201 research outputs found

    Confinement and Superconductivity at the LaAlO3/SrTiO3 Interface

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    The interface between LaAlO3 and SrTiO3, two good band insulators, was found to be conducting [1], and, in some doping range, superconducting with a maximum critical temperature of about 300 mK [2,3]. I will discuss in this presentation the electronic structure, superconductivity, and the Tc versus doping phase diagram of LaAlO3/SrTiO3 and ((LaAlO3)0.5-(SrTiO3)0.5)-SrTiO3 interfaces. I will also compare superconductivity at these interfaces with superconductivity in bulk doped SrTiO3 [4]. [1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004). [2] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007). [3] A. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008). [4] S. Gariglio, M. Gabay, and J.-M. Triscone, Review APL Materials, 4, 060701 (2016)

    Tuning of the depolarization field and nanodomain structure in ferroelectric thin films

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    The screening efficiency of a metal-ferroelectric interface plays a critical role in determining the polarization stability and hence the functional properties of ferroelectric thin films. Imperfect screening leads to strong depolarization fields that reduce the spontaneous polarization or drive the formation of ferroelectric domains. We demonstrate that by modifying the screening at the metal-ferroelectric interface through insertion of ultrathin dielectric spacers, the strength of the depolarization field can be tuned and thus used to control the formation of nanoscale domains. Using piezoresponse force microscopy, we follow the evolution of the domain configurations as well as polarization stability as a function of depolarization field strength.Comment: 19 pages, 7 figure

    Proposed cavity Josephson plasmonics with complex-oxide heterostructures

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    We discuss how complex-oxide heterostructures that include high-Tc superconducting cuprates can be used to realize an array of sub-millimeter cavities that support Josephson plasmon polaritons. These cavities have several attractive features for new types of light matter interaction studies and we show that they promote "ultrastrong" coupling between THz frequency radiation and Josephson plasmons. Cavity electrodynamics of Josephson plasmons allows to manipulate the superconducting order-parameter phase coherence. As an example, we discuss how it could be used to cool superconducting phase fluctuations with light

    Nanoscale studies of domain wall motion in epitaxial ferroelectric thin films

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    Atomic force microscopy was used to investigate ferroelectric switching and nanoscale domain dynamics in epitaxial PbZr0.2Ti0.8O3 thin films. Measurements of the writing time dependence of domain size reveal a two-step process in which nucleation is followed by radial domain growth. During this growth, the domain wall velocity exhibits a v ~ exp[-(1/E)^mu] dependence on the electric field, characteristic of a creep process. The domain wall motion was analyzed both in the context of stochastic nucleation in a periodic potential as well as the canonical creep motion of an elastic manifold in a disorder potential. The dimensionality of the films suggests that disorder is at the origin of the observed domain wall creep. To investigate the effects of changing the disorder in the films, defects were introduced during crystal growth (a-axis inclusions) or by heavy ion irradiation, producing films with planar and columnar defects, respectively. The presence of these defects was found to significantly decrease the creep exponent mu, from 0.62 - 0.69 to 0.38 - 0.5 in the irradiated films and 0.19 - 0.31 in the films containing a-axis inclusions.Comment: 13 pages, 15 figures, to be published in J. App. Phys. special issue on ferroelectric

    Probing Quantum Confinement and Electronic Structure at Polar Oxide Interfaces

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    Polar discontinuities occurring at interfaces between two different materials constitute both a challenge and an opportunity in the study and application of a variety of devices. In order to cure the large electric field occurring in such structures, a reconfiguration of the charge landscape sets in at the interface via chemical modifications, adsorbates or charge transfer. In the latter case, one may expect a local electronic doping of one material: one sparkling example is the two-dimensional electron liquid (2DEL) appearing in SrTiO3_3 once covered by a polar LaAlO3_3 layer. Here we show that tuning the formal polarisation of a (La,Al)1−x_{1-x}(Sr,Ti)x_xO3_3 (LASTO:xx) overlayer through chemical composition modifies the quantum confinement of the 2DEL in SrTiO3_3 and its electronic band structure. The analysis of the behaviour in magnetic field of superconducting field-effect devices reveals, in agreement with ab initioab\ initio calculations and self-consistent Poisson-Schr\"odinger modelling, that quantum confinement and energy splitting between electronic bands of different symmetries strongly depend on interface charge densities. These results not only strongly support the polar discontinuity mechanisms with a full charge transfer to explain the origin of the 2DEL at the celebrated LaAlO3_3/SrTiO3_3 interface, but also demonstrate an effective tool for tailoring the electronic structure at oxide interfaces.Comment: 18 pages, 4 figures, 1 ancillary file (Supporting Information

