Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X37a: Complex Oxide Interfaces and Heterostructures VIIFocus
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Sponsoring Units: DMP DCMP Chair: Alex Demkov, University of Texas Room: 383 |
Friday, March 17, 2017 8:00AM - 8:12AM |
X37a.00001: Electric-field-switchable modulation doping in a ferrroelectric-semiconductor heterostructure Xiaohui Liu, Evgeny Tsymbal, Karin Rabe In most ferroelectric field-effect transistor (FFET) configurations, the modulation of the conductivity by switching of the ferroelectric gate is dominated by the electrostatically-induced change in carrier concentration in the channel. However, recent work on PZT/LaNiO3 has shown that the conductivity of the on state can be dramatically increased by opening of conducting channels in the ferroelectric itself [REF]. In this work, we present first-principles calculations for ferroelectric BaTiO3 on n-type SrTiO3, showing an even larger effect of this type.~ We present a simple model that shows that conduction in the ferroelectric is largely controlled by the work function step between the ferroelectric and the channel material. Extensions of these investigations to other FFET material combinations will be discussed.~ \newline \newline [REF] M. S. J. Marshall, A. Malashevich, A. S. Disa, M.-G. Han, H. Chen, Y. Zhu, S. Ismail-Beigi, F. J. Walker and C. H. Ahn, Phys. Rev. Applied 2, 051001 (2014). [Preview Abstract] |
Friday, March 17, 2017 8:12AM - 8:24AM |
X37a.00002: Selective area growth of BaTiO$_{3}$ for ferroelectric field-effect transistor application Patrick Ponath, Agham Posadas, Michael Schmidt, Paul Hurley, Ryan Duffy, Jian Wang, Chadwing Yound, Alex Demkov Titanates are an important class of materials with many interesting functional properties and applications for non-volatile memory, i.e. BaTiO$_{3}$, which is a promising candidate for the realization of a ferroelectric field-effect transistor. However, the difficulty of chemically etching titanates has hindered their commercial use in device manufacturing so far. Here, we propose a technique to circumvent this problem. Using molecular beam epitaxy, we grew compressively strained ferroelectric BaTiO$_{3}$, within photolithographically defined openings of a sacrificial SiO$_{2}$ layer on germanium (001) and strontium titanate (001). Etching away the sacrificial SiO$_{2}$ can reveal isolated nanoscale gate stacks circumventing the need to etch the titanate thin film. Different SiO$_{2}$ processing techniques are compared for Ge(001) and Nb:STO(001) substrates and the thermal stability of the SiO$_{2}$ pattern as well as the resulting surface roughness after a thermal anneal will be reported. Using X-ray diffraction we find that the BaTiO$_{3}$ film is tetragonal with the longer $c$-axis being out of plane, which is a requirement for the ferroelectric field effect transistor. The crystal quality of the BaTiO$_{3}$ films grown in the openings is confirmed using RHEED and cross-sectional transmission electron microscopy. [Preview Abstract] |
Friday, March 17, 2017 8:24AM - 9:00AM |
X37a.00003: Tunable inversion symmetry in heterostructures of layered oxides Invited Speaker: James Rondinelli Traditional approaches to create and control functional electronic materials have focused on new phases in previously unknown bulk minerals. More recently, interlayer physics has spawned interest in known materials in unexplored atomic scale geometries, especially in complex transition metal oxides (TMO), where heterostructures can be created on demand. In this talk, I show that although epitaxial strain routinely induces (enhances) electric polarizations, biaxial strain can also induce an unanticipated polar-to-nonpolar (P-NP) structural transition in (001) thin films of naturally layered $A_{n+1}B_n$O$_{3n+1}$ ($n=1-\infty$) oxides. Density functional theory calculations and a complete phenomenological model for Ca$_3$Ti$_2$O$_7$ are used to show that the origin of the P-NP transition originates from the interplay of trilinear-related lattice mode interactions active in the layered oxides, and those