Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session G51: Kitaev Materials IIFocus Recordings Available
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Sponsoring Units: GMAG Chair: Yang Yang, University of Minnesota Room: McCormick Place W-474B |
Tuesday, March 15, 2022 11:30AM - 12:06PM |
G51.00001: Realization of a New Spin-Orbital Quantum State in the 4d Honeycomb System Ag3LiRh2O6 Invited Speaker: Faranak Bahrami The crystal electric field effect and spin-orbit coupling in 4d/5d honeycomb systems create an isospin-½ (Jeff=½) ground state and anisotropic Kitaev interactions. Notable examples of such systems are the iridate materials and α-RuCl3 [1, 2, 3]. Based on theoretical predictions, the ground states of such materials can be tuned to other exotic spin-orbital states such as an Ising spin-½ state. However, such a ground state has been considered inaccessible due to the overwhelming influence of spin-orbit coupling in these materials. Here, by using a topochemical reaction, we modified the inter-layer chemical bonds in a honeycomb rhodate, Li2RhO3, and synthesized Ag3LiRh2O6. This process induced a trigonal distortion in Ag3LiRh2O6 that led to a transition from the isospin-½ ground state (Kitaev limit) in the parent compound to an Ising spin-½ ground state (Ising limit) in the product. This change in spin orbital state resulted in a dramatic change in magnetism. Whereas Li2RhO3 shows a spin-freezing transition at 6 K, Ag3LiRh2O6 reveals a robust long-range antiferromagnetic transition at 94 K. Magnetization, heat capacity, and μSR measurements confirm the robust long-range antiferromagnetic transition at 94 K, and x-ray absorption spectroscopy measurements support our perturbation theory results and confirm that Ag3LiRh2O6 possesses an Ising spin-½ state. This is the first realization of a change of ground state between the Kitaev and Ising limits in a specific honeycomb structure. |
Tuesday, March 15, 2022 12:06PM - 12:18PM |
G51.00002: Microwave noise thermometry of graphene/α-RuCl3 heterostructures Jesse Balgley, Erik A Henriksen, Jackson Butler Recently, we discovered a strong charge transfer between graphene and the layered Mott insulator α-RuCl3, which yields hole densities as large as a few 1013 cm-2 in graphene and is tunable by modulating the graphene-RuCl3 separation with an insulating spacer. Beyond the potential for applications of graphene devices having tunable and patterned potential profiles, the charge transfer makes graphene a low-resistance electrical contact to α-RuCl3, which may be useful for accessing its electronic and thermal properties. We present microwave Johnson-Nyquist noise thermometry measurements of graphene/α-RuCl3 heterostructures to quantify the thermal transport mechanisms between graphene and α-RuCl3, and demonstrate tunability of the coupling with interface separation. We will present details of the fabrication process and measurement setup and discuss how this technique may be useful for probing the thermal conductance of exfoliated α-RuCl3. |
Tuesday, March 15, 2022 12:18PM - 12:30PM |
G51.00003: Machine learning model parameters of RuCl3 at high magnetic field Michael J Lawler, Kimberly A Modic, Brad J Ramshaw $alpha$-RuCl$_3$ is a magnetic insulator exhibiting quantum spin liquid phases possibly found in the Kitaev honeycomb model. Much of the effort towards determining Hamiltonian parameters has focused on low magnetic field ordered phases. We study this problem in the high magnetic field limit where mean-field theory becomes a good description. We do so by machine learning model parameters from 160,000 low dimensional data points that include magnetic field[1], torque[2], and torsion data[3]. Our machine, an MFT-ANN, maps thermodynamic conditions (temperature and field vector) to model parameters via a fully connected time-reversal covariant neural network and then predicts observable values using mean-field theory. To train the machine, we use PyTorch to enable backpropagation through mean-field theory with a pure PyTorch implementation of the Newton-Raphson method. The results provide a distribution of parameter values showing interaction parameters beyond the Kitaev coupling K are equally important to the physics of $\alpha$-RuCl$_3$. |
Tuesday, March 15, 2022 12:30PM - 12:42PM |
G51.00004: Experimental Measurement of Ru3+ Form Factor Colin Sarkis, Masaaki Matsuda, Jaime A Fernandez-Baca, Jiaqiang Yan, Arnab Banerjee, Christian Balz, David G Mandrus, Stephen E Nagler The need for fault tolerant quantum computation has resulted in a large push for materials realizing Kitaev’s model of bond dependent exchange on a honeycomb lattice. In this regard, α-RuCl3 remains one of the most promising candidate materials. A host of neutron scattering, thermal Hall, Raman, and THz spectroscopy experiments have shown evidence that α-RuCl3 is proximate to a Kitaev quantum spin liquid (QSL), with the QSL state stabilized through application of a magnetic field within the honeycomb plane. Despite the large amount of interest, several unanswered questions remain, including the true form of the low energy Hamiltonian of the system. Quantitative analysis of neutron scattering data would be aided by an accurate determination of the magnetic form factor of Ru3+, where the mixed spin-orbital doublet ground state is expected to show notable deviations from the spherical approximation. In this talk, I will discuss recent experimental results where we have directly measured the magnetic form factor through half-polarized neutron diffraction of α-RuCl3 under an applied magnetic field above TN , well into the paramagnetic regime. |
