Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session J2: Heterostructures of Correlated Oxide Materials |
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Sponsoring Units: DCMP Chair: Allen Goldman, University of Minnesota Room: LACC 151 |
Tuesday, March 22, 2005 11:15AM - 11:51AM |
J2.00001: Dielectric Superlattices with Broken Inversion Symmetry Invited Speaker: Dielectric phases with large electronic susceptibilities and high resistivity can be grown by ozone assisted atomic layer by layer molecular beam epitaxy. We have combined different perovskite titanate phases that have different unstrained lattice constants into fully strained superlattices in which inversion symmetry is broken. For example, a supercell consisting of one unit cell each of CaTiO$_{3}$, SrTiO$_{3}$ and BaTiO$_{3}$ is denoted 111+, while the opposite stacking ordering is called 111-. For testing, these superlattices are sandwiched between lattice matched conducting oxide electrodes and patterned into capacitor structures. In such superlattices, each layer is connected to two different phases. The TiO$_{3}$ octahedra in each layer asymmetrically distort because of the different strain each layer experiences. This results in a built-in polarization that persists at high temperatures in the paraelectric phases. The direction of the polarization is controlled by the molecular stacking architecture, ``plus'' or ``minus.'' At low temperatures an unusual ferro-like phase develops with asymmetric polarization states. This work has been done in collaboration with Maitri Warusawithana, Hao Chen, Michael Weissman and Jian-Min Zuo and supported by the US Department of Energy [Preview Abstract] |
Tuesday, March 22, 2005 11:51AM - 12:27PM |
J2.00002: Surface versus Bulk Coulomb Correlations in Photoemission Spectra of SrVO$_3$ and CaVO$_3$ Invited Speaker: Photoemission is a key spectroscopy for the study of the electronic properties of strongly correlated materials. Spectra taken at low photon energies tend to show significantly stronger correlation features than high energy spectra, suggesting that Coulomb correlations near the surface are enhanced compared to the bulk. To investigate these differences the dynamical mean field theory is used in combination with the multi-orbital Quantum Monte Carlo method in order to evaluate quasi-particle spectra which can be compared directly with photoemission distributions. In the case of perovskites like SrVO$_3$ and CaVO$_3$, the planar character of the partially filled $t_{2g}$ bands and the reduced coordination of surface atoms give rise to an effective narrowing of the surface density of states. As a result, the quasi-particle weight near $E_F$ is reduced and the amplitude of the lower and upper Hubbard bands is enhanced, in agreement with experiment [1]. Preliminary results for VO$_2$ surfaces will also be discussed. [1] A. Liebsch, Phys. Rev. Lett. {\bf 90}, 96401 (2003); Eur. Phys. J. B {\bf 32}, 477 (2003). [Preview Abstract] |
Tuesday, March 22, 2005 12:27PM - 1:03PM |
J2.00003: Artificial charge-modulation in atomic–scale perovskite titanate superlattices Invited Speaker: In research for exploring new phenomena and functioning devices based on oxide heterostructures and artificial superlattices, it is important to understand the interfacial electronic structure, which is quite distinct from the bulk electronic states because of epitaxial strain and charge/polarization discontinuity. This aspect is also related to the charge ordering phenomena seen in bulk perovskites. Thus, the atomic-scale study to measure and design the interfacial electronic structure is highly relevant. We have grown a number of perovskite titanate superlattices by pulsed laser deposition, specifically controlling oxidation, surface crystallization kinetics, surface termination and layer thickness. In an abrupt interface between band insulator, SrTiO$_{3}$, and Mott insulator, LaTiO$_{3}$, we found that the extra electrons on the Ti site distribute on a length scale exceeding Thomas-Fermi screening length, due to a large induced lattice polarization. This results in a metallic interface down to low temperature. We also found a conducting interface in SrTiO$_{3}$/LaAlO$_{3}$ superlattices. In this case, the conductivity can be controlled over a wide range by changing the composition of the interface. We have also grown solid solution films consisting of SrTiO$_{3}$, LaTiO$_{3}$, and LaAlO$_{3}$ within the framework of charge modulation in perovskite titanates. A variety of electronic states was developed and will be discussed in terms of optical absorption spectra and transport properties comparing with those of the superlattices. [Preview Abstract] |
