2,820 research outputs found
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
Field Induced Nodal Order Parameter in the Tunneling Spectrum of YBaCuO Superconductor
We report planar tunneling measurements on thin films of
YBaCuO at various doping levels under magnetic fields. By
choosing a special setup configuration, we have probed a field induced energy
scale that dominates in the vicinity of a node of the d-wave superconducting
order parameter. We found a high doping sensitivity for this energy scale. At
Optimum doping this energy scale is in agreement with an induced
order parameter. We found that it can be followed down to low fields at optimum
doping, but not away from it.Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev.
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
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
Origin of time reversal symmetry breaking in Y(1-y)Ca(y)Ba(2)Cu(3)O(7-x)
We have studied the Zero Bias Conductance Peak (ZBCP) of the tunneling
conductance measured on (1,1,0) oriented Y(1-y)Ca(y)Ba(2)Cu(3)O(7-x) thin films
as a function of doping and of magnetic field. A spontaneous (zero field) split
of the ZBCP was observed only in overdoped samples (either by O or by Ca). The
magnitude of this split was found to be linear in doping. All samples exhibited
a magnetic field splitting, also strongly doping dependent. The field
susceptibility chi=d(delta)/dH diverges at the point at which spontaneous ZBCP
splitting occurs, its inverse value, chi^(-1), following a linear doping
dependence on both the underdoped and overdoped sides. We discuss these results
in terms of recent theoretical models of Time Reversal Symmetry Breaking
(TRSB).Comment: 5 figure
Kinetic energy change with doping upon superfluid condensation in high temperature superconductors
In conventional BCS superconductors, the electronic kinetic energy increases
upon superfluid condensation (the change DEkin is positive). Here we show that
in the high critical temperature superconductor Bi-2212, DEkin crosses over
from a fully compatible conventional BCS behavior (DEkin>0) to an
unconventional behavior (DEkin<0) as the free carrier density decreases. If a
single mechanism is responsible for superconductivity across the whole phase
diagram of high critical temperature superconductors, this mechanism should
allow for a smooth transition between such two regimes around optimal doping.Comment: 3 pages, 2 figure
Influence of temperature dependent inelastic scattering on the superconducting proximity effect
We have measured the differential resistance of mesoscopic gold wires of
different lengths connected to an aluminum superconductor as a function of
temperature and voltage. Our experimental results differ substantially from
theoretical predictions which assume an infinite temperature independent gap in
the superconductor. In addition to taking into account the temperature
dependence of the gap, we must also introduce a temperature dependent inelastic
scattering length in order to fit our data
Spontaneous magnetization and Hall effect in superconductors with broken time-reversal symmetry
Broken time reversal symmetry (BTRS) in d wave superconductors is studied and
is shown to yield current carrying surface states. The corresponding
spontaneous magnetization is temperature independent near the critical
temperature Tc for weak BTRS, in accord with recent data. For strong BTRS and
thin films we expect a temperature dependent spontaneous magnetization with a
paramagnetic anomaly near Tc. The Hall conductance is found to vanish at zero
wavevector q and finite frequency w, however at finite q,w it has an unusual
structure.Comment: 7 pages, 1 eps figure, Europhysics Letters (in press
Remarkable change of tunneling conductance in YBCO films in fields up to 32.4T
We studied the tunneling density of states in YBCO films under strong
currents flowing along node directions. The currents were induced by fields of
up to 32.4T parallel to the film surface and perpendicular to the
planes. We observed a remarkable change in the tunneling conductance at high
fields where the gap-like feature shifts discontinuously from 15meV to a lower
bias of 11meV, becoming more pronounced as the field increases. The effect
takes place in increasing fields around 9T and the transition back to the
initial state occurs around 5T in decreasing fields. We argue that this
transition is driven by surface currents induced by the applied magnetic field.Comment: 4 pages, 7 figure
The arctic seasonal cycle of total column CO2 and CH4 from ground-based solar and lunar FTIR absorption spectrometry
Solar absorption spectroscopy in the near infrared has been performed in Ny-Ålesund (78.9° N, 11.9° E) since 2002; however, due to the high latitude of the site, the sun is below the horizon from October to March (polar night) and no solar absorption measurements are possible. Here we present a novel method of retrieving the total column dry-air mole fractions (DMFs) of CO2 and CH4 using moonlight in winter. Measurements have been taken during the polar nights from 2012 to 2016 and are validated with TCCON (Total Carbon Column Observing Network) measurements by solar and lunar absorption measurements on consecutive days and nights during spring and autumn. The complete seasonal cycle of the DMFs of CO2 and CH4 is presented and a precision of up to 0.5 % is achieved. A comparison of solar and lunar measurements on consecutive days during day and night in March 2013 yields non-significant biases of 0. 66 ± 4. 56 ppm for xCO2 and −1. 94 ± 20. 63 ppb for xCH4. Additionally a model comparison has been performed with data from various reanalysis models
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