Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session E23: Bulk IridatesFocus Session
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Sponsoring Units: GMAG DMP DCOMP Chair: Rolando Valdes Aguilar, Ohio State Univ - Columbus Room: LACC 402B |
Tuesday, March 6, 2018 8:00AM - 8:12AM |
E23.00001: Evolution of in-gap spectral weight in electron- and hole-doped Sr2IrO4 studied by time-resolved optical reflectivity Hao Chu, Alberto De la Torre, Liuyan Zhao, Nakheon Sung, B. J. Kim, Gang Cao, Jae-Suk Lee, David Hsieh The 5d transition metal oxide Sr2IrO4 realizes an unusual Jeff = 1/2 antiferromagnetic Mott insulating ground state that is stabilized by an interplay of strong spin-orbit coupling and on-site Coulomb repulsion. Motivated by its structural and magnetic similarities to La2CuO4 - the parent compound of cuprate high-Tc superconductors - there have recently been intensive efforts to study the electron-doped (Sr1-xLax)2IrO4 series and the hole-doped Sr2Ir1-xRhxO4 series. In both cases, light substitution levels trigger a collapse of long-range antiferromagnetic order and an emergence of unusual metallic states with residual pseudogaps. Here we study the doping evolution of the electronic density of states in both (Sr1-xLax)2IrO4 and Sr2Ir1-xRhxO4 using ultrafast time-resolved optical reflectivity. Our results reveal a similar development of in-gap states and a shifting of the chemical potential upon crossing over from Mott insulator to metal on both electron- and hole-doped sides of the phase diagram. |
Tuesday, March 6, 2018 8:12AM - 8:24AM |
E23.00002: Raman Spectroscopy Study of Charge Order Excitations in Metallic (Sr1-xLax)3Ir2O7 Wencan Jin, Siwen Li, Ibrahim Boulares, Zach Porter, Christi Peterson, Jianpeng Liu, Qiang Han, Stephen Wilson, Roberto Merlin, Liuyan Zhao Charge order has been universally observed in underdoped copper-oxide based high-Tc superconductors (cuprates) and shown to compete with superconductivity. In a strong spin orbit coupled cuprate analogue system, layered perovskite iridium oxides (iridates), Mott insulating, pseudogap and d-wave gap behaviors have been discovered in the single layer variant Sr2IrO4, while metal insulator transition and charge-order like Fermi surface instability has only been seen in the bilayer counterpart Sr3Ir2O7. However, a full symmetry characterization on the charge order in Sr3Ir2O7 is yet missing, making its analogy to the charge order in cuprates incomplete. In this talk, I will show our evidence of the charge order excitation and the identification of its symmetry. I will also discuss its significance in comparison with the charge order in cuprates. |
Tuesday, March 6, 2018 8:24AM - 8:36AM |
E23.00003: Phonon Renormalization by Dzyaloshinskii-Moriya Interactions in 5d Pyrochlore Iridate Y2Ir2O7 Jaeseok Son, Byungcheol Park, Choong Hyun Kim, Hwanbeom Cho, So Yeun Kim, Luke Sandilands, Je-Guen Park, Soonjae Moon, Tae Won Noh It may be possible that the optical phonon of the 5d pyrochlore iridates can be renormalized due to the Dzyloshinskii-Moriya (DM) Hamiltonian. In this study, the evidence for unconventional spin-phonon interaction in Y2Ir2O7 was identified by infrared spectroscopy and first-principles calculations. The measured infrared phonon frequencies were suddenly shifted near the Néel temperature of the exotic all-in-all-out magnetism indicating the presence of spin-phonon interaction. From calculations, this phonon renormalization was well-described as a modulation of the basis Ir-O-Ir bond angle. It was also found that the DM interaction was strongly modulated by the basis Ir-O-Ir bond angle. This indicates that DM interaction is the major origin of phonon renormalization. This spin-phonon interaction in Y2Ir2O7 is possible due to the inherently strong SOI and consequently can be realized in pyrochlore structures. This spin-phonon coupling via DM interaction is distinguished from the conventional picture of the spin-phonon coupling mechanism via super-exchange interaction. |
Tuesday, March 6, 2018 8:36AM - 8:48AM |
