Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C12: Focus Session: Complex Oxide Interfaces - Polar interfaces I |
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Sponsoring Units: DMP Chair: Mark Rzchowski, University of Wisconsin, Madison Room: 314 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C12.00001: Photo-sensitive Transport Properties of the Two-dimensional Electron Gas at LaAlO$_3$/SrTiO$_3$ Interfaces T. Hernandez, Sangwoo Ryu, C.W. Bark, C.B. Eom, M.S. Rzchowski Photoresistance has been previously well characterized in highly resistive ($>10^6$ $\Omega/\Box$) LaAlO$_3$/SrTiO$_3$ heterostructures, showing a decrease in resistance on exposure to light. In some cases insulating heterostructures with LaAlO$_3$ layer below the critical thickness have become conducting on exposure to light. Here we report on the effects of light exposure on much lower sheet resistance ($\sim10^4$ $\Omega/\Box$) LaAlO$_3$/SrTiO$_3$ interfaces, which we find to show a non-negligible increase in resistance. This effect is opposite to the behavior of our more resistive samples. We discuss temperature and magnetic field dependence, possible mechanisms for this behavior, and the implications for other transport properties. [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C12.00002: Ultrafast photoresponse of oxide nanostructures Lu Chen, Yanjun Ma, Mengchen Huang, Sangwoo Ryu, Chung Wung Bark, Chang-Beom Eom, Jeremy Levy Photoconductivity has been demonstrated for nanostructures at the interface of LaAlO$_3$/SrTiO$_3$ and spectral response shows signatures of in-gap states being responsible for photoresponse\footnote{Irvin, P. et al. Rewritable Nanoscale Oxide Photodetector. Nature Photon. 4, 849-852 (2010).}. However, as a wide bandgap material, SrTiO$_3$ shows large nonlinear optical coefficients. Here we discuss time-resolved measurements for exploring the nonlinearity of photoconductivity in the oxide nanostructures. It is found that a nonresonant $\chi^{(3)}$ process results in the observed tunable localized ultrafast response, as well as optical rectification, which can principally lead to the generation and detection of THz radiation. Due to the nanoscale nature of our device, these results foreshadow the control of THz field at single molecule scales. [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C12.00003: Real-time characterization of nanostructures written at the LaAlO$_3$/SrTiO$_3$ interface Alexandre Gauthier, Patrick Irvin, Jeremy Levy Nanostructures can be written on the LaAlO$_3$/SrTiO$_3$ interface using conductive AFM lithography\footnote{C. Cen, \textit{et al.}, \textit{Science} \textbf{323}, 1026 (2009)}. These structures can be configured into devices including photodetectors\footnote{P. Irvin, \textit{et al.}, \textit{Nat. Photonics} \textbf{4}, 849 (2010)} and transistors\footnote{G. Cheng, \textit{et al.}, \textit{Nat. Nanotechnol.} \textbf{6}, 343 (2011)}. Characterization of complex devices requires simultaneous measurements between several pairs of electrodes. We have developed a method to take measurements between all electrodes simultaneously by both measuring and applying a bias at a unique frequency to each electrode. Fourier analysis is then used to separate measured signals by source terminal. This allows us to efficiently characterize multi-terminal devices in real-time, as they are being created. This method will allow for the use of new experimental techniques. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C12.00004: Direct Patterning of Oxide Interface with High Mobility 2DEG without Physical Etching Nirupam Banerjee, Mark Huijben, Gertjan Koster, Guus Rijnders Discovery of highly mobile two dimensional electron gas (2DEG) at the atomically engineered interface between two wide band-gap perovskite insulators, SrTiO3 (STO) and LaAlO3 (LAO) has stimulated the research to apply oxide materials in electronic devices such as high mobility electron transistors (HMET). In spite of excellent interfacial transport properties manifested, challenges remained in structuring these heterointerfaces without damaging the STO single crystal. Top-down physical etching process was an unsuitable choice to serve the purpose since it induces substrate conductivity through creation of oxygen vacancies. Here, we will demonstrate development of a novel procedure for fabricating patterned functional interfaces based on epitaxial-lift-off technique. With its help devices incorporating patterned interfaces of LAO-STO was fabricated devoid of any physical etching process performed and temperature dependent magneto transport properties were investigated. The results demonstrated conservation of the high-quality interface properties in the