb model. Most candidate materials, however, order at low temperatures due to the presence of additional spin coupling interactions. Nonetheless, a wealth of theoretical and experimental work has shown that the Kitaev term is the dominant magnetic exchange. |
Wednesday, March 4, 2020 10:48AM - 11:00AM |
L47.00015: Anamolous, Anisotropic and Extended Nonlinear Susceptibility in a-RuCl3 Bellave Shivaram, Ludwig FW Holleis, Joseph Prestigiacomo, Zhije Fan, Satoshi Nishimoto, Jiaqiang Yan, Arnab Banerjee, Michael Osofsky, Jeroen Van den Brink, Gia-Wei Chern, David Mandrus, Stephen E Nagler In proximate Kitaev a-RuCl3, we establish a radically new behavior of the nonlinear susceptibility hitherto not established. For T < Tc and B || ab-plane, a dual nonlinear response is observed. The high field response above 2 T yields a third order nonlinear susceptibility which peaks near Tc = 7.5 K, reminiscent of behavior in a classical 2D antiferromagnet, but with a large positive value for lowest T. The low field response, for B less than 2T - on the other hand, is positive quadratic and shows a rapid rise below Tc. This dual response vanishes for T beyond Tc where the nonlinear response is dominated by only the cubic term. This term is significantly > 0 and turns negative only for a temperature T > 50 K >> Tc. Classical Monte Carlo (CMC) simulations capture the observed quadratic response, albeit only near Tc. The CMC however fail to reproduce the large quadratic term at low T.. This discrepancy indicates inadequacy of current models and points to the importance of multi-spin correlations in a-RuCl3. For B || c-axis, we measure no dual response with only the cubic term being apparent. We also compare our results with exact diagonalization calculations performed on 16 x 16 cluster. |
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