E23.00004: Correlation effects and hidden spin-orbit entangled electronic order in parent and electron-doped iridates Sr$_2$IrO$_4$ Ziqiang Wang, Sen Zhou, Kun Jiang, Hua Chen Analogs of the high-T$_c$ cuprates have been long sought after in transition metal oxides. Due to the strong spin-orbit coupling (SOC), the $5d$ perovskite iridates Sr$_2$IrO$_4$ exhibit a low-energy electronic structure remarkably similar to the cuprates. Recent experiments discovered hidden order in the parent and electron-doped iridates, some with striking analogies to the cuprates, including Fermi surface pockets, Fermi arcs, and pseudogap phenomena. We study the correlation and disorder effects on the spin-orbit entangled electronic states using a five-orbital model derived from the band theory. We find that the experimental observations are consistent with a $d$-wave spin-orbit density wave order that breaks the symmetry of a joint two-fold spin-orbital rotation followed by a lattice translation. There are a Berry phase and a staggered plaquette spin flux associated with circulating $J_{\rm eff}=1/2$ spin current as electrons hop between Ir atoms. This electronic order can emerge spontaneously from the intersite Coulomb interactions between the spatially extended iridium $5d$ orbitals, turning the metallic state into an electron-doped quasi-2D Dirac semimetal with important implications on the possible superconducting state suggested by recent experiments. |
Tuesday, March 6, 2018 8:48AM - 9:00AM |
E23.00005: Tunneling into Bilayer Iridate Sr3Ir2O7 Noah Bray-Ali Spin-orbit coupling in the iridates links magnetism to the flow of electric charge. We adapt the Eliashberg procedure and calculate the electron tunneling density of states in the bilayer iridate Sr3Ir2O7. Fits to measured tunneling conductance allow us to estimate the product of the electron-magnon coupling strength with the magnon density of states. We compare the results to expectations based on recent neutron and x-ray scattering from magnons in this material and find good agreement for the location of van Hove singularities in the magnon density of states. |
Tuesday, March 6, 2018 9:00AM - 9:12AM |
E23.00006: Raman Spectroscopy Study of Magnetic Excitation across Insulator Metal Transition in (Sr1-xLax)3Ir2O7 Siwen Li, Wencan Jin, Ibrahim Boulares, Roberto Merlin, Zach Porter, Christi Peterson, Stephen Wilson, Liuyan Zhao Double layer perovskite iridate Sr3Ir2O7 realizes a rare spin-orbit coupled Jeff=1/2 Mott insulating ground state. Upon electron doping by La substituting Sr, a first order insulator metal transition (IMT), together with a suppression of the long-range Jeff = ½ antiferromagnetic order, has been observed. Moreover, a charge-order like Fermi surface instability has been discovered in the metallic (Sr1-xLax)3Ir2O7. Inelastic light scattering provides a useful probe on magnetic excitations and charge ordering at the same time. In this talk, we will present our Raman spectroscopy studies on the evolution of the magnetic excitation across the IMT, from x = 0 to x = 0.04, and a temperature range of 80 K to 300 K. Furthermore, we will also discuss the relationship between the magnetic excitation and the charge order excitation. |
Tuesday, March 6, 2018 9:12AM - 9:24AM |
E23.00007: Exploring the Energy Landscape of Resistive Switching in Antiferromagnetic Sr3Ir2O7 Morgan Williamson, Shida Shen, Gang Cao, Jianshi Zhou, John Goodenough, Maxim Tsoi Antiferromagnetic (AFM) materials are expected to improve stability, scalability, and speed of magnetic memory applications thanks to the insensitivity of AFMs to magnetic fields and their high natural frequencies. Of particular interest are AFM transition metal oxides (TMO) as their properties can be tuned using various external stimuli, thus opening an entirely new dimension to the field of spintronics. We have previously demonstrated that the transport properties of AFM TMOs can be tuned by an externally applied electric field [1] – a reversible resistive switching induced by an electric bias was found in Sr3Ir2O7 and tentatively attributed to electric-field driven lattice distortions/structural transition. In this work, we probe the energy barrier associated with this transition using time-resolved measurements of the switching. We observed an exponential dependence of the switching probability on both applied bias and temperature consistent with thermal activation over an energy barrier. [1] C. Wang et al. Phys. Rev. B 92, 115136 (2015); H. Seinige et al. ibid 94, 214434 (2016) |
Tuesday, March 6, 2018 9:24AM - 9:36AM |
