617 research outputs found
Phase separation from electron confinement at oxide interfaces
Oxide heterostructures are of great interest both for fundamental and
applicative reasons. In particular the two-dimensional electron gas at the
LaAlO/SrTiO or LaTiO/SrTiO interfaces displays many different
physical properties and functionalities. However there are clear indications
that the interface electronic state is strongly inhomogeneous and therefore it
is crucially relevant to investigate possible intrinsic electronic mechanisms
underlying this inhomogeneity. Here the electrostatic potential confining the
electron gas at the interface is calculated self-consistently, finding that the
electron confinement at the interface may induce phase separation, to avoid a
thermodynamically unstable state with a negative compressibility. This provides
a generic robust and intrinsic mechanism for the experimentally observed
inhomogeneous character of these interfaces.Comment: 8 pages and 4 figure
Le reboisement au Sénégal : bilan des réalisations de 1993 à 1998
La production de plants au Sénégal, toutes espèces confondues, a régressé de 46% entre 1993 et 1998 et reste dominée par les espèces forestières (77% des productions). Les espèces exotiques à croissance rapide sont les plus demandées par les populations locales. Durant cette période, le taux annuel de reboisement est évalué à 13 679 ha. Ce taux est nettement inférieur au taux de déboisement estimé à 50 000 ha/an. Les plantations massives représentent l'essentiel des réalisations de ces dernières années et concernent essentiellement les plantations villageoises et communautaires. L'effort national de reboisement est en baisse constante et les superficies plantées ont diminué de 38%. Cette situation est en grande partie liée à la récession économique et à la régression de près de 60% des réalisations des projets de développement forestier qui assurent plus de 58% des superficies plantées. La situation actuelle du secteur forestier découles de contraintes d'ordre économique, méthodoloqique, technique et institutionnelle qui sont discutées dans cette étude. (Résumé d'auteur
c-axis Josephson Tunneling in Twinned YBCO Crystals
Josephson tunneling between YBCO and Pb with the current flowing along the
c-axis of the YBCO is persumed to come from an s-wave component of the
superconductivity of the YBCO. Experiments on multi-twin samples are not
entirely consistent with this hypothesis. The sign change of the s-wave order
parameter across the N_T twin boundaries should give cancelations, resulting in
a small tunneling current. The actual current is larger than this.
We present a theory of this unexpectedly large current based upon a surface
effect: disorder-induced supression of the d-wave component at the (001)
surface leads to s-wave coherence across the twin boundaries and a non-random
tunneling current. We solve the case of an ordered array of d+s and d-s twins,
and estimate that the twin size at which s-wave surface coherence occurs is
consistent with typical sizes observed in experiments. In this picture, there
is a phase difference of between different surfaces of the material. We
propose a corner junction experiment to test this picture.Comment: 5 pages, 4 eps figure
Inhomogeneous multi-carrier superconductivity at LaXO3/SrTiO3 (X=Al or Ti) oxide interfaces
Several experiments reveal the inhomogeneous character of the superconducting
state that occurs when the carrier density of the two-dimensional electron gas
formed at the LaXO3/SrTiO3 (X=Al or Ti) interface is tuned above a threshold
value by means of gating. Re-analyzing previous measurements, that highlight
the presence of two kinds of carriers, with low and high mobility, we shall
provide a description of multi-carrier magneto-transport in an inhomogeneous
two-dimensional electron gas, gaining insight into the properties of the
physics of the systems under investigation. We shall then show that the
measured resistance, superfluid density, and tunneling spectra result from the
percolative connection of superconducting "puddles" with randomly distributed
critical temperatures, embedded in a weakly localizing metallic matrix. We
shall also show that this scenario is consistent with the characteristics of
the superconductor-to-metal transition driven by a magnetic field. A
multi-carrier description of the superconducting state, within a weak-coupling
