Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session B12: Focus Session: Complex Oxide Interfaces - Titanates |
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Sponsoring Units: DMP Chair: Mikel Holcomb, West Virginia University Room: 314 |
Monday, March 18, 2013 11:15AM - 11:51AM |
B12.00001: First-principles modeling of titanate/ruthenate superlattices Invited Speaker: Javier Junquera The possibility to create highly confined two-dimensional electron gases (2DEG) at oxide interfaces has generated much excitement during the last few years. The most widely studied system is the 2DEG formed at the LaO/TiO$_{2}$ polar interface between LaAlO$_{3}$ and SrTiO$_{3}$, where the polar catastrophe at the interface has been invoked as the driving force. More recently, partial or complete delta doping of the Sr or Ti cations at a single layer of a SrTiO$_{3}$ matrix has also been used to generate 2DEG. Following this recipe, we report first principles characterization of the structural and electronic properties of (SrTiO$_{3}$)$_{5}$/(SrRuO$_{3}$)$_{1}$ superlattices, where all the Ti of a given layer have been replaced by Ru. We show that the system exhibits a spin-polarized two-dimensional electron gas extremely confined to the 4$d$ orbitals of Ru in the SrRuO$_{3}$ layer, a fact that is independent of the level of correlation included in the simulations. For hybrid functionals or LDA+U, every interface in the superlattice behaves as minority-spin half-metal ferromagnet, with a magnetic moment of $\mu$ = 2.0 $\mu_{\rm B}$/SrRuO$_{3}$ unit. The shape of the electronic density of states, half metallicity and magnetism are explained in terms of a simplified tight-binding model, considering only the $t_{2g}$ orbitals plus (i) the bi-dimensionality of the system, and (ii) strong electron correlations. Possible applications are discussed, from their eventual role in thermoelectric applications to the possible tuning of ferromagnetic properties of the 2DEG with the polarization of the dielectric. Work done in collaboration with P. Garc\'ia, M. Verissimo-Alves, D. I. Bilc, and Ph. Ghosez. [Preview Abstract] |
Monday, March 18, 2013 11:51AM - 12:03PM |
B12.00002: Polarization controlled Ohmic to Schottky transition at a metallic oxide-doped ferroelectric interface Xiaohui Liu, Yong Wang, J.D. Burton, Evgeny Tsymbal Recently the coexistence of ferroelectricity and conductivity was observed in electron-doped BaTiO$_3$ [1], opening an exciting avenue for novel ferroelectric device applications. A basic structure which may be used for future applications is the metal/ferroelectric hetero-junction. Using first-principles methods and taking the SrRuO$_3$/BaTiO$_3$ interface as a prototypical system, we investigate the effects of polarization reversal in BaTiO$_3$ on the electronic transport across this interface. Our studies show a significant change in the resistance by switching ferroelectric polarization. This arises due to the polarization driven conversion of the interface from the Ohmic to the Schottky regime, i.e. for one polarization orientation the interface exhibits a tunneling barrier, whereas the interface is metallic for the opposite polarization orientation. Our prediction represents a new path for ferroelectric devices and may lead to exciting new applications as non-volatile memories and logic. \\[4pt] [1] T. Kolodiazhnyi et al, Phys. Rev. Lett. 104, 147602 (2010). [Preview Abstract] |
Monday, March 18, 2013 12:03PM - 12:15PM |
B12.00003: Controlling the density of electrons in the 2DEG at complex oxide interfaces Chris Van de Walle, Lars Bjaalie, Luke Gordon, Anderson Janotti The formation of a two-dimensional electron gas (2DEG) at the interface between two insulators, SrTiO$_{\mathrm{3\thinspace }}$(STO) and LaAlO$_{\mathrm{3}}$ (LAO), has sparked huge interest in oxide electronics. In spite of almost a decade of research, the mechanisms that determine the density of this 2DEG have not yet been unravelled. The polar discontinuity at the STO/LAO interface can in principle sustain an electron density of 3.3x10$^{\mathrm{14\thinspace }}$cm$^{\mathrm{-2\thinspace }}$(0.5 electrons per unit cell). However, experimentally observed densities are more than an order of magnitude lower. Using a combination of first-principles and Schr\"{o}dinger-Poisson simulations we investigate the origin of the electrons in the 2DEG at the STO/LAO interface. We analyze the asymmetric nature of the heterostructures, i.e., the inability to form a second LAO/STO interface that is a mirror image of the first, and the effects of passivation of the LAO surface. Our results apply to oxide interfaces in general, and explain why the SrTiO$_{\mathrm{3}}$/GdTiO$_{\mathrm{3\thinspace }}$interface has been found to exhibit the full density of 0.5 electrons per unit cell. [Preview Abstract] |
Monday, March 18, 2013 12:15PM - 12:27PM |
