Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session V30: Charge Transfer and Electron Gases at Oxide InterfacesFocus
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Sponsoring Units: DMP Chair: Uli Aschauer, ETH Zurich Room: 329 |
Thursday, March 17, 2016 2:30PM - 3:06PM |
V30.00001: Charge transfer and emergent phenomena of oxide heterostructures Invited Speaker: Hanghui Chen Charge transfer is a common phenomenon at oxide interfaces. We use first-principles calculations to show that via heterostructuring of transition metal oxides, the electronegativity difference between two dissimilar transition metal ions can lead to high level of charge transfer and induce substantial redistribution of electrons and ions. Notable examples include i) enhancing correlation effects and inducing a metal-insulator transition [1]; ii) tailoring magnetic structures and inducing interfacial ferromagnetism [2]; iii) engineering orbital splitting and inducing a non-cuprate single-orbital Fermi surface [3]. Utilizing charge transfer to induce emergent electronic/magnetic/orbital properties at oxide interfaces is a robust approach. Combining charge transfer with quantum confinement and expitaxial strain provides an appealing prospect of engineering electronic structure of artificial oxide heterostructures. [1] H. Chen, A. J. Millis and C. A. Marianetti, PRL 111, 116403 (2013) [2] H. Chen, H. Park, A. J. Millis and C. A. Marianetti, PRB 90, 245138 (2014) [3] H. Chen, D. P. Kumah, A. S. Disa, F. J. Walker, C. H. Ahn, and S. Ismail-Beigi, PRL 110, 186402 (2013) [Preview Abstract] |
Thursday, March 17, 2016 3:06PM - 3:18PM |
V30.00002: Structural and electronic properties of Sr(Zr,Ti)O$_3$ alloys for use in oxide heterostructures Leigh Weston, Anderson Janotti, Xiangyuan Cui, Burak Himmetoglu, Catherine Stampfl, Chris G. Van de Walle Sr(Ti,Zr)O$_3$ alloys are promising materials for use in oxide heterostructures, however the fundamental properties of this system have not yet been characterized. Using hybrid density functional calculations, we study the electronic and structural properties of ordered SrTi$_x$Zr$_{1-x}$O$_3$ alloys at $x$=0, 0.25, 0.5, 0.75, and 1. As Ti is added to SrZrO$_3$, the lattice parameter is reduced according to Vegard's law, while the band gap shows a large bowing and is sensitive to the Ti distribution. For $x$=0.5, arranging the Ti and Zr atoms into a 1$\times$1 SrZrO$_3$/SrTiO$_3$ superlattice along the [001] direction leads to a highly dispersive single band at the conduction-band minimum (CBM) that is absent in the parent compounds, and a direct gap close to that of pure SrTiO$_3$. This is explained by the splitting of the Ti 3$d$ $t_{2g}$ states in the reduced symmetry of the superlattice, lowering the band originating from the Ti 3$d_{xy}$ orbitals. The lifting of the orbital degeneracy around the CBM suppresses scattering due to electron-phonon interactions. We propose that short-period SrZrO$_3$/SrTiO$_3$ superlattices could be exploited to engineer the band structure and improve carrier mobility compared to bulk SrTiO$_3$. [Preview Abstract] |
Thursday, March 17, 2016 3:18PM - 3:30PM |
V30.00003: Direct observation of induced ferromagnetism in SrTiO$_{\mathrm{3}}$ quantum wells confined in GdTiO$_{\mathrm{3}}$ heterostructures Ryan Need, Brandon Isaac, Brian Kirby, Julie Borchers, Susanne Stemmer, Stephen Wilson Complex oxide thin film heterostructures with charge transfer at the interface provide a uniquely tunable environment in which to study the physics of highly-correlated and quantum-confined electron systems. Previous studies using magnetoresistance measurements had suggested that thin SrTiO$_{\mathrm{3}}$ layers become ferromagnetic when grown between thicker GdTiO$_{\mathrm{3}}$ layers. Here we report the direct observation of induced ferromagnetism in SrTiO$_{\mathrm{3}}$ quantum wells using polarized neutron reflectometry (PNR). Four GdTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ superlattice structures with varying SrTiO$_{\mathrm{3}}$ layer thickness were grown epitaxially on LSAT (001) substrates by hybrid molecular beam epitaxy. Chemical and magnetic depth-profiles were refined using a combination of x-ray and polarized neutron reflectivity measurements taken at temperatures ranging from 4-300K. We observed a critical thickness below which the SrTiO$_{\mathrm{3}}$ layer have non-zero magnetism in the center of the well. These results are in excellent agreement with the previous magnetoresistance measurements and provide the first direct observation of induced-magnetism in this system. [Preview Abstract] |
