Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J4: Focus Session: Emergent Properties in Bulk Complex Oxides: Iridates II |
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Sponsoring Units: GMAG DMP Chair: Gabriel Ramirez, University of California, San Diego Room: 112/110 |
Tuesday, March 4, 2014 2:30PM - 3:06PM |
J4.00001: Time-Reversal Symmetry Breaking and Consequent Physical Responses Induced by All-In-All-Out Type Magnetic Order on the Pyrochlore Lattice Invited Speaker: Taka-hisa Arima Pyrochlore-type 5d transition-metal oxide compounds Cd$_{2}$Os$_{2}$O$_{7}$ and R$_{2}$Ir$_{2}$O$_{7}$ (R$=$rare earth) undergo a metal-insulator transition accompanied by a magnetic transition. Recently, the magnetic structures of Cd$_{2}$Os$_{2}$O$_{7}$ [1] and Eu$_{2}$Ir$_{2}$O$_{7}$ [2] were investigated by means of resonant x-ray magnetic scattering. The x-ray data indicated the all-in/all-out type magnetic order. The all-in/all-out order breaks the time-reversal symmetry, while the spontaneous magnetization is essentially absent. The magnetic order can be viewed as ferroic magnetic octupolar order. The magnetic order is expected to provide several unique physical properties like quadratic magnetization. linear magneto-capacitance, linear magneto-resistance, linear magneto-mechanical coupling and so on [3]. The symmetry breaking results in two non-equivalent domains, ``all-in/all-out'' and ``all-out/all-in.'' Interestingly, some theoretical works predict that a peculiar metallic state would appear on the domain wall. The observation and control of the domain distribution are essential for studying verious exotic physical responses. We have developed an x-ray technique for domain imaging and started studying the effects of external stimuli on the domain distribution [4]. This work was performed in collaboration with S. Tardif, S. Takeshita, H. Ohsumi, D. Uematsu, H. Sagayama, J. J. Ishikawa, S. Nakatsuji, J. Yamaura, and Z. Hiroi. \\[4pt] [1] J. Yamaura et al., Phys. Rev. Lett. 108, 247205 (2012).\\[0pt] [2] H. Sagayam et al., Phys. Rev. B 87, 100403R (2013).\\[0pt] [3] T. Arima, J. Phys. Soc. Jpn. 82, 013705 (2013).\\[0pt] [4] T. Samuel et al., submitted. [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J4.00002: Characterization of the Structural and Magnetic Symmetries of Sr$_{2}$IrO$_{4}$ via Nonlinear Optical Spectroscopy Darius Torchinsky, Hao Chu, Tongfei Qi, Gang Cao, David Hsieh The combination of strong electron-electron interactions and large spin-orbit coupling in the iridates provides a unique platform for realizing exotic electronic phases. A characterization of the structural and magnetic symmetries of these systems is important for understanding whether many of the predicted phases can be realized. Here we discuss how measurement of the nonlinear optical susceptibility using a novel rotational anisotropy technique can be used to study the structural and magnetic symmetries of iridates, which provides a complement to neutron and x-ray diffraction based probes. The perovskite iridate Sr$_{2}$IrO$_{4}$ in particular has been intensively studied recently owing to its novel $J_{eff} =$1/2 magnetic Mott insulating ground state, possible unconventional metal-to-insulator transition and potential for high-T$_{\mathrm{c}}$ superconductivity upon doping. We apply our technique to Sr$_{2}$IrO$_{4}$ to examine both its structural and magnetic symmetries across the Neel transition and discuss our observations in the context of the intriguing physics of these systems. [Preview Abstract] |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J4.00003: A Spatially Resolved Optical Second Harmonic Generation (SHG) Study of the Perovskite Iridate Sr$_{2}$IrO$_{4}$ with Bulk Sensitivity Liuyan Zhao, Hao Chu, Darius Torchinsky, Tongfei Qi, Gang Cao, David Hsieh There has been a lot of recent interest in the layered perovskite iridate, Sr$_{2}$IrO$_{4}$, owing to its novel spin-orbital entangled Mott insulator ground state and its potential to realize high-Tc superconductivity upon doping. Although its bulk structural and magnetic point group symmetries have been characterized by resonant x-ray and neutron diffraction, these measurements provide spatially integrated