patterned structures enabling future studies of low-dimensional confinement on high mobility interface conductivity as well as interfacial magnetism. [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C12.00005: Local characterization and charge modification of LaAlO$_{3}$/SrTiO$_{3}$ heterointerface: influence of gas environment Haeri Kim, Dong-Wook Kim, Seon Young Moon, Seung Hyup Baek, Ho Won Jang The discovery of high-mobility 2D electron gas (2DEG) at the interface between two band insulators, LaAlO$_{3}$ and SrTiO$_{3}$ (LAO/STO) has stimulated researches for both applications and fundamental understanding of the intriguing phenomena. Recent experimental and theoretical studies have elucidated roles of charged surface adsorbates on the modification of the resistance of the LAO/STO system. Thus, manipulation and characterization of the surface charges on the LAO/STO surface can be crucial step for unveiling the mechanism of the peculiar physical phenomena. In this work, we used scanning probe microscopy (SPM) to investigate how the ambient gas, such as H$_{2}$/Ar, Ar, and O$_{2}$, could influence the work function and resistance of the LAO/STO system. Also, we studied how the SPM tip-induced charge writing affected the surface potential, Vsurf, and resistance of the LAO/STO. Quantitative measurement of the influence of ambient gas and the charge writing on the surface potential led us to develop a model to explain the unique transport properties of the oxide-based 2DEG. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C12.00006: Piezoresponse force microscopy imaging of nanostructures created by conductive AFM lithography at oxide heterointerfac Mengchen Huang, Feng Bi, Sangwoo Ryu, Chang-Beom Eom, Jeremy Levy Nanoscale control of the metal-insulator transition in 3-unit cell (u.c.) LaAlO$_3$/SrTiO$_3$ heterostructures using conductive AFM (c-AFM) lithography allows the creation of conductive nanostructures \footnote{C. Cen, \textit{et al.} \textit{Nat. Mater}. \textbf{7}, 2136 (2008)}. Piezoelectric effects have recently been observed in planar LaAlO$_3$/SrTiO$_3$ heterostructures \footnote{C. W. Bark, \textit{et al.} \textit{Nano Letter}. 12(4), 1765 (2012)}, and the piezoresponse differs between the conducting and insulating states of 3-u.c. samples where c-AFM modulates the transition \footnote{M. Huang, \textit{et al.} arXiv: 1208.287 (2012)}. We have employed piezoresponse force microscopy (PFM) to detect and image the piezoresponse variations of nanostructures created by c-AFM lithography. PFM imaging allows visualization of the nanostructures, expanding capabilities for characterizing and studying individual devices. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C12.00007: Parallel conductive-AFM lithography of LaAlO$_3$/SrTiO$_3$ using 1D multiple-tip array Shuo Li, Mengchen Huang, Feng Bi, Sangwoo Ryu, Chang-Beom Eom, Jeremy Levy Nanoscale devices at the LaAlO$_3$/SrTiO$_3$ interface can be created by metastable charging of the top LaAlO$_3$ surface using a voltage-biased conductive-AFM tip.\footnote{C.Cen, \textit{et al.}, \textit{Nature Materials} \textbf{7}, 298 (2008).} In order to create scalable nanoelectronic circuits, it will be important to develop a process to allow multiple tips to write nanostructures in parallel. Here we demonstrate a parallel writing scheme using a 1D multiple-tip array. Independent control over the writing process for each tip is achieved by holding the tip array at a fixed potential and varying the voltage applied to individual electrodes. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C12.00008: Channel-like conduction in LaAlO$_3$/SrTiO$_3$ heterointerfaces Beena Kalisky, Eric Spanton, Hilary Noad, John Kirtley, Christopher Bell, Hiroki Sato, Yanwu Xie, Yasuyuki Hikita, Carsten Woltmann, Georg Pfanzelt, Rainer Jany, Harold Hwang, Jochen Mannhart, Kathryn Moler LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures exhibit metallic conduction at the interface. Many studies of LAO/STO properties are done by transport measurements which measure conductance over macroscopic areas of the sample or device. Local information about the electronic transport is crucial to the understanding of such new materials. We use scanning SQUID microscopy to map the magnetic field locally generated by current flowing at the interface in several LAO/STO samples. We find that the conduction is non-homogeneous and channel-like on the scale of microns and that the stripes/channels are related to tetragonal domains formed in the STO below $\sim$ 105K. We will describe the details of this exciting observation and its impact on transport studies of the LAO/STO interface. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C12.00009: Investigation of Current Channels at the Interface between Complex Oxide Heterostructures Aaron Rosenberg, John Kirtley, Eric Spanton, Christopher Watson, Emiliano Di Gennaro, Umberto Scotti Di Uccio, Carmela Aruta, Francesco Tafuri, Fabio Miletto Granozio, Kathryn Moler The interface between SrTiO$_3$ and LaAlO$_3$, both perovskite oxide insulators, supports metallic and superconducting states under certain conditions. Previous unpublished data by Kalisky et al. shows spatial variation in the current flow in these interfaces, including enhanced conductivity associated with structural domains. The microscopic origin of this variation in conductivity is unknown. We extend the previous work to LaGaO$_{3}$/SrTiO$_{3}$, and NdGaO$_3$/ SrTiO$_{3}$ interfaces, observe similar stripe-like modulations in the current flow, and study their temperature and frequency dependence. Additionally, we plan to study how the current channels in LaAlO$_{3}$/SrTiO$_{3}$ change under a uniaxial strain. Investigation of these spatial variations may improve our understanding of the relationship between structure and conductivity in complex oxide interfaces. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:30PM |
C12.00010: Broadband THz Generation and Detection at 10 nm Scale Yanjun Ma, Mengchen Huang, Jeremy Levy, Sangwoo Ryu, Chung Wung Bark, Chang-Beom Eom The terahertz region of the electromagnetic spectrum (0.1 THz-10 THz) probes a wealth of information relevant for material, biological, medical and pharmaceutical sciences, as well as applications in chemical sensing and homeland security. To date, there have been no methods capable of controlling THz radiation at scales relevant for single molecules. Here we report the generation and detection of broadband terahertz radiation from 10-nm-scale nanojunctions which are ''sketched'' at the interface of LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) heterostructure with a conductive atomic force microscope (c-AFM) tip. The nonresonant $\chi^{(3)}$ process is characterized for a single nanojunction structure, which is nonlienar electronic response to both the static field cross the junction and the optical field illuminated the junction. The same mechanism can result in the generation and detection of broadband THz radiation. This unprecedented control of terahertz radiation, on a scale of four orders of magnitude smaller than the diffraction limit, creates a pathway toward ultra-high-resolution THz imaging, single-molecule fingerprinting, spectroscopic characterization of catalysts, and other applications. [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C12.00011: Extraordinary two-dimensional charge transport at Co$_{2}$YSi (Y$=$Mn,Fe)-SrTiO$_{3}$ Interface P.K. Rout, Himanshu Pandey, Anupam Guleria, P.C. Joshi, Z. Hossain, R.C. Budhani We present extraordinary charge transport in epitaxial thin films of Co$_{2}$MnSi and Co$_{2}$FeSi grown on SrTiO$_{3}$, which shows remarkably low residual resistivity ($\approx $ 10$^{-7}\Omega $cm), giant residual resistivity ratio (as high as 1680) and high mobility ($\approx $ 10$^{4}$ cm$^{\mathrm{2}}$V$^{-1}$s$^{-1})$. Furthermore, such unusual behavior is not observed in films deposited on other cubic oxide substrates of comparable lattice parameters. The thickness dependent study establishes the presence of an electrically more conducting interfacial layer. We believe that a possible mechanism for the electronic behavior of the interface lies in a significant band bending at the interface in addition to the defects due to redox reaction of energetic particles during film growth. We compare our results with the behavior of recently discovered two dimensional electron gas (2DEG) at LaAlO$_{3}$/SrTiO$_{3}$ interface. The strong magnetic character of Heusler alloys combined with their metallicity adds a new dimension to 2DEG problem and makes it potentially important for spintronics applications. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C12.00012: Epitaxial Photoactive CoO/SrTiO$_{3}$ on Si(001) Alex Demkov, Hosung Seo, Thong Ngo, Agham Posadas, Son Hoang, Martin MacDaniel, Dirk Utess, Dina Tryiyso, Buddie Mullins, John Ekerdt Cobalt oxide (CoO) films were grown epitaxially on Si(001) by atomic layer deposition (ALD) using a thin (1.6 nm) buffer layer of strontium titanate (STO) grown by molecular beam epitaxy (MBE). Reflection high-energy electron diffraction, X-ray diffraction, and cross sectional scanning transmission electron microscopy were performed to characterize the crystalline structure of the films. The CoO films were found to be crystalline as-deposited even at the low growth temperature with no evidence of Co diffusion into Si. \textit{In-situ} X-ray photoelectron spectroscopy (XPS) was used to measure the band alignment of the two heterojunctions, CoO/STO and CoO/TiO$_{2}$. The experimental band alignment is compared to electronic structure calculations using density functional theory. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C12.00013: Two Dimensional Electron Gas and Rashba Effect at the Perovskite Polar Surface of KTaO$_3$ S. Satpathy, K.V. Shanavas Using density-functional calculations, we study the formation of the 2DEG and the Rashba effect at the polar surface of the perovskite oxide KTaO$_3$, in which the 2DEG has been recently observed [1]. While the formation of the subbands are similar to the polar interface of LaAlO$_3$/ SrTiO$_3$ [2], we find that atomic relaxations play a significant role here in determining its properties. The relaxations substantially weaken the electric field due to the polar structure, reducing electron density at the surface layer. Quite significantly, we find that the lattice relaxations suppress the surface induced asymmetry in the electronic wavefunctions close to the surface, which can explain the lack of significant Rashba splitting in experiments, despite the presence of heavy elements. With a tight-binding model that includes the asymmetry-controlled hopping, we find that the Rashba effect is present only for bands with certain orbital character, allowing for its possible gate control by tuning the occupancies of the various subbands. Density-functional studies with an applied electric field support these results.\\[4pt] [1] P. King {\it et. al.}, Phys. Rev. Lett., 108, 11602 (2012)\\[0pt] [2] Z. Popovi\'c, S. Satpathy, and R. Martin, Phys. Rev. Lett., 101, 256801 (2008) [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C12.00014: 2 dimensional electron gas in chemically stable SrSnO$_3$/KTaO$_3$ interface Hyukwoo Kwon, Chulkwon Park, Kookrin Char Recent 2DEGs are mostly formed at the interface of LaAlO$_{3}$/SrTiO$_{3}$(LAO/STO) system, which is explained by the polar catastrophe mechanism. Because of large propensity of oxygen vacancy formation in SrTiO$_{3}$, there remains a possibility that the origin of 2DEG of LAO/STO system may stem from extra charge of oxygen vacancy of SrTiO$_{3}$, not from the polar layer of LaAlO$_{3}$. In this presentation, we report the realization of 2DEG at the interface of SrSnO$_{3}$/KTaO$_{3}$(SSO/KTO), which is chemically stable due to extremely stable oxygen stoichiometry. This SrSnO$_{3}$/KTaO$_{3}$ heterosturcture was epitaxially grown by puled laser deposition and the interface was found atomically matched by transmission electron microscope and reciprocal space mapping. We measured the magnetic property of SSO/KTO heterostructure and acquired a large ferromagnetic signal, which is unchanged in the temperature range of 4$\sim$300 K by SQUID magnetometer measurement. As SrSnO$_{3}$ and KTaO$_{3}$ are non-magnetic materials, this ferromagnetic signal may result from 2DEG at the interface of SSO/KTO. We hope our results can shed lights on the exact mechanism of 2DEGs that are formed at oxide interfaces. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C12.00015: Correlation enhanced effective mass of two-dimensional electrons in Mg$_x$Zn$_{1-x}$O/ZnO heterostructures Yuichi Kasahara, Yugo Oshima, Joseph Falson, Yusuke Kozuka, Atsushi Tsukazaki, Masashi Kawasaki, Yoshihiro Iwasa Mg$_x$Zn$_{1-x}$O/ZnO provides extremely clean two-dimensional electron systems (2DESs) that exhibit the integer and fractional quantum Hall effects, as in GaAs-based heterostructures. The uniqueness of Mg$_x$Zn$_{1-x}$O/ZnO, compared with the GaAs-based heterostructures, lies in the fact that such clean 2DESs emerge with effective mass of electrons in ZnO, which is over four times higher than that in GaAs, indicating that the effects of electron correlation are expected to be much more pronounced than their GaAs counterparts. Here we show the results of combined magnetotransport and cyclotron resonance experiments on 2DESs confined in Mg$_x$Zn$_{1-x}$O/ZnO heterostructures. We have observed a steep enhancement of transport masses ($m^*_{tr}$) with decreasing carrier density, whereas the effective masses determined by the cyclotron resonance ($m^*_{CR}$) are independent of the carrier density and are comparable to the effective mass of bulk ZnO. The discrepancies between $m^*_{tr}$ and $m^*_{CR}$ directly gauges the strength of the electron-electron interactions. Therefore, observed enhancement of $m^*_{tr}$, which exceeds $m^*_{CR}$ by nearly 60\%, is a direct consequence the electron-electron interactions. [1] Y. Kasahara et al., Phys. Rev. Lett., Accepted. [Preview Abstract] |
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