Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R21: Focus Session: Coupling Phenomena in Oxides and Optical and Electronic Properties |
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Sponsoring Units: DMP Chair: Pavlo Zubko, Universite de Geneve Room: 323 |
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R21.00001: A Transport Perspective on Local Manipulation of Ferroelectric and Correlated Electron Surfaces Invited Speaker: Petro Maksymovych The majority of transport studies aim to identify intrinsic electronic properties of materials, thus avoiding large electric fields, hysteresis, chemical reactions and hot electrons. In this talk, I will discuss the electron transport signatures of the opposite regime, where a complex oxide surface is subjected to strong local field and/or force gradients. Most notably, we have established an insulator-metal transition within an insulating perovskite oxide controlled solely by ferroelectric switching at the nanoscale [1]. This was the first time metallic conductivity has been found in a ferroelectric, despite a variety of theoretical scenarios dating back to the 70's that hypothesized such a behavior. Equally intriguing is the ability to tune the type and magnitude of metallic conductivity of ferroelectric nanodomains by orders of magnitude using applied electric field. Landau-Ginzburg-Devonshire (LGD) formalism captures the essence of these effects, by describing carrier accumulation or depletion at inclined and charged domain walls. On the other hand, local transport measurements on the surfaces of nominally conducting surfaces (such as manganites and nickel oxide) have induced an insulating state, the effect we refer to as `piezochemistry' and assign to strain-induced redistribution of oxygen vacancies [2]. These coupled transport phenomena in oxides have practical implications, while transport itself appears to be a highly sensitivie probe of ferroic transitions and ionic effects. Experiments were conducted at the Center for Nanophase Materials Sciences, sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy. Work was also supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division.\\[4pt] [1] P. Maksymovych, A. N. Morozovska, P. Yu, E. A. Eliseev, Y.-H. Chu, R. Ramesh, A. P. Baddorf, S. V. Kalinin, Nano Lett.,12, 209 (2012).\\[0pt] [2] Y. Kim, S. Kelly, E. Strelcov, A. Morozovska, E. Eliseev, S. Jesse, N. Balke, I. Hwang, T. Choi, B. Ho Park, P. Maksymovych, S. V. Kalinin, Submitted (2012) [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R21.00002: Interplay of strain and oxygen vacancies in CaMnO$_3$ Ulrich Aschauer, Reto Pfenninger, Sverre M. Selbach, Tor Grande, Nicola A. Spaldin Application of strain through heteroepitaxy has become an established route to engineering novel material properties such as multiferroism in perovskites. First principles calculations have been shown to accurately describe material properties as the in-plane lattice constants are changed by strain, and often indicate that large strain magnitudes ($>$4\%) are required to induce new functionalities. At such large values, however, it is unclear whether strain will be accommodated primarily by changes in intrinsic lattice constants as usually assumed, or by the formation of point defects. Conversely, the use of strain to engineer point-defect concentrations and stoichiometry is largely unexplored. Here we use first-principles calculations to investigate the stability of the $Pnma$ perovskite CaMnO$_3$ under bi-axial strain towards the formation of oxygen vacancies. We discuss the underlying mechanism for strain-vacancy coupling as well as the implications of our results for the growth of highly strained epitaxial films. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R21.00003: Effect of Surface Engineering on Tunneling Across Ferroelectric Thin Films Le Zhang, Shijie Li, Haidong Lu, Alexei Gruverman, Andrei Sokolov For a practical implementation of ferroelectric based devices, it is indispensable to obtain reliable switching characteristic of ME/FE/ME heterostructures. Electrostatic force microscopy shows that polar surfaces are achievable without top ME electrode and ionic adsorbates delivers enough screening ions, it is intriguing enough that top metal electrode appears to be less effective than such screening by adsorbates. This observation emphasizes the importance of surface engineering [1] heterojunctions in order to retain or enhance ferroelectric response. We report on our study of the role of top electrodes in ferroelectric stability and tunneling properties of heterojunctions containing BaTiO$_3$ thin film, grown epitaxially on Ti-terminated SrTiO$_3$ substrate with LaSrMnO$_3$ and SrRuO$_3$ as bottom electrodes by pulsed laser deposition. Epitaxial top electrodes are formed by the same method. Alternatively, Pt and Ni-based metallic films are deposited by electron beam evaporation. PFM response and HRTEM analysis of obtained interface are presented. Transport and magneto-transport measurement are discussed in the frame of ferroelectric polarization induced effects.