Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session J6: Focus Session: Phonons, Spins, Charge and Orbital ordering in Complex Oxides |
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Sponsoring Units: DMP DCOMP Chair: Roman Engel-Herbert, Pennylvania State University Room: 006A |
Tuesday, March 3, 2015 2:30PM - 3:06PM |
J6.00001: Nonlinear phononics and ultrafast control at complex interfaces Invited Speaker: Andrea Cavalleri |
Tuesday, March 3, 2015 3:06PM - 3:18PM |
J6.00002: Capturing photoinduced structural distortion within a unit cell of BiFeO$_3$ Haidan Wen, Michel Sassi, Zhenlin Luo, Carolina Adamo, Darrell Schlom, Rosso Kevin, Xiaoyi Zhang The interaction of light with correlated materials has been an intensely studied research forefront in which the coupling of radiation energy to selective degrees of freedom offers a novel contact-free control knob to tune functionalities on ultrafast time scales. We studied a photoexcited multiferroic BiFeO$_3$ thin film using time-resolved x-ray absorption spectroscopy aided by density functional theory calculations. The study revealed a uniaxial deformation of the unit cell at a constant volume with a minimal oxygen octahedron rotation, consistent with the influence of photoinduced carriers to the ferroelectric polarization. These important findings illustrate a microscopic picture of local structural reconfiguration around iron atoms at atomistic length scales during a photocarrier-mediated nonequilibrium process in polar materials. [Preview Abstract] |
Tuesday, March 3, 2015 3:18PM - 3:30PM |
J6.00003: Neutron Scattering Study of Low Energy Phonons and Spin Waves in Multiferroic BiFeO3 Zhijun Xu, Guangyong Xu, John Shneelock, Jinsheng Wen, Peter Gehring, Chris Stock, Barry Winn, Masaaki Matsuda, Ma Jie, Genda Gu, Toshimitsu Ito, Stephen Shapiro, Robert Birgeneau We have performed inelastic neutron scattering studies of the low-energy phonon and spin-wave modes in the multiferroic BiFeO3. The low-energy phonon and spin wave dispersion relations along different directions have been mapped out over a broad temperature range from 100K to 750K. The temperature dependence of the intensities, dispersion relations, and lifetimes (inverse energy width) of these phonon and spin wave modes will be presented. Possible interactions between the lattice and spin dynamics will also be discussed. This work is supported by the Office of Basic Energy Sciences, DOE. [Preview Abstract] |
Tuesday, March 3, 2015 3:30PM - 4:06PM |
J6.00004: Engineering orbital polarization in three-component nickelate heterostructures Invited Speaker: Charles Ahn The interplay between the structural and physical properties of transition metal complex oxides allows for the engineering of their functional properties by tuning atomic-scale structure using interfacial coupling. We show how this principle can be used to achieve orbital polarization and two-dimensional electronic conductivity in nickelate heterostructures [1, 2]. Using a combination of first principles theory and synchrotron-based experiments, we identify key structural features in LaNiO3 thin films grown using molecular beam epitaxy, allowing one to correlate differences in physical structure with electronic transport properties and x-ray absorption spectroscopy measurements. By applying general design principles, such as charge transfer and inversion symmetry breaking, which lead to degeneracy breaking of the Ni 3d orbitals, one can design and fabricate tri-component superlattices to engineer the electronic and orbital properties of rare-earth nickelate compounds, achieving a two-dimensional single band electronic surface at the Fermi energy.\\[4pt] [1] H. Chen et. al. Phys. Rev. Lett. 110, 186402 (2013)\\[0pt] [2] D. Kumah et. al. Adv. Materials 26, 1935 (2014) [Preview Abstract] |
Tuesday, March 3, 2015 4:06PM - 4:18PM |
J6.00005: Time-resolved hard X-ray nano-diffraction study on the long-lived strain in BiFeO$_{3}$ induced by optical transient grating Yi Zhu, Qingteng Zhang, Pice Chen, Carolina Adamo, Zhonghou Cai, Donald Walko, Eric Dufresne, Darrell Schlom, Paul Evans, Haidan Wen Optical transient grating excitation was applied to induce spatially modulated strain in a BiFeO$_{3}$ thin film. The spatial profile and the dynamics of the photo-induced strain were measured by time-resolved hard X-ray nano-diffraction at beamline 7-ID of the Advanced Photon Source. By interfering two ultrafast laser beams at 400nm wavelength and $\sim$ 100fs pulse duration, an optical transient grating with $\sim$ 5micron period sinusoidal profile was created and used to excite the BiFeO$_{3}$ sample. By using the nano-focused ultrafast X-ray pulses, both the strain profile and the dynamics induced by the photo-generated carriers were mapped out. We found the out-of-plane strain profile remained sinusoidal while the strain amplitude modulation lasted tens of microseconds, indicating the non-thermal nature of this photo-induced strain in BiFeO$_{3}$. [Preview Abstract] |
Tuesday, March 3, 2015 4:18PM - 4:30PM |
