Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F46: 4d/5d Transition Metal Systems -- Perovskite IridatesFocus
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Sponsoring Units: DMP GMAG Chair: Gang Cao, University of Colorado, Boulder Room: BCEC 212 |
Tuesday, March 5, 2019 11:15AM - 11:51AM |
F46.00001: “Molecules” in solids against magnetism in 4d and 5d compounds Invited Speaker: Daniel Khomskii Close to Mott transition several novel states can appear [1]. In particular, “molecular clusters” can be formed in the solid, such as dimers, trimers, etc. [2]. Especially important for these phenomena is the role of different d-orbitals, which leads to different orbital-selective effects. In my talk I will formulate the main ideas and will illustrate such phenomena, especially dimer formation, on many examples, especially for systems with 4d and 5d electrons. The concept of orbital-selective Peierls transitions will be proposed and justified [3, 4]. In systems containing structural metal dimers there may exist in the presence of different orbitals a special state with partial formation of singlets by electrons on one orbital, while others are effectively decoupled and may give e.g. long-range magnetic order or stay paramagnetic. Similar situation can be realized in dimers spontaneously formed at structural phase transitions, which can be called orbital-selective Peierls transition [5]. Yet another consequence of this picture is that for odd number of electrons per dimer there exist competition between double exchange mechanism of ferromagnetism and the formation of singlet dimer by electron on one orbital. Such molecular states can strongly reduce and effectively suppress double exchange ferromagnetism[6]. I will discuss some implications of these phenomena, and consider examples of real systems, in which orbital-selective phase are realized. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F46.00002: Mott transition and collective charge pinning in electron doped Sr2IrO4 Kai Wang, Nimrod Bachar, Jeremie Teyssier, Weiwei Luo, Willem Rischau, Gernot Scheerer, Alberto De la Torre, Robin S. Perry, Felix Baumberger, Dirk Van Der Marel We studied the in-plane dynamic and static charge conductivity of electron doped Sr2IrO4 using optical spectroscopy and DC transport measurements. The optical conductivity indicates that the pristine material is an indirect semiconductor with a direct Mott gap of 0.55 eV. Upon substitution of 2% La per formula unit the Mott gap is suppressed except in a small fraction of the material (15%) where the gap survives, and overall the material remains insulating. Instead of a zero energy mode (or Drude peak) we observe a soft collective mode (SCM) with a broad maximum at 40 meV. Doping to 10% increases the strength of the SCM, and a zero-energy mode occurs together with metallic DC conductivity. Further increase of the La substitution doesn't change the spectral weight integral up to 3 eV. It does however result in a transfer of the SCM spectral weight to the zero-energy mode, with a corresponding reduction of the DC resistivity for all temperatures from 4 to 300 K. The presence of a zero-energy mode signals that at least part of the Fermi surface remains ungapped at low temperatures, whereas the SCM appears to be caused by pinning a collective frozen state involving part of the doped electrons. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F46.00003: electric control of the structural properties of spin-orbit coupled 5d iridate Sr2IrO4 Feng Ye, Christina Hoffmann, Wei Tian, Jieming Sheng, Hengdi Zhao, Hao Zheng, Gang Cao The unique competition between spin-orbit interactions (SOI) and Coulomb correlation in 5d elements compounds drives unusual physical behaviors. Due to the entanglement of spin and orbital degrees of freedom, the form of magnetic interactions depends on the underlying lattice geometry. In the case square-lattice Sr2IrO4, the Hamiltonian is governed by an isotropic Heisenberg plus a dipolar-like anisotropy term and leads to a weak canted antiferromagnetic order. Recent bulk measurement identified a novel coupling between the applied electric current and the canted IrO6 octahedra. This drives a large nonlinear structural response closely tracking the magnetization. Neutron diffraction study on the structural evolution reveals an anomalous response of local IrO6 distortion with respect to the applied electric current in the basal plane. Our results indicate the combination of SOI and the canted antiferromagnetic order provides a new paradigm for simultaneous electric control of the physics properties. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F46.00004: Fracturing the inter-plane magnetic correlation in Sr2IrO4 with single laser pulse Ruitang Wang, Jiaqi Lin, Haidan Wen, Derek Meyers, Jian Liu, Xuerong Liu Pump and probe type experiments have been generally performed to study the non-equilibrium dynamics of the charge, spin and lattice. Besides the generally expected full recovery to the ground states after long enough time, meta-stable states, which are not available from thermal equilibrium evolution, can emerge. We show that, upon single laser pulse pumping, the 3D AFM ordering in Sr2IrO4 is fractured into lower dimension and the system enters a meta-stable state. Such effect was studied as function of laser fluence. Our results show that the in-plane spin ordering correlation length is macroscopic (micron size), and the same before and after the pumping. While the out-of-plane correlation in the meta-stable state is quickly suppressed with the increasing of laser fluence, and saturates to be about ~1 unit cell without further reduction. