Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W32: Focus Session: Iridates |
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Sponsoring Units: GMAG DMP Chair: Sae-Hwan Chun, Argonne National Laboratory Room: 207B |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W32.00001: Temperature and bias dependence of anisotropic magnetoresistance in antiferromagnetic Sr$_{2}$IrO$_{4}$ Heidi Seinige, Cheng Wang, Gang Cao, Jian-Shi Zhou, John B. Goodenough, Maxim Tsoi We study anisotropic magnetoresistance (AMR) in antiferromagnetic (AFM) Mott insulator Sr$_{2}$IrO$_{4}$ [1]. Such AMR is a promising candidate for monitoring the magnetic order parameter in AFM spintronics. Here we present temperature- and electrical bias-dependent measurements of the point-contact AMR in single crystals of Sr$_{2}$IrO$_{4}$. The point-contact technique allows to probe very small volumes and, therefore, look for electronic transport in Sr$_{2}$IrO$_{4}$ on a microscopic scale. Point-contact measurements at liquid nitrogen temperature revealed a large negative magnetoresistance (MR) for magnetic fields applied within IrO$_{2}$ a-b plane and electric currents flowing perpendicular to the plane. The observed MR decreases with increasing temperature and falls to zero at T$_{\mathrm{N\acute{e}el}}$ $\sim$ 240 K. Interestingly, the temperature dependence of MR ratios differs qualitatively from that of the resistivity. The point-contact measurements also show a strong dependence of MR on the dc bias applied to the contact. The latter can be associated with correlations between electronic transport and magnetic order in Sr$_{2}$IrO$_{4}$.\\[4pt] [1] C. Wang et al., Phys. Rev. X, November 2014. [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W32.00002: The Evolution of Electronic Structure in Electron and Hole-Doped Sr2IrO4 Yue Cao, Xiuwen Zhang, Haoxiang Li, Xiaoqing Zhou, Rajendra Dhaka, Nicholas Plumb, Tongfei Qi, Jasminka Terzic, Alex Zunger, Gang Cao, D. S. Dessau How the electronic structure evolves in doped Mott insulators remains debated after decades of study, and affects the interpretations of many bulk and spectroscopic properties, including dc-conductance, quantum oscillations, etc. The recent discovery of the spin-orbital coupled J$=$1/2 Mott insulator Sr2IrO4 provides a new perspective into the above question. Combining angle-resolved photoemission spectroscopy and first-principles calculations, we present a unified description how the band dispersion, Fermi surface, chemical potential, and Mott gap changes in electron and hole doped Sr2IrO4. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W32.00003: Electrically tunable transport in antiferromagnetic Sr$_{2}$IrO$_{4}$ Cheng Wang, Heidi Seinige, Gang Cao, Jian-Shi Zhou, John B. Goodenough, Maxim Tsoi Electronic transport in antiferromagnetic (AFM) Mott insulator Sr$_{2}$IrO$_{4}$ is studied under high electric fields. Our goal is to address the question of electronic conduction in nano-scale AFM spintronic applications [1] where high biases and associated electric fields are routinely present. We use nano-scale contacts between a sharpened Cu tip and single crystal of Sr$_{2}$IrO$_{4}$ to achieve electric fields up to a few MV/m. When an electrical bias is applied to such a point contact, the electric potential drops essentially in a small contact region, thus leading to high electric fields and providing a means to probe electronic transport on a microscopic scale. Detailed measurements of point-contact current-voltage characteristics revealed that the contact resistance decreased significantly (50-70{\%}) with an increasing dc bias. The observed bias dependence can be well fitted by an activation energy model that involves band structure modifications under strong electric fields. Our findings suggest a promising path towards band-gap engineering in 5d transition metal oxides, which may lead to appealing technical solutions in developing next generation's electronic devices.\\[4pt] [1] C. Wang et al., Phys. Rev. X, November 2014. [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W32.00004: A hidden non-dipolar magnetic order parameter in Sr$_{2}$IrO$_{4}$ observed using nonlinear optical measurements Liuyan Zhao, Vsevolod Ivanov, Darius Torchinsky, Hao Chu, Ron Lifshitz, Rebecca Flint, Tongfei Qi, Gang Cao, David Hsieh Iridium oxides are predicted to host a variety of exotic electronic phases arising from the interplay of electron correlations and spin-orbit coupling. There is particular interest in Sr$_{2}$IrO$_{4}$ owing to its striking structural and electronic similarities to the parent compound of high-Tc cuprates La$_{2}$CuO$_{4}$, which is further strengthened by the recent observation of Fermi arcs with a pseudogap behavior in doped Sr$_{2}$IrO$_{4}$ [1]. In this talk we report evidence of a previously hidden non-dipolar order parameter in Sr$_{2}$IrO$_{4}$ using low temperature nonlinear optical generation techniques [2,3]. We will discuss the significance of this novel order parameter in the context of cuprate high-Tc superconductivity and present comparative studies on non-perovskite families of iridium oxides. \\[4pt] [1] Y.K. Kim et. al, Science, 345, 187(2014)\\[0pt] [2] D. H. Torchinsky et. al, Rev. Sci. Instrum. 