Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session J64: Thermoelectricity, Ferroelectricity, and Dielectric Behavior of Complex Oxide Films and HeterostructuresFocus
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Sponsoring Units: DMP Chair: Bharat Jalan, University of Minnesota Room: Mile High Ballroom 4E |
Tuesday, March 3, 2020 2:30PM - 3:06PM |
J64.00001: Tuning the electronic and thermoelectric response of oxide superlattices by confinement, strain and interface polarity Invited Speaker: Rossitza Pentcheva Transition metal oxides are prospective candidates for energy conversion applications e.g. as thermoelectrics owing to their chemical and thermal stability and in particular to their complex correlated nature. Nanostructuring and reduced dimensionality can lead to further performance enhancement. By combining DFT+U calculations and Boltzmann transport theory we explore the implications of interface polarity, confinement and strain on the thermoelectric properties of perovskite superlattices. Taking as an example LaNiO3/SrTiO3(001), we demonstrate that compatible n- and p-type materials can be realized by selective choice of the layer stacking at polar interfaces [1]. On the other hand, a strongly enhanced thermoelectric response is obtained in nonpolar LaNiO3/LaAlO3(001) superlattices due to the confinement-driven metal-to-insulator transition [2]. This concept is further extended to (SrXO3)1/(SrTiO3)n(001) SL with X = V, Cr, and Mn [3]. Last but not least, the thermoelectric response of topologically nontrivial phases is discussed. |
Tuesday, March 3, 2020 3:06PM - 3:18PM |
J64.00002: MBE-grown BaTiO3 films for waste heat energy conversion using first order phase transformation William Nunn, Ashley Bucsek, Maike Wegner, Eckhard Quandt, Richard D. James, Bharat Jalan A new method for heat to electricity conversion which takes advantage of the first order phase transformation of ferroelectric materials will be presented. Low temperature waste heat can be utilized with the proposed device in which a transfer of energy was successfully shown using a BaTiO3 single crystal heated and cooled repeatedly through its phase transformation. [1] We use these results as a motivation for our discussion on the growth of ferroelectric thin films using the hybrid molecular beam epitaxy approach. High quality, epitaxial single crystal BaTiO3 thin films were grown using this technique and showed a bulk-like phase transformation with thickness of 350 nm, an encouraging behavior for our device. By alloying with BaSnO3, we also explore the BaTiO3-BaSnO3 system for its potential advantages for energy conversion including control over dielectric constant and Curie temperature. We will analyze structure and composition through high resolution x-ray diffraction and x-ray photoelectron spectroscopy as well discuss their effect on dielectric and ferroelectric properties. |
Tuesday, March 3, 2020 3:18PM - 3:30PM |
J64.00003: Artificial two-dimensional ferroelectric metal at room temperature Mikhail Kareev, Yanwei Cao, Zhen Wang, Se Young Park, Yakun Yuan, Xiaoran Liu, Sergey M Nikitin, Hirofumi Akamatsu, Derek Meyers, Srimanta Middey, Paul Thompson, Philip Ryan, Padriac Shafer, Alpha T. N'Diaye, Elke Arenholz, Venkatraman Gopalan, Yimei Zhu, Karin M Rabe, Jak Chakhalian Polar metals, commonly defined by the coexistence of polar crystal structure and metallicity, are thought to be scarce because the long-range electrostatic fields are expected to be fully screened by the conduction electrons of a metal. Moreover, reducing from three to two dimensions, it remains an open question whether a polar metal can exist. Here we report on the realization of a room temperature two-dimensional ferroelectric metal in a tri-color superlattice BaTiO3/SrTiO3/LaTiO3. A combination of advanced probes and DFT calculations have revealed the microscopic mechanisms of unusual periodic electric polarization, charge distribution, and orbital symmetry. Our results provide a route to create all-oxide artificial non-centrosymmetric quasi-two-dimensional metals with exotic quantum and topological states including potentially coexisting ferroelectric, ferromagnetic, and superconducting phases. |
Tuesday, March 3, 2020 3:30PM - 3:42PM |
J64.00004: Tuning the correlation strength via cation order in double perovskites Turan Birol, Arpita Paul A2BB'O6 double perovskites often behave very differently than their parent compounds. This can lead to phenomena such as frustrated magnetism or half metallicity, which are often driven by inter-cationic charge transfer. In this talk, we study the V-Nb double perovskites, where V and Nb ions attain different valences despite coming from the same column in the periodic table. By performing first principles Density Functional Theory + Dynamical Mean Field Theory (DFT+DMFT) calculations, we show that the interplay of cation order and this charge transfer affect the electronic structure of Sr2VNbO6 in a very interesting way, and gives rise to a phase diagram that includes correlated Hund's metallic, as well as Mott insulating phases. |