interactions are directly strain tunable. Moreover these layered oxides exhibit a quasi-two dimensional phonon mode---an acoustic branch with quadratic dispersion, enabling unusual membrane effects such as tunable negative thermal expansion. I conclude by emphasizing that broken inversion symmetric structures offer a plentiful playground for realizing new functionalities in thin films, including new multiferroics from polar metals. [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X37a.00004: First-principles Studies of Ferroelectricity in $ \rm BiMnO_3 $ Thin Films Yun-Peng Wang, Hai-Ping Cheng The ferroelectricity in $ \rm BiMnO_3 $ thin films is a long-standing problem. We employed a first-principles density functional theory with inclusion of the local Hubbard Coulomb (U) and exchange (J) terms. The parameters U and J are optimized to reproduce the atomic structure and the energy gap of bulk C2/c $ \rm BiMnO_3 $. With these optimal U and J parameters, the calculated ferromagnetic Curie temperature and lattice dynamics properties agree with experiments. We then studied the ferroelectricity in few-layer $ \rm BiMnO_3 $ thin films on $ \rm SrTiO_3 $(001) substrates. Our calculations identified ferroelectricity in monolayer, bilayer and trilayer $ \rm BiMnO_3 $ thin films. We find that the energy barrier for $90^\circ$ rotation of electric polarization is about 3 -- 4 times larger than that of conventional ferroelectric materials. [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X37a.00005: Electric field control of magnetism and metal-insulator transition in multiferroic superlattices from nonmagnetic polar metals SAURABH GHOSH, Albina Y. Borisevich, Sokrates T. Pantelides LiOsO$_3$ is a ferroelectric metal and LiNbO$_3$ is a prototype ferroelectric insulator, whereas (LiOsO$_3$)$_1$/ (LiNbO$_3$)$_1$ is predicted to be a correlated mott multiferroic [ Danilo Puggioni et.al. Phys. Rev. Lett. 115, 087202 (2015)]. According to this paper, tuning the degree of electronic correlation in this system leads to a phase transition from an antiferromagnetic polar metal (APM) phase to an antiferromagnetic ferroelectric insulating (AFI) phase. In view of the interest for technological applications and for fundamental understanding, the important question that remains open is whether electric-field control of such an APM-AFI phase transition is possible. Here, using first-principles density functional theory (DFT) and DFT+U (static $d-d$ Coulomb interaction), and considering (LiOsO$_3$)$_m$ / (LiNbO$_3$)$_n$ superlattices, we have investigated possible electric-field control of magnetism and band gap in the AFI phase. We found that these systems are weak ferromagnetic (with net magnetic moment of $\sim$ 0.15$\mu_B$/Os) and strong magnetoelectric. Thus, electric field control of magnetism is possible in these systems. Further, we discuss the possibility of tuning the band gap with an electric field. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X37a.00006: First-Principles Prediction of Two-Dimensional Electron Gas Driven by Polarization Discontinuity in Nonpolar/Nonpolar AHfO3/SrTiO3 (A=Ca, Sr, and Ba) Heterostructures Jianli Cheng, Safdar Nazir, Kesong Yang By using first-principles electronic structure calculations, we explored the possibility of producing two-dimensional electron gas (2DEG) in nonpolar/nonpolar AHfO$_3$/SrTiO$_3$ (A = Ca, Sr, and Ba) heterostructures (HS). Two types of interfaces, AO/TiO$_2$ and HfO$_2$/SrO, each with AO and HfO$_2$ surface terminations, are modeled, respectively. The polarization domain and resulting interfacial electronic property are found to be more sensitive to the surface termination of the film rather than the interface model. As film thickness increases, an insulator-to-metal transition (IMT) is found in all the HS with HfO$_2$ surface termination: for AO/TiO$_2$ interfaces, predicted critical film thickness for an IMT is about 7, 6, and 3 unit cells for CaHfO$_3$/SrTiO$_3$, SrHfO$_3$/SrTiO$_3$, and BaHfO$_3$/SrTiO$_3$, respectively; for HfO$_2$/SrO interfaces, the critical film thickness is about 7.5, 5.5, and 4.5 unit cells, respectively. In contrast, for the HS with AO surface termination, only CaHfO$_3$/SrTiO$_3$ exhibits an IMT with a much larger critical film thickness about 11 - 12 unit cells. This work is expected to stimulate further experimental investigation to the interfacial conductivity in the nonpolar/nonpolar AHfO$_3$/SrTiO$_3$ HS. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X37a.00007: Reconstruction of Polarization Vortices by Diffraction Mapping of Ferroelectric PbTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$~Superlattice Using a High Dynamic Range Pixelated Detector Kayla Nguyen, Yi Jiang, Michael Cao, Prafull Purohit, Celesta Chang, Ajay Yadav, Margaret McCarter, Javier Hong, Mark Tate, Ramamoorthy Ramesh, Sol Gruner, David Muller Ferroelectric polarization vortices realized in PbTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ superlattice of complex oxides$^{\mathrm{\thinspace }}$are emerging phenomena that can be used for low-power memory. Here, we apply a high dynamic range electron microscopy pixel array detector (EMPAD) developed at Cornell where convergent beam electron diffraction (CBED) pattern is recorded at 1 ms. Using the EMPAD, we measured \textless P\textgreater of individual diffracted spots where vectorization of \textless Px\textgreater and \textless Py\textgreater reconstructs the polarization vortices at 0.5-1 nm, over an arbitrarily large field of view. Applications of ptychography and careful inspection of \textless P\textgreater help us explore chirality in 3-dimensions and measure the orbital angular momentum of the polarization vortices. [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X37a.00008: Predicting two-dimensional electron gas mediated magnetoelectric coupling at ferroelectric PbTiO$_{3}$/SrTiO$_{3}$ heterostructure Lan-ying Wei, Chao Lian, Sheng Meng We predict the emergence of interfacial ferromagnetism in the two-dimensional electron gas (2DEG) at the ferroelectric PbTiO$_{3}$/SrTiO$_{3}$ heterostructure from first principles calculations. Free electrons provided by naturally existing oxygen vacancies in SrTiO$_{3}$ are driven to the heterostructure interface under the polarizing electric field of ferroelectric PbTiO$_{3}$. These spin-polarized electrons are captured by interfacial Ti atoms, forming a 2DEG, which surprisingly exhibits ferromagnetism even at room temperature, once the density of 2DEG reaches a relatively small critical value of $\sim$60 $\mu$C/cm$^2$. This critical value can be easily achieved in practice. The ferroelectricity-controlled ferromagnetism mediated by 2DEG via interface shows exceptional magnetoelectric coupling strength several times larger than that of other systems, which enables us to control the magnetism by electric field conveniently. Besides, the PbTiO$_{3}$/SrTiO$_{3}$ heterostructure is cheap, easy-growing and controllable, facilitating great promising applications in low cost spintronics and information storage at room temperature. [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X37a.00009: Local modulation of carrier density in graphene-ferroelectric field effect transistors through flexoelectric switching Anna Gura, Hsiang C. Hsing, Mohammed Yusuf, Xu Du, Mattew Dawber We use a ferroelectric (FE) material to harness the electric functionalities of graphene (Gr) by engineering Gr-FE Field Effect Transistors. In these devices, the underlying FE superlattice layer is used to control the charge state of the Gr channel. By using artificially layered FE superlattices and optimizing parameters during growth and Gr deposition, we have obtained ideal interfaces that result in hysteretic devices. However, our successful devices using PbTiO$_{3}$/SrTiO$_{3}$ as the FE layer display a shift of the gating and C-V curves towards positive gate voltages, making the polarization state unstable. We believe this is caused by ordered structural defects that arise during growth of the superlattice. To overcome this obstacle we have designed a hybrid superlattice system consisting of PbTiO$_{3}$/SrTiO$_{3}$/PbTiO$_{3}$/SrRuO$_{3}$ alternating layers. In these samples