85, 083102(2014)\\[0pt] [3] D. H. Torchinsky et. al, arXiv 1411.0783(2014) [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W32.00005: Fermi arcs vs Fermi pockets in Perovskite Iridates: A first principles study Hasnain Hafiz, Junfeng He, T.R. Mion, T. Hogan, C. Dhital, X. Chen, Q. Lin, M. Hashimoto, D.H. Lu, Y. Zhang, R.S. Markiewicz, S.D. Wilson, Rui-hua He, A. Bansil The Ruddlesden-Popper series of iridates (Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$) and 3d transition metal copper oxides (cuprates) share key features in their structural and electronic properties. A recent angle-resolved photoemission (ARPES) study\footnote{Science 345, 187-190 (2014)} of electron-doped Sr$_{2}$IrO$_{4}$ reported similar features in underdoped cuprates, including the presence of disconnected gapless segments of the Fermi surface or Fermi arcs. We report first principles calculations on electron-doped Sr$_{3}$Ir$_{2}$O$_{7}$ to gain insight into the evolution of the Fermi surface with doping. We discuss Fermi arcs and Fermi pockets in single and bilayer iridates, and show that even though Fermi arcs are a generic feature of the cuprates, this is not the case in the iridates. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W32.00006: $^{23}$Na and $^{17}$O NMR studies of hyperkagome Na$_4$Ir$_3$O$_8$ Abigail Shockley, Fabrice Bert, Jean-Christophe Orain, Yoshihiko Okamoto, Philippe Mendels Na$_4$Ir$_3$O$_8$ is a unique case of a 3D corner sharing triangular lattice which can be decorated with quantum spins. It has spurred a lot of theoretical interest as a spin liquid candidate of a new kind where the Hamiltonian might not be thought in terms of a simple Heisenberg case because of spin orbit coupling on the Ir 5d element. We present a comprehensive set of NMR data taken on both the $^{23}$Na and $^{17}$O sites. We have found that magnetic freezing of all Ir sites sets in below $T_f\sim$ 7.5K $\sim 0.019J$ with a clear hyperfine field transferred from Ir moments and a drastic decrease of $1/T_1$ . Above $T_f$, physical properties are expected to be a landmark of frustration in this exotic geometry. We will discuss our shift and relaxation data in the temperature range of 300K to 7.5 K in the light of published thermodynamic measurements (Y. Okamotoa et al, PRL 99 137207, 2007 and Y. Singh et al, PRB 88 220413(R), 2013) and comment on their implications for the already existing large body of theoretical work. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W32.00007: 55~Tesla coercive magnetic field in frustrated Sr$_3$NiIrO$_6$ John Singleton, Jae-Wook Kim, Craig Topping, Anders Hansen, Eun-Deok Mun, Saman Ghannadzadeh, Paul Goddard, Xuan Luo, Yoon Seok Oh, Sang-Wook Cheong, Vivien Zapf We have measured extremely large coercive magnetic fields of up to 55~T in Sr$_3$NiIrO$_6$, with a switched magnetic moment $\approx 0.8~\mu_{\rm B}$ per formula unit. As far as we are aware, this is the largest coercive field observed thus far. This extraordinarily hard magnetism has a completely different origin from that found in conventional ferromagnets. Instead, it is due to the evolution of a frustrated antiferromagnetic state in the presence of strong magnetocrystalline anisotropy due to the overlap of spatially-extended Ir$^{4+}$ 5$d$ orbitals with oxygen 2$p$ and Ni$^{2+}$ 3$d$ orbitals. This work highlights the unusual physics that can result from combining the extended $5d$ orbitals in Ir$^{4+}$ with the frustrated behaviour of triangular lattice antiferromagnets. [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W32.00008: Canted Antiferromagnetism and Unusual Intermediate Phase in Double Perovskite Iridates (La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$)$_{2}$ZnIrO$_{6}$ Wenka Zhu, Wei Tong, Chi-Ken Lu, Jinmei Wang, Shixiong Zhang Iridates represent a unique material system that possesses both strong spin-orbit coupling (SOC) and electron correlation. The interplay between SOC and correlation could facilitate the emergence of novel electronic and magnetic states. In this work, we have performed a systematic study of the