Thursday, March 17, 2016 3:30PM - 3:42PM |
V30.00004: Intrinsic Electronic Confinement at Conducting Oxide Interfaces Danfeng Li, Stefano Gariglio, Wei Liu, Alexandre Fête, Margherita Boselli, Marc Gabay, Jean-Marc Triscone The discovery of a two-dimensional electron liquid (2DEL), confined at the interface between the two band insulators LaAlO$_{\mathrm{3}}$ (LAO) and SrTiO$_{\mathrm{3}}$ (STO) has generated tremendous research interest [1]. The 2DEL confinement lifts the degeneracy of Ti t$_{\mathrm{2g}}$ orbitals and promotes exotic physical properties [2]. A previous study [3] has demonstrated that a 2DEL is also observed when LAO is alloyed with STO (La,Al)$_{\mathrm{1-}}_{x}$(Sr,Ti)$_{x}$O$_{\mathrm{3}}$ (LASTO:$x)$. The threshold thickness required for the onset of conductivity scales with $x$. We present here a study of magnetotransport and superconductivity at the (LASTO:0.5)/STO interface. The thickness of the 2DEL, measured using perpendicular and parallel critical fields is larger than the one at the LAO/STO interface. This change is due to a modification on the confining potential linked to a reduced charge transfer that is scaling as 1/x. This study supports an intrinsic origin to the formation of the 2DEL in the LAO/STO system. [1] A. Ohtomo, H. Y. Hwang, Nature \textbf{427}, 423 (2004). [2] P. Zubko, S. Gariglio, M. Gabay, P. Ghosez, and J.-M. Triscone, Annual Review of Condensed Matter Physics \textbf{2}, 141 (2011). [3] M.L. Reinle-Schmitt, C. Cancellieri, D. Li, D. Fontaine, M. Medarde, E. Pomjakushina, C.W. Schneider, S. Gariglio, P. Ghosez, J.-M. Triscone, and P.R. Willmott, Nature Communications \textbf{3}, 932 (2012). [Preview Abstract] |
Thursday, March 17, 2016 3:42PM - 3:54PM |
V30.00005: Beyond GaAs: Room-Temperature Intersubband Absorption in SrTiO$_{\mathrm{3}}$/LaAlO$_{\mathrm{3}}$ Multiple Quantum Wells John Ortmann, Nish Nookala, Qian He, Agham Posadas, Albina Borisevich, Mikhail Belkin, Alex Demkov With the recent advancements in oxide thin film fabrication, it is possible to design and grow oxide quantum well heterostructures whose well depths far exceed those of traditional GaAs-based quantum wells. Here, we discuss the design, fabrication, structural quality, and optical properties of MBE-grown SrTiO$_{\mathrm{3}}$/LaAlO$_{\mathrm{3}}$ multiple quantum wells. These oxide quantum wells have a conduction band offset of greater than 2eV, as measured by X-ray photoelectron spectroscopy. We present simulations of the confined states within the wells and demonstrate the feasibility of driving intersubband transitions whose energies exceed 1eV. Furthermore, we demonstrate the excellent crystalline quality of these heterostructures via X-ray diffraction spectra and STEM-HAADF imaging and present evidence of atomic-scale control of the structures. Finally, we present room-temperature FTIR spectra demonstrating the first-reported evidence of intersubband absorption in SrTiO$_{\mathrm{3}}$/LaAlO$_{\mathrm{3}}$ multiple quantum wells and discuss the possibility of oxide quantum well-based devices. [Preview Abstract] |
Thursday, March 17, 2016 3:54PM - 4:06PM |
V30.00006: Constriction based superconducting quantum interference devices at the LaAlO$_3$/SrTiO$_3$ interface Emre Mulazimoglu, Srijit Goswami, Ana M. R. V. L. Monteiro, Roman Woelbing, Dieter Koelle, Reinhold Kleiner, Yaroslav Blanter, Lieven M. K. Vandersypen, Andrea D. Caviglia The two-dimensional (2D) superconductor formed at the interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) has been studied extensively and shows many intriguing properties. However, to date there exist no measurements which are sensitive to the phase of the superconducting order parameter, a fundamental prerequisite to understand the microscopic mechanism of the superconductivity. Here, we realize superconducting quantum interference devices (SQUIDs) at the LAO/STO interface. Using nanoscale patterning, we define sub-100 nm physical constrictions, which serve as weak links between superconducting reservoirs. The SQUIDs show clear flux-periodic oscillations in the critical current. Back gate and temperature dependent studies, in combination with numerical simulations, show that the low superfluid density of this 2D superconductor results in an exceptionally large, gate controllable kinetic inductance of the SQUID. This ability to perform phase-sensitive measurements opens up a completely new approach to study this unique interfacial superconductor. [Preview Abstract] |