Tuesday, March 3, 2020 3:42PM - 3:54PM |
J64.00005: Ferroelectric Polarization Induced Magnetic Order and Topological Hall Effect at the PbZr0.2Ti0.8O3/SrIrO3 Heterointerface Le Zhang, Myung-Geun Han, Yimei Zhu, Xia Hong The 5d Iridate SrIrO3 (SIO) is a paramagnetic semimetal with strong spin-orbit coupling (SOC) when prepared in the orthorhombic phase. In this study, we explore the possibility of inducing magnetic order and nontrivial chiral spin structures in ultrathin SIO films using the polarization field of a neighboring ferroelectric layer. We have fabricated high quality epitaxial PbZr0.2Ti0.8O3 (PZT)/SIO (1.6-2 nm) heterostructures and demonstrated nonvolatile ferroelectric field-effect modulation of channel resistance. For the 1.6 nm SIO channel, we can reversibly control SIO between the metallic and insulating phases by switching the polarization field of PZT. More interestingly, in the insulating phase, the Hall effect measurements reveal features of hysteresis and topological Hall effect, suggesting a polarization-induced antiferromagnetic order. We ascribe it to the emerging interfacial Dzyaloshinskii–Moriya interaction due to strong SOC and polarization-enhanced inversion symmetry breaking. Our results demonstrate a feasible way to achieve electrically controlled magnetic order and design topological electronics at the heterointerface of complex oxides. |
Tuesday, March 3, 2020 3:54PM - 4:06PM |
J64.00006: Charge Engineering in Nickelate-based Mott Transistors Gated by Ferroelectrics Yifei Hao, Xuegang Chen, Le Zhang, Xia Hong We present a comprehensive study of ferroelectric-gated Mott transistors based on high quality epitaxial heterostructures composed of a ferroelectric Pb(Zr,Ti)O3 (PZT) gate and a correlated oxide RNiO3 (R = La, Nd, Sm) channel. For single layer nickelate channels, including Sm0.5Nd0.5NiO3, NdNiO3 (NNO), and LaNiO3 (LNO), the resistance switching ratio ΔR/Ron increases exponentially with decreasing channel thickness until it approaches the electrical dead layer thickness, with the maximum ΔR/Ron~194% observed in 1 nm LNO channel. Inserting a La1-xSrxMnO3 (LSMO) buffer layer results in up to two orders of magnitude increase in ΔR/Ron for devices with the same total channel thickness, with the maximum ΔR/Ron reaching 1,225% in the NNO/LSMO channel. The giant enhancement is attributed to the interfacial charge transfer effect between RNiO3 and LSMO, which effectively reduces the carrier density in the active channel. Our studies address the key materials challenges that limit the application potential of epitaxial complex oxide-based field effect transistors. |
Tuesday, March 3, 2020 4:06PM - 4:18PM |
J64.00007: Spin-dependent charge transport in crystalline BaTiO3-germanium tunnel junctions Yichen Jia, Charles H Ahn, Frederick J Walker The epitaxial integration of functional oxides on conventional semiconductors using advanced molecular beam epitaxy opens new opportunities for coupling their unique properties with semiconductors for post-CMOS computing paradigms. Here we measure spin-dependent transport in Permalloy-BaTiO3-Ge tunnel junctions using the three-terminal Hanle technique. We show that the aligned conduction bands at the epitaxial BaTiO3-Ge interface allow controllable charge transport between quantum tunneling and trap-assisted conduction through an external bias. In the quantum tunneling region, we observe a negative magnetoresistance with a full width at half maximum of ~100 mT when an external magnetic field is applied perpendicular to the BaTiO3-Ge interface. The negative magnetoresistance evolves into a superposition of two peaks with opposite polarity and distinct linewidth when the trap-assisted conduction becomes dominant. Possible mechanisms include a spin-dependent transport model involving defect states in the heterostructure. The correlation between charge transport and magneto-resistive response is the basis of a sensitive technique to measure defects in oxide semiconductor heterostructures. |
Tuesday, March 3, 2020 4:18PM - 4:30PM |
J64.00008: In plane strained Barium Titanate thin films directly integrable on Silicon Marc Reynaud, Pei-Yu Chen, Sunah Kwon, Bryce Edmonson, Moon Kim, John G. Ekerdt, Agham Posadas, Alexander Demkov Integrating ferroelectric materials onto Si is a critical part of the design new photonic devices such as optical modulators and switches. However, a difficult question to answer thus far has been how to optimize the crystalline orientation in these materials. One interesting material is Barium Titanate (BTO), because it exhibits the largest known Pockels effect which is the modulation of the refractive index by an electric field. However, many applications and device geometries require BTO with the polarization pointing in plane with the substrate, but so far it has proven difficult to optimize BTO to maximize in plane domains. Here, we present a method for the growth of single crystal BTO films that are entirely in plane and strain tunable. In this study, we use extensive X-ray characterization, TEM imaging and direct measurement of the Pockels coefficient in these films to show that the films are entirely in plane oriented BTO. This provides a method for high quality in plane oriented BTO to be directly integrated on Silicon. |
