Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session Y10: 4d/5d materials IIFocus

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Sponsoring Units: DMP Chair: Natalia Perkins, Univ of Minnesota  Twin Cities Room: LACC 301B 
Friday, March 9, 2018 11:15AM  11:51AM 
Y10.00001: Comprehensive Study of the Dynamics of a Classical Kitaev Spin Liquid Invited Speaker: Cristian Batista We will discuss the differences and similarities in the dynamical structure factors of the spin1/2 and the classical Kitaev liquids [1]. The quantum behavior is restricted to low temperatures where a gap protects visons from decohering the system. Once this quantum gap is breached, at low temperatures compared to the coupling constant, significant entropic disorder decoheres the Majorana fermions and the system is described quantitatively by classical dynamics. The lowtemperature and lowenergy spectrum of the classical model exhibits a finiteenergy peak, which is the precursor of the one produced by the Majorana modes of the S=1/2 model. The classical peak is spectrally narrowed compared to the quantum result and can be explained by magnon excitations within fluctuating onedimensional manifolds (loops). Hence the difference from the classical limit to the quantum limit can be understood by the fractionalization of magnons propagating in onedimensional manifolds. Moreover, we show that the momentumspace distribution of the lowenergy spectral weight of the S=1/2 model follows the distribution of zero modes of the classical model. 
Friday, March 9, 2018 11:51AM  12:03PM 
Y10.00002: Electron Doping in Sr_{3}Ir_{2}O_{7}: MetalInsulator Transition and Collapse of Magnetic Order Michael Swift, Zach Porter, Stephen Wilson, Chris Van de Walle The spinorbitassisted Mott insulator Sr_{3}Ir_{2}O_{7 }undergoes a phase transition from an antiferromagnetic insulator into a paramagnetic metal via electron doping. We present the results of ab initio electronic structure calculations which identify the transition and provide insight into its origins. Our results describe both the paramagnetic metallic state and the antiferromagnetic insulating state. The charge gap and staggered magnetization are observed to decrease with increasing doping, and a metalinsulator phase transition occurs at an electron concentration corresponding to 4.8% substitution of Sr with La, in good agreement with experiment. We also describe the structural effects of La doping in Sr_{3}Ir_{2}O_{7 }as a competition between deformationpotential effects driven by electron doping and steric effects from the substitution of smaller La atoms. Curiously, our firstprinciples calculations fail to capture the lowtemperature structural distortion reported in the metallic regime, supporting the notion that this distortion arises as a secondary manifestation of an unconventional electronic order parameter in this material. 
Friday, March 9, 2018 12:03PM  12:15PM 
Y10.00003: Observation of halfinteger thermal Hall conductance in a Kitaev quantum magnet Yuichi Kasahara, Kaori Sugii, Takafumi Ohnishi, Masaaki Shimozawa, Minoru Yamashita, Nobuyuki Kurita, Hidekazu Tanaka, Joji Nasu, Yukitoshi Motome, Takasada Shibauchi, Yuji Matsuda The systems composed of interacting 1/2 spins on a honeycomb lattice with bonddirectional exchange interactions are of vital interest, as they host a ground state of Kitaev quantum spin liquid (QSL), where spins are fractionalized into emergent Majorana fermions. Here we report on the measurements of thermal Hall effect in a candidate Kitaev magnet αRuCl_{3}. Although αRuCl_{3} exhibits antiferromagnetic (AFM) order in zero field, the application of inplane magnetic field melts the AFM order, leading to the Kitaev QSL. We discover that in the Kitaev QSL state the 2D thermal Hall conductance reaches a quantum plateau as a function of applied magnetic field, which is exactly half of the quantization value reported for the integer quantum Hall system. The observed halfinteger thermal Hall conductance in a bulk material is a direct signature of topologically protected chiral edge current of Majorana fermion. The thermal conductivity vanishes rapidly upon entering the forced ferromagnetic state, suggesting the topological quantum phase transition between the states with and without chiral Majorana edge states. 
Friday, March 9, 2018 12:15PM  12:27PM 
Y10.00004: Tuning the electronic properties of SrIrO_{3} by epitaxial constraints Danilo Puggioni, Hongbin Zhang, Jian Liu, James Rondinelli 5d transition metal oxides provide an ideal playground where strong crystal field splitting encounters strong spinorbit coupling. Here, using first principles density functional theory (DFT) calculations, we study the strain responses of the atomic structure and electronic properties in the correlated perovskite metal SrIrO_{3}. We examine the proposed breakdown of the J_{eff} states using DFT calculations compared with available experimental data. We find that although DFT and DFT+U methods qualitatively reproduce the iridate band structure, these methods result in incorrect orbital projections, which lead to ambiguity of the J_{eff} picture. Our results suggest that epitaxial strain is a useful way for tailoring the 5d spinorbit coupling in perovskite iridates. 
Friday, March 9, 2018 12:27PM  12:39PM 
Y10.00005: Ultrafast Magnetic Control in the Spinorbit Coupled Iridate Sr_{2}IrO_{4} GuFeng Zhang, Jingdi Zhang, Xiang Chen, Stephen Wilson, Richard Averitt The spinorbit coupled Mott insulator Sr_{2}IrO_{4} has attracted considerable attention because of its exotic J_{eff}=1/2 Mott state arising from the interplay of onsite Coulomb repulsion and strong spinorbit coupling. It is of interest to investigate how the magnetization dynamically evolves in response to electromagnetic excitation. We measured the dynamic properties of the J_{eff}=1/2 Mott state using strong subgap excitation (THz and midIR), monitoring the induced Kerr signal. THzinduced Kerr rotation is observed below T_{N} (230K), following the THz pulse shape. Notably, for midIR circularly polarized excitation, we observe a strong temperature dependent Kerr rotation below 100K. We will discuss the possible origin of the observed dynamics in this intriguing material. 
