46 research outputs found
Critical point network for drainage between rough surfaces
In this paper, we present a network method for computing two-phase flows between two rough surfaces with significant contact areas. Low-capillary number drainage is investigated here since one-phase flows have been previously investigated in other contributions. An invasion percolation algorithm is presented for modeling slow displacement of a wetting fluid by a non wetting one between two rough surfaces. Short-correlated Gaussian process is used to model random rough surfaces.The algorithm is based on a network description of the fracture aperture field. The network is constructed from the identification of critical points (saddles and maxima) of the aperture field. The invasion potential is determined from examining drainage process in a flat mini-channel. A direct comparison between numerical prediction and experimental visualizations on an identical geometry has been performed for one realization of an artificial fracture with a moderate fractional contact area of about 0.3. A good agreement is found between predictions and observations
Very high upper critical fields in MgB2 produced by selective tuning of impurity scattering
We report a significant enhancement of the upper critical field of
different samples alloyed with nonmagnetic impurities. By studying
films and bulk polycrystals with different resistivities , we show a
clear trend of increase as increases. One particular high
resistivity film had zero-temperature well above the
values of competing non-cuprate superconductors such as and Nb-Ti. Our
high-field transport measurements give record values and for high resistivity films and
for untextured bulk polycrystals. The highest
film also exhibits a significant upward curvature of , and
temperature dependence of the anisotropy parameter opposite to that of single crystals: decreases as the
temperature decreases, from to .
This remarkable enhancement and its anomalous temperature dependence
are a consequence of the two-gap superconductivity in , which offers
special opportunities for further increase by tuning of the impurity
scattering by selective alloying on Mg and B sites. Our experimental results
can be explained by a theory of two-gap superconductivity in the dirty limit.
The very high values of observed suggest that can be made
into a versatile, competitive high-field superconductor.Comment: An updated version of the paper (12/12/2002)that was placed on
cond-mat on May 7 200
57Fe Mossbauer spectroscopy and magnetic measurements of oxygen deficient LaFeAsO
We report on the magnetic behavior of oxygen deficient LaFeAsO1-x (x-0.10)
compound, prepared by one-step synthesis, which crystallizes in the tetragonal
(S.G. P4/nmm) structure at room temperature. Resistivity measurements show a
strong anomaly near 150 K, which is ascribed to the spin density wave (SDW)
instability. On the other hand, dc magnetization data shows paramagnetic-like
features down to 5 K, with an effective moment of 0.83 mB/Fe. 57Fe Mossbauer
studies (MS) have been performed at 95 and 200 K. The spectra at both
temperatures are composed of two sub-spectra. At 200 K the major one (88%), is
almost a singlet, and corresponds to those Fe nuclei, which have two oxygen
ions in their close vicinity. The minor one, with a large quadrupole splitting,
corresponds to Fe nuclei, which have vacancies in their immediate neighborhood.
The spectrum at 95 K, exhibits a broadened magnetic split major (84%)
sub-spectrum and a very small magnetic splitting in the minor subspectrum. The
relative intensities of the subspectra facilitate in estimating the actual
amount of oxygen vacancies in the compound to be 7.0(5)%, instead of the
nominal LaFeAsO0.90. These results, when compared with reported 57Fe MS of
non-superconducting LaFeAsO and superconducting LaFeAsO0.9F0.1, confirm that
the studied LaFeAsO0.93 is a superconductivity-magnetism crossover compound of
the newly discovered Fe based superconducting family.Comment: 7 pages text + Figs : Comments/suggestions welcome
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Evidence for two distinct scales of current flow in polycrystalline Sm and Nd iron oxypnictides
Early studies have found quasi-reversible magnetization curves in
polycrystalline bulk rare-earth iron oxypnictides that suggest either
wide-spread obstacles to intergranular current or very weak vortex pinning. In
the present study of polycrystalline samarium and neodymium rare-earth iron
oxypnictide samples made by high pressure synthesis, the hysteretic
magnetization is significantly enhanced. Magneto optical imaging and study of
the field dependence of the remanent magnetization as a function of particle
size both show that global currents over the whole sample do exist but that the
intergranular and intragranular current densities have distinctively different
temperature dependences and differ in magnitude by about 1000. Assuming that
the highest current density loops are restricted to circulation only within
grains leads to values of ~5 MA/cm2 at 5 K and self field, while whole-sample
current densities, though two orders of magnitude lower are 1000-10000 A/cm2,
some two orders of magnitude higher than in random polycrystalline cuprates. We
cannot yet be certain whether this large difference in global and intragrain
current density is intrinsic to the oxypnictides or due to extrinsic barriers
to current flow, because the samples contain significant second phase, some of
which wets the grain boundaries and produces evidences of SNS proximity effect
in the whole sample critical current.Comment: 28 pages, 14 figure
Generation and annealing of splayed columnar defects in (Bi,Pb)(2)Sr2Ca2Cu3Ox silver-sheathed tapes after 600 MeV proton irradiation
Splayed columnar defects are efficient pinning centers for magnetic vortices in High-T-c Superconductors. Irradiation with 600 MeV protons as primary projectiles generates heavy fission fragments via a pBi nuclear reaction. These secondary projectiles create randomly oriented columnar defects with important effects on the magnetic and transport critical current densities of (Bi, Pb)(2)Sr2Ca2Cu3Ox Ag-sheathed tapes
Enhanced critical current densities in (Bi, Pb)(2)Sr2Ca2Cu3O10 silver-sheathed tapes by splayed columnar defects after 600 MeV proton irradiation
Splayed columnar defects are efficient pinning centers for magnetic vortices in High-T-c Superconductors. Irradiation with 000 MeV protons as primary projectiles generates heavy fission fragments in (Bi, Pb)(2)Sr2Ca2Cu3O10 via a p-Bi nuclear reaction. The high-Z secondary projectiles are sufficiently energetic to create randomly oriented columnar defects with important effects on the magnetic and transport critical current densities of (Bi, Pb)(2)Sr2Ca2Cu3O10 Ag-sheathed tapes. These effects are discussed on the basis of the splayed defect topology