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 spin-1/2 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 low-temperature and low-energy spectrum of the classical model exhibits a finite-energy 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 one-dimensional manifolds (loops). Hence the difference from the classical limit to the quantum limit can be understood by the fractionalization of magnons propagating in one-dimensional manifolds. Moreover, we show that the momentum-space distribution of the low-energy 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 Sr3Ir2O7: Metal-Insulator Transition and Collapse of Magnetic Order Michael Swift, Zach Porter, Stephen Wilson, Chris Van de Walle The spin-orbit-assisted Mott insulator Sr3Ir2O7 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 metal-insulator 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 Sr3Ir2O7 as a competition between deformation-potential effects driven by electron doping and steric effects from the substitution of smaller La atoms. Curiously, our first-principles calculations fail to capture the low-temperature 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 half-integer 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 bond-directional 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 α-RuCl3. Although α-RuCl3 exhibits antiferromagnetic (AFM) order in zero field, the application of in-plane 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 half-integer 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 SrIrO3 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 spin-orbit 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 SrIrO3. We examine the proposed breakdown of the Jeff 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 Jeff picture. Our results suggest that epitaxial strain is a useful way for tailoring the 5d spin-orbit coupling in perovskite iridates. |
Friday, March 9, 2018 12:27PM - 12:39PM |
Y10.00005: Ultrafast Magnetic Control in the Spin-orbit Coupled Iridate Sr2IrO4 Gu-Feng Zhang, Jingdi Zhang, Xiang Chen, Stephen Wilson, Richard Averitt The spin-orbit coupled Mott insulator Sr2IrO4 has attracted considerable attention because of its exotic Jeff=1/2 Mott state arising from the interplay of on-site Coulomb repulsion and strong spin-orbit coupling. It is of interest to investigate how the magnetization dynamically evolves in response to electromagnetic excitation. We measured the dynamic properties of the Jeff=1/2 Mott state using strong sub-gap excitation (THz and mid-IR), monitoring the induced Kerr signal. THz-induced Kerr rotation is observed below TN (230K), following the THz pulse shape. Notably, for mid-IR 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 X-ray Scattering Study of Metallic Sr3(Ir1-xRux)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 B-site leads to a percolative metal-insulator transition near 33% Ru-substitution, 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 x-ray scattering results on Sr3(Ir1-xRux)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 longer-range 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 Na2IrO3 Daniel Eth, Vidvuds Ozolins We report on DFT(+U+SOC) calculations of Na2IrO3. 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 Na2IrO3 is a Mott insulator. Investigating the band structures yields support for the quasi-molecular orbital theory over the Jeff = 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 Reig-i-Plessis, 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 t2g state will split into the occupied Jeff = 3/2 and the partially occupied Jeff = ½ states. It is this Jeff = 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 Jeff = ½ Ir compounds are to this limit. In many cases, non-cubic crystal fields are of the same energy scale as the spin orbit coupling and cause the Jeff = ½ and the Jeff = 3/2 states to mix. We present data on a several Iridium halide materials, M2IrX6 M = K, Na and NH3 and X = Cl, Br, which have the antifluorite structure and the Ir atom inside separated halide octahedra. We present resonant inelastic X-ray scattering and X-ray absorption data that show record low splitting of the t2g 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 spin-orbit coupling Invited Speaker: Tomohiro Takayama Honeycomb is a bipartite lattice and free from geometrical frustration. Nevertheless, exotic magnetic ground states have been predicted in honeycomb-based transition-metal oxides with strong spin-orbit coupling. We will present two honeycomb-based materials realizing such unconventional magnetism. |
Friday, March 9, 2018 1:51PM - 2:03PM |
Y10.00010: A Theory for Pump-Probe Resonant Inelastic X-ray 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 X-ray 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, time-resolved RIXS (tr-RIXS) 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 tr-RIXS. Through the evaluation of tr-RIXS for a few important physical models, we show that tr-RIXS has the capability to elucidate the dynamics of the particle-hole excitations. As a function of pump intensity, frequency, and time, tr-RIXS exhibits Floquet band replicas and renormalization even at this multi-particle 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 core-hole potential in the intermediate state. |
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