information. In fact, recent neutron diffraction studies on Sr$_{2}$IrO$_{4}$ suggest that such measurements may be averaging over crystallographic domains of reduced symmetry that in turn generate distinct magnetic domains [1]. Therefore, spatial resolution is desirable in order to gain full understanding of the point group symmetries of Sr$_{2}$IrO$_{4}$. Here, we show that optical SHG can provide a bulk sensitive measurement of the point group symmetries. By performing such SHG measurements in an imaging mode, we study the possible microscopic domain structures recently suggested. More generally, our SHG imaging technique provides an alternative way to probe the point group symmetries of iridate crystals, which are not always amenable to neutron scattering due to their small sample sizes and strong neutron absorption cross section. \\[4pt] [1] F. Ye et al., Phys. Rev. B 87, 140406 (R) (2013); C. Dhital et al., Phys. Rev. B 87, 144405 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J4.00004: Polaronic absorption in Sr$_{2}$IrO$_{4}$ Chang Hee Sohn, Tong-Fei Qi, Kyung Joo Noh, Hyun-Ju Park, Hyang Keun Yoo, Gang Cao, Kyung Wan Kim, Deok-Yong Cho, Soon Jae Moon, Tae Won Noh Sr$_{2}$IrO$_{4}$ has received much attention as a novel $J_{\mathrm{eff}} =$ 1/2 Mott insulator. Many theorists have supposed that exotic novel ground state such as superconductivity, topological insulator, and quantum spin liquid could emerge in $J_{\mathrm{eff}} =$ 1/2 state. However, despite of great interests on Sr$_{2}$IrO$_{4}$, the ground state of this material is elusive up to now. Unlike previous Mott scenario, recent reports support that Sr$_{2}$IrO$_{4}$ can be described as Slater insulator rather than Mott insulator. The origin of temperature evolutions of electronic structure shown in many experiments also remains vague until now. Here, we investigated the detail temperature evolution of electronic structure of Sr$_{2}$IrO$_{4}$ using infrared spectroscopy. We couldn't observe any anomaly in optical conductivity near the $T_{N}$, which is not consistent with recent reports. Instead, we observed the continuous changes in our optical data which can be explained in terms of polaronic behavior, closely related to La$_{2}$CuO$_{4}$. [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J4.00005: Ferromagnetic Resonance of the Weak Ferromagnet Sr$_2$IrO$_4$ Hua Chen, Allan MacDonald We derive a pseudospin model for the strongly spin-orbit coupled 5$d$ oxide Sr$_2$IrO$_4$ that is based on a standard $t/U$ expansion and Slater's theory of atomic multiplets. Using this model, we use linear spin-wave theory to evaluate the material's spin wave spectrum and address the influence of quantum fluctuation on the saturation magnetization. We find that the ferromagnetic resonance (FMR) frequency has an unusual square root dependence on magentic field, with a prefactor related to the strength of the exchange coupling between $j=1/2$ pseudospins. FMR can thus be used as an alternative probe of the nontrivial magnetism of Sr$_2$IrO$_4$. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J4.00006: Phonon assisted optical excitation in narrow bandgap spin-orbit insulator Sr$_{3}$Ir$_{2}$O$_{7}$ Deok-Yong Cho, Hyun-Ju Park, Chang Hee Sohn, Da Woon Jeong, Gang Cao, Kyung Wan Kim, Soon Jae Moon, Tae Won Noh We examined the temperature (T) evolution of the optical conductivity spectra of Sr$_{3}$Ir$_{2}$O$_{7}$ crystal. We found that the features of low energy $d$-$d$ excitation ($\hbar\omega$ $<$ 300 meV) between two $J_{\mathrm{eff}}$ = 1/2 states, evolve drastically in a wide temperature range (4 K $<$ T $<$ 400 K). This large T evolution in the low energy feature is not observed in O K-edge x-ray absorption spectra, suggesting that it is presumably originated from phonon-assisted indirect optical transitions. The results of the simulation in which the phonon-absorbing and phonon-emitting processes is considered, show a consistency with the experimental spectra. The peak energy of the transition between two $J_{\mathrm{eff}}$ = 1/2 bands also decreases apparently by $\sim$100 meV along with the abundance of phonon-assisted charge excitations in the narrow bandgap semiconductor Sr$_{3}$Ir$_{2}$O$_{7}$. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J4.00007: Static and time-resolved optical spectroscopy on Lithium Iridate Jake Koralek, Jamie Hinton, Shreyas Patankar, Joe Orenstein, Tess Smidt, Nicholas Breznay, Nityan Nair, James Analytis We use FTIR and pump-probe spectroscopy to study lithium iridates. The IR spectrum shows an anomalous peak which emerges as temperature is reduced and is highly anisotropic in the ab-plane polarization. In the time-domain we observe similarly anisotropic reflectivity transients whose multiple dynamic components evolve as temperature is reduced. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J4.00008: Possible Superconductivity Induced by Strong Spin-Orbit Coupling in Carrier Doped Iridium Oxides Insulators Kazutaka Nishiguchi, Tomonori Shirakawa, Hiroshi Watanabe, Ryotaro Arita, Seiji Yunoki $5d$ transition metal oxide Sr$_{2}$IrO$_{4}$ and its relevant Iridium oxides have attracted much interest because of exotic properties arising from highly entangled spin and orbital degrees of freedom due to strong spin-orbit coupling (SOC). Sr$_{2}$IrO$_{4}$ crystalizes in the layered perovskite structure, similar to cuprates. Five $5d$ electrons in Ir occupy its $t_{2g}$ orbitals which are split by strong SOC, locally inducing an effective total angular momentum $J_{\textrm{eff}}=1/2$, analogous to a $S=1/2$ state in cuprates. Because of the similarities to cuprates, the possibility of superconductivity (SC) in Iridium oxides has been expected theoretically once mobile carriers are introduced into the $J_{\textrm{eff}}=1/2$ antiferromagnetic insulator [1]. To study theoretically possible SC in carrier doped Sr$_{2}$IrO$_{4}$, we investigate a three-orbital Hubbard model with SOC. By solving the Eliashberg equation in the random phase approximation, we find that $J_{\textrm{eff}}=1/2$ antiferromagnetic fluctuations favor $d_{x^{2}-y^{2}}$-wave SC with a mixture of singlet and triplet Cooper pairings. We will also discuss the particle-hole asymmetry of the SC induced by electron and hole doping. [1] H. Watanabe, et. al., Phys. Rev. Lett. {\bf 110}, 027002 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J4.00009: Quantum oscillations in a metallic pyrochlore irridate, Bi2Ir2O7 Amalia Coldea, Matthew Watson, Samuel Blake, Prabhakaran Dharmalingam, Alix McCollam We report quantum oscillations observed in single crystals of Bi2Ir2O7 using torque magnetometry in high magnetic fields up to 33T and low temperatures to 0.3K. Quantum oscillations allow to determine the extremal areas of the Fermi surface perpendicular to the magnetic field, the quasiparticle masses and also the scattering time. Our results are compared with first-principle band structure calculations that take into account the effects of the spin-orbit coupling. We find evidence both for small and large Fermi surfaces and the effective masses are unexpectedly light. The effect of the magnetic field on the electronic structure of Bi2Ir2O7 will be also discussed. [Preview Abstract] |
Tuesday, March 4, 2014 4:42PM - 4:54PM |
J4.00010: Metal-Insulator Transition and Topological Phases of Pyrochlore Iridates Ru Chen, Eun-Gook Moon, Leon Balents The 4d and 5d transition metal oxides are interesting because these materials incorporate both strong spin-orbit coupling and strong correlations, and consequently display distinct physical properties and the tantalizing possibility of novel topological phases. A prominent family in this class, the rare earth pyrochlore iridates, shows a metal-insulator transition and non-collinear complex magnetic ordering in the insulating state. We carry out magnetic band structure calculations using the GGA+U method, which reproduce the systematic trend that stronger order and larger gaps occur with decreasing rare earth radius. A corresponding paramagnetic band calculation shows that Pr$_2$Ir$_2$O$_7$ is a strong candidate for a nodal quadratic band touching state, in which the doubly degenerate conduction and valence bands touch at the zone center, right at the Fermi level. This suggests that Pr$_2$Ir$_2$O$_7$ is very sensitive to perturbations, such as time reversal symmetry or cubic symmetry breaking terms, giving rise to the possibility of many novel phases. Indeed, we demonstrate using first-principles calculations that uniaxial strain applied along the (111) direction opens a band gap and converts the material to a strong topological insulator. [Preview Abstract] |
