Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H30: Novel Behavior at Oxide InterfacesFocus
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Sponsoring Units: DMP Chair: Beatriz Noheda, University of Groningen Room: 329 |
Tuesday, March 15, 2016 2:30PM - 3:06PM |
H30.00001: Interfaces between strongly correlated oxides : controlling charge transfer and induced magnetism by hybridization Invited Speaker: Manuel Bibes At interfaces between conventional materials, band bending and alignment are controlled by differences in electrochemical potential. Applying this concept to oxides in which interfaces can be polar and cations may adopt a mixed valence has led to the discovery of novel two-dimensional states between simple band insulators such as LaAlO$_{\mathrm{3}}$ and SrTiO$_{\mathrm{3}}$. However, many oxides have a more complex electronic structure, with charge, orbital and/or spin orders arising from correlations between transition metal and oxygen ions. Strong correlations thus offer a rich playground to engineer functional interfaces but their compatibility with the classical band alignment picture remains an open question. In this talk we will show that beyond differences in electron affinities and polar effects, a key parameter determining charge transfer at correlated oxide interfaces is the energy required to alter the covalence of the metal-oxygen bond. Using the perovskite nickelate (RNiO$_{\mathrm{3}})$ family as a template, we have probed charge reconstruction at interfaces with gadolinium titanate GdTiO$_{\mathrm{3}}$ using soft X-ray absorption spectroscopy and hard X-ray photoemission spectroscopy. We show that the charge transfer is thwarted by hybridization effects tuned by the rare-earth (R) size. Charge transfer results in an induced ferromagnetic-like state in the nickelate (observed by XMCD), exemplifying the potential of correlated interfaces to design novel phases. Further, our work clarifies strategies to engineer two-dimensional systems through the control of both doping and covalence. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H30.00002: Electric field effect on magneto-thermopower in oxide interface LaAlO$_{3}$/SrTiO$_{3}$ Tomoya Asaba, Gang Li, Pascal Wittlich, Jochen Mannhart, Lu Li Oxide interface LaAlO$_{3}$/SrTiO$_{3}$ has been attracting huge interest as it shows 2-dimensional electron liquid behavior, negative compressibility and coexistence of magnetism and superconductivity. In this study we measured the magneto-thermoelectric effect and report the observation of the quantum oscillation in the thermopower. Backing gating through SrTiO$_3$ tunes the carrier density and the oscillation pattern in the thermoelectric signal. At low carrier density, the oscillation frequency decreases when more electrons are populated, which directly indicates the negative capacitance. At high carrier density regime, both of magneto-thermopower and Nernst effect show the nonlinear behavior which suggests a Lifshitz transition. These results point to the unique multiband electronic structure of oxide interface. [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H30.00003: Spin transport in LaAlO$_{3}$/SrTiO$_{3}$ heterostructures Thach D. N. Ngo, Jungwon Chang, Kyujoon Lee, Seungju Han, Joonsung Lee, Youngheon Kim, Myunghwa Jung, Yongjoo Doh, Mahnsoo Choi, Jonghyun Song, Jinhee Kim Since the discovery in 2004, the 2-dimensional electron gas at the LaAlO$_{3}$/SrTiO$_{3}$ heterointerfaces has attracted considerable attentions because of the fascinating physical phenomena and their strong tuneability. However, the manipulation of the spin degree of freedom in this oxide structure is still unattainable. Here, we report the spin-dependent electrical transport in hybrid magnetic tunnel junctions based on the ferromagnetism at the oxide interface. The Ohmic spin injection into the LaAlO$_{3}$/SrTiO$_{3}$ heterostructure is feasible due to the insertion of an approximately thin Ti layer between the oxide and ferromagnetic metal (Co). The observed tunnel magnetoresistive effect shows such a strong anisotropy that the magnitude and even sign of the tunneling magnetoresistance ratio are dramatically modulated by a rotational, in-plane magnetic field. This is attributable to the strong Rashba-type spin-orbit coupling in the oxide structure. In addition, the spin transport is also associated with the tetragonal domain configuration of the SrTiO$_{3}$ substrate. These results provide a further support for the existence of the macroscopic ferromagnetism at LaAlO$_{3}$/SrTiO$_{3}$ interface. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H30.00004: Scanning SQUID measurements of magnetism in LaAlO$_3$/SrTiO$_3$ heterostructures H. Noad, E. M. Spanton, J. A. Bert, B. Kalisky, K. C. Nowack, C. Bell, M. Kim, Y. Hikita, M. Hosoda, H. K. Sato, Y. Xie, P. Wittlich, H. Y. Hwang, J. Mannhart, K. A. Moler LaAlO$_3$ and SrTiO$_3$ are both nonmagnetic band insulators. It is therefore surprising that signatures of magnetism in LaAlO$_3$/SrTiO$_3$ heterostructures (LAO/STO) have been seen in a wide variety of experiments. Using scanning superconducting quantum interference device (SQUID) microscopy we previously found that, above a critical thickness of three unit cells of LAO, there can be heterogeneous patches of ferromagnetism. The observed ferromagnetic patches are sparse, and many samples show very few ferromagnetic patches. Scanning SQUID observations suggest that, although ferromagnetic patches can arise, the ground state of LAO/STO is not strongly ferromagnetic. In the few samples studied at millikelvin temperatures, we also observed a relatively homogeneous paramagnetic response with a 1/T-like dependence, suggesting a landscape of localized spins. Theoretical proposals for the origin of magnetism in LAO/STO include intrinsic spin polarization near the interface and cation or oxygen vacancy defects. Measurements on samples with deliberately tuned oxygen content will help us to evaluate these proposals. [Preview Abstract] |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H30.00005: Correlations between magnetic and piezoelectric response at gated LaAlO$_3$/SrTiO$_3$ interfaces Qing Guo, Jianan Li, Mengchen Huang, Hyungwoo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy The interface between perovskite oxide semiconductors LaAlO$_3$ and SrTiO$_3$ exhibits remarkable conducting, superconducting, magnetic, and spintronic properties that are strongly influenced by electron density. Scanning probe methods have the ability to probe local properties of interest. For example, magnetic force microscopy (MFM) has be used to measure magnetism at the LaAlO$_3$/SrTiO$_3$ interface\footnote{F. Bi, \textit{et al.}, Nat. commun. \textbf{5}, 5019 (2014); F. Bi, \textit{et al.}, Appl. Phys. Lett.. \textbf{107}, 082402 (2015). }, while piezoelectric force microscopy has been used to measure the local electron density\footnote{M. Huang, \textit{et al.}, APL Mater. \textbf{1}, 052110 (2013).}. Here we directly compare these two methods to provide further insight into the relationship between electron density and magnetic properties. [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H30.00006: Magneto-optical Kerr probing of LAO/STO interface ferromagnetism Jianan Li, Qing Guo, Feng Bi, Mengchen Huang, Hyungwoo Lee, Chang-Beom Eom, Patrick Irvin, Jeremy Levy Interfacial ferromagnetism in LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures has been probed by a variety of techniques. Recently, magnetic force microscopy (MFM) was used to image ferromagnetic domains that are electrically tunable at room temperature\footnote{F. Bi, \textit{et al}. Nat. commun. \textbf{5}, 5019 (2014)} when the samples were grown in certain conditions. Optical techniques provide powerful tools for probing magnetic phenomena, and recently magnetic circular dichroism has been observed in reduced bulk STO crystals\footnote{W. D. Rice, \textit{et al}. Nat. mater. \textbf{13}, 481–487 (2014)}. Here we describe a scanning magneto-optical Kerr imaging system that could achieve sub-micrometer precision and 10$^{-4}$ rad/$\sqrt{\mathrm{Hz}}$ sensitivity with a 150 fs pulsed-laser centered at 425 nm. Such capability would make pump and probe measurement on the gate-tunable LAO/STO ferromagnetism and ultrafast imaging of domain dynamics possible. [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H30.00007: Surface chemically-switchable ultraviolet luminescence from interfacial two-dimensional electron gas Jonathan E Spanier, Mohammad I Islam, Diomedes Saldana-Greco, Zongquan Gu, Fenggong Wang, Eric Breckenfeld, Qingyu Lei, Ruijuan Xu, Christopher J Hawley, Xiaoxing Xi, Lane W Martin, Andrew M Rappe \noindent We report intense, narrow-linewidth, surface chemisorption-activated and reversible ultraviolet (UV) photoluminescence from radiative recombination of the two-dimensional electron liquid with photo-excited holes at the LaAlO$_3$/SrTiO$_3$ interface. The switchable luminescence arises from an electron transfer-driven modification of the electronic structure via H-chemisorption onto AlO$_2$-terminated LaAlO$_3$, at least 2 nm away from the interface. Control of the onset of emission and its intensity are functionalities that go beyond the luminescence of compound semiconductor quantum wells. Connections between reversible chemisorption, fast electron transfer, and quantum-well luminescence suggest a new model for surface chemically reconfigurable solid-state UV optoelectronics and molecular sensing.