Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A13: Focus Session: Titanate Interfaces and Heterostructures |
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Sponsoring Units: DMP Chair: Roman Engel-Herbert, Pennsylvania State University Room: 007D |
Monday, March 2, 2015 8:00AM - 8:36AM |
A13.00001: Thermal and photoconductivity at the La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CrO$_{3}$/SrTiO$_{3}$(001) interface Invited Speaker: Scott Chambers We have investigated the electronic and photophysical properties of interfaces between La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CrO$_{3}$ and SrTiO$_{3}$(001) as prepared by molecular beam epitaxy. LaCrO$_{3}$ is a III-III antiferromagnetic insulator whereas SrCrO$_{3}$ is a II-IV metallic oxide. Substituting Sr$^{2+}$ for La$^{3+}$ in LaCrO$_{3}$ effectively dopes holes into the top of valence band, reducing the band gap and the resistivity, and generating a $p$-type oxide semiconductor In contrast, SrTiO$_{3}$ is a wide-gap II-IV semiconductor that is readily made $n$-type by La doping. Therefore, the La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CrO$_{\mathrm{3}}$/SrTiO$_{3}$(001) system has much potential for interesting interface physics with regard to studying intrinsic conductivity via electronic reconstruction and electron-hole pair separation upon light irradiation. However, there are inherent physical and chemical complexities at these interfaces and within the bulk of the La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$CrO$_{3}$ films which can have profound effects on the associated functional properties. In this talk, we present some of our most recent results from this ongoing investigation.\\[4pt] In collaboration with Kelvin Zhang, Du Yingge, Sushko Peter, Bowden Mark, Shutthanandan V, Pacific Northwest National Laboratory; and Shawn Sallis, Louis Piper, Binghamton University. [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A13.00002: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 8:48AM - 9:00AM |
A13.00003: Extreme high-density electron gas using band engineered complex oxide interfaces Peng Andrew Xu, Timothy C. Droubay, Jong Seok Jeong, Scott A. Chambers, Andre K. Mkhoyan, Bharat Jalan The study of interfaces between polar and non-polar complex oxides has seen unprecedented growth due to their unique ability to display interface-stabilized ground states including high-density two-dimensional electron gas (equivalent to 0.5 electron/u.c./interface). In this talk, we will present detailed thickness dependent structural and electronic transport study of the MBE-grown NdTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ and SrTiO$_{\mathrm{3}}$/NdTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ heterostructures. High-resolution x-ray diffraction, atomic force microscopy, reflection high-energy electron diffraction, scanning transmission electron microscopy and different spectroscopy techniques reveal nearly stoichiometric composition and abrupt interfaces. We will review the long-standing question on the origin of carriers at these interfaces and will present novel routes to achieve carrier density in excess of 0.5 electron/unit cell/interface using band engineered oxide interfaces. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A13.00004: Band offset engineering of 2DEG oxide systems on Si Eric Jin, Lior Kornblum, Divine Kumah, Ke Zou, Christine Broadbridge, Joseph Ngai, Charles Ahn, Fred Walker The discovery of 2-dimensional electron gases (2DEGs) at perovskite oxide interfaces has sparked much interest in recent years due to their large carrier densities when compared with semiconductor heterostructures. For device applications, these oxide systems are plagued by low room temperature electrical mobilities. We present an approach to combine the high carrier density of 2DEG oxides with a higher mobility medium in order to realize the combined benefits of higher mobility and carrier density. We grow epitaxial films of the interfacial oxide system LaTiO$_3$/SrTiO$_3$ (LTO/STO) on silicon by molecular beam epitaxy. Magnetotransport measurements show that the sheet carrier densities of the heterostructures scale with the number of LTO/STO interfaces, consistent with the presence of a 2DEG at each interface. Sheet carrier densities of 8.9 x 10$^{14}$ cm$^{-2}$ per interface are measured. Band offsets between the STO and Si are obtained, showing that the conduction band edge of the STO is close in energy to that of silicon, but in a direction that hinders carrier transfer to the silicon substrate. Through modification of the STO/Si interface, we suggest an approach to raise the band offset in order to move the 2DEG from the oxide into the silicon. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A13.00005: High-carrier-density phase in LaTiO$_{3}$/SrTiO$_{3}$ superlattices Se Young Park, Karin Rabe, Andrew Millis We investigate superlattices composed of alternating layers of Mott insulating LaTiO$_{3}$ and band insulating SrTiO$_{3}$ from first principles, using the density functional theory plus U (DFT+U) method. For values of U above a critical threshold, we find that melting of the Mott-insulating phase can extend from the interface into the LaTiO$_{3}$ layer, resulting in a sheet carrier density exceeding the density of 0.5 electrons per in-plane unit cell found in previous studies. The critical U for the melting transition is larger than the critical Coulomb correlation required for the insulating LaTiO$_{3}$, suggesting the existence of a high sheet carrier density phase in LaTiO$_{3}$/SrTiO$_{3}$ superlattices. The effects of in-plane strain and varying layer thickness on the melting transition are discussed. For insulating superlattices, we study the strain and thickness dependence of the polarization and its relation to near-interface local atomic distortions. [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A13.00006: Phase diagram and high density two-dimensional electron gas at the LaAlO$_{3}$/La$_{0.5}$Sr$_{0.5}$TiO$_{3}$/SrTiO$_{3}$ heterostructures Haijiao Ma, Zhen Huang, Shengwei Zeng, Anil Annadi, Thirumalai Venky Venkatesan, Ariando Ariando We report a two dimensional electron gas with a high carrier density at the LaAlO$_{\mathrm{3}}$/La$_{\mathrm{0.5}}$Sr$_{\mathrm{0.5}}$TiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ heterostructures, reaching a value of about five times higher than that observed at the LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ interface. The La$_{\mathrm{0.5}}$Sr$_{\mathrm{0.5}}$TiO$_{\mathrm{3}}$ polar layer is introduced to preserve the degeneracy of the Ti t$_{\mathrm{2g}}$ orbitals and minimize the disorder at the La$_{\mathrm{0.5}}$Sr$_{\mathrm{0.5}}$TiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ interface. Various thickness combinations of La$_{\mathrm{0.5}}$Sr$_{\mathrm{0.5}}$TiO$_{\mathrm{3}}$and LaAlO$_{\mathrm{3}}$ layers are used for tuning the total internal potential of the polar layer responsible for the charge transfer. Experimental data showed that the carrier density increases by raising the total internal potential, and this is in a good agreement with a simple electrostatic model. A complete metal-insulator phase diagram is obtained, which shows that at least 3.15 eV polar potential is needed to form the metallic interface at the SrTiO$_{\mathrm{3}}$, providing an estimate for the critical thickness needed for the metallic phase. Nonlinear Hall effect was observed below 60 K which can be understood by multiple filling of the degenerated orbitals responsible for multiple band electronic conductions. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A13.00007: Induced Itinerant Antiferromagnetism in SrTiO3 Brandon Isaac, Evgeny Mikheev, Chris Freeze, Susanne Stemmer Interfaces between RTiO3 (R $=$ Gd or Sm) and SrTiO3 grown by molecular beam epitaxy induce a high-density two-dimensional electron gas (2DEG). Such 2DEGs show evidence of strong electron correlation effects, including non-Fermi liquid behavior, and a non-trivial magnetoresistance. Here we present magneto-transport measurements of SmTiO3/SrTiO3/SmTiO3 quantum well structures to investigate induced, itinerant, antiferromagnetic ordering in the SrTiO3 through proximity to the SmTiO3. At low temperatures, the Hall effect, the longitudinal resistance, and the magnetoresistance all show evidence of itinerant antoferromagnetism. For example, the longitudinal resistance shows deviations from the logarithmic correction of weak localization, and the Hall effect indicates opening of a gap and loss of carriers. The results will be discussed in terms of a possible spin-density wave formation and gap opening on the Fermi surface. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A13.00008: Pseudogaps in SrTiO$_{3}$ Quantum Wells Patrick Marshall, Santosh Raghavan, Evgeny Mikheev, Susanne Stemmer A departure from Fermi liquid behavior appears in molecular beam epitaxy grown SrTiO$_{3}$ quantum wells embedded in the antiferromagnetic insulator SmTiO$_{3,\, }$suggesting proximity to a quantum critical point. We will report on the observation of pseudogap behavior in SmTiO$_{3}$/SrTiO$_{3}$/SmTiO$_{3}$ quantum wells via tunneling spectroscopy measurements. Tunnel junction devices with SrZrO$_{3}$ barriers grown in-situ were fabricated from quantum wells of varying thickness. The tunneling conductance spectra of these devices revealed the formation of a pseudogap in the density of states upon cooling, indicating the onset of non-Fermi liquid behavior. The pseudogap state was most pronounced in thin quantum wells, persisting up to nearly 200 K in the well containing only 2 SrO layers. The pseudogap was absent in the thickest wells, which showed only a small suppression of the density of states with a logarithmic dependence on bias resulting from disorder. The results are compared to tunneling spectra of GdTiO$_{3}$/SrTiO$_{3}$/GdTiO$_{3}$ quantum wells, providing insight into the role of structural distortion and octahedral tilts on the electronic structure and quantum critical behavior in oxide heterostructures. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A13.00009: Tunable Orbital-Selective Magnetic Interaction in Tricolor Oxide Interfaces Yanwei Cao, Michael Kareev, Xiaoran Liu, Debraj Choudhury, Srimanta Middey, Derek Meyers, Jak Chakhalian Recently, several theoretical scenarios of orbital-selective magnetic interactions were proposed to understand the emergence of the unexpected interfacial magnetism in the archetypical SrTiO$_{\mathrm{3}}$-based two-dimensional electron gas systems, the origin of which is still intriguing and not an entirely understood phenomenon in oxide interface physics. Experimentally, however, there thus far lacks a material system to directly demonstrate the magnetic interaction with orbital-selection (dxy vs. dxz/dyz) and eventually manipulate this magnetic interaction. To address this, here we induced 2DEG and localized magnetism into the same SrTiO$_{\mathrm{3}}$ layer by devising the heterostructure LaTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$/YTiO$_{\mathrm{3}}$. Combined electrical transport and atomic-resolved scanning transmission electron microscope with electron energy loss spectroscopy revealed that the magnetic localized electrons are formed by the spin transfer from the YTiO3 layer into 2DEG formed at the LaTiO$_{\mathrm{3}}$ /SrTiO$_{\mathrm{3}}$ interface, with the orbital occupancy and strength of the magnetic interaction controlled by the SrTiO$_{\mathrm{3}}$ layer thickness. Our work provides an ideal platform to explore the orbital physics driven by the interfacial magnetism with prospects for exciting spintronic applications. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A13.00010: High electron density 2DEGs at 111 SrTiO$_{3}$/SmTiO$_{3}$ interfaces Santosh Raghavan, Susanne Stemmer (001) SrTiO$_{3}$ quantum wells formed in structures such as RTiO$_{3}$/SrTiO$_{3}$/RTiO$_{3}$ (R $=$ Gd or Sm) exhibit two dimensional electron gases (2DEGs) that exhibit ferromagnetism and non-Fermi-liquid behavior. 2DEGs are also expected for (111) quantum wells, which furthermore form a honeycomb lattice that is susceptible to geometric frustration and nontrivial band structures. In this work, we present the growth of high quality SrTiO$_{3}$ and SmTiO$_{3}$ layers on (111) LSAT substrates using a hybrid molecular beam epitaxy technique. Structural and electrical characterization show atomically sharp (111) interfaces between SrTiO$_{3}$ and SmTiO$_{3}$, and the presence of a high charge carrier density of $\sim$ 3 $\times$ 10$^{14}$ cm$^{-2}$. We will discuss results of magneto-transport studies in highly confined quantum wells that are only a few atomic planes thick and compare them with results from (001) interfaces. [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A13.00011: ABSTRACT WITHDRAWN |
Monday, March 2, 2015 10:36AM - 10:48AM |
A13.00012: Thickness dependent metal-insulator transition in GdTiO$_{3}$/SrTiO$_{3}$ superlattices Anderson Janotti, Lars Bjaalie, Burak Himmetoglu, Chris Van de Walle SrTiO$_{3}$ (STO) is at the core of recent discoveries of two-dimensional electron gas (2DEG) formation at complex oxide interfaces, with the 2DEG residing on the STO side. Experimental results for ultrathin STO layers inserted in GdTiO$_{3}$ reveal a transition from metallic to insulating behavior, and suggest a strong interplay between electron-electron interaction and lattice distortions. Using first-principles calculations we investigate the evolution of the electronic structure of STO/GTO superlattices as a function of the thickness of the STO layer. We show that the metal-to-insulator transition is a bulk property of STO that emerges at extreme doping levels [1]. For thick STO layers, we find a two-dimensional electron gas with a density of 1/2 electron per unit-cell area per interface within the STO. However, once the STO layer thickness is reduced below three layers, we find that the electrons localize on every second interface Ti atom, giving a charge-ordered Mott-insulating phase. This onset of localization is analyzed in terms of the electron density in the STO layer and octahedral distortions at the interface. The Mott-insulator phase is shown to occur both in STO/LaAlO$_{3}$ and STO/GdTiO$_{3}$ heterostructures with ultrathin STO layers. \\[4pt] [1] L. Bjaalie, A. Janotti, B. Himmetoglu, and C. G. Van de Walle, Phys. Rev. B \textbf{90}, 195117 (2014). [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A13.00013: Mixed-valence magnetism in TiO$_{2}$/TiO$_{\mathrm{x}}$ superlattices W.C. Hsieh, P.V. Wadekar, H.C. Huang, C.F. Chang, M.S. Wong, H.W. Seo, F.C. Chuang, Q.Y. Chen Epitaxial TiO$_{2}$ and TiO$_{\mathrm{x}}$ superlattices, $\sim$ 1-nm thick per layer by sputtering at 570$^{\circ}$C using pure argon on sapphire substrates. From HR-TEM, the periodically alternating layers are well-defined. XPS analyses based on the binding energy of Ti 2p$_{3/2}$ peaks suggest the co-existence of Ti$^{+3}$ and Ti$^{+4}$, thus verifying the mixed-valence nature. The M(H) curves measured at room temperature using SQUID showed hysteretic loops typical of ferromagnetism. Electrical transport measurements were done at zero field demonstrate transition of charge ordering at low temperatures, reminiscent of what was found in Ti-rich Ti$_{\mathrm{1+x}}$O$_{2}$ single-layer thin films, made by Ti ion implantation into TiO$_{2}$ crystals, in which randomly distributed TiO$_{2}$, Ti$_{2}$O$_{3}$ and TiO were found to coexist. Preliminary First-principle (\textit{ab initio}) calculations to understand the roles of oxygen vacancies in various TiO$_{2}$ super-cells could indeed lead to spontaneous magnetizations. We thus argue that mixed-valence titanium ions are responsible for the magnetism [Preview Abstract] |
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