5,169 research outputs found
Current Distribution in the Three-Dimensional Random Resistor Network at the Percolation Threshold
We study the multifractal properties of the current distribution of the
three-dimensional random resistor network at the percolation threshold. For
lattices ranging in size from to we measure the second, fourth and
sixth moments of the current distribution, finding {\it e.g.\/} that
where is the conductivity exponent and is the
correlation length exponent.Comment: 10 pages, latex, 8 figures in separate uuencoded fil
Resistance and Resistance Fluctuations in Random Resistor Networks Under Biased Percolation
We consider a two-dimensional random resistor network (RRN) in the presence
of two competing biased percolations consisting of the breaking and recovering
of elementary resistors. These two processes are driven by the joint effects of
an electrical bias and of the heat exchange with a thermal bath. The electrical
bias is set up by applying a constant voltage or, alternatively, a constant
current. Monte Carlo simulations are performed to analyze the network evolution
in the full range of bias values. Depending on the bias strength, electrical
failure or steady state are achieved. Here we investigate the steady-state of
the RRN focusing on the properties of the non-Ohmic regime. In constant voltage
conditions, a scaling relation is found between and , where
is the average network resistance, the linear regime resistance
and the threshold value for the onset of nonlinearity. A similar relation
is found in constant current conditions. The relative variance of resistance
fluctuations also exhibits a strong nonlinearity whose properties are
investigated. The power spectral density of resistance fluctuations presents a
Lorentzian spectrum and the amplitude of fluctuations shows a significant
non-Gaussian behavior in the pre-breakdown region. These results compare well
with electrical breakdown measurements in thin films of composites and of other
conducting materials.Comment: 15 figures, 23 page
Gap and subgap tunnelling in cuprates
We describe strongly attractive carriers in cuprates in the framework of a
simple quasi-one dimensional Hamiltonian with a local attraction. In contrast
with the conventional BCS theory there are two energy scales, a temperature
independent incoherent gap and a temperature dependent coherent gap
combining into one temperature dependent global gap
. The temperature dependence of the gap
and single particle (Giaver) tunnelling spectra in cuprates are quantitatively
described. A framework for understanding of two distinct energy scales observed
in Giaver tunnelling and electron-hole reflection experiments is provided.Comment: 9 pages (RevTex), 4 postscript figures, typos correcte
Theory of SIS tunnelling in cuprates
We show that the single-particle polaron Green's function describes SIS
tunnelling in cuprates, including the absence of Ohm's law at high voltages,
the dip/hump features in the first derivative of the current, a substantial
incoherent spectral weight beyond quasiparticle peaks and unusual shape of the
peaks.
The theory allows us to determine the characteristic phonon frequencies,
normal and superconducting gaps, impurity scattering rate, and the
electron-phonon coupling from the tunnelling data.Comment: 10 pages, 2 figure
Small and large polarons in nickelates, manganites, and cuprates
By comparing the optical conductivities of La_{1.67}Sr_{0.33}NiO_{4} (LSNO),
Sr_{1.5}La_{0.5}MnO_4 (SLMO), Nd_2CuO_{4-y} (NCO), and
Nd_{1.96}Ce_{0.04}CuO_{4} (NCCO), we have identified a peculiar behavior of
polarons in this cuprate family. While in LSNO and SLMO small polarons localize
into ordered structures below a transition temperature, in those cuprates the
polarons appear to be large, and at low T their binding energy decreases. This
reflects into an increase of the polaron radius, which may trigger coherent
transport.Comment: File latex, 15 p. incl. 4 Figs. epsf, to appear on the Journal of
Superconductivity - Proc. "Stripes 1996" - Roma Dec 199
The over-representation of binary DNA tracts in seven sequenced chromosomes