Thursday, March 17, 2016 4:06PM - 4:18PM |
V30.00007: Electrostatically defined superconducting quantum interference devices at the LaAlO$_3$/SrTiO$_3$ interface Srijit Goswami, Emre Mulazimoglu, Ana Monteiro, Roman Woelbing, Dieter Koelle, Reinhold Kleiner, Yaroslav Blanter, Lieven Vandersypen, Andrea Caviglia Two-dimensional superconductivity at the LaAlO$_3$ (LAO)/ SrTiO$_3$ (STO) interface can be controlled via the field effect, whereby a global back gate can be used to systematically tune the critical temperature (T$_c$) of the bulk superconductor. Here, we exploit this sensitivity of T$_c$ to the field effect to create an electrostatically defined superconducting quantum interference device (SQUID) at the LAO/STO interface. The device consists of a superconducting loop with nanoscale local top gates on each arm. By controllably depleting carriers below the gates we create a SQUID with two identical Josephson junctions (JJs), giving rise to flux-periodic oscillations in the critical current. Furthermore, by independently tuning the two JJs we create an asymmetric SQUID, which allows us to accurately estimate an extremely large kinetic inductance of about 60nH. While other examples of gate-tunable JJs do exist, they necessarily involve physical interfaces between two dissimilar materials. In contrast, our gate-defined SQUIDs are unique in that the entire device is made from a single superconductor with purely electrostatic interfaces. [Preview Abstract] |
Thursday, March 17, 2016 4:18PM - 4:30PM |
V30.00008: Aharanov-Bohm quantum interference in LaAlO$_3$/SrTiO$_3$ Hall bar structures Patrick Irvin, Shicheng Lu, Anil Annadi, Guanglei Cheng, Michelle Tomczyk, Mengchen Huang, Jeremy Levy, Hyungwoo Lee, Chang-Beom Eom Aharanov-Bohm (AB) interference can arise in transport experiments when magnetic flux threads through two or more transport channels. The existence of this behavior requires long-range ballistic transport and is typically observed only in exceptionally clean materials. We observe AB interference in wide ($w \sim 100$ nm) channels created at the LaAlO$_3$/SrTiO$_3$ interface using conductive AFM lithography. Interference occurs above a critical field $B \sim 4$ T and increases in magnitude with increasing magnetic field. The period of oscillation implies a ballistic length that greatly exceeds the micron-scale length of the channel, consistent with Fabry-Perot interference in 1D channels. The conditions under which AB oscillations are observed will be discussed in the context of the electron pairing mechanism in LaAlO$_3$/SrTiO$_3$. [Preview Abstract] |
Thursday, March 17, 2016 4:30PM - 4:42PM |
V30.00009: Dimensionality effects on superconductivity in LaAlO$_3$/SrTiO$_3$ nano-structures Anil Annadi, Shicheng Lu, Guanglei Cheng, Michelle Tomczyk, Mengchen Huang, Patrick Irvin, Hyungwoo Lee, Chang-Beom Eom, Jeremy Levy We investigate electron transport, especially superconductivity, in LaAlO$_3$/SrTiO$_3$ nanostructures with respect to the dimensionality by creating channels with widths varying from nanometers to micrometers using c-AFM lithography. Superconducting properties such as upper critical magnetic field and critical current are compared as a function of electron doping (gating) and temperature. The superconducting properties among these devices show a marked deviation from naive scaling expectations. We discuss the results in regard to ferroelastic domains and possible edge/boundary mode transport scenarios. [Preview Abstract] |
Thursday, March 17, 2016 4:42PM - 4:54PM |