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F46.00005: Relativistic Quasi-particle Nature Emerging in semi-metallic Sr2IrO4. Jeong Woo Han, Sun Woo Kim, Wonshik Kyung, Changyoung Kim, Gang Cao, Jinchen Wang, Sangmo Cheon, Jongseok Lee Whereas Dirac carrier responses have been extensively investigated in weakly-correlated-systems, it is rare to observe the Dirac physics in strongly-correlated-materials. Here, we provide experimental and theoretical evidences of an emergence of a Dirac dispersion in Tb-doped Sr2IrO4 which exhibits a semi-metallic and paramagnetic ground state. From the symmetry analyses, we find that the multiple crystal symmetries including nonsymmorphic symmetry play a pivotal role in the development of the Dirac dispersion, and furthermore, an intrinsic d-wave gap order collapse the line-nodal Dirac dispersion into the point-nodal dispersion. Linear dispersion of the energy band near the Fermi level is confirmed by angle-resolved photoemission spectroscopy, and the Dirac carrier responses are further manifested by an extremely low-scattering rate (~6 cm-1) of free carriers. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F46.00006: Evidence of resistive switching into a dynamical state in antiferromagnetic iridates Morgan Williamson, Shida Shen, Gang Cao, Jianshi Zhou, Maxim Tsoi Resistive switching, which is targeted for antiferromagnetic (AFM) memory applications [1], has recently been observed in the AFM iridates Sr2IrO4 [2] and Sr3Ir2O7 [3, 4]. Here we demonstrate that the switching state at high electrical biases displays an increased noise pattern, which is indicative of a dynamical state at high biases. We employ a spectrum analyzer to characterize the noise pattern associated with the high-bias switching state and investigate the dependence of the noise spectrum on the magnitude of applied bias and magnetic field. The emergence of the noise was found to be strongly correlated with the onset of resistive switching. The noise power density displayed a 1/f2 frequency dependence with an amplitude mimicking the hysteretic behavior of the resistance switching. We argue that the observed noise spectrum could be associated with a random switching between different states and discuss various interpretations of its origin. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F46.00007: Magnetic frustration revealed by the two-magnon Raman scattering in Sr2IrO4 and Sr3Ir2O7 Ekaterina Pärschke, Yao Wang, Thomas Devereaux, Cheng-Chien Chen Exploring the physics of spin-orbit Mott insulators such as iridates is expected to shed light onto high-temperature superconductivity as well as provide valuable insights into the interplay of spin-orbit coupling, Hund's and Coulomb interactions. Of particular interest are low-energy magnetic excitations in one- and two-layer perovskites Sr2IrO4 and Sr3Ir2O7. While one-magnon excitations in these materials are fairly well understood, their two-magnon Raman energies have been reported to vary greatly and lack simple correlation with single-magnon bandwidth. Here, we employ exact diagonalization to compute the Raman response of both systems and show that magnetic frustration and finite spin gap are responsible for the unexpectedly low two-magnon peak in Sr3Ir2O7. We also present a modified spin wave approach for an intuitive explanation of the numeric results. Our analysis highlights magnetic frustration as one of the important differences between single- and double-layer perovskites iridates and emphasizes its impact on the two-magnon Raman response in general. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F46.00008: Angle-resolved photoemission spectroscopy studies on Rh-doped Sr2IrO4 Jimin Kim, Sunwook Park, Jonathan Denlinger, Bumjoon Kim Doped Mott insulator feature a plethora of rich physics, most of which remain unclear, owing to strong correlation between electrons therein. Sr2IrO4, a spin-orbit coupled Mott insulator, is a prominent candidate to host such emergent phenomena, especially, similar to those in cuprate high-temperature superconductors. Therefore, it is worthwhile to study a phase diagram of Sr2IrO4 as a function of dopant density to unveil possible instabilities or exotic phases. To this end, we performed angle-resolved photoemission spectroscopy (ARPES) studies on hole-doped Sr2IrO4, achieved by ionic substitution of Rh for Ir, due to the relatively weak spin-orbit coupling of Rh. Obtained ARPES spectra and related phenomena will be discussed in terms of the phase diagram. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F46.00009: Correlation Effects and Hidden Spin-Orbit Entangled Electronic Order in Parent and Electron-Doped Iridates Sr2IrO4 Sen Zhou, Kun Jiang, Hua Chen, Ziqiang Wang Recent experiments discovered hidden order in the parent and electron-doped iridates Sr2IrO4, some with striking analogies to the cuprates, including Fermi surface pockets, Fermi arcs, and pseudogap. Here, we study the correlation and disorder effects in 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 twofold spin-orbital rotation followed by a lattice translation. The associated staggered circulating Jeff = 1/2 spin current can be probed by advanced techniques of spin-current detection in spintronics. 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 5, 2019 1:27PM - 1:39PM |