BACKGROUND: DNA tracts composed of only two bases are possible in six combinations: A+G (purines, R), C+T (pyrimidines, Y), G+T (Keto, K), A+C (Imino, M), A+T (Weak, W) and G+C (Strong, S). It is long known that all-pyrimidine tracts, complemented by all-purines tracts ("R.Y tracts"), are excessively present in analyzed DNA. We have previously shown that R.Y tracts are in vast excess in yeast promoters, and brought evidence for their role in gene regulation. Here we report the systematic mapping of all six binary combinations on the level of complete sequenced chromosomes, as well as in their different subregions. RESULTS: DNA tracts composed of the above binary base combinations have been mapped in seven sequenced chromosomes: Human chromosomes 21 and 22 (the major contigs); Drosophila melanogaster chr. 2R; Caenorhabditis elegans chr. I; Arabidopsis thaliana chr. II; Saccharomyces cerevisiae chr. IV and M. jannaschii. A huge over-representation, reaching million-folds, has been found for very long tracts of all binary motifs except S, in each of the seven organisms. Long R.Y tracts are the most excessive, except in D. melanogaster, where the K.M motif predominates. S (G, C rich) tracts are in excess mainly in CpG islands; the W motif predominates in bacteria. Many excessively long W tracts are nevertheless found also in the archeon and in the eukaryotes. The survey of complete chromosomes enables us, for the first time, to map systematically the intergenic regions. In human and other chromosomes we find the highest over-representation of the binary DNA tracts in the intergenic regions. These over-representations are only partly explainable by the presence of interspersed elements. CONCLUSIONS: The over-representation of long DNA tracts composed of five of the above motifs is the largest deviation from randomness so far established for DNA, and this in a wide range of eukaryotic and archeal chromosomes. A propensity for ready DNA unwinding is proposed as the functional role, explaining the evolutionary conservation of the huge excesses observed
r.hu-Erythropoietin (EPO) treatment of pre-ESRD patients slows the rate of progression of renal decline
BACKGROUND: As EPO treatment of chronic anemia of advanced renal disease is now the standard of care we examined if such treatment may slow the progression of renal function decline. METHODS: Data of 18 pre-ESRD patients were analyzed retrospectively 12 months prior and prospectively 12 months after the initiation of EPO. Mean creatinine was 5.0 ± 1.8 mg/dL (Mean ± SEM) when starting EPO at a weekly dose of 5000 ± 500 units once the hematocrit was below 30 %. EPO dose was titrated monthly for a hematocrit between 33.0% and 37.0%. Metabolic complications and hypertension were controlled. RESULTS: At month_0 the average blood pressure was 148/76 ± 5/4 mmHg and at month_12 it was 145/73 ± 6/3 mmHg (p = 0.75 by 2 tailed paired Student's t test). 12/18 patients were on an ACE-i or ARB before month_0 and 14/18 were on it after (p = 0.71 by Fisher's 2 tailed exact test). The average hematocrit rose from 26.9% ± 0.6 to 33.1 % ± 0.1. When linear regression analysis was applied to pre- and post-EPO 1/creatinine data the mean rate of decline was -0.0140 ± 0.0119 (mean ± SD) and -0.0017 ± 0.0090 (non-parametric Wilcoxon matched pairs signed rank sum test: Z value: -2.91; P = 0.004) respectively. 5/18 patients did not require dialysis 12 months after starting EPO (month_0). CONCLUSION: Treatment of the anemia of chronic renal failure with erythropoietin, when instituted together with vigorous metabolic control may slow the rate of renal function decline
Infrared response of ordered polarons in layered perovskites
We report on the infrared absorption spectra of three oxides where charged
superlattices have been recently observed in diffraction experiments. In
LaSrNiO, polaron localization is found to suppress the
low-energy conductivity through the opening of a gap and to split the
- vibrational manifold of the oxygen octahedra. Similar effects
are detected in SrLaMnO and in LaNiO, with
peculiar differences related to the type of charge ordering.Comment: File latex, 11 p. + 3 Figures, to appear on Phys. Rev. B (Rapid
Commun.), 1 Oct. 1996. The figures will be faxed upon request.
E-mail:[email protected] Fax: +39-6-446315
Identification of the bulk pairing symmetry in high-temperature superconductors: Evidence for an extended s-wave with eight line nodes
we identify the intrinsic bulk pairing symmetry for both electron and
hole-doped cuprates from the existing bulk- and nearly bulk-sensitive
experimental results such as magnetic penetration depth, Raman scattering,
single-particle tunneling, Andreev reflection, nonlinear Meissner effect,
neutron scattering, thermal conductivity, specific heat, and angle-resolved
photoemission spectroscopy. These experiments consistently show that the
dominant bulk pairing symmetry in hole-doped cuprates is of extended s-wave
with eight line nodes, and of anisotropic s-wave in electron-doped cuprates.
The proposed pairing symmetries do not contradict some surface- and
phase-sensitive experiments which show a predominant d-wave pairing symmetry at
the degraded surfaces. We also quantitatively explain the phase-sensitive
experiments along the c-axis for both Bi_{2}Sr_{2}CaCu_{2}O_{8+y} and
YBa_{2}Cu_{3}O_{7-y}.Comment: 11 pages, 9 figure
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