\\[4pt] [1] H. Lu et all, Adv. Mater., 24, 1209 (2012). [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R21.00004: Graphene Field Effect Sensors for the Study of Nanoscale Ferroelectric Thin Films Anil Rajapitamahuni, Vijay Raj Singh, Zhiyong Xiao, Xia Hong We have constructed graphene field effect devices as sensors to study the dielectric and pyroelectric properties of nanoscale ferroelectric thin films. Using off$-$axis radio frequency magnetron sputtering, we have grown epitaxial single crystalline Pb(Zr,Ti)O$_{3}$(PZT) and (Ba,Sr)TiO$_{3}$ (BSTO) films of 30$-$100 nm thick on (001) Nb:SrTiO$_{3}$ substrates. X$-$Ray and AFM characterizations show the films have high crystallinity and smooth surface. Piezo-response force microscopy studies show that the as-grown PZT films have uniform polarization pointing towards the substrate. Graphene flakes are mechanically exfoliated on PZT and BSTO thin films and single to few layers are fabricated into field effect devices. We extract the carrier density in graphene from Hall Effect measurements, and use it to probe the polarization change of the ferroelectric gate layer. From the gating efficiency we found the dielectric constant of 100 nm PZT film to be 50. Its pyroelectric coefficient is $\sim$15 nC/cm$^2$K at 300 K and the polarization saturates below 100 K. We have also studied the effect of film thickness on the dielectric and pyroelectric properties of the ferroelectric thin films. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R21.00005: Investigation of laser induced space charge fields in lithium niobate at low temperature with Raman spectroscopy Greg Stone, Volkmar Dierolf We report the measurement of space charges fields generated by a laser beam at low temperatures using Raman spectroscopy. Raman spectra obtained with a focused laser exhibits frequency shifts of certain Raman peaks that appear as a function of time. Analysis of these shifts reveals that they originate from changes in the local electric field that are predominately parallel to the z-axis of the crystal. The magnitude of the frequency shifts and the corresponding maximum space charge field established inside the crystal are dependent on the defect concentration. Above a certain threshold field, the built-up space charge field is drastically reduced by discharges and builds up again afterwards. The changes in the Raman spectrum remain after the laser is turned off but disappear upon heating the sample above 200K. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R21.00006: First-principles Investigations of Fe Impurities in KNbO$_{3}$ Mohua Bhattacharya, Steven Lewis, William Dennis The perovskite based material KNbO$_{3}$ has been studied extensively for its photorefractive properties, where the electro-optic effect combined with photoconductivity changes the local refractive index of the material in response to the incident intensities. The presence of a transition metal impurity like Fe is required for efficient photorefractive performance of this material. To shed light on the physical mechanism of this behavior, we perform first-principles calculations within the density functional theory framework. In this talk, we present the geometric and electronic structures of KNbO$_{3}$:Fe super cells and compare two cases: one in which the Fe$^{3+}$ impurity on the Nb$^{5+}$ site is compensated by an O vacancy in the first coordination shell and one in which the O vacancy is in the second coordination shell. Connections of this work to recent experimental measurements are discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R21.00007: Far infrared studies of PrFe$_{3}$(BO$_{3})_{4}$ Kirill Boldyrev, Marina Popova, Taras Stanislavchuk, Andrei Sirenko, Leonard Bezmaternykh We present results on polarized far infrared reflectance, transmittance and ellipsometry measurements of PrFe$_{3}$(BO$_{3})_{4}$ single crystals in a wide temperature range (5 - 300K). Rare-earth iron borates RFe$_{3}$(BO$_{3})_{4}$ undergo an antiferromagnetic phase transition at temperatures below 40 K and all of them demonstrate magnetoelectric and magnetoelastic effects. PrFe$_{3}$(BO$_{3})_{4}$ orders antiferromagnetically at T$_{N}$~$=$~32 K. Pronounced changes in the low-frequency phonon spectra of PrFe$_{3}$(BO$_{3})_{4}$ are observed at T$_{N}$ which points to a significant spin-lattice interaction. Below 90 K, a new feature at 48 cm$^{-1}$ appears in the pi-polarized reflectance spectra. We attribute this feature to a Pr$^{3+}$ crystal-field transition that becomes observable in reflectance due to interaction with a nearby phonon 60cm$^{-1}$. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R21.00008: Band gap hierarchy of single crystal CoFe$_2$O$_4$ thin films from optical absorption spectroscopy Brian Holinsworth, Hunter Sims, Dipanjan Mazumdar, Qi Sun, Mehmet Yurtisigi, Sanjoy Sarker, Arun Gupta, Bill Butler, Janice Musfeldt Thin film materials have a wide variety of applications and also serve as an useful bridge between bulk single crystals and the nanoscale. In this work, we report temperature-dependent optical absorption spectroscopy of single crystal CoFe$_2$O$_4$ thin-films along with complimentary electronic structure analysis. This magnetic insulator has one of the highest Curie temperature among complex oxides and potentially useful in areas such as spintronics. Similar to its Nickel analogue,\footnote{Q.C. Sun, H. Sims, D.Mazumdar, J.X.Ma, B. Holinswoth, K.O'Neal, G.Kim, W.H.Butler, A.Gupta, and J.Musfeldt (accepted to Phys. Rev. B).} our work reveals CoFe$_2$O$_4$ to be an indirect band gap material (1.2 eV) with a direct gap much higher (2.8eV) at 300K. These gap values are robust down to 4.2K. Electronically, both chemical tuning and inversion fraction are found to be important factors in lowering of the band gap compared to NiFe$_2$O$_4$. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R21.00009: Electrocaloric Properties of Epitaxial Strontium Titanate Films Jialan Zhang, Burc Misirlioglu, Pamir Alpay, George Rossetti The pyroelectric and electrocaloric effects in polar dielectric solids result from the coupling between the electrical and thermal properties. Although STO crystals or polycrystalline ceramics remain paraelectric down to 0 K, the ferroelectric phase can be induced by uniaxial stress, an external electrical field, or by doping. Here we develop a nonlinear thermodynamic theory to compute the electrocaloric response of strontium titanate thin films as a function of misfit strain, temperature, electric field strength, and electrode configuration. Our results show that the adiabatic temperature change $\Delta T$ of epitaxial (001) STO films can be controlled by the misfit strain and by varying the thermal and electrical boundary conditions. For films in a capacitor configuration on compressive substrates, the transition between paraelectric and strain-induced ferroelectric tetragonal phases produces a large adiabatic temperature change at room temperature. For films on tensile substrates, the transition between the paraelectric and strain-induced ferroelectric orthorhombic phases can also be accessed using inter-digitated electrodes, and the maximum EC response occurs with a [110] orientation. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R21.00010: Combined Angle-Resolved Photoemission Spectroscopy and Theoretical Study of the Surface Electronic Structure of SrTiO$_{3}$ Richard C. Hatch, Kurt Fredrickson, Chungwei Lin, Miri Choi, Agham B. Posadas, Hosung Seo, Alexander A. Demkov The surface electronic structure of the O 2$p$-derived valence band states of (001)-oriented, TiO$_{2}$-terminated SrTiO$_{3}$ is measured along various crystallographic directions using angle-resolved photoemission spectroscopy (ARPES). A comparison of ARPES spectra to \textit{ab initio}, density functional theory (DFT) band structure calculations as well as the theoretical band structure calculated at the tight binding level are in excellent agreement. ARPES measurements also reveal a mid-gap state located roughly 0.5 eV above the valence band which we interpret as a surface state. This interpretation is supported by DFT calculations of an SrTiO$_{3}$ slab which reveals the existence of a surface state located in the gap roughly 0.5 eV above the projected valence band. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R21.00011: Resonant inelastic soft x-ray scattering as a site-specific probe of electron-phonon coupling in one-dimensional edge-shared cuprates S. Johnston, W.S. Lee, B. Moritz, J. van den Brink, Z.-X. Shen, T. P. Devereaux Resonant inelastic x-ray scattering (RIXS) is a powerful probe for studying excitations in strongly correlated systems. With continued advancements of the technique the overall energy resolution has improved to the point of probing low-energy boson excitations near the elastic line. In this talk we present evidence for coupling to an optical oxygen phonon in the RIXS spectrum at the oxygen K-edge of the quasi-1D edge shared cuprate Ca$_{2+x}$Y$_{2-x}$Cu$_5$O$_{10}$. This mode is identified as a compressive mode polarized perpendicular to the chain direction, modulating the Cu-O charge transfer energy and setting the size exchange interaction. By comparing to small cluster calculations we extract a sizable electron-phonon coupling strength in a site-resolved manner, implying a strong integration of the lattice degrees of freedom into the electronic structure. [Preview Abstract] |
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