J6.00006: Phonon dispersion relation in cubic BaTiO$_3$ Izumi Tomeno, Jaime Fernandez-Baca, Riki Kobayashi, Songxue Chi, Kunihiko Oka, Yorihiko Tsunoda The lattice dynamics of cubic BaTiO$_3$ ($T_c=405$ K) has been investigated along the [100], [110], and [111] directions at $T=$453, 700 and 890 K using inelastic neutron scattering. The transverse acoustic (TA) phonon dispersion is relatively isotropic. This indicates that all the measured TA branches are mainly governed by the Ba atom motion against the O atoms. The zone-boundary TA phonon energies for BaTiO$_3$ are approximately twice as large as those for cubic PbTiO$_3$. These differences cannot be explained by the A-site atomic mass alone. The large Ba-O force-constant reflects the ionic nature of Ba atom in BaTiO$_3$. The softening of the transverse optical (TO) $\Delta_5$ and $\Sigma_4$ branches in a wide $q$ range at $T=453$ K is in contrast to the steep softening of the TO $\Lambda_3$ branch toward the zone center. The TO $\Sigma_4$ mode is heavily damped in the range $q\le$0.25 at 453 K, whereas the well-defined TO $\Sigma_4$ peak for $q=$0.25 is observed at 890 K. The low-lying TO $\Delta_5$ and $\Sigma_4$ branches are primary due to the low-frequency Ti-O-Ti chain-like motion along [001]. The soft mode behavior in BaTiO$_3$ is discussed in comparison with the previous study on PbTiO$_3$. [Preview Abstract] |
Tuesday, March 3, 2015 4:30PM - 4:42PM |
J6.00007: [001] electric field effect on phonons in PMN-32PT John Schneeloch, Zhijun Xu, Barry Winn, Chris Stock, Peter Gehring, Guangyong Xu We report inelastic neutron scattering measurements on a single crystal of the relaxor ferroelectric 68\%Pb(Mg$_{1/3}$Nb$_{2/3}$)O$_{3}$-32\%PbTiO$_{3}$ (PMN-32PT) under an external [001] electric field (0.5 kV/cm). In addition to the partial suppression of diffuse scattering intensities, as previously reported, we also see a field-induced change in acoustic phonon intensities and energies. The change is anisotropic, with clear differences between the [100] and [001] directions. We will discuss these results and their relation to possible changes in the domain structure under field and coupling between diffuse scattering and phonon modes. [Preview Abstract] |
Tuesday, March 3, 2015 4:42PM - 4:54PM |
J6.00008: Magnetic field induced directional dichroism of spin waves in multiferroic BiFeO$_3$ at THz frequencies Urmas Nagel, T. R\~o\~om, S. Bord\'acs, I. K\'ezsm\'arki, H.T. Yi, S.-W. Cheong, Jun Hee Lee, Randy S. Fishman Using far infrared spectroscopy in high magnetic fields we show that spin excitations in BiFeO$_3$ simultaneously interact with the electric and magnetic field components of light resulting in directional dichroism (DD) of absorption. DD in BiFeO$_3$ arises because an applied static magnetic field induces a toroidal moment in the cycloidal spin structure. Strong DD is observed even in the room-temperature state of the material. The results are explained on the microscopic level as an interplay of five different interactions: isotropic exchange couplings between nearest and next nearest neighbors, an easy-axis anisotropy along the ferroelectric polarization, Dzyaloshinskii-Moriya (DM) interaction that creates the cycloid and DM interaction that causes spin canting. [Preview Abstract] |
Tuesday, March 3, 2015 4:54PM - 5:06PM |
J6.00009: Charge Density Wave Behavior of Ionic Liquid Gated Strontium Titanate Nanowires* Terence Bretz-Sullivan, Allen Goldman Measurements of the current-voltage characteristics of ionic liquid gated nanometer scale channels of strontium titanate have been carried out. These characteristics exhibit a large voltage threshold for conduction and a nonlinear power law behavior at all temperatures measured. The source-drain current of these nanowires scales as a power law of the difference between the source-drain voltage and the threshold voltage. The temperature dependence of the threshold voltage appears to be related to the inverse of the temperature dependent dielectric constant of strontium titanate in qualitative agreement with a simple model of charge density wave depinning. These observations, when taken together, are evidence that a gate induced charge density wave has been induced, and is depinned by strong electric fields. * This work was supported by DOE Basic Energy Sciences Grant DE-FG02-02ER46004. Samples were fabricated at the Minnesota Nanofabrication Center. Parts of this work were carried out in the University of Minnesota Characterization Facility, a member of the Materials Research Facilities Network (www.mrfn.org) funded via the NSF MRSEC program. [Preview Abstract] |
Tuesday, March 3, 2015 5:06PM - 5:18PM |
J6.00010: Neutron and X-ray studies in suppressing orbital order in FeV2O4 with Cr doping. Dalmau Reig-i-Plessis, Zhangsu Wen, Alexander Thaler, Vasile O. Garlea, Haidong Zhou, Jacob Ruff, Gregory Macdougall FeV$_2$O$_4$ is a spinel compound with an orbitally active V$^{3+}$ cation on a frustrated pyrochlore sublattice and Jahn-Teller active Fe$^{3+}$ on a diamond sublattice. Previous studies show that this material has three structural and two magnetic transitions, and that orbital order leads to coupling between the spin and lattice degrees-of-freedom. The opposite end of the doping series is the multiferroic, FeCr$_2$O$_4$, which has spin, but no orbital degree of freedom on the Cr$^{3+}$ and only two structural transitions. Although both materials show a higher temperature collinear ferrimagnetic state and a non-collinear phase at lower temperature, the physics must be different since the canting transition in FeV$_2$O$_4$ is associated with the orbital order at the lowest structural transition. In this talk, I will present the results of synchrotron X-ray and neutron powder diffraction studies of the structural and magnetic transitions in the doping series FeV$_{2-x}$Cr$_x$O$_4$. Specifically, I will comment on the doping-temperature phase diagram we extract from these measurements, and the region of co-existence between distinct non-collinear spin orders which exist at finite doping. [Preview Abstract] |
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