Tuesday, March 22, 2005 1:03PM - 1:39PM |
J2.00004: Electronic Reconstruction in Correlated Electron Heterostructures Invited Speaker: Understanding of surface/interface properties of correlated electron materials is an important scientific question and is necessary for possible devices utilizing these materials. As a first step, we focus on the ``charge leakage'' between the different materials inspired by a recent experiment by Ohtomo {\it et al}.[1] We present the following theoretical studies of heterostructures comprised of a Mott insulator and a band insulator; (1) Hartree-Fock analysis of a realistic three-band model for a heterostructure between LaTiO$_3$ and SrTiO$_3$, the structure grown and measured by Ohtomo {\it et al}.,[2] (2) Dynamical-mean-field approximation analysis of a simplified single-band Hubbard model heterostructure.[3] In each case, the heterostructure is defined by placing charge +1 at La sites (charge difference between La$^{3+}$ and Sr$^{2+}$ ions), and the long-range Coulomb repulsion between conduction (Ti $d$) electrons is treated by Hartree approximation. We show that spin/orbital orderings in thin heterostructures are generically different from the bulk and that the interface region, $\sim$ 3 unit cell wide, is always metallic. Prediction for photoemission experiments are made to show how the electronic properties change as a function of position through the interface. Optical conductivity measurements are proposed to investigate the nature of orderings and quasiparticle subbands. We also give general discussion of the correlated electron surface/interface problem. This work has been done in collaboration with Andrew J. Millis, and is supported by JSPS, NSF DMR 0338376 and DOE ER46169. [1]Ohtomo et al., Nature \textbf{419}, 378 (2002). [2]Okamoto and Millis, Nature {\bf 428}, 630 (2004), and Phys. Rev. B {\bf 70}, 195120 (2004). [3]Okamoto and Millis, Phys. Rev. B (in press), cond- mat/0407592. [Preview Abstract] |
Tuesday, March 22, 2005 1:39PM - 2:15PM |
J2.00005: Enhancing Ferroelectrics using Strain Invited Speaker: We have used epitaxy and the misfit strain imposed by an underlying substrate to shift the paraelectric-to-ferroelectric transition temperature ($T_{c})$ by \textit{hundreds} of degrees and to enhance the ferroelectric properties of SrTiO$_{3}$ and BaTiO$_{3}$. Although SrTiO$_{3}$ is normally not ferroelectric at any temperature, predictions based on thermodynamic analysis imply that a biaxial strain of order 1{\%} will shift its $T_{c}$ to the vicinity of room temperature. Such strains are also predicted to elevate the $T_{c}$ of BaTiO$_{3}$ by comparable amounts. In practice, the synthesis of uniformly strained ferroelectric films is challenging. Epitaxial ferroelectric films are often grown to thicknesses greatly exceeding their critical values, resulting in undesirable relaxation toward a zero-strain state by the introduction of dislocations. Dislocation densities of $\sim $10$^{11}$~cm$^{-2}$ are common in epitaxial ferroelectric films grown on lattice-mismatched substrates, and the resulting inhomogeneous strain smears out the ferroelectric phase transition. Our approach to controlling the properties of ferroelectric SrTiO$_{3}$ and BaTiO$_{3}$ films centers on the development of new substrates (DyScO$_{3}$ and GdScO$_{3})$ that enable the growth of uniformly strained films below, or at least far closer to, the critical thickness for relaxation. Our results$^{1,2}$ demonstrate not only the largest strain-induced shift in $T_{c}$ ever achieved, but also manifest a paradigm shift in how to manipulate the properties of ferroelectric thin films. Strain is a viable alternative to the traditional method of chemical substitutions for shifting $T_{c}$ by large amounts. These strained SrTiO$_{3}$ and BaTiO$_{3}$ films have better structural perfection (narrower rocking curve widths) than SrTiO$_{3}$ and BaTiO$_{3}$ single crystals. An unexpected surprise is that the strained SrTiO$_{3}$ films exhibit a frequency dependence of their dielectric constant consistent with \textit{relaxor} ferroelectricity. $^{1 }$J.H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y.L. Li, S. Choudhury, W. Tian, M.E. Hawley, B. Craigo, A.K. Tagantsev, X.Q. Pan, S.K. Streiffer, L.Q. Chen, S.W. Kirchoefer, J. Levy, and D.G. Schlom, \textit{Nature} \textbf{430} (2004) 758-761. $^{2 }$K.J. Choi, M. Biegalski, Y.L. Li, A. Sharan, J. Schubert, R. Uecker, P. Reiche, Y.B. Chen, X.Q. Pan, V. Gopalan, L.-Q. Chen, D.G. Schlom, and C.B. Eom, \textit{Science} \textbf{306} (2004) 1005-1009. [Preview Abstract] |
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