1,174 research outputs found
Transition from a mixed to a pure d-wave symmetry in superconducting optimally doped YBaCuO thin films under applied fields
We have probed the Landau levels of nodal quasi-particles by tunneling along
a nodal direction of (110) oriented YBaCuO thin films with a
magnetic field applied perpendicular to the planes, and parallel to the
film's surface. In optimally doped films and at low temperature, finite energy
nodal states are clearly observed in films thinner than the London penetration
depth. Above a well defined temperature the order parameter reverts to a pure
\emph{d}-wave symmetry.Comment: 4 pages, 4 figure
Phase transition between d-wave and anisotropic s-wave gaps in high temperature oxides superconductors
We study models for superconductivity with two interactions: due to
antiferromagnetic(AF) fluctuations and due to phonons, in a weak coupling
approach to the high temperature superconductivity. The nature of the two
interactions are considerably different; is positive and sharply peaked
at (,) while is negative and peaked at () due to
weak phonon screening. We numerically find (a) weak BCS attraction is enough to
have high critical temperature if a van Hove anomaly is at work, (b) (AF)
is important to give d-wave superconductivity, (c) the gap order parameter
is constant(s-wave) at extremely overdope region and it
changes to anisotropic s-wave as doping is reduced, (d) there exists a first
order phase transition between d-wave and anisotropic s-wave gaps. These
results are qualitatively in agreement with preceding works; they should be
modified in the strongly underdope region by the presence of antiferromagnetic
fluctuations and ensuing AF pseudogap.Comment: 4 pages in RevTex (double column), 4 figure
Proximity effect in granular superconductor-normal metal structures
We fabricated three-dimensional disordered Pb-Cu granular structures, with
various metal compositions. The typical grain size of both metals is smaller
than the superconductor and normal metal coherence lengths, thus satisfying the
Cooper limit. The critical temperature of the samples was measured and compared
with the critical temperature of bilayers. We show how the proximity effect
theories, developed for bilayers, can be modified for random mixtures and we
demonstrate that our experimental data fit well the de Gennes weak coupling
limit theory in the Cooper limit. Our results indicate that, in granular
structures, the Cooper limit can be satisfied over a wide range of
concentrations.Comment: 15 pages, 4 figure
Superfluid density of superconductor-ferromagnet bilayers
We report the first measurements of the effective superfluid density n_S(T)
\propto \lambda^{-2}(T) of Superconductor-Ferromagnet (SC/FM) bilayers, where
\lambda is the effective magnetic field penetration depth. Thin Nb/Ni bilayers
were sputtered in ultrahigh vacuum in quick succession onto oxidized Si
substrates. Nb layers are 102 A thick for all samples, while Ni thicknesses
vary from 0 to 100 A. T_C determined from \lambda^{-2}(T) decreases rapidly as
Ni thickness d_Ni increases from zero to 15 A, then it has a shallow minimum at
d_Ni \approx 25 A. \lambda^{-2}(0) behaves similarly, but has a minimum several
times deeper. In fact, \lambda^{-2}(0) continues to increase with increasing Ni
thickness long after T_C has stopped changing. We argue that this indicates a
substantial superfluid density inside the ferromagnetic Ni films.Comment: 13 pages, 2 figures, MMM 2007 proceeding
Towards understanding the variability in biospheric CO2 fluxes:Using FTIR spectrometry and a chemical transport model to investigate the sources and sinks of carbonyl sulfide and its link to CO2
Understanding carbon dioxide (CO2) biospheric processes is of great importance because the terrestrial exchange drives the seasonal and interannual variability of CO2 in the atmosphere. Atmospheric inversions based on CO2 concentration measurements alone can only determine net biosphere fluxes, but not differentiate between photosynthesis (uptake) and respiration (production). Carbonyl sulfide (OCS) could provide an important additional constraint: it is also taken up by plants during photosynthesis but not emitted during respiration, and therefore is a potential means to differentiate between these processes. Solar absorption Fourier Transform InfraRed (FTIR) spectrometry allows for the retrievals of the atmospheric concentrations of both CO2 and OCS from measured solar absorption spectra. Here, we investigate co-located and quasi-simultaneous FTIR measurements of OCS and CO2 performed at five selected sites located in the Northern Hemisphere. These measurements are compared to simulations of OCS and CO2 using a chemical transport model (GEOS-Chem). The coupled biospheric fluxes of OCS and CO2 from the simple biosphere model (SiB) are used in the study. The CO2 simulation with SiB fluxes agrees with the measurements well, while the OCS simulation reproduced a weaker drawdown than FTIR measurements at selected sites, and a smaller latitudinal gradient in the Northern Hemisphere during growing season when comparing with HIPPO (HIAPER Pole-to-Pole Observations) data spanning both hemispheres. An offset in the timing of the seasonal cycle minimum between SiB simulation and measurements is also seen. Using OCS as a photosynthesis proxy can help to understand how the biospheric processes are reproduced in models and to further understand the carbon cycle in the real world
Scattering by magnetic and spin-orbit impurities and the Josephson current in superconductor-ferromagnet-superconductor junctions
We analyze the Josephson current in a junction consisting of two
superconductors (S) and a ferromagnetic layer (F) for arbitrary impurity
concentration. In addition to non-magnetic impurities, we consider also
magnetic ones and spin-orbit scattering. In the limit of weak proximity effect
we solve the linearized Eilenberger equation and derive an analytical
expression for the Josephson critical current valid in a broad range of
parameters. This expression enables us to obtain not only known results in the
dirty and clean limits but also in a intermediate region of the impurity
concentration, which may be very important for comparison with experimental
data.Comment: revised versio
Magnetoresistance of Junctions made of Underdoped YBCO Separated by a Ga-doped YBCO Barrier
We report magnetoresistance measurements of ramp type
superconductor-normal-superconductor (SNS) junctions. The junctions consist of
underdoped (YBCO) electrodes separated by a barrier of
. We observe a large positive magnetoresistance,
linear in the field. We suggest that this unusual magnetoresistance originates
in the field dependence of the proximity effect. Our results indicate that in
underdoped YBCO/N/YBCO SNS structures, the proximity effect does not exhibit
the anomalously long range found in optimally doped YBCO structures. From our
data we obtain the diffusion coefficient and relaxation time of quasiparticles
in underdoped YBCO.Comment: 5 figures, accepted for publication in Physical Review
Proximity Effect Enhancement Induced by Roughness of SN Interface
Critical temperature reduction is considered for a thin film of
a layered superconductor (S) with a rough surface covered by a thick layer of a
normal metal (N). The roughness of the SN interface increases the penetration
of electrons from the normal metal into the superconductor and leads to an
enhancement of the proximity effect. The value of induced by the
roughness of the SN interface can be much higher than for a film
with a plain surface for an extremely anisotropic layered superconductor with
the coherence lengths .Comment: 2 page
Magnetic Interference Patterns and Vortices in Diffusive SNS junctions
We study theoretically the electronic and transport properties of a diffusive
superconductor-normal metal-superconductor (SNS) junction in the presence of a
perpendicular magnetic field. We show that the field dependence of the critical
current crosses over from the well-known Fraunhofer pattern in wide junctions
to a monotonous decay when the width of the normal wire is smaller than the
magnetic length \xi_H = \sqrt{\Phi_0/H}, where H is the magnetic field and
\Phi_0 the flux quantum. We demonstrate that this behavior is a direct
consequence of the magnetic vortex structure appearing in the normal region and
predict how such structure is manifested in the local density of states.Comment: 6 pages, 3 figure
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