Friday, March 9, 2018 12:39PM  12:51PM 
Y10.00006: Resonant Inelastic Xray Scattering Study of Metallic Sr3(Ir1xRux)2O7 Julian Schmehr, Michael Aling, Mary Upton, Stephen Wilson Sr3Ir2O7 is a correlated Jeff=1/2 Mott insulator which is extremely susceptible to perturbation by chemical substitution. Doping with Ru on the Bsite leads to a percolative metalinsulator transition near 33% Rusubstitution, but antiferromagnetic order persists well into the metallic phase with a pronounced maximum of TN around 45% doping (Dhital et al., Nat. Commun. 5, 3377 (2014)). Here we report resonant inelastic xray scattering results on Sr3(Ir1xRux)2O7 samples within the metallic antiferromagnetic regime. We observe that spin wave excitations reminiscent of the undoped parent compound survive well into the metallic phase. The bandwidth is reduced as expected for the case where disorder quenches longerrange exchange interactions; however the zone center spin gap is strongly increased with continued Ru substitution. Potential explanations for this behavior and their implications for the exchange in the parent system are discussed. 
Friday, March 9, 2018 12:51PM  1:03PM 
Y10.00007: DFT(+U+SOC) Investigation of Na_{2}IrO_{3} Daniel Eth, Vidvuds Ozolins We report on DFT(+U+SOC) calculations of Na_{2}IrO_{3}. We find that miniscule perturbations to the crystal structure often lead to significantly different relaxed energies and band gaps – corresponding to different combinations of Ir 5d orbitals. The tendency of DFT+U to get trapped in such local energy minima suggests an explanation for discrepancies in the literature. Comparing ground state solutions with and without U and SOC shows that inclusion of SOC is necessary and sufficient to stabilize the experimental zigzag order and to open a band gap, casting doubt on the theory that Na_{2}IrO_{3} is a Mott insulator. Investigating the band structures yields support for the quasimolecular orbital theory over the J_{eff} = 1/2 model. 
Friday, March 9, 2018 1:03PM  1:15PM 
Y10.00008: Near Ideal Realization of the Jeff = ½ State in Ir Antifluorite Compounds. Dalmau ReigiPlessis, Adam Aczel, Patrick Clancy, Jacob Ruff, Mary Upton, Greg MacDougall Strong spin orbit coupling in 5d materials can lead to a particular type of Mott physics. For a 5d atom with octahedral coordination, the triply degenerate t_{2g} state will split into the occupied J_{eff} = 3/2 and the partially occupied J_{eff} = ½ states. It is this J_{eff} = ½ state that can host a large range of exotic phases such as quantum spin liquids and superconductivity. However it is debated how close many of the currently studied J_{eff} = ½ Ir compounds are to this limit. In many cases, noncubic crystal fields are of the same energy scale as the spin orbit coupling and cause the J_{eff} = ½ and the J_{eff} = 3/2 states to mix. We present data on a several Iridium halide materials, M_{2}IrX_{6} M = K, Na and NH_{3} and X = Cl, Br, which have the antifluorite structure and the Ir atom inside separated halide octahedra. We present resonant inelastic Xray scattering and Xray absorption data that show record low splitting of the t_{2g} orbitals, suggesting that these materials are a better realization of the Jeff = ½ state than any previously studied material. We combine these results with neutron scattering and muon spin spectroscopy to additionally explore their magnetic states. 
Friday, March 9, 2018 1:15PM  1:51PM 
Y10.00009: Exotic honeycomb magnets with strong spinorbit coupling Invited Speaker: Tomohiro Takayama Honeycomb is a bipartite lattice and free from geometrical frustration. Nevertheless, exotic magnetic ground states have been predicted in honeycombbased transitionmetal oxides with strong spinorbit coupling. We will present two honeycombbased materials realizing such unconventional magnetism. 
Friday, March 9, 2018 1:51PM  2:03PM 
Y10.00010: A Theory for PumpProbe Resonant Inelastic Xray scattering Yuan Chen, Yao Wang, Chunjing Jia, Brian Moritz, Thomas Devereaux Nonequilibrium approaches are widely used as powerful techniques in condensed matter physics to characterize the evolution of excitations. In recent years, resonant inelastic Xray scattering (RIXS) has become an important tool to investigate specific collective excitations in correlated materials with momentum and energy resolution. Combined with the progress in ultrafast instrumentation, timeresolved RIXS (trRIXS) holds the promise to detect nonequilibrium dynamics of numerous collective modes, which play significant roles in emergent phenomena. Here we present a theoretical and numerical study of trRIXS. Through the evaluation of trRIXS for a few important physical models, we show that trRIXS has the capability to elucidate the dynamics of the particlehole excitations. As a function of pump intensity, frequency, and time, trRIXS exhibits Floquet band replicas and renormalization even at this multiparticle level. The evolution of other collective excitations coupled to charge degrees of freedom can be revealed in the spectra, with distinct modulation and relaxation due to the corehole potential in the intermediate state. 
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