67 research outputs found
Pulsed laser deposition of SrTiO3/LaGaO3 and SrTiO3/LaAlO3: plasma plume effects
Pulsed laser deposition of SrTiO3/LaGaO3 and SrTiO3/LaAlO3 interfaces has
been analyzed with a focus on the kinetic energy of the ablated species. LaGaO3
and LaAlO3 plasma plumes were studied by fast photography and space-resolved
optical emission spectroscopy. Reflection high energy electron diffraction was
performed proving a layer-by-layer growth up to 10-1 mbar oxygen pressure. The
role of the energetic plasma plume on the two-dimensional growth and the
presence of interfacial defects at different oxygen growth pressure has been
discussed in view of the conducting properties developing at such
polar/non-polar interfaces
Transport in strongly-coupled graphene-LaAlO3/SrTiO3 hybrid systems
We report on the transport properties of hybrid devices obtained by
depositing graphene on a LaAlO3/SrTiO3 oxide junction hosting a 4 nm-deep
two-dimensional electron system. At low graphene-oxide inter-layer bias the two
electron systems are electrically isolated, despite their small spatial
separation, and very efficient reciprocal gating is shown. A pronounced
rectifying behavior is observed for larger bias values and ascribed to the
interplay between electrostatic depletion and tunneling across the LaAlO3
barrier. The relevance of these results in the context of strongly-coupled
bilayer systems is discussed.Comment: 10 pages, 3 figure
Charge density waves enhance the electronic noise of manganites
The transport and noise properties of Pr_{0.7}Ca_{0.3}MnO_{3} epitaxial thin
films in the temperature range from room temperature to 160 K are reported. It
is shown that both the broadband 1/f noise properties and the dependence of
resistance on electric field are consistent with the idea of a collective
electrical transport, as in the classical model of sliding charge density
waves. On the other hand, the observations cannot be reconciled with standard
models of charge ordering and charge melting. Methodologically, it is proposed
to consider noise-spectra analysis as a unique tool for the identification of
the transport mechanism in such highly correlated systems. On the basis of the
results, the electrical transport is envisaged as one of the most effective
ways to understand the nature of the insulating, charge-modulated ground states
in manganites.Comment: 6 two-column pages, 5 figure
Polar catastrophe and electronic reconstructions at the LaAlO3/SrTiO3 interface: evidence from optical second harmonic generation
The so-called "polar catastrophe", a sudden electronic reconstruction taking
place to compensate for the interfacial ionic polar discontinuity, is currently
considered as a likely factor to explain the surprising conductivity of the
interface between the insulators LaAlO3 and SrTiO3. We applied optical second
harmonic generation, a technique that a priori can detect both mobile and
localized interfacial electrons, to investigating the electronic polar
reconstructions taking place at the interface. As the LaAlO3 film thickness is
increased, we identify two abrupt electronic rearrangements: the first takes
place at a thickness of 3 unit cells, in the insulating state; the second
occurs at a thickness of 4-6 unit cells, i.e., just above the threshold for
which the samples become conducting. Two possible physical scenarios behind
these observations are proposed. The first is based on an electronic transfer
into localized electronic states at the interface that acts as a precursor of
the conductivity onset. In the second scenario, the signal variations are
attributed to the strong ionic relaxations taking place in the LaAlO3 layer
Probing charge transfer during metal-insulator transitions in graphene-LaAlO3/SrTiO3 systems
Two-dimensional electron systems (2DESs) at the interface between LaAlO3 (LAO) and SrTiO3 (STO) perovskite oxides display a wide class of tunable phenomena ranging from superconductivity to metal-insulator transitions. Most of these effects are strongly sensitive to surface physics and often involve charge transfer mechanisms, which are, however, hard to detect. In this work, we realize hybrid field-effect devices where graphene is used to modulate the transport properties of the LAO/STO 2DES. Different from a conventional gate, graphene is semimetallic and allows us to probe charge transfer with the oxide structure underneath the field-effect electrode. In LAO/STO samples with a low initial carrier density, graphene-covered regions turn insulating when the temperature is lowered to 3 K, but conduction can be restored in the oxide structure by increasing the temperature or by field effect. The evolution of graphene's electron density is found to be inconsistent with a depletion of LAO/STO, but it rather points to a localization of interfacial carriers in the oxide structure.Two-dimensional electron systems (2DESs) at the interface between LaAlO3 (LAO) and SrTiO3 (STO) perovskite oxides display a wide class of tunable phenomena ranging from superconductivity to metal-insulator transitions. Most of these effects are strongly sensitive to surface physics and often involve charge transfer mechanisms, which are, however, hard to detect. In this work, we realize hybrid field-effect devices where graphene is used to modulate the transport properties of the LAO/STO 2DES. Different from a conventional gate, graphene is semimetallic and allows us to probe charge transfer with the oxide structure underneath the field-effect electrode. In LAO/STO samples with a low initial carrier density, graphene-covered regions turn insulating when the temperature is lowered to 3 K, but conduction can be restored in the oxide structure by increasing the temperature or by field effect. The evolution of graphene's electron density is found to be inconsistent with a depletion of LAO/STO, but it rather p..