BCS-like model, will be finally discussed.Comment: 12 pages 10 figure
Improving HTc Josephson Junctions (HTc JJ) by annealing: the role of vacancy-interstitial annihilation
We have studied the annealing effect in transport properties of High
temperature Josephson Junctions (HTc JJ) made by ion irradiation. Low
temperature annealing (80 degrees Celsius) increases the JJ transition
temperature (TJ) and the Ic.Rn product, where Ic is the critical current and Rn
the normal resistance. We found that the spread in JJ characteristics can be
lowered by sufficient long annealing times. Using random walk numerical
simulations, we showed that the characteristic annealing time and the evolution
of the spread in JJ characteristics can be explained by a vacancy-interstitial
annihilation process rather than by an oxygen diffusion one.Comment: 7 pages and 3 figures submitted to Applied Physics Letter
Study and optimization of ion-irradiated High-Tc Josephson nanoJunctions by Monte Carlo simulations
High Tc Josephson nanoJunctions (HTc JnJ) made by ion irradiation have
remarkable properties for technological applications. However, the spread in
their electrical characteristics increases with the ion dose. We present a
simple model to explain the JnJ inhomogeneities, which accounts quantitatively
for experimental data. The spread in the slit's width of the irradiation mask
is the limiting factor.Monte Carlo simulations have been performed using
different irradiation conditions to study their influence on the spread of the
JnJ charcateristics. A "universal" behavior has been evidenced, which allows to
propose new strategies to optimize JnJ reproducibility.Comment: 14 pages, 6 Figures. accepted in Journal of Applied Physic
Physical Mechanism of the d->d+is Transition
We discuss the basic physical mechanism of the d->d+is transition, which is
the currently accepted explanation for the results of tunneling experiments
into planes. Using the first-order perturbation theory, we show that the
zero-bias states drive the transition. We present various order-of-magnitude
estimates and consistency checks that support this picture.Comment: 7 pages, 2 figure
Multi-band superconductivity and nanoscale inhomogeneity at oxide interfaces
The two-dimensional electron gas at the LaTiO3/SrTiO3 or LaAlO3/SrTiO3 oxide
interfaces becomes superconducting when the carrier density is tuned by gating.
The measured resistance and superfluid density reveal an inhomogeneous
superconductivity resulting from percolation of filamentary structures of
superconducting "puddles" with randomly distributed critical temperatures,
embedded in a non-superconducting matrix. Following the evidence that
superconductivity is related to the appearance of high-mobility carriers, we
model intra-puddle superconductivity by a multi-band system within a weak
coupling BCS scheme. The microscopic parameters, extracted by fitting the
transport data with a percolative model, yield a consistent description of the
dependence of the average intra-puddle critical temperature and superfluid
density on the carrier density.Comment: 7 pages with 3 figures + supplemental material (4 pages and 5
figures
Quantized conductance in a one-dimensional ballistic oxide nanodevice
Electric-field effect control of two-dimensional electron gases (2-DEG) has
enabled the exploration of nanoscale electron quantum transport in
semiconductors. Beyond these classical materials, transition metal-oxide-based
structures have d-electronic states favoring the emergence of novel quantum
orders absent in conventional semiconductors. In this context, the
LaAlO3/SrTiO3 interface that combines gate-tunable superconductivity and
sizeable spin-orbit coupling is emerging as a promising platform to realize
topological superconductivity. However, the fabrication of nanodevices in which
the electronic properties of this oxide interface can be controlled at the
nanoscale by field-effect remains a scientific and technological challenge.
Here, we demonstrate the quantization of conductance in a ballistic quantum
point contact (QPC), formed by electrostatic confinement of the LaAlO3/SrTiO3
2-DEG with a split-gate. Through finite source-drain voltage, we perform a
comprehensive spectroscopic investigation of the 3d energy levels inside the
QPC, which can be regarded as a spectrometer able to probe Majorana states in
an oxide 2-DEG
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