Tuesday, March 4, 2014 4:54PM - 5:06PM |
J4.00011: Magnetic order in pyrochlore iridate Y$_2$Ir$_2$O$_7$ probed by electron paramagnetic resonance W.K. Zhu, W. Tong, L. Pi, L.S. Ling, Y. Zhang, S.X. Zhang We performed electron paramagnetic resonance (EPR) studies of the magnetic properties of the pyrochlore iridate Y$_2$Ir$_2$O$_7$ compound. The resonance line in the EPR spectrum shifts towards lower fields when the temperature is below 100 K, suggesting the appearance of a local field generated by the ordered magnetic moments at low temperatures. The temperature dependent local field and magnetization both show hysteresis between zero field cooling and field cooling, however, the magnetic ordering temperature $T_O$ ($\sim$ 100 K) determined by EPR is about 50 K lower than the transition temperature $T_M$ ($\sim$ 150 K) in the $M-T$ measurements, indicating a possible short-range order state at the intermediate temperature. A hyperfine structure was detected below the long-range ordering temperature, suggesting the co-existence of isolated paramagnetic Ir$^{4+}$ moments with the magnetic order state. [Preview Abstract] |
Tuesday, March 4, 2014 5:06PM - 5:18PM |
J4.00012: Microscopic Mechanism of Giant Non-reciprocal Directional Dichroism in CuB$_2$O$_4$ Shingo Toyoda, Nobuyuki Abe, Shojiro Kimura, Taka-hisa Arima CuB$_2$O$_4$ shows giant Non-reciprocal Directional Dichroism (NDD) at 1.405 eV, which corresponds to the intratomic d$_{x^2-y^2 }$-d$_{xy}$ transition of Cu$^{2+}$ hole. To clarify the origin of NDD, we performed measurements of the optical absorption in high magnetic fields up to 15 T. The g-factor estimated from the Zeeman splitting was as large as 2.8. This result suggests that orbital angular momentum contributes to the magnetic moments. To make sure of this estimation, we conducted an exact-diagonalization calculation considering crystal field, spin-orbit coupling, and Zeeman energy. The calculation reveals that d$_{xy}$ hybridizes with d$_{yz}$, d$_{zx}$ to form a spin-orbital coupled state. The calculated g-factor of this state is 2.6, which is comparable with the observed g-factor. We further calculated the oscillator strength of the transition from d$_{x^2-y^2}$ to the spin-orbital coupled state. The resulting oscillator strength successfully explains the experimental results of the magnetic field direction dependence of NDD. [Preview Abstract] |
Tuesday, March 4, 2014 5:18PM - 5:30PM |
J4.00013: Multiferroicity with coexisting isotropic and anisotropic spins in Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ Jae Wook Kim, Y. Kamiya, E. Mun, M. Jaime, N. Harrison, J. Thompson, G. Chern, C. Batista, V. Zapf, V. Kiryukhin, H. Yi, Y. Oh, S.-W. Cheong We study physical properties in Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ (CCMO) by magnetization, magnetostriction, electric polarization, and magnetocaloric measurements under high magnetic fields. On the controversial topic of the spin state of Co, we find evidence for high spin state with $S$ = 3/2 with no field-induced spin-state crossover up to 97 T. In addition, our data also indicate that Mn spins are quasi-isotropic and develop components in the $ab$-plane in applied magnetic fields of 10 T and cant until saturation at 85 T whereas the Ising Co spins saturate by 25 T. We also find transient steps that appear only within a range of magnetic field sweep rates. This feature resembles the behavior observed in isostructural compound Ca$_3$Co$_2$O$_6$ (CCO) where metastable steps appear in magnetization in non-equilibrium condition. However, CCMO has more complex magnetic interactions due to Mn spins and a different ground state compared to CCO. Our results give a different view to the magnetoelectric coupling in this material, namely the spin-flop of Mn$^{4+}$ destabilizes electric polarization instead of a spin flip. The origin of the the transient behavior in CCMO is also discussed. [Preview Abstract] |
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