Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U38: Focus Session: Ferroelectric Oxide Superlattices and Oxide Thermoelectrics |
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Sponsoring Units: DCMP Chair: Jean-Mare Triscone, University of Geneva Room: Morial Convention Center 230 |
Thursday, March 13, 2008 8:00AM - 8:12AM |
U38.00001: Thermoelectric response of Lanthanum-doped SrTiO$_{3}$ thin-films grown under various oxygen partial pressures Matthew L. Scullin, Jayakanth Ravichandran, Mark Huijben, Choongho Yu, Subroto Mukerjee, Joel Moore, Arun Majumdar, R. Ramesh Doped strontium titanate is a strong candidate for the next-generation high-$Z$ bulk thermoelectric material due to both its wide tunability in transport properties and very large carrier effective mass. Thermoelectric thin-films of Sr$_{1-x}$La$_{x}$TiO$_{3-\delta }$ with various La content were grown via pulsed-laser deposition (PLD) on (001)-oriented LSAT substrates under various oxygen partial pressures. We find that electron transport is dominated by carriers from oxygen vacancies in samples grown at low $p_{O2} \quad <$ 10$^{-6}$ Torr, and that thermopower as high as 1000 \textit{$\mu $}V/K can be achieved even in heavily La-doped samples. Doping combinations that yield resistivities as low as 5 m$\Omega $-cm yield power factors @ 300K as high as 0.7 W/m-K, implying \textit{ZT} $>$ 0.1. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U38.00002: ``Pudding mold"-type band dispersion as the origin of large thermopower in Na$_x$CoO$_2$ Kazuhiko Kuroki, Ryotaro Arita Na$_x$CoO$_2$ is an interesting material in that it has large thermopower, non-trivial magnetic property, and becomes superconducting when water molecules are intercalated. Recently, in Ref. 1 and 2, we proposed that the magnetism and the superconductivity can have the same root. Namely they can originate from the peculiar band dispersion of a$_{1g}$ band. In this study, by using Boltzmann's equation, we calculated the thermopwer of this system. We found that the origin of the coexistence of the large thermopower and the low resistivity in this material is that not just the density of states, the effective mass, nor the band width, but indeed again the peculiar ``pudding mold''-type band dispersion of the a$_{1g}$ band which consists of a dispersive portion below the Fermi level and a dispersionless portion above the Fermi level[3]. \newline [1] K. Kuroki et al., Phys. Rev. B 73, 184503 (2006). \newline [2] K. Kuroki et al., Phys. Rev. Lett. 98, 13601 (2007). \newline [3] K. Kuroki and R. Arita, J. Phys. Soc. Jpn. 76 (2007) 083707 [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U38.00003: Thermoelectric properties of Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{y}$ thin films grown by pulsed laser deposition Shufang Wang, Venimadhav Adyam, Shengming Guo, Qi Li, Xiaoxing Xi Epitaxial and c-axis preferred oriented oxide thermoelectric Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{y}$ thin films have been deposited on LaAlO$_{3}$ (100), Al$_{2}$O$_{3 }$(0001) and fused silica substrates using pulsed laser deposition. At room temperature, the Seebeck coefficient and resistivity are of the order of 125 $\mu $V/K, 120 $\mu $V/K, 110 $\mu $V/K and 3 m$\Omega $ cm, 2 m$\Omega $ cm, 14 m$\Omega $ cm for the films on LaAlO$_{3}$ (100), Al$_{2}$O$_{3 }$(0001) and silica substrates respectively. A large negative in-plane magnetoresistance (MR) is observed in the films at low temperatures, with a MR reaching 41{\%} at 9 T and T=1.8 K in films on LaAlO$_{3}$ (100). We also observed a large bias current-dependent resistivity in the films at low temperature, which has been attributed to the suppression of spin-density-wave by electric field. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U38.00004: Localization of holes at oxygen sites in a thermoelectric rhodate revealed by photoemission spectroscopy Ishida Yukiaki, Baba Teruhisa, Eguchi Ritsuko, Matsunami Masaharu, Taguchi Munetaka, Chainani Ashish, Senba Yasunori, Ohashi Haruhiko, Okamoto Yoshihiko, Takagi Hidenori, Shin Shik We have performed soft-x-ray absorption and photoemission studies on Sr1-xRh2O4 [1], a thermoelectric material structurally and electronically analogous to NaxCoO2. Metal-insulator transition occurs with hole doping into the Rh 4d t2g band via introducing Sr vacancies [1]. Valence-band spectra of Sr1-xRh2O4 showed satellite structures $\sim $11 eV below the Fermi level, which cannot be explained by LDA or LDA+U calculations. The O 1s2p2p Auger peak in the photoemission spectra appeared from the 11-eV satellite in the vicinity of the O 1s absorption edge, indicating that the satellite is a sign of holes localized at oxygen sites. Concomitantly, doping dependent changes appeared mainly in the O 1s absorption spectra rather than in the Rh 3p3/2 absorption spectra. We discuss inhomogeneous evolution of the system with hole doping along with Sr-vacancy potential and d-p hybridization. [1] Y. Okamoto, et al., J. Phys. Soc. Jpn . 