Tuesday, March 15, 2022 12:42PM - 12:54PM |
G51.00005: Low-temperature field dependence of thermal conductivity in α-RuCl3 Jordan Baglo, Etienne Lefrancois, Quentin Barthélemy, Subin Kim, Young-June Kim, Louis Taillefer The layered honeycomb compound α-RuCl3 continues to attract great interest as a leading candidate material in which to experimentally realize Kitaev quantum spin liquid physics. Early thermal transport measurements [1] reported a plateau region of half-integer-quantized 2D thermal Hall conductance ascribed to protected chiral Majorana edge currents of the Kitaev quantum spin liquid; however, attempts to reproduce this result have seen conflicting reports from different groups [2, 3]. More recently, 1/H-periodic oscillatory behavior was reported in the in-plane field dependence of longitudinal thermal conductivity [3], strongly reminiscent of quantum oscillations in a metal, and hypothesized here to derive from a spinon Fermi surface: a tantalizing possibility begging further experimental confirmation. |
Tuesday, March 15, 2022 12:54PM - 1:06PM |
G51.00006: ElectronicTransport inModulation-doped α-RuCl3 Jackson Butler, Jesse Balgley, Erik A Henriksen The layered Mott insulator α-RuCl3 is a prominent candidate to host a quantum spin liquid phase. Recently, we found that monolayer graphene donates 4∗1013 electrons/cm2 to α-RuCl3when the two materials are placed in direct contact. The charge transfer persists, though to a lesser extent, even when the two materials are spatially separated by a thin insulating spacer, which realizes modulation doping of the α-RuCl3. In light of models for high-Tc superconductivity based on charge-doped Mott insulators, we are motivated to explore the electronic transport of charge-doped α-RuCl3samples. We fabricate van der Waals heterostructures comprising thin flakes of α-RuCl3, modulation-doped by graphene across a thin insulating spacer, and also contacted by graphene leads. We will present results of electronic transport in these devices as a function of electron doping, temperature, and magnetic field. |
Tuesday, March 15, 2022 1:06PM - 1:18PM |
G51.00007: Stabilizing the Kitaev spin liquid state in Ag3LiIr2O6 with high magnetic fields Shengzhi Zhang, Minseong Lee, Faranak Bahrami, Fazel Tafti, Vivien Zapf A spin-liquid system doesn’t form static long-range order even at the lowest temperature despite the existence of strong magnetic interactions. In some materials, this happens because the nearest-neighbor antiferromagnetic interactions in a frustrated geometry can’t all be satisfied. Among the spin liquid systems, “Kitaev” spin liquids are of particular interest because an analytically solvable model is available and they are predicted to form a gapless quantum spin liquid with Majorana fermions. Recently, Ag3LiIr2O6 was proposed to be a potential “Kitaev” spin liquid [1]. Evidences include the lack of long-range order, a two-step release of magnetic entropy and a scaling behavior in the AC-susceptibility as a function of temperature over magnetic field. However, crystals with higher quality were later found to order antiferromagnetically at 8 K [2]. Nevertheless, a spin liquid state may still be present in a portion of the phase diagram. In this talk, I will discuss how we seek a field-stabilized spin liquid state in high magnetic fields in Ag3LiIr2O6, similar to the case of α-RuCl3 [3]. |
Tuesday, March 15, 2022 1:18PM - 1:30PM |
G51.00008: Ground state in proximity to a possible Kitaev spin liquid: The undistorted honeycomb iridate NaxIrO3 (0.60 ≤ x ≤ 0.80) Hengdi Zhao, Bing Hu, Feng Ye, Minhyea Lee, Pedro Schlottmann, Gang Cao We report the results of our study of a recently synthesized honeycomb iridate NaxIrO3 (0.60 ≤ x ≤ 0.80). Single-crystal NaxIrO3 adopts a honeycomb lattice noticeably without distortions and stacking disorder inherently existent in its sister compound Na2IrO3. The oxidation state of the Ir ion is a mixed-valence state resulting from a majority Ir5+(5d4) ion and a minority Ir6+(5d3) ion. NaxIrO3 is a Mott insulator likely with a predominant pseudospin = 1 state. It exhibits an effective moment of 1.1μB/Ir and a Curie-Weiss temperature of −19 K but with no discernible long-range order above 1 K. The physical behavior below 1 K features two prominent anomalies at Th = 0.9 K and Tl = 0.12 K in both the heat capacity and AC magnetic susceptibility. Intermediate between Th and Tl lies a pronounced temperature linearity of the heat capacity with a large slope of 77 mJ/mole/K2, a feature expected for highly correlated metals but not at all for insulators. These results along with a comparison drawn with the honeycomb lattices Na2IrO3 and (Na0.2Li0.8)2IrO3 point to an exotic ground state in proximity to a possible Kitaev spin liquid. |