F46.00010: Pseudogap and electronic structure of electron-doped Sr2IrO4 Alice Moutenet, Antoine Georges, Michel Ferrero We present [1] a theoretical investigation of the effects of correlations on the electronic structure of the Mott insulator Sr2IrO4 upon electron doping. A rapid collapse of the Mott gap upon doping is found, and the electronic structure displays a strong momentum-space differentiation at low doping level: The Fermi surface consists of pockets centered around (π/2,π/2), while a pseudogap opens near (π,0). Its physical origin is shown to be related to short-range spin correlations. The pseudogap closes upon increasing doping, but a differentiated regime characterized by a modulation of the spectral intensity along the Fermi surface persists to higher doping levels. These results, obtained within the cellular dynamical mean-field-theory framework, are discussed in comparison to recent photoemission experiments and an overall good agreement is found. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F46.00011: Magnetic excitations in Sr2IrO4 measured with inelastic neutron scattering Stuart Calder, Daniel Pajerowski, Matthew Brandon Stone, Andrew May The widespread current interest in 5d materials stemmed from the observation that relativistic spin-orbit coupling drives a Mott-like insulating ground state with pseudospin Jeff=1/2 magnetic moments in the iridate compound Sr2IrO4. One surprising and enduring aspect of the physics of Sr2IrO4 is the observation of similarities to the parent unconventional cuprate La2CuO4. The degree to which this analogy holds, and if this suggests a proximate unconventional superconducting regime in Sr2IrO4, stands as an important outstanding question. In particular, a central issue in Sr2IrO4 is to understand how the strong spin-orbit coupling affects the magnetic excitations. Inelastic neutron scattering (INS) allows direct access to the magnetic excitations, but measurements have so far been lacking due to several hurdles posed by iridates. Utilizing time-of-flight INS with a gram sized single crystal array we successfully overcame these challenges. The results reveal direct evidence for two-dimensional in-plane magnetic interactions in Sr2IrO4 and allows a definition of the spin-gap. Modelling of the INS results with an isotropic 2D Hamiltonian supports a robust analogy with the physics of unconventional cuprates. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F46.00012: Ferroelectric field and magnetic field effect on the spin-orbit coupled Mott insulator Sr2IrO4 Arnoud Everhardt, Yun-Long Tang, Xiaoxi Huang, Jay LeFebvre, Shane Cybart, R Ramesh The 4d and 5d transition metals are commonly characterized by a decreased Hubbard repulsion U which diminishes correlation effects, but simultaneously by an increased spin orbit coupling to create a new type of correlation effects which have been leading to such as spin-orbit coupled Mott insulators, Weyl semimetals, axion insulators and spin liquids. This rich physics allows small perturbations to create large effects in these strongly correlated materials. The Ruddlesden-Popper series of Srx+1IrxO3x+1 shows large differences in conductive behavior, where the n=∞ perovskite SrIrO3 is metallic while the n=1 Sr2IrO4 is an insulator due to a spin-orbit coupling band splitting to a Jeff=1/2 state. This state has many similarities to the high TC cuprate superconductors which show an S=1/2 state, which loses its antiferromagnetism and becomes superconducting upon hole doping. Likewise, theoretically it is argued that under electron doping it is possible to drive Sr2IrO4 superconducting. Here a ferroelectric field effect is used on ultrathin Sr2IrO4 films to drive it to a more metallic state, by playing on the competition between electron doping and the canted antiferromagnetic transition at ~200K. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F46.00013: Effects of spin-orbit coupling on the metal-insulator transition in Sr2Ir(1-x)T(x)O4 (T = Rh,Ru) Berend Zwartsenberg, Ryan P Day, Elia Razzoli, Matteo Michiardi, Nan Xu, Ming Shi, Jonathan Denlinger, Stuart Calder, Bumjoon Kim, Hidenori Takagi, Ilya Elfimov, Andrea Damascelli We study the role of spin-orbit coupling (SOC) in the metal-insulator transition of Rh- and Ru-substituted Sr2IrO4. Although similar, Rh and Ru have significantly different effects in driving the insulator to metal transition. Whereas Rh substitution realizes a Fermi surface at concentrations of 0.16 and higher, Ru substituted samples are insulating for similar values. We study the progression of effective SOC in the Rh substituted samples through a method based on photoemission matrix elements. We find that Rh reduces the effective SOC. This interpretation is substantiated by cluster models which show that SOC mixes into an average of the two subspecies. This mixing however, is strongly dependent on orbitals having similar energies. We argue that this quantitatively explains the difference between Ru and Rh substitution: since Ru has one less electron, the orbital energies are different from Ir, hence SOC mixing is ineffective. The model thus does not only explain the observed reduction of SOC in the Rh doped samples, it also shows why this reduction does not occur in Ru doped samples, thereby demonstrating that SOC is a key parameter in describing the MIT in Rh substituted Sr2IrO4. |
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