93 research outputs found
High-Quality Planar high-Tc Josephson Junctions
Reproducible high-Tc Josephson junctions have been made in a rather simple
two-step process using ion irradiation. A microbridge (1 to 5 ?m wide) is
firstly designed by ion irradiating a c-axis-oriented YBa2Cu3O7-? film through
a gold mask such as the non-protected part becomes insulating. A lower Tc part
is then defined within the bridge by irradiating with a much lower fluence
through a narrow slit (20 nm) opened in a standard electronic photoresist.
These planar junctions, whose settings can be finely tuned, exhibit
reproducible and nearly ideal Josephson characteristics. This process can be
used to produce complex Josephson circuits.Comment: 4 pages, 5 figures, to be published in 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
Spin-polarized tunneling spectroscopy in tunnel junctions with half-metallic electrodes
We have studied the magnetoresistance (TMR) of tunnel junctions with
electrodes of La2/3Sr1/3MnO3 and we show how the variation of the conductance
and TMR with the bias voltage can be exploited to obtain a precise information
on the spin and energy dependence of the density of states. Our analysis leads
to a quantitative description of the band structure of La2/3Sr1/3MnO3 and
allows the determination of the gap delta between the Fermi level and the
bottom of the t2g minority spin band, in good agreement with data from
spin-polarized inverse photoemission experiments. This shows the potential of
magnetic tunnel junctions with half-metallic electrodes for spin-resolved
spectroscopic studies.Comment: To appear in Physical Review Letter
Phase Separation and the Phase Diagram in Cuprates Superconductors
We show that the main features of the cuprates superconductors phase diagram
can be derived considering the disorder as a key property of these materials.
Our basic point is that the high pseudogap line is an onset of phase separation
which generates compounds made up of regions with distinct doping levels. We
calculate how this continuous temperature dependent phase separation process
occurs in high critical temperature superconductors (HTSC) using the
Cahn-Hilliard approach, originally applied to study alloys. Since the level of
phase separation varies for different cuprates, it is possible that different
systems with average doping level pm exhibit different degrees of charge and
spin segregation. Calculations on inhomogeneous charge distributions in form of
stripes in finite clusters performed by the Bogoliubov-deGennes superconducting
approach yield good agreement to the pseudogap temperature T*(pm), the onset of
local pairing amplitudes with phase locked and concomitantly, how they develop
at low temperatures into the superconducting phase at Tc(pm) by percolation.Comment: 9 pages, 9 figures. Submitted to Phys. Rev.
Nearly total spin polarization in La2/3Sr1/3MnO3 from tunneling experiments
We have performed magnetotransport measurements on La2/3Sr1/3MnO3 / SrTiO3 /
La2/3Sr1/3MnO3 magnetic tunnel junctions. A magnetoresistance ratio of more
than 1800 % is obtained at 4K, from which we infer an electrode spin
polarization of at least 95 %. This result strongly underscores the
half-metallic nature of mixed-valence manganites and demonstrates its
capability as a spin analyzer. The magnetoresistance extends up to temperatures
of more than 270K. We argue that these improvements over most previous works
may result from optimizing the patterning process for oxide heterostructures.Comment: to appear in Applied Physics Letter
Co-doped (La,Sr)TiO3-d: a high-Curie temperature diluted magnetic system with large spin-polarization
We report on tunneling magnetoresistance (TMR) experiments that demonstrate
the existence of a significant spin polarization in Co-doped (La,Sr)TiO3-d
(Co-LSTO), a ferromagnetic diluted magnetic oxide system (DMOS) with high Curie
temperature. These TMR experiments have been performed on magnetic tunnel
junctions associating Co-LSTO and Co electrodes. Extensive structural analysis
of Co-LSTO combining high-resolution transmission electron microscopy and Auger
electron spectroscopy excluded the presence of Co clusters in the Co-LSTO layer
and thus, the measured ferromagnetism and high spin polarization are intrinsic
properties of this DMOS. Our results argue for the DMOS approach with complex
oxide materials in spintronics
Conductivity of underdoped YBa2Cu3O7-d : evidence for incoherent pair correlations in the pseudogap regime
Conductivity due to superconducting fluctuations studied in optimally doped
YBa2Cu3O7-d films displays a stronger decay law in temperature than explainable
by theory. A formula is proposed, which fits the data very well with two
superconductive parameters, Tc and the coherence length ksi_c0, and an energy
scale Delta*. This is also valid in underdoped materials and enables to
describe the conductivity up to 300 K with a single-particle excitations
channel in parallel with a channel whose contribution is controlled by ksi_c0,
Tc and Delta*.
This allows to address the nature of the pseudogap in favour of incoherent
pairing.Comment: 14 pages, 4 figures, 1 tabl
Spin-dependent tunneling through high-k LaAlO3
We report on the use of the LaAlO3 (LAO) high-k dielectric as a tunnel
barrier in magnetic tunnel junctions. From tunnel magnetoresistance (TMR)
measurements on epitaxial La2/3Sr1/3MnO3/LAO/La2/3Sr1/3MnO3 junctions, we
estimate a spin polarization of 77% at low temperature for the
La2/3Sr1/3MnO3/LAO interface. Remarkably, the TMR of La2/3Sr1/3MnO3/LAO/Co
junctions at low bias is negative, evidencing a negative spin polarization of
Co at the interface with LAO, and its bias dependence is very similar to that
of La2/3Sr1/3MnO3/STO/Co junctions. We discuss possible reasons for this
behaviour.Comment: 14 pages, 3 figure
Full oxide heterostructure combining a high-Tc diluted ferromagnet with a high-mobility conductor
We report on the growth of heterostructures composed of layers of the
high-Curie temperature ferromagnet Co-doped (La,Sr)TiO3 (Co-LSTO) with
high-mobility SrTiO3 (STO) substrates processed at low oxygen pressure. While
perpendicular spin-dependent transport measurements in STO//Co-LSTO/LAO/Co
tunnel junctions demonstrate the existence of a large spin polarization in
Co-LSTO, planar magnetotransport experiments on STO//Co-LSTO samples evidence
electronic mobilities as high as 10000 cm2/Vs at T = 10 K. At high enough
applied fields and low enough temperatures (H < 60 kOe, T < 4 K) Shubnikov-de
Haas oscillations are also observed. We present an extensive analysis of these
quantum oscillations and relate them with the electronic properties of STO, for
which we find large scattering rates up to ~ 10 ps. Thus, this work opens up
the possibility to inject a spin-polarized current from a high-Curie
temperature diluted oxide into an isostructural system with high-mobility and a
large spin diffusion length.Comment: to appear in Phys. Rev.
- …