\\ Work supported by NSF under DMR 1124696, DoE BES under DE-FG02-07ER46453, DE-FG-2-07ER15920, and DE-SC0004764, and by ONR under N00014-12-1-1033 and N00014-11-1-0664. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H30.00008: Ferroelectric Tunnel Junction with a Semiconductor Electrode Xiaohui Liu, J.D. Burton, Evgeny Tsymbal Realizing a large tunneling electroresistance (TER) is crucial for the application of ferroelectric tunnel junctions (FTJs) for device applications. FTJs are typically composed of a thin ferroelectric layer sandwiched by two metallic electrodes, where TER generally results from the dependence of the effective tunneling barrier height on the ferroelectric polarization. Since the resistance depends exponentially not only on barrier height but also on barrier width, TER is expected to be greatly enhanced when one of the electrodes is a semiconductor where the depletion region near the interface can be controlled via ferroelectric polarization. To explore this possibility, we perform studies on a SrRuO3/BaTiO3/n-SrTiO3 FTJ using first-principles density functional theory (DFT). We analyze the effect of ferroelectric polarization on the barrier width and transport properties. Our studies show that, in addition to modulation of the depletion region in n-SrTiO3, layers in the BaTiO3 barrier near the interface become conducting for one polarization orientation leading to a large TER effect. Additional DFT$+$U calculations reveal that this effect is insensitive to the bandgap of the barrier, indicating that this should be a robust mechanism to realize large TER in FTJs. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H30.00009: Effect of epitaxial strain on tunneling electroresistance in ferroelectric tunnel junctions Andrei Sokolov, Ohheum Bak, Haidong Lu, Evgeny Tsymbal, Alexei Gruverman We present the effect of compressive strain on the tunneling electroresistance (TER) effect in BaTiO3/SrRuO3 (BTO/SRO) heterostructures. The films were prepared using PLD technique and characterized by XRD and RHEED methods. We performed comprehensive study of ferroelectric and transport properties of obtained structure and find that epitaxial strain imposed by the mismatch of NdGaO3 and SrTiO3 lattice parameters with the BTO and SRO layers improves ferroelectric polarization of BTO and concurrently promotes the metallicity of the SRO films. While the enhanced polarization is beneficial for the TER magnitude, the reduced asymmetry in the tunneling barrier due to the shortened screening length of SRO is detrimental for the effect. Thus, a combined effect of strain on the polarization of the ferroelectric barrier and the screening properties of the electrodes needs to be taken into account when considering and predicting the TER effect in ferroelectric tunnel junctions [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H30.00010: Reversible Magnetoelectric Control of Exchange Coupling in Monodomain BiFeO$_3$ Heterostructures J Irwin, W Saenrang, B.A. Davidson, J.E. Podkaminer, KJ Reierson, F Maccherozzi, S Dhesi, J. W. Freeland, M.S. Rzchowski, C. B. Eom The electric field control of ferromagnetism has exciting applications in spintronic devices such as magnetic tunnel junctions. We investigate reversible rotation of the magnetization of a Co overlayer on a heterostructure with a monodomain thin film of the multiferroic BiFeO$_3$ (BFO). For different electric polarization directions of the BFO film we observe a rotation in Co magnetization direction by photoelectron emission microscopy (PEEM). This effect is robust over at least 100 cycles of the BFO electric polarization. According to anisotropic magnetoresistance (AMR) and magneto-optic Kerr effect (MOKE) measurements performed at room temperature, the easy magnetic direction for the Co layer rotates in plane when the electric polarization of the BFO is switched. Additionally, X-ray linear dichroism (XLD) measurements verify the presence of a magnetoelectric effect in the BFO and suggest magnetic coupling