E23.00008: Anisotropic antiferromagnetic order in triangle lattice Ca2Sr2IrO6 Feng Ye, Jieming Sheng, Christina Hoffmann, Valentino Cooper, Satoshi Okamoto, Hao Zheng, Hengdi Zhao, Jasminka Terzic, Gang Cao The spin-orbit coupling (SOC) in the 5d iridates gives rise to a large variety of quantum phases such as Jeff=1/2 Mott insulating state. Due to the entanglement of spin and orbital degrees of freedom, the form of magnetic interactions depends closely on the underlying lattice geometry and could lead to novel magnetic ground states, for example, the weak ferromagnetism in the square lattice Sr2IrO4 with 180-degree Ir-O-Ir bond angle and proximity to the Kitaev model in the honeycomb lattice Na2IrO3 with 90-degree bond angle. We report neutron and x-ray diffraction studies of the magnetic order of the triangle lattice Ca2Sr2IrO6. In this system, individual Ir ions are well separated from each other and form nearly undistorted IrO6 octahedron. An antiferromagnetic order is observed below 13 K, with magnetic configuration breaking the 3-fold symmetry of the triangle lattice. The ordering moment is about 0.6 µB/site, larger than those of other iridates with strong connectivity between neighboring Ir ions. The anisotropic magnetic behavior highlights the important role of SOC in determining the magnetic ground state in this triangle lattice iridate. |
Tuesday, March 6, 2018 9:36AM - 9:48AM |
E23.00009: Large, non-saturating magnetoresistivity and magnetic anisotropy in the iridate BaIrO3 Hengdi Zhao, Jasminka Terzic, Hao Zheng, David Graf, Wenhai Song, Yifei Ni, Yu Zhang, Gang Cao The hexagonal BaIrO3 is an antiferromagnetic (AFM) insulator with a charge gap of 0.1 eV. It undergoes a sharp magnetic transition at 182 K with an order magnetic moment smaller than 0.1 Bohr magneton. This compound is known for decades, but its physical properties are largely unexplored. Results of our recent study of this system reveal an unusually large, non-saturating magnetoresistivity up to 36 Tesla and a strong magnetic anisotropy. In addition, a close examination of structural properties of this material, along with results of early studies, confirms the existence of a charge density wave (CDW) that may co-exist with the AFM state. We present and discuss structural, transport, magnetic, and thermal properties of this and other related compounds as functions of temperature, magnetic field, and pressure. |
Tuesday, March 6, 2018 9:48AM - 10:00AM |
E23.00010: Decoupling of the magnetic and insulating states in single-crystal (Sr1-xAx)2IrO4 (A=Ca or Ba) Jasminka Terzic, Hengdi Zhao, Hao Zheng, Wenhai Song, Yifei Ni, Yu Zhang, Gang Cao Sr2IrO4 is the archetype Jeff=1/2 Mott insulator with a Néel temperature TN = 240 K and an energy gap of 0.62 eV. Here we report results of our recent study of Ca or Ba doped Sr2IrO4. Substituting Ca2+ (up to 15%) or Ba2+ (up to 3%) ion for the Sr2+ ion provides no additional charge carriers, but alters the lattice parameters and reduces IrO6 octahedra distortions in Sr2IrO4. This study reveals that Ca or Ba doping retains the Néel temperature, however, drastically reduces the electrical resistivity by more than five orders of magnitude or even precipitates a sharp insulator-to-metal transition at lower temperatures. In essence, the vanishing insulating state due to Ca or Ba doping is accompanied by an unchanged Néel temperature. This study provides strong evidence that there is an unusual correlation between the magnetic and insulating state in this spin-orbit tuned insulator. |
Tuesday, March 6, 2018 10:00AM - 10:12AM |
E23.00011: Unidirectional spin density wave state in electron doped Sr2IrO4 Xiang Chen, Julian Schmehr, Zahirul Islam, Zach Porter, Eli Zoghlin, Kenneth Finkelstein, Jacob Ruff, Stephen Wilson Materials that exhibit both strong spin orbit coupling and appreciable electron correlation effects are predicted to host numerous new electronic states. One prominent example is the Jeff =1/2 Mott state in the monolayer iridates, where introducing electrons is predicted to manifest phase behaviors such as high temperature superconductivity analogous to the hole-doped S=1/2 Mott state of the monolayer cuprates. Here we investigate the influence of electron doping on the magnetic ground state of the monolayer spin-orbit assisted Mott material Sr2IrO4. By utilizing resonant x-ray scattering our data establish the gradual transition from the long-range antiferromagnetic order intrinsic the unperturbed parent state into to short-range antiferromagnetic state under the light electron substitution. Coincident with the doping threshold where a coherent Fermi surface is established, we observe the formation of a unidirectional spin density wave phase modulated along the Ir-Ir bond diagonal that coexists with a short-range antiferromagnetic background. The potential origins of this phase will be discussed, and our results support the conjecture that the partially quenched Mott phases in electron-doped Sr2IrO4 and hole-doped La2CuO4 share common competing electronic states. |