75, 023704 (2006). [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U38.00005: Quantum Critical Paraelectrics and the Casimir Effect in Time Lucia Palova, Premala Chandra, Piers Coleman We present a study of the quantum paraelectric-ferroelectric transition (QPFT) with special emphasis on temperature near a quantum critical point where it acts as a boundary condition in imaginary time. The effect of temperature on quantum critical fluctuations can be likened to the Casimir effect where critical electromagnetic fluctuations are probed through their sensitivity to boundary conditions in space. Exploiting this analogy, we use finite-size scaling methods to study the finite-temperature properties of a quantum paraelectic in the vicinity of a QPFT. Simple arguments are used to derive the $1/T^{2}$ divergence of the paraelectric susceptibility ($\chi$) previously found with diagrammatic techniques. Self-consistent mean-field theory is used to probe the classical-quantum crossover in $\chi$ and the resulting temperature-pressure phase diagram is presented. Observable consequences of our scaling approach for high pressure measurements on strontium titanate and potassium tantalate at low temperatures will also be discussed. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U38.00006: Effective Hamiltonian Approach to Ferroelectric Ordering in Oxide Superlattices Jun Hee Lee, Umesh V. Waghmare, Jaejun Yu We developed an effective Hamiltonian approach to the ferroelectric ordering in oxide superlattices. Despite that recent experiments have made a huge progress in making layer-by- layer heterostructures of ferroelectric superlattices, direct first-principles calculations are so far limited to the finite- size calculations, and electrostatic models are too simple to reflect intricate interactions of superlattices in atomic scale. We took the effective Hamiltonian method as one of candidates for the calculations of ferroelectric superlattices not only because it has enough degrees-of-freedom of energy terms to reflect complex interactions such as dipole-dipole interaction and short-range repulsion, but because it needs small amount of computational load compared with first- principles. All the parameters in the effective Hamiltonian are predetermined from the first-principles of constituent bulk components. As an application of the model parameters, we calculated the polarization of (BaTiO$_3$)$_n$/(SrTiO$_3$)$_m$, the results of which are in good agreement with those of previous first-principles calculations for not only of average polarizations but also of local polarizations. This effective Hamiltonian procedure can provide for developing a model for other kinds of oxide superlattices. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U38.00007: Predicting polarization and nonlinear dielectric response of arbitrary perovskite superlattice sequences Invited Speaker: A complete theory of epitaxial perovskite superlattices requires an understanding both of epitaxial strain effects and of electrostatic boundary conditions. Here, focusing on the latter issue, we\footnote{In collaboration with Massimiliano Stengel, Karin M. Rabe and David Vanderbilt.} have carried out first-principles calculations of the nonlinear dielectric properties of short-period ``bicolor'' and ``tricolor'' CaTiO$_3$/SrTiO$_3$/BaTiO$_3$ superlattices having the in-plane lattice constant of SrTiO$_3$. In particular, we have calculated the layer polarizations $p_j$ as defined using the Wannier-based method of Wu, Di\'eguez, Rabe and Vanderbilt\footnote{X. Wu, O. Di\'eguez, K. Rabe and D. Vanderbilt, Phys.\ Rev.\ Lett.\ {\bf 97}, 107602 (2006).} for each neutral BaO, SrO, CaO, or TiO$_2$ layer. We use a cluster expansion (CE) technique to model the layer polarizations $p_j$ of a selected set of bicolor superlattices as a function of the displacement field $D$ (which is uniform throughout the insulating superlattice), the chemical identity of the layer itself, and the chemical identity of its neighboring layers. We find that $p_j$ is a strongly localized function of its chemical environments at fixed $D$ field, i.e., the dependence on the identity of the neighboring layers decays rapidly with distance. This localized property enables us to arrive at a truncated and simplified CE model which can accurately predict $p_j(D)$ in arbitrary layer sequences, both bicolor and tricolor. A similar approach is used to model the dependence of the $c$ lattice constant. With all this information in hand, we can predict the polarization, piezoelectric and nonlinear dielectric response of arbitrary superlattice sequences. The power of the approach is demonstrated by showing that a model fitted only to calculations on inversion-symmetric bi-color superlattices can successfully predict the inversion symmetry breaking in tricolor superlattices such as 2SrTiO$_3$/1BaTiO$_3$/1CaTiO$_3$. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U38.00008: First-principles study of piezoelectric response in PbTiO$_{3}$/SrTiO$_{3}$ 1 x 1 superlattice Yanpeng Yao, Huaxiang Fu Artificial superlattices composed of two materials of similar structure but distinct properties are a promising group of new materials to achieve modified properties. For ferroelectric materials, it is well known that strain effects due to lattice mismatch can considerably alter the properties of ferroelectric thin films. Here we perform first-principles density functional calculations to study the structure and polarization response of PbTiO$_{3}$/SrTiO$_{3}$ (PT/ST) 1x1 superlattice to varied inplane strain. The superlattice is chosen to be grown along the [001] crystallographic direction, and is assumed to be tetragonal. For a given inplane strain, the out-of-plane lattice constant and atomic positions are relaxed. Similar calculations on PT, ST single materials are also performed for comparison. We find that by increasing strain, the superlattice has combined features as those of PT and ST single materials. Being unpolar without strain, the superlattice becomes ferroelectric at a critical inplane lattice constant larger than that of ST. The PT/ST 1x1 superlattice is shown to have a much larger piezoelectric response than PT, a property similar to ST. But, compared to ST, the large piezoelectric response in PT/ST is easier to realize, since a smaller inplane strain is required. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U38.00009: Improper ferroelectricity in perovskite oxide artificial superlattices Matthew Dawber, Nicolas Stucki, Celine Lichtensteiger, Jean-Marc Triscone, Eric Bousquet, Patrick Hermet, Philippe Ghosez In paraelectric/ferroelectric heterostructures with thick constituent layers electrostatics is the dominant interaction between layers and we have previously demonstrated that the key ferroelectric parameters, polarization and critical temperature can be tuned over a very large range in PbTiO$_{3}$/SrTiO$_{3}$ superlattices by varying the ratio of the layer thicknesses [1]. However, as the layers become thinner, a departure from the electrostatic model is observed, which manifests itself as an unusually high ferroelectric polarization and transition temperature and a high, but temperature independent, dielectric constant. Detailed examination of the phase transitions with temperature reveal that along with these enhanced characteristics there is a fundamental change in the nature of the ferroelectricity. The microscopic origin of this change, a form of improper ferroelectricity, is revealed by first principles calculations to occur through a coupling of oxygen rotations and the polarization mode at the interfaces in the material. \\ $[1]$ M. Dawber et al., Adv. Mat. (2007) [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U38.00010: A new hybrid exchange-correlation functional for accurate prediction of the electronic and structural properties of ferroelectric oxide bulks and nanostructures D.I. Bilc, R. Shaltaf, J. \'I\~niguez, Ph. Ghosez We report a systematic comparison of various DFT and hybrid exchange-correlation functionals for the prediction of the electronic and structural properties of prototypical ferroelectric oxides. We find that some, although not all, usual DFT functionals predict the structure with acceptable accuracy, but always underestimate the electronic band gaps. Conversely, common hybrids yield an improved description of the band gaps, but overestimate the volume and atomic distortions associated to ferroelectricity, giving rise to an unacceptably large $c/a$ ratio for the tetragonal phases of BaTiO$_3$ and PbTiO$_3$. This {\it super-tetragonality} is found to be induced mainly by the GGA exchange energy. We thus propose an alternative hybrid functional, B1-WC, that mixes exact exchange following the B1 scheme with the recently improved GGA proposed by Wu and Cohen. This B1-WC renders accurate description of both the structural and electronic properties of typical ferroelectric oxide at the bulk level. The case of Pt/BaTiO$_3$ heterostructures is also considered and a comparison of the of B1-WC and LDA results for atomic relaxation and electronic band alignment at the interface is presented. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U38.00011: First principles calculations at fixed dielectric displacement Massimiliano Stengel, David Vanderbilt, Nicola Spaldin With the experimental advances in growth and characterization of superlattices and thin-film capacitors based on ferroelectric perovskites, it is becoming more and more crucial to achieve a fundamental understanding of these structures by accurate first-principles modeling. Unfortunately, difficulties in the treatment of finite electric fields and macroscopic polarization in periodic systems - in particular when metallic electrodes are present - make the application of \emph{ab-initio} techniques to such systems particularly challenging. To address these issues, we present here a method to perform first-principles calculations of periodic systems at a fixed value of the macroscopic dielectric displacement. This technique, which complements previously established strategies to fix $P$ or $\mathcal{E}$, provides a simple and natural way to explore the phase diagram of a ferroelectric system as a function of a single electrical order parameter. We demonstrate the power of our approach by computing the electrical equations of state for two symmetrical ferroelectric capacitors which are based on the same combination of materials, Au and BaZrO$_3$, but are characterized by inequivalent interface geometries and stoichiometries. In particular, we use the information extracted from the centrosymmetric systems to accurately predict the electrical equation of state for an asymmetric capacitor of analogous composition. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U38.00012: Broadband Permittivity Measurements of Ruddlesden-Popper Sr$_{n+1}$Ti$_{n}$O$_{3n+1 }$(n=1,2,3) Thin Films N. Orloff, W. Tian, D. Schlom, J. Booth, I. Takeuchi In order to explore the microwave dielectric response of Sr$_{2}$TiO$_{4}$, Sr$_{3}$Ti$_{2}$O$_{7}$, and Sr$_{4}$Ti$_{3}$O$_{10}$ thin films, we have performed broadband in-plane quantitative complex permittivity($\varepsilon )$ measurements on Sr$_{n+1}$Ti$_{n}$O$_{3n+1 }$(n=1,2,3) thin films in the frequency range 100Hz-40GHz. The films, of approximately 160 nm thickness, were fabricated by molecular beam epitaxy[1], and standard lithographic techniques were used to define coplanar waveguide transmission lines and interdigitated capacitors using gold. We extracted $\varepsilon $ from the measured complex S-parameters (.01-40GHz) and the complex impedance (100Hz-.001GHz), which were measured at 70K, 150K, 200K, and 250K using a cyrogenic probe station. We found that below $\sim $10GHz the $\varepsilon $'s of these thin films were approximately constant with frequency: $\varepsilon \approx $38, 48, and 100 for Sr$_{n+1}$Ti$_{n}$O$_{3n+1 }$(n=1,2,3) respectively. In addition, the measured value for $\varepsilon $ of Sr$_{2}$TiO$_{4}$ is consistent with recent theoretical calculations [2]. We will discuss in detail the temperature and electric field dependence of the measured complex $\varepsilon $ for these material systems. [1] J.H. Haeni, \textit{et al} APL, \textbf{78}, 21 (2001) [2] C.J. Fennie and M.K. Rabe, PRB, \textbf{68}, 184111 \textbf{(}2003) [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U38.00013: Competing structural instabilities in Ti-based layered-perovskite-oxide superlattices Serge Nakhmanson Utilizing first-principles computational techniques, we have mapped out structural instabilities in the Ruddlesden-Popper homologous oxide superlattice families with a general chemical formula A$_{n-1}$A$^{\prime}_{2}$Ti$_{n}$O$_{3n+1}$, A = Sr, Ba, Pb (perovskite-type block) and A$^{\prime}$ = Sr (rocksalt-type block), for $n$ = 1--5. Our calculations show that each superlattice family has a unique set of ``instability footprints'' --- including the ferroelectric, antiferroelectric and antiferrodistortive types --- which may or may not have a strong coupling to epitaxial strain. Furthermore, the existence and strength of structural instabilities within each particular family change dramatically with an increasing number of perovskite-type layers $n$, granting us wide flexibility to fine-tune the properties of these materials for various device applications or, e.g., for integration into composites with magnetic Ru- or Mn-based layered-perovskite-oxide compounds. [Preview Abstract] |
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