1,779 research outputs found
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.
Coexistence of a triplet nodal order-parameter and a singlet order-parameter at the interfaces of ferromagnet-superconductor Co/CoO/In junctions
We present differential conductance measurements of Cobalt / Cobalt-Oxide /
Indium planar junctions, 500nm x 500nm in size. The junctions span a wide range
of barriers, from very low to a tunnel barrier. The characteristic conductance
of all the junctions show a V-shape structure at low bias instead of the
U-shape characteristic of a s-wave order parameter. The bias of the conductance
peaks is, for all junctions, larger than the gap of indium. Both properties
exclude pure s-wave pairing. The data is well fitted by a model that assumes
the coexistence of s-wave singlet and equal spin p-wave triplet fluids. We find
that the values of the s-wave and p-wave gaps follow the BCS temperature
dependance and that the amplitude of the s-wave fluid increases with the
barrier strength.Comment: 5 pages, Accepted to Phys. Rev.
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
Proximity Effect in Normal Metal - High Tc Superconductor Contacts
We study the proximity effect in good contacts between normal metals and high
Tc (d-wave) superconductors. We present theoretical results for the spatially
dependent order parameter and local density of states, including effects of
impurity scattering in the two sides, s-wave pairing interaction in the normal
metal side (attractive or repulsive), as well as subdominant s-wave paring in
the superconductor side. For the [100] orientation, a real combination d+s of
the order parameters is always found. The spectral signatures of the proximity
effect in the normal metal includes a suppression of the low-energy density of
states and a finite energy peak structure. These features are mainly due to the
impurity self-energies, which dominate over the effects of induced pair
potentials. For the [110] orientation, for moderate transparencies, induction
of a d+is order parameter on the superconductor side, leads to a proximity
induced is order parameter also in the normal metal. The spectral signatures of
this type of proximity effect are potentially useful for probing time-reversal
symmetry breaking at a [110] interface.Comment: 10 pages, 10 figure
Quantitative Simulation of the Superconducting Proximity Effect
A numerical method is developed to calculate the transition temperature of
double or multi-layers consisting of films of super- and normal conductors. The
approach is based on a dynamic interpretation of Gorkov's linear gap equation
and is very flexible. The mean free path of the different metals, transmission
through the interface, ratio of specular reflection to diffusive scattering at
the surfaces, and fraction of diffusive scattering at the interface can be
included. Furthermore it is possible to vary the mean free path and the BCS
interaction NV in the vicinity of the interface. The numerical results show
that the normalized initial slope of an SN double layer is independent of
almost all film parameters except the ratio of the density of states. There are
only very few experimental investigations of this initial slope and they
consist of Pb/Nn double layers (Nn stands for a normal metal). Surprisingly the
coefficient of the initial slope in these experiments is of the order or less
than 2 while the (weak coupling) theory predicts a value of about 4.5. This
discrepancy has not been recognized in the past. The autor suggests that it is
due to strong coupling behavior of Pb in the double layers. The strong coupling
gap equation is evaluated in the thin film limit and yields the value of 1.6
for the coefficient. This agrees much better with the few experimental results
that are available.
PACS: 74.45.+r, 74.62.-c, 74.20.F
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
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
Local and macroscopic tunneling spectroscopy of Y(1-x)CaxBa2Cu3O(7-d) films: evidence for a doping dependent is or idxy component in the order parameter
Tunneling spectroscopy of epitaxial (110) Y1-xCaxBa2Cu3O7-d films reveals a
doping dependent transition from pure d(x2-y2) to d(x2-y2)+is or d(x2-y2)+idxy
order parameter. The subdominant (is or idxy) component manifests itself in a
splitting of the zero bias conductance peak and the appearance of subgap
structures. The splitting is seen in the overdoped samples, increases
systematically with doping, and is found to be an inherent property of the
overdoped films. It was observed in both local tunnel junctions, using scanning
tunneling microscopy (STM), and in macroscopic planar junctions, for films
prepared by either RF sputtering or laser ablation. The STM measurements
exhibit fairly uniform splitting size in [110] oriented areas on the order of
10 nm2 but vary from area to area, indicating some doping inhomogeneity. U and
V-shaped gaps were also observed, with good correspondence to the local
faceting, a manifestation of the dominant d-wave order parameter
CcpA represses the expression of the divergent cit operons of Enterococcus faecalis through multiple cre sites
<p>Abstract</p> <p>Background</p> <p>In <it>Enterococcus faecalis </it>the genes encoding the enzymes involved in citrate metabolism are organized in two divergent operons, <it>citHO </it>and <it>oadHDB-citCDEFX-oadA-citMG </it>(<it>citCL </it>locus). Expression of both operons is specifically activated by adding citrate to the medium. This activation is mediated by binding of the GntR-like transcriptional regulator (CitO) to the <it>cis</it>-acting sequences located in the <it>cit </it>intergenic region. Early studies indicated that citrate and glucose could not be co-metabolized suggesting some form of catabolite repression, however the molecular mechanism remained unknown.</p> <p>Results</p> <p>In this study, we observed that the <it>citHO </it>promoter is repressed in the presence of sugars transported by the Phosphoenolpyruvate:carbohydrate Phosphotranserase System (PTS sugars). This result strongly suggested that Carbon Catabolic Repression (CCR) impedes the expression of the activator CitO and the subsequent induction of the <it>cit </it>pathway. In fact, we demonstrate that CCR is acting on both promoters. It is partially relieved in a <it>ccpA</it>-deficient <it>E. faecalis </it>strain indicating that a CcpA-independent mechanism is also involved in regulation of the two operons. Furthermore, sequence analysis of the <it>citH</it>/<it>oadH </it>intergenic region revealed the presence of three putative catabolite responsive elements (<it>cre</it>). We found that they are all active and able to bind the CcpA/P-Ser-HPr complex, which downregulates the expression of the <it>cit </it>operons. Systematic mutation of the CcpA/P-Ser-HPr binding sites revealed that <it>cre1 </it>and <it>cre2 </it>contribute to <it>citHO </it>repression, while <it>cre3 </it>is involved in CCR of <it>citCL</it></p> <p>Conclusion</p> <p>In conclusion, our study establishes that expression of the <it>cit </it>operons in <it>E. faecalis </it>is controlled by CCR via CcpA-dependent and -independent mechanisms.</p
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
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