magnetic properties of a double perovskite iridate La$_{2}$ZnIrO$_{6}$ and its hole-doped compounds (La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$)$_{2}$ZnIrO$_{6}$ via dc magnetization measurements and electron spin resonance (ESR) spectroscopy. The magnetic ground state of La$_{2}$ZnIrO$_{6}$ was demonstrated to be a canted antiferromagnetic (AFM) phase based on the observation of magnetic hysteresis loops and antiferromagnetic resonance in the ESR spectra. The nature of the canted AFM state can be understood with the Heisenberg exchange model plus the Dzyaloshinskii-Moriya interaction. Additionally, an intermediate superparamagnetic-like phase was observed between the high-temperature paramagnetic state and the low-temperature canted-AFM state. With the introduction of Ir$^{5+}$ by Sr doping, the AFM interaction is weakened accompanied by an enhancement of electrical conductivity. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W32.00009: Mapping the phase diagram of (Sr$_{1-x}$ La$_x$)$_{3}$ Ir$_2$ O $_7$ Tom Hogan, Zahra Yamani, Zac Ward, Stephen Wilson Here we present an experimental study of the (Sr$_{1-x}$ La$_x$)$_{3}$ Ir$_2$ O $_7$ phase diagram, exploring the evolution of magnetic, charge, and structural degrees of freedom as the system undergoes a rapid insulator-to-metal phase transition. The parent J$_{eff}$=$\frac{1}{2}$ Mott state melts, revealing a first order-like phase boundary between a localized antiferromagnetic metallic state and a correlated metal. The evidence of a rich interplay of correlated effects upon doping the bilayer Sr$_{3}$Ir$_{2}$O$_{7}$ will be discussed and presents an argument for the persistent influence of correlation physics once the metallic regime is reached in this and related spin-orbit Mott materials. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W32.00010: Electronic Structure near Metal-Insulator Transition in Sr$_{3-x}$La$_{x}$Ir$_{2}$O$_{7}$ Gregory Affeldt, Tom Hogan, Christopher Smallwood, Tanmoy Das, Sung-Kwan Mo, Stephen Wilson, Alessandra Lanzara The bilayer perovskite iridate Sr$_3$Ir$_2$O$_7$ exhibits an insulating state at low temperature driven by the cooperation of spin-orbit coupling and moderate Coulomb correlations. Transport measurements have shown a metal-insulator transition with electron doping in Sr$_{3-x}$La$_{x}$Ir$_{2}$O$_{7}$ near x = 0.12. We will show how the electronic structure evolves through the metal to insulator transition and discuss these results in terms of strong correlations and how these evolve with doping. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W32.00011: Direct Observation of Magnetic Frustration via Bond-directional Interactions in a Honeycomb Lattice Iridate Na$_{2}$IrO$_{3}$ Sae Hwan Chun, H. Zheng, C. Stoumpos, C. Malliakas, J.F. Mitchell, Jong-Woo Kim, Jungho Kim, Y. Choi, T. Gog, Kavita Mehlawat, Yogesh Singh, A. Al-Zein, M. Morreti Sala, M. Krisch, J. Chaloupka, G. Jackeli, G. Khaliullin, B.J. Kim Despite its long-range zigzag magnetic ground state, the honeycomb Na2IrO3 is considered as a model system for approaching a Kitaev quantum spin liquid due to a proposed bond-directional magnetic frustration. Using resonant x-ray magnetic scattering, we find direct evidence of this frustration and follow its temperature dependence. We observe that three zigzag magnetic states with short-range correlation are displayed in the diffuse magnetic peaks as a function of scattering geometry up to 6T$_{\mathrm{N}}$. Each zigzag state breaks C3 symmetry individually, but the simultaneous C3 rotation in both real and spin spaces remains invariant as consequence of the distinct magnetic anisotropies tied to three Ir-Ir bonds in the lattice. This result confirms the dominant role of the bond-directional interactions in the frustration. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W32.00012: Short-range correlations in the magnetic ground state of $Na_{4} Ir_{3} O_{8} $ Rebecca Dally, Tom Hogan, Alex Amato, Hubertus Luetkens, Chris Baines, Jose Rodriguez-Rivera, Michael Graf, Stephen Wilson The magnetic ground state of the candidate three-dimensional quantum spin liquid $Na_{4} Ir_{3} O_{8} $ has been studied through bulk magnetization, muon spin relaxation and neutron scattering measurements. $Na_{4} Ir_{3} O_{8} $ possesses a unique hyper-Kagome lattice of Ir moments that is potentially accompanied by a novel realization of Heisenberg-Kitaev exchange. This fact combined with the absence of previously reported magnetic ordering has led to its candidacy as a three-dimensional quantum spin liquid. Our combined experimental data show that a short-range, frozen, ground state comprised of quasi-static moments develops in this material below a characteristic temperature $T_{F} =6K$, persisting down until at least 20 mK. The expected dynamical ground state of a quantum spin liquid was not observed but rather an inhomogeneous quasi-static spin state that survives with persistent long timescale fluctuations. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W32.00013: Ultrafast reflectivity dynamics in the honeycomb iridates James Hinton, Shreyas Patankar, Eric Thewalt, Jake Koralek, Alejandro Ruiz, Gilbert Lopez, Nicholas Breznay, James Analytis, Joseph Orenstein The combination of strong spin orbit coupling and Mott physics in the iridium oxides produces a variety of interesting phenomena. In the A$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ compounds, this is thought to give rise to spin-anisotropic magnetic interactions described by the Kitaev model. While Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ displays simple zigzag antiferromagnetism, the complex, incommensurate spiral magnetic order observed in $\gamma $-Li$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ suggests that Kitaev exchange is the dominant spin interaction in this system. In this work, we performed ultrafast pump-probe reflectivity measurements on single crystals of Na$_{\mathrm{2}}$IrO$_{\mathrm{3\thinspace }}$ and $\gamma $-Li$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ using nJ pulses at 1.5 eV photon energy. At high temperatures, we observe a reflectivity transient corresponding to electronic heating and excitation across the Mott gap which is isotropic with respect to probe polarization. In $\gamma $-Li$_{\mathrm{2}}$IrO$_{\mathrm{3}}$ , a small anisotropy emerges upon cooling close to the Ne\'{e}l transition at 38 K, followed by an abrupt onset of a long lived, highly anisotropic photo-induced increase in the reflectivity at T$_{\mathrm{N}}$. The temperature dependent dynamics of this signal indicate that it corresponds to non-thermal destruction of the magnetic order. Although similar reflectivity dynamics are observed at low temperature in Na$_{\mathrm{2}}$IrO$_{\mathrm{3}}$, there are no clear features related to the Ne\'{e}l transition at 16 K. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W32.00014: Magnetic order in the frustrated Ising-like chain compound Sr$_3$NiIrO$_6$ E. Lefran\c{c}ois, L.C. Chapon, V. Simonet, P. Lejay, R. Ballou, S. Rayaprol, E.V. Sampathkumaran, D. Khalyavin, D.T. Adroja Oxides of the family A$_3$MM'O$_6$ (A = alkaline-earth metal, M, M'= transition metal) attracted a lot of attention because of their unconventional magnetic properties due to the interplay between low dimensionality, magnetic frustration and magnetocrystalline anisotropy. In these compounds, the M and M' ions form chains which are distributed on a triangular lattice. We studied the 5d-based system Sr$_3$NiIrO$_6$, which is in the strong spin-orbit coupling regime, by single crystal magnetization measurements and neutron powder diffraction. The magnetization revealed a large easy-axis of anisotropy confining the Ni$^{2+}$ and Ir$^{4+}$ magnetic moments along the chains. Besides, the zero-field-cooled and field-cooled measurements show that there are two characteristic temperatures: T$_1$ = 75 K and T$_2$ = 17 K. The first one is associated with the appearance of a magnetic order with a propagation vector k = (0, 0, 1). At T$_2$, the susceptibility reaches a maximum followed by a sudden drop. The magnetic structure was determined from neutron powder diffraction only up to a global phase. However, symmetry arguments allowed determining the exact nature of the magnetic ground state below T$_2$, thus clarifying the universal magnetic properties of this family of compounds. [Preview Abstract] |
Thursday, March 5, 2015 5:18PM - 5:30PM |
W32.00015: Magnetotransport in BaIrO$_{3}$ Jennifer Trinh, Joshua Flynn, Mas Subramanian, Arthur Ramirez The quasi-one-dimensional monoclinic semiconductor BaIrO$_{3}$ possesses a transition at 177 K characterized by both charge-ordering and ferromagnetism. We have measured the magneto-transport on single crystal samples in fields of up to 7T. We find that the strongly hysteretic positive magnetoresistance (MR) possesses both linear and quadratic contributions. The linear contribution is positive in the full temperature range studied, with a peak below the transition temperature at 150 K. The quadratic MR changes sign at T$_{\mathrm{C}}$, from positive above to negative below. The Hall effect is non-monotonic as a function of field below T$_{\mathrm{C}}$ and we will relate this to the FM order parameter. [Preview Abstract] |
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