2,969 research outputs found
Superconductivity and superconducting order parameter phase fluctuations in a weakly doped antiferromagnet
The superconducting properties of a recently proposed phenomenological model
for a weakly doped antiferromagnet are analyzed, taking into account
fluctuations of the phase of the order parameter. In this model, we assume that
the doped charge carriers can't move out of the antiferromagnetic sublattice
they were introduced. This case corresponds to the free carrier spectra with
the maximum at , as it was observed in ARPES
experiments in some of the cuprates in the insulating state [1]. The doping
dependence of the superconducting gap and the temperature-carrier density phase
diagram of the model are studied in the case of the pairing
symmetry and different values of the effective coupling. A possible relevance
of the results to the experiments on high-temperature superconductors is
discussed.Comment: 16 pages, 4 figure
The Generalized Graetz Problem in Finite Domains
We consider the generalized Graetz problem associated with stationary convection-diffusion inside a domain having any regular three-dimensional translationally invariant section and finite or semi-infinite extent. Our framework encompasses any previous “extended” and “conjugated” Graetz generalizations and provides theoretical bases for computing the orthogonal set of generalized two-dimensional Graetz modes. The theoretical framework includes both heterogeneous and possibly anisotropic diffusion tensors. In the case of semi-infinite domains, the existence of a bounded solution is shown from the analysis of two-dimensional operator eigenvectors which form a basis of L2 . In the case of finite domains a similar basis can be exhibited, and the mode’s amplitudes can be obtained from the inversion of newly defined finite domain operator. Our analysis includes both the theoretical and practical issues associated with this finite domain operator inversion as well as its interpretation as a multireflection image method. Error estimates are provided when numerically truncating the spectrum to a finite number of modes. Numerical examples are validated for reference configurations and provided in nontrivial cases. Our methodology shows how to map the solution of stationary convection-diffusion problems in finite three-dimensional domains into a two-dimensional operator spectrum, which leads to a drastic reduction in computational cost
Isotope effect on superconductivity in Josephson coupled stripes in underdoped cuprates
Inelastic neutron scattering data for YBaCuO as well as for LaSrCuO indicate
incommensurate neutron scattering peaks with incommensuration away
from the point. can be replotted as a linear function of
the incommensuration for these materials. This linear relation implies that the
constant that relates these two quantities, one being the incommensuration
(momentum) and another being (energy), has the dimension of velocity
we denote : . We argue that this
experimentally derived relation can be obtained in a simple model of Josephson
coupled stripes. Within this framework we address the role of the isotope effect on the . We assume that the incommensuration is
set by the {\em doping} of the sample and is not sensitive to the oxygen
isotope given the fixed doping. We find therefore that the only parameter that
can change with O isotope substitution in the relation
is the velocity . We predict an oxygen isotope effect on and expect
it to be .Comment: 4 pages latex file, 2 eps fig
The cosine law at the atomic scale: Toward realistic simulations of Knudsen diffusion
We propose to revisit the diffusion of atoms in the Knudsen regime in terms
of a complex dynamical reflection process. By means of molecular dynamics
simulation we emphasize the asymptotic nature of the cosine law of reflection
at the atomic scale, and carefully analyze the resulting strong correlations in
the reflection events. A dynamical interpretation of the accomodation
coefficient associated to the slip at the wall interface is also proposed.
Finally, we show that the first two moments of the stochastic process of
reflection non uniformly depend on the incident angle
Chemical complexity in the Horsehead photodissociation region
The interstellar medium is known to be chemically complex. Organic molecules
with up to 11 atoms have been detected in the interstellar medium, and are
believed to be formed on the ices around dust grains. The ices can be released
into the gas-phase either through thermal desorption, when a newly formed star
heats the medium around it and completely evaporates the ices; or through
non-thermal desorption mechanisms, such as photodesorption, when a single
far-UV photon releases only a few molecules from the ices. The first one
dominates in hot cores, hot corinos and strongly UV-illuminated PDRs, while the
second one dominates in colder regions, such as low UV-field PDRs. This is the
case of the Horsehead were dust temperatures are ~20-30K, and therefore offers
a clean environment to investigate what is the role of photodesorption. We have
carried-out an unbiased spectral line survey at 3, 2 and 1mm with the IRAM-30m
telescope in the Horsehead nebula, with an unprecedented combination of
bandwidth high spectral resolution and sensitivity. Two positions were
observed: the warm PDR and a cold condensation shielded from the UV field
(dense core), located just behind the PDR edge. We summarize our recently
published results from this survey and present the first detection of the
complex organic molecules HCOOH, CH2CO, CH3CHO and CH3CCH in a PDR. These
species together with CH3CN present enhanced abundances in the PDR compared to
the dense core. This suggests that photodesorption is an efficient mechanism to
release complex molecules into the gas-phase in far-UV illuminated regions.Comment: 15 pages, 7 figures, 7 tables, Accepted in Faraday discussions 16
Mechanisms for Lasing with Cold Atoms as the Gain Medium
We realize a laser with a cloud of cold rubidium atoms as gain medium, placed
in a low-finesse cavity. Three different regimes of laser emission are observed
corresponding respectively to Mollow, Raman and Four Wave Mixing mechanisms. We
measure an output power of up to 300 W and present the main properties of
these different lasers in each regime
Isotope Effect in the Superfluid Density of HTS Cuprates: Stripes, Pseudogap and Impurities
Underdoped cuprates exhibit a normal-state pseudogap, and their spins and
doped carriers tend to spatially separate into 1- or 2-D stripes. Some view
these as central to superconductivity, others as peripheral and merely
competing. Using LaSrCuZnO we show that an oxygen
isotope effect in and in the superfluid density can be used to
distinguish between the roles of stripes and pseudogap and also to detect the
presence of impurity scattering. We conclude that stripes and pseudogap are
distinct, and both compete and coexist with superconductivity.Comment: Revised submission to PRL with added appendix on a possible isotope
effect in the effective mass, 4 pages, 3 figure
Cotunneling through a magnetic single-molecule transistor based on N\atC60
We present an experimental and theoretical study of a magnetic
single-molecule transistor based on N@C60 connected to gold electrodes.
Particular attention is paid to the regime of intermediate molecule-lead
coupling, where cotunneling effects manifest themselves in the Coulomb-blockade
regime. The experimental results for the differential conductance as a function
of bias, gate voltage, and external magnetic field are in agreement with our
analysis of the tunneling rates and provide evidence of magnetic signatures in
single-N@C60 devices arising from an antiferromagnetic exchange interaction
between the C60 spin and the nitrogen spin.Comment: Accepted for publication in PRB Rapid Com, 4 pages, 4 figures, with
supplementary information (6 pages, 3 figures
Influence of Ablation on Differential Arc Resistance
The influence of ablation on the du/di behavior of an arc in a model gas circuit breaker was examined. Specifically the transition from a state without ablation in the nozzle towards states with ablation was of interest, since prior work indicated that for high currents the voltage becomes constant or du/di gets even positive if ablation is present. Measurements with different blow pressures and rectangular DC currents of varying amplitude were compared, using PMMA-nozzles and dry air as blowing gas. Ablation was measured by weighing the nozzle, scanning the cross section, and using a coordinate measuring machine. The results agreed well, and confirmed that higher pressure shifts the du/di curve towards more favorable values
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