Tuesday, March 3, 2020 4:30PM - 4:42PM |
J64.00009: Switching Rashba spin texture via interface with a ferroelectric compound Xiaoli Zhang, Carlos Mera, Alex Zunger We study the microscopic factors controlling the switching of a Rashba(R) spin texture by switching the ferroelectric (FE) polarization of an underlying FE substrate. The spin polarization (SP) in R band depends on the direction of the intrinsic R electric dipole. Thus, an electric field changing the ferroelectic polarization should be able to reverse the Rashba SP. We study a heterojunction between FE PbTiO3 (with TiO2 layers termination), and a R film of Pt, Pb, Rh, Ir, Au and Ag. We determine the role of FE slab thickness, passivation and atom relaxation on both the creation of R bands and SP switching. The FE polarization up (down) corresponds to the structural configuration with Ti above (below) O. Therefore, the R material, mainly interacts with Ti (O) for polarization up (Down). The R material then receives (donate) electrons for polarization up (down). Since Ti and O atoms mainly form the conduction and valence bands, respectively, we find that for up (down) polarization, R bands appear at the conduction (valence) bands. We find these R bands exhibit opposite helical spin texture for opposite ferroelectic polarization directions, allowing the possibility of control the SP direction with electric fields with an external electric field. |
Tuesday, March 3, 2020 4:42PM - 4:54PM |
J64.00010: Reversal of Tunneling Electroresistance in Ferroelectric Tunnel Junctions by Interface Engineering Ming Li, Lingling Tao, Evgeny Y Tsymbal A ferroelectric tunnel junction (FTJ) consists of two metal electrodes separated by a nm-thick ferroelectric barrier which allows quantum-mechanical tunneling through it. A tunneling electroresistance (TER) effect is a sizable change in resistance of a FTJ with reversal of ferroelectric polarization. Depending on the energy alignment between electrodes and barrier, the tunneling conductance can be dominated by either electrons or holes. For electron (hole) tunneling, the Fermi energy lies closer to the conduction band minimum (valence band maximum) of the barrier. Low (high) resistance is achieved when polarization is pointing to the electrode with longer screening length for electron (hole) tunneling, referred to as positive (negative) TER. In this work, using density functional theory calculations, we demonstrate the crossover between the electron-like and hole-like tunneling in a practical FTJ La1-xSrxMnO3/BaTiO3/Pt with positive (La1-xSrxO)1-x/(TiO2)0 and negative (MnO2)x-1/(BaO)0 interface terminations. The positively (negatively) charged interface pulls down (up) the electrostatic potential energy at the interface and thus shifts Fermi energy of La1-xSrxMnO3 closer to the conduction band minimum (valence band maximum) of BaTiO3, leading to positive (negative) TER. |
Tuesday, March 3, 2020 4:54PM - 5:06PM |
J64.00011: Effect of Strain and Doping on the Magnetoelectric Coupling at LSMO/PZT Multiferroic
Interfaces: An Ab Initio Study Krishna Acharya, Igor Vasiliev The magnetic properties of multiferroic materials can be controlled by the applied |
Tuesday, March 3, 2020 5:06PM - 5:18PM |
J64.00012: Effect of confinement and octahedral rotations on the electronic, magnetic, and thermoelectric properties of SrXO3/SrTiO3(001) superlattices, (X = V, Cr, and Mn) Manish Verma, Benjamin Geisler, Rossitza Pentcheva
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Tuesday, March 3, 2020 5:18PM - 5:30PM |
J64.00013: Depth-resolved electronic structure of a BiFeO3/La0.7Sr0.3MnO3 superlattice investigated by standing-wave hard X-ray photoemission spectroscopy (SW-HXPS) Henrique Martins, Alexander Gray, Giuseppina Conti, Inna Vishik, Slavomir Nemsak Studies of artificially designed interfaces of transition metal oxide heterostructures enable opportunities to engineer new stable phases of matter, leading to novel complex devices. However, it is a challenge to identify the features in the electronic structure that induce these emergent interfacial phenomena. Here we have studied a prototypical ferroelectric/ferromagnetic interface between two perovskite oxides, BiFeO3 and La0.7Sr0.3MnO3, by Bragg-reflection standing-wave hard X-ray photoemission spectroscopy (SW-HXPS). The superior depth precision of the SW-HXPS provides information on interfacial and “bulk” signals, and it allows us to identify the character of the interfacial features in the electronic structure in correlation with the detailed depth information on chemical states. |
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