Tuesday, March 15, 2022 1:30PM - 1:42PM |
G51.00009: Towards Quantum Magnetism with Pr4+ Ions: A Crystal-Field Study Jensen M Kaplan, Arun Ramanathan, Martin P Mourigal, Henry S La Pierre, Alexander I Kolesnikov The Pr4+ based honeycomb antiferromagnet Na2PrO3 has been of recent interest due to its potential to realize Kitaev interactions with rare-earth ions. In this context, the Pr4+ ion has a 4f1 configuration making it a Kramers ion. The single-ion properties of Pr4+ ions were first investigated in PrO2 and BaPrO3, which revealed the equal importance of spin-orbit coupling and crystal-field effects, reducing the separation between the 2F5/2 and 2F7/2 electronic multiplets and requiring an intermediate coupling approach. Overall, few Pr4+ compounds are known. We extended these efforts to explore the single-ion properties of Pr4+ in different compounds such as Na2PrO3, Sr2PrO4, and Li8PrO6. We will present inelastic neutron scattering data for Sr2PrO4 and Li8PrO6 and the resulting Crystal Electric Field analysis that characterize the nature of the magnetic moments in these systems. |
Tuesday, March 15, 2022 1:42PM - 1:54PM |
G51.00010: Non-collinear magnetic order in honeycomb Na2PrO3 revealed by single crystal torque measurements Kylie J MacFarquharson, Ryutaro Okuma, Radu Coldea The honeycomb antiferromagnet Na2PrO3 has recently attracted interest as a candidate to display unconventional magnetic behaviour from bond-dependent Kitaev interactions between 4f Pr4+ (jeff = 1/2) moments [1], complementary to much explored Kitaev iridates and α-RuCl3. We have succeeded in growing single crystals with no evidence for stacking faults or site mixing, and observe a sharp transition to long-range magnetic order at low temperatures, consistent with previously reported powder measurements [2]. Using highly-sensitive piezo-cantilever torque measurements on single-grain crystals we observe strongly contrasting angular dependence of the torque in orthogonal crystallographic planes, which can be naturally explained by a non-collinear, canted magnetic structure and we propose a minimal anisotropic exchange model to explain its stability, compatible with the crystal structure symmetry. |
Tuesday, March 15, 2022 1:54PM - 2:06PM |
G51.00011: Spin-phonon coupling and phonon(-Hall) transport in α-RuCl3 from first principles David Kaib, Sananda Biswas, Dominik Lessnich, Roser Valenti, Stephen Winter Recent experimental studies on α-RuCl3 have pointed to the presence of significant magnetoelastic coupling. For example, the longitudinal thermal conductivity, which is expected to be dominated by phonons, shows a strong dependence on magnetic field strength and on the presence/absence of the magnetic order. This raises the question whether the heavily discussed thermal Hall conductivity in α-RuCl3 could contain a significant contribution from a phonon-Hall effect. |
Tuesday, March 15, 2022 2:06PM - 2:18PM |
G51.00012: Field-angle anisotropy of proximate Kitaev systems under an in-plane magnetic field: Magnon contribution in the polarized phase Beom Hyun Kim α-RuCl3 is a proximate Kitaev system, in which dominant bond-directional Ising interaction (Kitaev interaction) is allowed among nearest neighboring sites in the honeycomb lattice. Its magnetic phase transition under the magnetic field has recently drawn much attention because of putative intermediate spin liquid phase. In the presence of the magnetic field, the antiferromagnetic zigzag order of α-RuCl3 is transferred into the polarized phase through the intermediate phase (IP). The non-Abelian spin liquid (NASL) phase has been proposed as the IP because of observed half-integer plateau of thermal Hall conductivity and field-angle anisotropy of specific heat under the magnetic field. |
Tuesday, March 15, 2022 2:18PM - 2:30PM |
G51.00013: Field-induced first-order topological phase transition in a Kitaev spin liquid candidate α-RuCl3 Shota Suetsugu, Ukai Yuzuki, Masaki Shimomura, Yuichi Kasahara, Hinako Murayama, Tomoya Asaba, Nobuyuki Kurita, Hidekazu Tanaka, Yuta Mizukami, Kenichiro Hashimoto, Takasada Shibauchi, Joji Nasu, Yukitoshi Motome, Yuji Matsuda The honeycomb magnet α-RuCl3 is a prime candidate for the Kitaev quantum spin liquid as revealed by the half-integer quantized thermal Hall (HIQTH) conductivity κxy in the field-induced paramagnetic state, indicating the emergence of non-abelian anyon. However, the fate of this topologically nontrivial state at intense fields remains largely elusive. Here, we report longitudinal thermal conductivity κxx in in-plane magnetic fields. Remarkably, κxx at very low temperatures exhibits a discontinuous jump at μ0H* = 11 T for bond perpendicular field direction along which the HIQTH effect is observed. We find that this H* is close to the field at which the HIQTH effect vanishes, demonstrating the emergent first-order topological phase transition. |
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