between the BFO and Co layers. [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H30.00011: Rashba type band splittings in ferroelectric semiconductor $\alpha $-GeTe(111) Beomyoung Kim, Hyungju Oh, Yeongkwan Kim, Jonathan Denlinger, Changyoung Kim There has been significant increase in the research of spin orbit coupling (SOC) induced exotic phenomena. The Rashba effect, theoretically predicted to exist, is one of the SOC related phenomena. The phenomenon was later experimentally observed in the surface states of metals and topological insulators as well as interfaces of hetero structures that have inversion symmetry breaking (ISB). Even bulk states with intrinsic ISB such as BiTeI is found to have Rashba split bands. It was very recently proposed that ferroelectric GeTe has Rashba effect in the bulk. This is a unique situation where ISB is provided not by the structure ISB but by an electrical polarization. We have performed angle-resolved photoemission spectroscopy (ARPES) on GeTe single crystals to investigate the unique bulk Rashba state. Our results indeed show the existence of a Rashba-type band splitting as theoretically predicted. We discuss various aspect of the Rashba states in ferroelectric semiconductor $\alpha $-GeTe(111). [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H30.00012: Effect of charge on the ferroelectric field effect in strongly correlated oxides Xuegang Chen, Zhiyong Xiao, Xiaozhe Zhang, Le Zhang, Weiwei Zhao, Xiaoshan Xu, Xia Hong We present a systematic study of the effect of charge on the ferroelectric field effect modulation of various strongly correlated oxide materials. We have fabricated high quality epitaxial heterostructures composed of a ferroelectric Pb(Zr,Ti)O$_{3}$ (PZT) gate and a correlated oxide channel, including Sm$_{0.5}$Nd$_{0.5}$NiO$_{3}$ (SNNO), La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO), SNNO/LSMO bilayers, and NiCo$_{2}$O$_{4}$ (NCO). The Hall effect measurements reveal a carrier density of \textasciitilde 4 holes/u.c. (0.4 cm$^{2}$V$^{-1}$s$^{-1})$ for SNNO to \textasciitilde 2 holes/u.c. (27 cm$^{2}$V$^{-1}$s$^{-1})$ for NCO. We find the magnitude of the field effect is closely related to both the intrinsic carrier density and carrier mobility of the channel material. For devices employing the SNNO/LSMO bilayer channel, we believe the charge transfer between the two correlated oxides play an important role in the observed resistance modulation. The screening capacitor of the channel materials and the interfacial defect states also have significant impact on the retention characteristics of the field effect. Our study reveals the critical role of charge in determining the interfacial coupling between ferroelectric and magnetic oxides, and has important implications in developing ferroelectric-controlled Mott memory devices. [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H30.00013: Band offset measurement of oxygen annealed SrTiO$_{\mathrm{3}}$/Si Eric Jin, Lior Kornblum, Charles Ahn, Fred Walker Integration of the perovskite oxide SrTiO$_{\mathrm{3}}$ (STO) with silicon by molecular beam epitaxy (MBE) was initially developed for new high-K gate dielectrics, and more recently as a means to combine the multifunctional properties of oxide heterostructures with the well-established silicon platform. The band alignment at an oxide-semiconductor junction is critical in determining its electrical properties, and control over the conduction band offset is a fundamental goal of materials science. Density functional theory calculations have shown that an interface dipole forms at the STO-Si interface, with the magnitude of this dipole determined by the exact composition of the interface. If the interface is oxygen deficient, the band alignment is type II. If the interface is modified via the addition of a monolayer of oxygen atoms, the predicted heterojunction becomes type-I. We characterize the band alignment of MBE-grown STO-Si films by x-ray photoemission spectroscopy and show that the conduction band offset is negative between STO and Si. We demonstrate an experimental \textasciitilde 0.5 eV increase in the conduction band offset for in-situ oxygen-annealed films, in close agreement with theoretical predictions. By careful control of the interface atomic structure, we show an approach toward tuning the band offset of the STO-Si system to modify electronic transport for a variety of device applications. [Preview Abstract] |
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