    High sensitivity variable-temperature infrared nanoscopy of conducting oxide interfaces

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    Probing the local transport properties of two-dimensional electron systems (2DES) confined at buried interfaces requires a non-invasive technique with a high spatial resolution operating in a broad temperature range. In this paper, we investigate the scattering-type scanning near field optical microscopy as a tool for studying the conducting LaAlO3/SrTiO3 interface from room temperature down to 6 K. We show that the near-field optical signal, in particular its phase component, is highly sensitive to the transport properties of the electron system present at the interface. Our modelling reveals that such sensitivity originates from the interaction of the AFM tip with coupled plasmon-phonon modes with a small penetration depth. The model allows us to quantitatively correlate changes in the optical signal with the variation of the 2DES transport properties induced by cooling and by electrostatic gating. To probe the spatial resolution of the technique, we image conducting nano-channels written in insulating heterostructures with a voltage-biased tip of an atomic force microscope.Comment: 19 pages, 5 figure

    Fabricating Superconducting Interfaces between Artificially-Grown LaAlO3_3 and SrTiO3_3 Thin Films

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    Realization of a fully metallic two-dimensional electron gas at the interface between artificially-grown LaAlO3_3 and SrTiO3_3 thin films has been an exciting challenge. Here we present for the first time the successful realization of a superconducting 2DEG at interfaces between artificially-grown LaAlO3_3 and SrTiO3_3 thin films. Our results highlight the importance of two factors-the growth temperature and the SrTiO3_3 termination. We use local friction force microscopy and transport measurements to determine that in normal growth conditions the absence of a robust metallic state at low temperature in the artificially-grown LaAlO3_3/SrTiO3_3 interface is due to the nanoscale SrO segregation occurring on the SrTiO3_3 film surface during the growth and the associated defects in the SrTiO3_3 film. By adopting an extremely high SrTiO3_3 growth temperature, we demonstrate a way to realize metallic, down to the lowest temperature, and superconducting 2DEG at interfaces between LaAlO3_3 layers and artificially-grown SrTiO3_3 thin films. This study paves the way to the realization of functional LaAlO3_3/SrTiO3_3 superlattices and/or artificial LaAlO3_3/SrTiO3_3 interfaces on other substrates

    Monodomain to polydomain transition in ferroelectric PbTiO3 thin films with La0.67Sr0.3MnO3 electrodes

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    Finite size effects in ferroelectric thin films have been probed in a series of epitaxial perovskite c-axis oriented PbTiO3 films grown on thin La0.67Sr0.33MnO3 epitaxial electrodes. The film thickness ranges from 480 down to 28 A (7 unit cells). The evolution of the film tetragonality c/a, studied using high resolution x-ray diffraction measurements, shows first a decrease of c/a with decreasing film thickness followed by a recovery of c/a at small thicknesses. This recovery is accompanied by a change from a monodomain to a polydomain configuration of the polarization, as directly demonstrated by piezoresponse atomic force microscopy measurements

    Artificial quantum confinement in LAO3/STO heterostructure

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    Heterostructures of transition metal oxides (TMO) perovskites represent an ideal platform to explore exotic phenomena involving the complex interplay between the spin, charge, orbital and lattice degrees of freedom available in these compounds. At the interface between such materials, this interplay can lead to phenomena that are present in none of the original constituents such as the formation of the interfacial 2D electron system (2DES) discovered at the LAO3/STO3 (LAO/STO) interface. In samples prepared by growing a LAO layer onto a STO substrate, the 2DES is confined in a band bending potential well, whose width is set by the interface charge density and the STO dielectric properties, and determines the electronic band structure. Growing LAO (2 nm) /STO (x nm)/LAO (2 nm) heterostructures on STO substrates allows us to control the extension of the confining potential of the top 2DES via the thickness of the STO layer. In such samples, we explore the dependence of the electronic structure on the width of the confining potential using soft X-ray ARPES combined with ab-initio calculations. The results indicate that varying the thickness of the STO film modifies the quantization of the 3d t2g bands and, interestingly, redistributes the charge between the dxy and dxz/dyz bands
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