Session W18: Focus Session: Interfaces in Complex Oxides - Devices and Nanostructures

Mesoscopic transport properties of LaAlO$_{3}$/SrTiO$_{3}$ devices

The conducting interface between the two band insulators LaAlO$_{3}$ and SrTiO$_{3}$ [1], with its unique electronic properties, stands as a prominent example of an oxide heterostructure for the realization of multifunctional devices. Using the electric field effect the ground state of this system can be tuned from insulating to superconducting [2]. Recently we demonstrated also the possibility to tune the carriers mobility by changing the deposition conditions of the LaAlO$_{3}$ film (3) and measured Shubnikov-de Haas oscillations, whose analyses demonstrate the 2D nature of the electronic states at the interface [3,4]. We are currently focusing on the realization of devices where the 2D quantum nature of the electronic states can be fully exploited. We show the feasibility of structures with lateral dimensions down to few hundreds nanometers, using an electron beam lithography based process. The lateral confinement of the electron gas in these devices is demonstrated by a phase coherent transport regime, which can be tuned by electric field.\\[4pt] [1] A.Ohtomo et al., Nature 427, 423 (2004)\\[0pt] [2] A.D.Caviglia et al., Nature 456, 624 (2008); C.Bell et al., PRL 103, 226802 (2009)\\[0pt] [3] A.D.Caviglia et al., PRL 105, 236802 (2010)\\[0pt] [4] M.Ben Shalom et al., PRL 105,206401 (2010)   

Sketched Oxide Single-Electron Transistor

Devices that confine and process single electrons represent an important scaling limit of electronics. Such devices have been realized in a variety of materials and exhibit remarkable electronic, optical and spintronic properties. Here, we use an atomic force microscope tip to reversibly ``sketch'' single-electron transistors by controlling a metal-insulator transition at the interface of two oxides.\footnote{Cheng \textit{et al.}, Nature Nanotechnology \textbf{6}, 343 (2011).} In these devices, single electrons tunnel resonantly between source and drain electrodes through a conducting oxide island with a diameter of $\sim$1.5 nm. We demonstrate control over the number of electrons on the island using bottom- and side-gate electrodes, and observe hysteresis in electron occupation that is attributed to ferroelectricity within the oxide heterostructure. These single-electron devices may find use as ultradense non-volatile memories, nanoscale hybrid piezoelectric and charge sensors, as well as building blocks in quantum information processing and simulation platforms.   

Unexpected magnetic Exchange Interaction between Epitaxial La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ film and a two Dimensional Electron Gas at an LaAlO$_{3}$/SrTiO$_{3}$ Interface

We have examined the electrical and magnetic properties of LSMO films epitaxially grown on STO, LAO and LAO/STO substrates. Compared to LAO and STO the LSMO films on LAO/STO shows a higher metal-insulator transition temperature and also an order of magnitude larger magnetization. The magnetic hysteresis loops measured in pinning fields show a large exchange coupling for the case of the LAO/STO substrate and the sign of the coupling supports a ferromagnet to a ferromagnet exchange. The magnetization measured arises from a combination of substrate magnetism and that of the film and the enhanced conductivity, metal-insulator transition temperature and magnetization can be accounted for by the exchange coupling between the magnetic phase observed in the LAO/STO interfaces and the LSMO layer. The decay length of this interaction in LSMO is 90 nm which is surprisingly much longer than has been observed in ferromagnetic metals. Furthermore, the discovery of a long range oscillatory magnetic exchange coupling with LAO thickness suggests a role for the LAO layer beyond a simple insulator in this magnetic heterostructure.   

Quantum spin hall effect in LaAlO$_3$/SrTiO$_3$ nanostructures

LaAlO$_3$/SrTiO$_3$ heterostructures are known to exhibit strong spin-orbit coupling. We investigate local and non-local transport behavior of nanoscale Hall crosses created by conductive AFM lithography. The four-terminal resistance of these structures is consistently found to be $\sim$$h/e^2$, independent of the length of the channel. We also observe large (1-10 k$\Omega$) non-local resistances and zero-field Hall resistance that are attributed to quantum spin Hall phase with a spin-orbit derived pseudo-magnetic fields $B_{eff}\sim 15$ T. The pure spin current is blocked by Cooper pairs that form below Tc~200 mK, leading to a collapse of the non-local and zero-field Hall resistances.   

