99 research outputs found
Time-resolved photoluminescence of n-doped SrTiO_3
Following the recent surge of interest in n-doped strontium titanate as a
possible blue light emitter, a time-resolved photoluminescence analysis was
performed on nominally pure, Nb-doped and oxygen-deficient single-crystal
SrTiO3 samples. The doping-effects on both the electronic states involved in
the transition and the decay mechanism are respectively analyzed by comparing
the spectral and dynamic features and the yields of the emission. Our
time-resolved analysis, besides shedding some light on the basic recombination
mechanisms acting in these materials, sets the intrinsic bandwidth limit of the
proposed blue light emitting optoelectronic devices made of Ti-based
perovskites heterostructures in the GHz range
Renormalization of Coulomb interactions in s-wave superconductor NaCoO
We study the renormalized Coulomb interactions due to retardation effect in
NaCoO. Although the Morel-Anderson's pseudo potential for
orbital is relatively large because the direct Coulomb repulsion
is large, that for interband transition between and
orbitals is very small since the renormalization factor for
pair hopping is square of that for . Therefore, the s-wave
superconductivity due to valence-band Suhl-Kondo mechanism will survive against
strong Coulomb interactions. The interband hopping of Cooper pairs due to shear
phonons is essential to understand the superconductivity in NaCoO.Comment: 2pages, 2figures, Proceedings of ICM in Kyoto, 200
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
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
Magneli-like phases in epitaxial anatase TiO2 thin films
Using high-resolution transmission electron microscopy and image simulation techniques in combination with ab initio calculations, we show the existence of two different superlattices of crystallographic shear planes, analogous to the Magn\ue9li phases of rutile, in oxygen-deficient films of anatase TiO2 epitaxially grown on LaAlO3 substrates. (103)- and (101)-oriented shear plane structures are detected in the outer film region and in proximity of the film/substrate interface, respectively. We show that these shear planes are characterized by TiO-like cubic local structures, which can deviate from the TinO2n 121 stoichiometry of the classical rutile-derived Magn\ue9li phases, particularly in the outer part of the film. Computed formation energies provide insights into the thermodynamic stability of the observed structures and their relations to the growth dynamics
High Curie temperature for La0.7Sr0.3MnO3 thin films deposited on CeO2/YSZ-based buffered silicon substrates
International audienceTwo kinds of epitaxial structures were grown by standard pulsed laser deposition on (001) Si, namely La0.7Sr0.3MnO3/Bi4Ti3O12/CeO2/YSZ/Si (BTO-based), and La0.7Sr0.3MnO3/SrTiO3/ CeO2 /YSZ/Si (STO-based) multilayers. The samples were investigated by means of x-ray diffraction, transmission electron microscopy, magnetic and transport measurements. The Curie temperature TC of the BTO-based samples was found to be higher (360 K) than for the typical reference epitaxial LSMO film grown on (001) SrTiO3 single crystal (345 K), due to high compressive in-plane strain. The STO-based samples show high structural quality, low roughness and high TC (350 K), making them interesting candidates for use in innovative LSMO-based bolometers or spintronic devices operating at room temperature. (Some figures in this article are in colour only in the electronic version
Magnetic properties of pseudomorphic epitaxial films of Pr_{0.7}Ca_{0.3}MnO_3 under different biaxial tensile stresses
In order to analyse the effect of strain on the magnetic properties of
narrow-band manganites, the temperature and field dependent susceptibilities of
about 8.5 nm thick epitaxial Pr0.7Ca0.3MnO3 films, respectively grown on (001)
and (110) SrTiO3 substrates, have been compared. For ultrathin samples grown on
(001) SrTiO3, a bulk-like cluster-glass magnetic behaviour is found, indicative
of the possible coexistence of antiferromagnetic and ferromagnetic phases. On
the contrary, ultrathin films grown on (110) substrates show a robust
ferromagnetism, with a strong spontaneous magnetization of about 3.4 mB /Mn
atom along the easy axis. On the base of high resolution reciprocal space
mapping analyses performed by x-ray diffraction, the different behaviours are
discussed in terms of the crystallographic constraints imposed by the epitaxy
of Pr0.7Ca0.3MnO3 on SrTiO3. We suggest that for growth on (110) SrTiO3, the
tensile strain on the film c-axis, lying within the substrate plane, favours
the ferromagnetic phase, possibly by allowing a mixed occupancy and
hybridization of both in-plane and out-of-plane eg orbitals. Our data allow to
shed some physics of inhomogeneous states in manganites and on the nature of
their ferromagnetic insulating state.Comment: 9 pages, 9 figure
Observation of a two-dimensional electron gas at the surface of annealed SrTiO3 single crystals by scanning tunneling spectroscopy
Scanning tunneling spectroscopy suggests the formation of a two dimensional
electron gas (2DEG) on the TiO2 terminated surface of undoped SrTiO3 single
crystals annealed at temperature lower than 400 {\deg}C in ultra high vacuum
conditions. Low energy electron diffraction indicates that the 2D metallic
SrTiO3 surface is not structurally reconstructed, suggesting that non-ordered
oxygen vacancies created in the annealing process introduce carriers leading to
an electronic reconstruction. The experimental results are interpreted in a
frame of competition between oxygen diffusion from the bulk to the surface and
oxygen loss from the surface itself.Comment: 9 pages, 3 figures, submitted to Physical Review
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