127 research outputs found
Interplay between density and superconducting quantum critical fluctuations
We consider the case of a density-driven metal-superconductor transition in
the proximity of an electronic phase separation. In particular we investigate
the interplay between superconducting fluctuations and density fluctuations,
which become quantum critical when the electronic phase separation vanishes at
zero temperature into a quantum critical point. In this situation the critical
dynamical density fluctuations strongly affect the dynamics of the Cooper pair
fluctuations, which acquire a more singular character with a z=3 dynamical
critical index. This gives rise to a scenario that possibly rules the
disappearance of superconductivity when the electron density is reduced by
elecrostatic gating at the LaAlO3/SrTiO3 interface.Comment: 5 pages, 4 figure
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
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
Josephson oscillation linewidth of ion-irradiated YBaCuO junctions
We report on the noise properties of ion-irradiated YBaCuO
Josephson junctions. This work aims at investigating the linewidth of the
Josephson oscillation with a detector response experiment at 132 GHz.
Experimental results are compared with a simple analytical model based on the
Likharev-Semenov equation and the de Gennes dirty limit approximation. We show
that the main source of low-frequency fluctuations in these junctions is the
broadband Johnson noise and that the excess () noise contribution
does not prevail in the temperature range of interest, as reported in some
other types of high-T superconducting Josephson junctions. Finally, we
discuss the interest of ion-irradiated junctions to implement frequency-tunable
oscillators consisting of synchronized arrays of Josephson junctions
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
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
Scanning Tunneling Spectroscopy on the novel superconductor CaC6
We present scanning tunneling microscopy and spectroscopy of the newly
discovered superconductor CaC. The tunneling conductance spectra, measured
between 3 K and 15 K, show a clear superconducting gap in the quasiparticle
density of states. The gap function extracted from the spectra is in good
agreement with the conventional BCS theory with = 1.6 0.2
meV. The possibility of gap anisotropy and two-gap superconductivity is also
discussed. In a magnetic field, direct imaging of the vortices allows to deduce
a coherence length in the ab plane 33 nm
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