B12.00004: Two-dimensional superconductivity induced by high-mobility carrier doping in LaTiO3/SrTiO3 hetero-structures Johan Biscaras, S. Hurand, C. Palma, J. Lesueur, N. Bergeal, D. Leboeuf, C. Proust, A. Rastogi, R.C. Budhani Transition metal oxides display a great variety of quantum electronic behaviors where correlations often play an important role. The achievement of high quality epitaxial interfaces involving such materials gives a unique opportunity to engineer artificial materials where new electronic orders take place. It has been shown recently that a two-dimensional electron gas 2DEG could form at the interface of two insulators such as LaAlO3 and SrTiO3, or LaTiO3 (a Mott insulator) and SrTiO3 [1,2]. We show that a superconducting two-dimensional electron gas is formed at the LaTiO3/SrTiO3 interface whose properties can be modulated by field effect using a metallic gate on the back of the substrate [3,4]. The gas consists of two types of carriers : a majority of low-mobility carriers always present, and a few high-mobility ones that can be injected by electrostatic doping. The calculation of the electrons spatial distribution in the confinement potential shows that the high-mobility electrons responsible for superconductivity set at the edge of the gas whose extension can be tuned by field effect [4].\\[4pt] [1] N. Reyren et al, Science 317, 1196 (2007)\\[0pt] [2] A. Ohtomo et al, Nature 419, 378 (2002)\\[0pt] [3] J. Biscaras et al, Nature Commun 1,89 (2010)\\[0pt] [4] J. Biscaras et al, PRL 108, 247004 (2012) [Preview Abstract] |
Monday, March 18, 2013 12:27PM - 12:39PM |
B12.00005: Incipient 2D Mott insulators in extreme high electron density, ultra-thin GdTiO3/SrTiO3/GdTiO3 quantum wells S. James Allen, Daniel G. Ouellette, Pouya Moetakef, Tyler Cain, Ru Chen, Leon Balents, Susanne Stemmer By reducing the number of SrO planes in a GdTiO$_3$ /SrTiO$_3$/ GdTiO$_3$ quantum well heterostructure, an electron gas with $\sim$ fixed 2D electron density can be driven close to the Mott metal insulator transition - a quantum critical point at $\sim$1 electron per unit cell. A single interface between the Mott insulator GdTiO$_3$ and band insulator SrTiO$_3$ has been shown to introduce $\sim$ 1/2 electron per interface unit cell. Two interfaces produce a quantum well with $\sim$ 7 10$^{14}$ cm$^{-2}$ electrons: at the limit of a single SrO layer it may produce a 2D magnetic Mott insulator. We use temperature and frequency dependent (DC - 3eV) conductivity and temperature dependent magneto-transport to understand the relative importance of electron-electron interactions, electron-phonon interactions, and surface roughness scattering as the electron gas is compressed toward the quantum critical point. Terahertz time-domain and FTIR spectroscopies, measure the frequency dependent carrier mass and scattering rate, and the mid-IR polaron absorption as a function of quantum well thickness. At the extreme limit of a single SrO plane, we observe insulating behavior with an optical gap substantially less than that of the surrounding GdTiO$_3$, suggesting a novel 2D Mott insulator. [Preview Abstract] |
Monday, March 18, 2013 12:39PM - 12:51PM |
B12.00006: Possible Mott physics in SrTiO$_3$/GdTiO$_3$ superlattices Ru Chen, SungBin Lee, Leon Balents We perform generalized gradient approximation (GGA) + Hubbard U to study the thickness-dependent metal to insulator transition in SrTiO$_3$/GdTiO$_3$ superlattices. A full structural optimization procedure is applied, showing significant electronic and structural reconstruction near the interface between the band insulator SrTiO$_3$ and Mott insulator GdTiO$_3$. In addition, we find high charge density at the interface, close to half electron per interface unit cell (pseudo-cubic notation). For the insulating ultra-thin SrTiO$_3$ layer case, we are able to describe it by a low energy effective Hamiltonian. Using Hartree-Fock approximation, we find the combining effect of the hopping parameters and the correlation in the $d$ orbitals of Ti can lead to possible Mott insulating state. Finally, magnetism is also studied and compared with the GGA+U result. [Preview Abstract] |
Monday, March 18, 2013 12:51PM - 1:27PM |
B12.00007: Engineering new properties in PbTiO$_{3}$ based superlattices: compositionally broken inversion symmetry and polarization rotation Invited Speaker: Matthew Dawber In this talk I will present results on two superlattice systems which contain ultra fine layers of PbTiO$_{3}$ and another perovskite material. In recent years, much work has been done on the PbTiO$_{3}$/SrTiO$_{3}$ system, with a focus on improper ferroelectricity and the arrangement of ferroelectric domains. Here, we consider two different partner materials for PbTiO$_{3}$, each of which introduces markedly different behavior in the resulting superlattice. PbTiO$_{3}$/SrRuO$_{3}$ superlattices with ultra-thin SrRuO$_{3}$ layers were studied both experimentally and using density functional theory. Due to the superlattice geometry, the samples show a large anisotropy in their electrical resistivity, which can be controlled by