Tuesday, March 6, 2018 10:12AM - 10:24AM |
E23.00012: A systematic transport and thermodynamic study of metallic states in single-crystal iridates Yifei Ni, Hao Zheng, Hengdi Zhao, Jasminka Terzic, Wenhai Song, Yu Zhang, Gang Cao It is widely recognized that a combined effect of strong spin-orbit and on-site Coulomb interactions generally causes an insulating ground state in most iridates. The Jeff = ½ Mott state first observed in Sr2IrO4 is a unique, intriguing consequence of such an effect and has been extensively studied in recent years. This work, as an extension of our research on iridates, focuses on metallic states in iridates that either naturally occur (e.g., in Bi2Ir2O7, SrIrO3, etc.), or are induced via chemical substitution (e.g., in La doped Sr2IrO4, Sr doped BaIrO3, etc.). We emphasize transport and thermodynamic properties of metallic states for a wide range of temperature from 50 mK to 1000 K as functions of magnetic field and pressure. These properties along with empirical trends in a group of metallic iridates will be summarized and discussed. |
Tuesday, March 6, 2018 10:24AM - 10:36AM |
E23.00013: Magnetization reversal and metal-insulator transition in rare earth doped (Sr1-xPrx)2IrO4* Yu Zhang, Jasminka Terzic, Hengdi Zhao, Hao Zheng, Wenhai Song, Yifei Ni, Gang Cao Sr2IrO4 is a spin-orbit-coupled insulator with an antiferromagnetic transition TN = 240 K. Due to delicate balance of energies that drive the unique ground state, chemical doping has strong effect on physical properties of iridates. We will discuss results of our recent study of Pr doped Sr2IrO4 and will compare those with our earlier work on Tb doping of the same compound. Substituting Pr3+ ion for the Sr2+ ion doesn’t alter the lattice parameters significantly, however even with slight Pr doping it induces several orders of magnitude reduction in resistivity as well as suppression of antiferromagnetic transition to lower temperatures. In addition to suppression of magnetic ordering temperature, field cooled magnetization reveals magnetization reversal below 30K which cannot be observed in zero-field cool magnetization. This behavior is quite different from that of Tb doped Sr2IrO4 and the comparison between the two systems will be discussed. |
Tuesday, March 6, 2018 10:36AM - 10:48AM |
E23.00014: Optical second harmonic generation study of an insulating parent cuprate Alberto De la Torre, Liuyan Zhao, Biqiong Yu, Martin Greven, David Hsieh Signatures of bulk inversion symmetry breaking have recently been reported across the pseudogap transition temperature in under-, optimal- and over-doped YBa2Cu3Oy single crystals using optical second harmonic generation rotational anisotropy (SHG-RA) measurements [1]. However it is presently unknown how this signal evolves upon approaching the un-doped insulating parent compound. Here we report a temperature dependent SHG-RA study of the parent cuprate Sr2CuO2Cl2 and discuss its relationship to recent reports of inversion symmetry breaking in the Mott insulator Sr2IrO4 [2], a strongly spin-orbit coupled analogue of La2CuO4. |
Tuesday, March 6, 2018 10:48AM - 11:00AM |
E23.00015: Strain Effect on Low Temperature Hall Measurements of Thin Film SrIrO3 Neil Campbell, Trevor Anderson, Tianxiang Nan, Chang-Beom Eom, Mark Rzchowski 5d transition-metal oxides have been of interest due to their strong spin-orbit interaction, and proposed Dirac crossings in the electronic structure. Such materials, including SrIrO3, show large changes in electronic structure and properties due to strain. Epitaxial metastable perovskite SrIrO3 thin films were grown by pulsed laser deposition with in situ high-pressure RHEED on various perovskite oxide substrates producing epitaxial strain from -2 to 1%. We find that the room temperature SrIrO3 resistivity increases by up to 3x with 3% change in strain. At low temperatures the Hall coefficient changes drastically. This low-temperature Hall signal is increasingly nonlinear with tensile strain, and the Hall resistivity does not scale with thickness. A multiband model reproducing this nonlinearity requires two electron bands, in disagreement with ARPES studies. We therefore propose a magnetic origin and consider the role of octahedral rotations on the electronic transport properties of SrIrO3 thin films. |
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