Study of equilibrium carrier transfer in LaAlO3/SrTiO3 from an epitaxial La1 12x Sr x MnO3 ferromagnetic layer
Using x-ray magnetic circular dichroism and ab-initio calculations, we explore the La1-xSrxMnO3/LaAlO3/SrTiO3 (001) heterostructure as a mean to induce transfer of spin polarized carriers from ferromagnetic La1-xSrxMnO3 layer into the 2DEG (two-dimensional electron gas) at the LaAlO3/SrTiO3 interface. By out-of-plane transport measurements, the tunneling across the LaAlO3 barrier is also analyzed. Our results suggest small or vanishing spin-polarization for the 2DEG: magnetic dichroism does not reveal a neat signal on Ti atoms, while calculations predict, for the pristine stoichiometric interface, a small spin-resolved mobile charge of 2.5 x 10(13) cm(-2) corresponding to a magnetic moment of 0.038 mu(B) per Ti atom, tightly confined within the single SrTiO3 layer adjacent to LaAlO3. Such a small magnetization is hard to be detected experimentally and perhaps not robust enough to survive to structural disorder, native doping, or La1-xSrxMnO3 dead-layer effects. Our analysis suggests that, while some spin-diffusion cannot be completely ruled out, the use of ferromagnetic La1-xSrxMnO3 epilayers grown on-top of LaAlO3/SrTiO3 is not effective enough to induce robust spin-transport properties in the 2DEG. The examined heterostructure is nevertheless an excellent test-case to understand some fundamental aspects of the spin-polarized charge transfer in 2D wells
Critical influence of target-to-substrate distance on conductive properties of LaGaO3/SrTiO3 interfaces deposited at 10-1 mbar oxygen pressure
We investigate pulsed laser deposition of LaGaO3/SrTiO3 at 10-1 mbar oxygen
background pressure, demonstrating the critical effect of the
target-to-substrate distance, dTS, on the interface sheet resistance, Rs. The
interface turns from insulating to metallic by progressively decreasing dTS.
The analysis of the LaGaO3 plume evidences the important role of the plume
propagation dynamics on the interface properties. These results demonstrate the
growth of conducting interfaces at an oxygen pressure of 10-1 mbar, an
experimental condition where a well-oxygenated heterostructures with a reduced
content of oxygen defects is expected.Comment: 16 pages, 3 figure
Energy and symmetry of excitations in undoped layered cuprates measured by Cu resonant inelastic x-ray scattering
We measured high resolution Cu edge resonant inelastic x-ray scattering
(RIXS) of the undoped cuprates LaCuO, SrCuOCl, CaCuO
and NdBaCuO. The dominant spectral features were assigned to
excitations and we extensively studied their polarization and scattering
geometry dependence. In a pure ionic picture, we calculated the theoretical
cross sections for those excitations and used them to fit the experimental data
with excellent agreement. By doing so, we were able to determine the energy and
symmetry of Cu-3 states for the four systems with unprecedented accuracy and
confidence. The values of the effective parameters could be obtained for the
single ion crystal field model but not for a simple two-dimensional cluster
model. The firm experimental assessment of excitation energies carries
important consequences for the physics of high superconductors. On one
hand, having found that the minimum energy of orbital excitation is always
eV, i.e., well above the mid-infrared spectral range, leaves to
magnetic excitations (up to 300 meV) a major role in Cooper pairing in
cuprates. On the other hand, it has become possible to study quantitatively the
effective influence of excitations on the superconducting gap in cuprates.Comment: 22 pages, 11 figures, 1 tabl
Self-formed Micro-Membranes
Oxide heterostructures represent a unique playground for triggering the
emergence of novel electronic states and for implementing new device concepts.
The discovery of 2D conductivity at the interface has been
linking for over a decade two of the major current research fields in Materials
Science: correlated transition-metal-oxide systems and low-dimensional systems.
A full merging of these two fields requires nevertheless the realization of
heterostructures in the form of freestanding membranes. Here
we show a completely new method for obtaining oxide hetero-membranes with
micrometer lateral dimensions. Unlike traditional thin-film-based techniques
developed for semiconductors and recently extended to oxides, the concept we
demonstrate does not rely on any sacrificial layer and is based instead on pure
strain engineering. We monitor through both real-time and post-deposition
analyses, performed at different stages of growth, the strain relaxation
mechanism leading to the spontaneous formation of curved hetero-membranes.
Detailed transmission electron microscopy investigations show that the
membranes are fully epitaxial and that their curvature results in a huge strain
gradient, each of the layers showing a mixed compressive/tensile strain state.
Electronic devices are fabricated by realizing ad hoc circuits for individual
micro-membranes transferred on silicon chips. Our samples exhibit metallic
conductivity and electrostatic field effect similar to 2D-electron systems in
bulk heterostructures. Our results open a new path for adding oxide
functionality into semiconductor electronics, potentially allowing for
ultra-low voltage gating of a superconducting transistors, micromechanical
control of the 2D electron gas mediated by ferroelectricity and
flexoelectricity, and on-chip straintronics.Comment: 8 pages, 4 figure
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