changing the thickness of the PbTiO$_{3}$ layers. Therefore, along the ferroelectric direction, SrRuO$_{3}$ layers can act as dielectric, rather than metallic, elements. We show that, by reducing the thickness of the PbTiO$_{3}$ layers, an increasingly important effect of polarization asymmetry due to compositional inversion symmetry breaking occurs. The compositional inversion symmetry breaking is seen in this bi-color superlattice due to the combined variation of A and B site ions within the superlattice. We have also achieved an experimental enhancement of the piezoelectric response and dielectric tunability in artificially layered epitaxial PbTiO$_{3}$/CaTiO$_{3}$ superlattices through an engineered rotation of the polarization direction. As the relative layer thicknesses within the superlattice were changed from sample to sample we found evidence for polarization rotation in multiple x-ray diffraction measurements. Associated changes in functional properties were seen in electrical measurements and piezoforce microscopy. These results demonstrate a new approach to inducing polarization rotation under ambient conditions in an artificially layered thin film. [Preview Abstract] |
Monday, March 18, 2013 1:27PM - 1:39PM |
B12.00008: Observation of the cubic Rashba effect in a SrTiO$_{3}$ two-dimensional electron gas Hiroyuki Nakamura, Hisashi Inoue, Minu Kim, Chris Bell, Masayuki Hosoda, Yasuyuki Hikita, Hiroshi Kohno, Takaaki Koga, Harold Hwang, Tsuyoshi Kimura Induced spin-orbit coupling effects at oxide interfaces, where $d$-orbitals form the conduction bands, are recently attracting much interest [1-4]. Here, we report magnetotransport of normally-off SrTiO$_{3}$ field-effect transistors with parylene gate insulator at a dilution refrigerator temperatures (50 mK - 1 K). An enlarged contribution of the weak antilocalization /weak localization (WL/WAL) effect in the magnetoconductance compared to that at 2 K [4] is used to analyze the Rashba effect in detail. It will be shown that a theoretical model with effective magnetic field configuration based on the cubic Rashba term perfectly matches the observed WL/WAL data.\\[4pt] [1] H. Nakamura \textit{et al}., Phys. Rev. B, \textbf{80}, 121308(R) (2009).\\[0pt] [2] M. Ben Shalom \textit{et al}., Phys. Rev. Lett. \textbf{104}, 126802 (2010). \\[0pt][3] A. D. Caviglia \textit{et al}., Phys. Rev. Lett. \textbf{104}, 126803 (2010).\\[0pt][4] H. Nakamura \textit{et al}., Phys. Rev. Lett., \textbf{108}, 206601 (2012). [Preview Abstract] |
Monday, March 18, 2013 1:39PM - 1:51PM |
B12.00009: Ionic liquid gating of strontium titanate nanostructures Patrick Gallagher, Sam Stanwyck, Menyoung Lee, James Williams, David Goldhaber-Gordon We present electronic transport measurements of two-dimensional electron systems (2DES) induced on strontium titanate surfaces. Using a combination of ionic liquid gates and nanopatterned metallic gates, we demonstrate the ability to isolate a nanoscale puddle of the 2DES and modulate its conductance over several orders of magnitude. Finally, we discuss the apparently gate-tunable superconducting behavior in these devices. [Preview Abstract] |
Monday, March 18, 2013 1:51PM - 2:03PM |
B12.00010: Transport Measurements of Mesoscopic Hall Bars on Strontium Titanate Sam Stanwyck, Patrick Gallagher, James Williams, David Goldhaber-Gordon We report low-temperature transport measurements of a two-dimensional electron system (2DES) at the surface of Strontium Titanate. We use electrolyte gating to create the 2DES, and then use nanopatterning techniques to define submicron constrictions with gate tunability. We observe features characteristic of superconducting transport through these small constrictions, including a critical current and critical field, but measure a nonzero resistance at zero bias. We consider possible explanations in light of these results, including large spatial inhomogeneities in the order parameter, as well as finite-size effects. [Preview Abstract] |
Monday, March 18, 2013 2:03PM - 2:15PM |
B12.00011: Importance of oxygen vacancies for the two-dimensional metallic state at the surface of SrTiO$_3$ Juan Shen, Harald O. Jeschke, Roser Valenti We analyze by means of density functional theory (DFT) the electronic structure of various oxygen-deficient SrTiO$_3$ surface slabs. We find a significant surface reconstruction after introducing oxygen vacancies and we show that the charges resulting from surface-localized oxygen vacancies --independently of the oxygen concentration-- redistribute in the surface region and deplete rapidly within a few layers from the surface suggesting the formation of a two-dimensional electron system (2DES). We also investigate possible oxygen-vacancy clustering effects and discuss our results in the context of recent angle-resolved photoemission spectroscopy observations of a highly metallic 2DES at the (001) vacuum-cleaved surface of SrTiO$_3$. [Preview Abstract] |
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