the C-V measurements are centered on 0V, providing retention of the polarization state without any applied compensation bias and enabling non-volatile polarization switching as a result of strain applied by an AFM Tip. We studied local changes in conductivity of the Gr and demonstrate the use this technique to design re-writable circuit elements on the graphene-FE hybrid devices. [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X37a.00010: Emergence of complex polar order in asymmetric PbTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ superlattices Margaret McCarter, Ajay Yadav, Shang-Lin Hsu, Zijian Hong, Anoop Rama Damodaran, Christopher Nelson, Julia Mundy, Long-Qing Chen, Lane Martin, Ramamoorthy Ramesh Recent work has shown the formation of three-dimensional polar vortices in ferroelectric/paraelectric (PbTiO$_{\mathrm{3}})_{n}$/(SrTiO$_{\mathrm{3}})_{n} $superlattices. This novel phase is stabilized by the interplay of lattice, charge, and orbital degrees of freedom. In this work, we employ a systematic study of the electrical polarization in asymmetric superlattices of PbTiO$_{\mathrm{3}}$ and SrTiO$_{\mathrm{3}}$ grown by pulsed laser deposition. In (PbTiO$_{\mathrm{3}})_{m}$/(SrTiO$_{\mathrm{3}})_{n}$ superlattices, phase-field modeling predicts a systemic evolution of the polarization state in the superlattice as a function of SrTiO$_{\mathrm{3}}$ thickness. We will present results of a combined experimental-theoretical study of the emergence of complex topologies of electrical polarization. To study superlattices as a function of SrTiO$_{\mathrm{3}}$ thickness, we map the electrical polarization using high-resolution scanning transmission electron microscopy in conjunction with piezoresponse force microscopy studies of the ferroelectric domain structures. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X37a.00011: Vortex-antivortex structures in PbTiO$_{3}$/SrTiO$_{3}$ superlattices Javier Junquera, Pablo Garc\'{\i}a-Fern\'andez, Pablo Aguado-Puente, Jorge \'I\~niguez When ultrathin ferroelectric layers of PbTiO$_{3}$ are embedded in superlattices with an incipient ferroelectric, such as SrTiO$_{3}$, the interplay between elastic, electrostatic, and gradient energies produce complex patterns of the electrical polarization. In particular, nanometer scale of vortex-antivortex arrays have been recently detected,\footnote{Yadav {\it et al.}, Nature {\bf 530}, 198 (2016)} and exotic properties such as the emergence of a negative capacitance have been measured.\footnote{\label{Zubko}P. Zubko {\it et al.}, Nature {\bf 534}, 524 (2016)} A realistic atomic simulation of these structures is difficult due to the large number of atoms required and the small differences in energies between the relevant phases. Here we use a recently developed second-principles method~$^{3,}$ \footnote{J. Wojdel {\it et al.}, J. Phys.: Condens. Matter {\bf 25}, 305401 (2013)} that treats all the lattice degrees of freedom with high accuracy at a modest computational cost. The effect of the periodicity, strain, temperature, and external electric fields in the formation of vortex-antivortex pairs is explored. We predict that some of the structures are chiral, that would make them optically active, supporting x-ray circular dichroism. [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X37a.00012: Understanding polarization bias in PbTiO3/SrTiO3 and PbTiO3/SrRuO3 superlattices Simon Divilov We use first principles density functional theory to explain how Pb-O divacancies (DV) and other symmetry breaking factors create a preferred polarization state leading to a bias as seen in experiments. Our results for PbTiO$_3$/SrTiO$_3$ (PTO/STO) indicate that Pb-O DV prefer to form on one side of the interface than the other during growth, leading to a preferred polarization state. For PbTiO$_3$/SrRuO$_3$ (PTO/SRO) the situation is more complicated due to intrinsic compositional inversion symmetry breaking, which competes with the tendency of DV to form at different sites within the superlattice structure. [Preview Abstract] |
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