Nonvolatile Ferroelectric Manipulation of Electronic Structure at LAO/STO Heterointerface

Hetero-interfaces between different oxide insulators have attracted a lot of interests. One of the most important system is the 2D electron gas at LaAlO$_{3}$(LAO) and SrTiO$_{3}$ (STO), which had been reported to possess metallic conduction and superconductivity. In this study, the top-patterned Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ ferroelectric layer epitaxially grown on LAO/STO was proposed as a nonvolatile electronic modulation, and the interface band deformation was investigated using photoelectron spectroscopy (PES). Result showed different thickness and polarization state of top PZT significantly affected the band structure and its corresponding valence band offset at the LAO/STO hetero-interface. The transport data indicated that the as-grown PZT would deplete the conducting interface of LAO/STO, while switching the polarization of PZT would enhance the interface conduction.   

Non-local piezoresponse in 3 u.c. LaAlO$_3$/SrTiO$_3$

Nanoscale control of the metal-insulator transition in 3 unit cell (u.c.) LaAlO$_3$/SrTiO$_3$ heterostructures can be achieved by the conducting AFM lithography,\footnote{C. Cen, S. Thiel, G. Hammerl, C. W. Schneider, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, Nat. Mater. \textbf{7}, 2136 (2008)} however the mechanism behind this transition is still not well understood. One proposed mechanism invokes ionic transport through the LaAlO$_3$ layer.\footnote{A. Kumar, S. Jesse, A. Gruverman, C. B. Eom, S.V.Kalinin, unpublished} We have performed a variety of local and non-local piezoforce measurements on 3 u.c. LaAlO$_3$/SrTiO$_3$ heterostructures. The existence and nature of the non-local piezoelectric effect places strong constraints on the origin of the piezoelectric response. This work is supported by NSF DMR-1104191.   

Piezoresponse Force Microscopy of Gated LaAlO$_3$/SrTiO$_3$ Heterostructures

The quasi-two-dimensional (q-2DEG) electron liquid at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface can be tuned through the metal-insulator transition using a metallic top gate. At low carrier densities, the capacitance between the top gate and q-2DEG is significantly enhanced beyond the geometric capacitance.\footnote{Lu Li, C. Richter, S. Paetel, T. Kopp, J. Mannhart, R.C. Ashoori, Science \textbf{332}, 825 (2011)} In order to understand the origins of this enhancement in capacitance, we have performed spatially resolved piezo-force microscopy (PFM) on a top-gated 5u.c. LAO/STO structure. A large enhancement in piezoresponse is observed as the interface is switched to the conducting phase. Within the transition region, spatial structures or domains are observed. We propose that such measurement can provide new insights into the metal-insulator transition of the interface, and the associated capacitance enhancement.   

Ferroelectric control of two dimensional electron gas in oxide heterointerface

Oxide heterointerfaces are emerging as one of the most exciting materials systems in condensed-matter science. One remarkable example is the LaAlO$_{3}$ /SrTiO$_{3}$ (LAO/STO) interface, a model system in which a highly mobile electron gas forms between two band insulators. Our study to manipulate the conductivity at this interface by using ferroeletricity of Pb(Zr,Ti)O$_{3}$. Our transport data strongly suggests that down polarization direction depletes the conducting interface of LAO/STO. After switching the polarization direction (up), it becomes accumulation. In addition, our experiments show there is obvious the band structure changed by cross-sectional scanning tunneling microscopy and combining with X-ray photoelectron spectroscopy (XPS) measurements. The transport properties are measured to build up the connection between macroscopic properties and local electronic structures that have been applied to study this structure. Controlling the conductivity of this oxide interface suggests that this technique may not only extend more generally to other oxide systems but also open much potential to ferroelectric field effect transistors.   

Anomalous High Mobility in LaAlO$_3$/SrTiO$_3$ Nanowires

Nanoscale control over the LaAlO$_3/$SrTiO$_3$ interface\footnote{C. Cen, S. Thiel, K. E. Andersen, C. S. Hellberg, J. Mannhart, and J. Levy, Nature Materials \textbf{7}, 2136 (2008).} provides a possible pathway for reconfigurable oxide-based nanoelectronics at densities that exceed conventional silicon electronics. One of the central challenges in replacing silicon relates to energy dissipation, which in turn depends on the carrier mobility. We have performed four-terminal transport measurements of LaAlO$_3/$SrTiO$_3$ nanowires at room temperature (300 K) and at low temperature ($\sim 500$ mK). We find that the equivalent 2D mobility of nanowires greatly exceeds that of bulk SrTiO$_3$ ($\mu_{STO}=6$ cm$^2$/Vs), and approaches that of optimally doped Si at room temperature. Low-temperature mobilities can exceed 30,000 cm$^2$/Vs. We discuss possible physical mechanisms to explain the anomalously high mobility and the implications for future device technologies.   