V30.00010: Quantum Oscillations at LaTiO$_3$/SrTiO$_3$ Interfaces Michael Veit, Yuri Suzuki Emergent metallic behavior at the interface of the Mott insulator LaTiO$_3$ and the band insulator SrTiO$_3$ was observed for the first time more than a decade ago. Since then the metallicity has been explained in terms of charge redistribution at the interface combined with lattice relaxation. However to date, Shubnikov de Haas oscillations have not been reported in this two dimensional metallic system. For ultrathin (3-4 unit cells) LaTiO$_3$ thin films on SrTiO$_3$, we report the observation of Shubnikov-de Haas oscillations whose frequency corresponds to a small Fermi pocket. Surprisingly the oscillation are only observed between 1 and 4 T. Above this range, the quantum limit is reached for this pocket so no more oscillations are observed. A Berry's phase of $\pi$ is also detected in these oscillations. Additionally a strong in-plane anisotropic magnetoresistance was measured in the heterostructures which, along with the Berry's phase, is attributed to a giant Rashba coupling at the interface. [Preview Abstract] |
Thursday, March 17, 2016 4:54PM - 5:06PM |
V30.00011: Impact of electric-field-dependent dielectric constants on two-dimensional electron gases in complex oxides Chris Van de Walle, Hartwin Peelaers, Karthik Krishnaswamy, Luke Gordon, Daniel Steiauf, Anna Sarwe, Anderson Janotti A high-density two-dimensional electron gas (2DEG) can be formed at interfaces of complex oxides. The electric field in the vicinity of the interface depends on the dielectric properties of the material as well as on the electron distribution. However, electric fields can strongly modify the dielectric constant of SrTiO$_3$ (STO) as well as other complex oxides. Solving the electrostatic problem thus requires a self-consistent approach in which the dielectric constant varies according to the local magnitude of the field. We have implemented the field dependence of the dielectric constant in a Schr\”odinger-Poisson solver and use the SrTiO$_3$/GdTiO$_3$ interface as an example to discuss the importance of taking this field dependence into account when modeling interfaces of complex oxides [1].\\ \newline [1] H. Peelaers, K. Krishnaswamy, L. Gordon, D. Steiauf, A. Sarwe, A. Janotti, and C. G. Van de Walle, Appl. Phys. Lett. {\bf 107}, 183505 (2015). [Preview Abstract] |
Thursday, March 17, 2016 5:06PM - 5:18PM |
V30.00012: Nanomechanical probes of sketched LaAlO$_3$/SrTiO$_3$ single-electron transistors Jessica Montone, Feng Bi, Mengchen Huang, Patrick IRVIN, Jeremy Levy, Hyungwoo Lee, Chang-Beom Eom The interface of LaAlO$_3$/SrTiO$_3$ presents a locally tunable metal-insulator transition that can be utilized to create complex nanostructures. Using conducting AFM lithography techniques, we can create a variety of nanoscale devices such as sketched single-electron transistors (SketchSETs)\footnote{G. Cheng, \textit{et. al}, Nature Nanotechnology \textbf{6}, 343 (2011).}. Due to the piezoelectric properties of LaAlO$_3$/SrTiO$_3$, there exists the possibility of locally modulating the local electron density using the pressure applied by an AFM tip. Some of the most interesting properties are only observed at cryogenic temperature. For this purpose we utilize a cryogenic AFM system. I will describe our efforts to perform nanomechanical imaging of conductive structures, which can be helpful in mapping the electronic properties of oxide nanostructures. [Preview Abstract] |
Thursday, March 17, 2016 5:18PM - 5:30PM |
V30.00013: Density-functional modelling of electron energy-loss spectra for LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ quantum well Miri Choi, Lingyuan Gao, Qian He, Albina Borisevich, Alexander A. Demkov LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ quantum wells (QW) are grown S by molecular beam epitaxy (MBE). The conduction band alignment is investigated using electron energy loss spectroscopy (EELS). We model the EELS spectrum using first-principle calculations. To account for the core-hole effect, the Z$+$1 approximationis adopted. Site-projected unoccupied p and d densities of states (pDOS) are extracted and compared with the experimental O K and Ti L edges that correspond to 1s to 2p and 2p to 3d transitions, respectively. Results fro bulk LaAlO$_{\mathrm{3\thinspace }}$(LAO) and$_{\mathrm{\thinspace }}$SrTiO$_{\mathrm{3\thinspace }}$(STO) are discussed first and then the quantum well case is analyzed. We investigate the orbital character of the conduction band states in a QW. We find that in LAO/STO QW, there are evanescent Ti-originated states in the LAO layer and elate them with the peak at EELS front edge. [Preview Abstract] |
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