First-principles modeling of Pt/LaAlO$_{3}$/SrTiO$_{3}$ nanocapacitors under an external bias potential

We study the electronic, structural and electrical properties of Pt/LaAlO$_{3}$/SrTiO$_{3}$ nanocapacitors under the action of an external applied bias, using first-principles calculations performed at fixed electric displacement $D$. In particular, we deduce a complete set of \emph{ab initio} band diagrams and a simple analytical expression for the electric field within the LaAlO$_{3}$~(LAO) film as a function of thickness and applied potential. In addition, we investigate the capacitance of the metal-oxide heterostructure in a field-effect transistor setup. We find that the electric field within LAO is a non-intrinsic quantity that monotonically decreases with increasing LAO thickness. The occurrence of spontaneous Zener tunneling in this system, therefore, is ruled out. We discuss the implications of our results in the light of recent experimental observations involving biased LAO/STO junctions and metallic top electrodes.   

THz rectified optical response of LaAlO$_{3}$/SrTiO$_{3}$ nanojunctions

Conducting AFM lithography can be used to create a variety of nanoscale devices at the LaAlO$_{3}$/SrTiO$_{3}$ interface\footnote{C.Cen et al., Nature Material, \textbf{7}, 298 (2008)}\footnote{C.Cen et al., Science, \textbf{323}, 1026 (2009)}. Nanoscale junctions have been shown to exhibit strongly localized photoconductivity over a range of wavelengths spanning the visible and near-infrared regime\footnote{P.Irvin et al., Nature Photonics, \textbf{4}, 849 (2010)}. Power-dependent and interferometric measurements of these nanostructures with ultrafast laser pulses reveal a nonlinear photoconductive response attributed to second-order nonlinear susceptibility of SrTiO$_{3}$. The breaking of inversion symmetry comes from the strong local electric field that extends across the junction. The ultrafast response of these nanojunctions make them attractive candidates for generation and detection of THz radiation at molecular scales. This work is supported by NSF DMR-1104191.   

Tuning the two-dimensional electron gas at the LaAlO$_{3}$/SrTiO$_{3}$(001) interface by metallic contacts

Density functional theory calculations reveal that adding a metallic overlayer on LaAlO$_{3}$/SrTiO$_{3}$(001) reduces/eliminates the electric field within the polar LaAlO$_{3}$ film and thus suppresses the thickness-dependent insulator-to-metal transition observed in uncovered films. Independent of the LaAlO$_{3}$ thickness both the surface and the interface are metallic, with an enhanced interface carrier density relative to LaAlO$_{3}$/SrTiO$_{3}$ (001) after the metallization transition. Moreover, a monolayer thick metallic Ti-contact exhibits a finite magnetic moment and for a thin SrTiO$_{3}$-substrate induces a spin-polarized 2D electron gas at the $n$-type interface due to confinement effects. The height of the Schottky barrier formed between the metal contact and LaAlO$_{3}$ depends strongly on the choice of the overlayer and allows to tune the carrier density at the interface [1]. \\[4pt] [1] V. Ruiz L\'{o}pez, R. Arras, W. E. Pickett, and R. Pentcheva, arXiv:1106.4205v1.   

Spatial Analogue of Quantum Spin Dynamics via Spin-Orbit Interaction

We theoretically demonstrate the mapping of electron spin dynamics from time to space in quantum wires with built-in spatially uniform and oscillating Rashba spin-orbit coupling in orthogonal directions. The presence of the spin-orbit interaction introduces pseudo-Zeeman couplings of the electron spins to effective magnetic fields. By periodically modulating the spin-orbit coupling along the quantum wire axis, it is possible to create the spatial analogue of spin resonance, without the need for real magnetic fields. The mapping of time-dependent operations onto a spatial axis suggests a new mode for quantum information processing in which gate operations are encoded into the band structure of the material. We describe a potential realization of such a material within nanowires at the interface of LaAlO$_{3}$/SrTiO$_{3}$ heterostructures.