220 research outputs found
Percolation of Superconductivity
In case of superconductors whose electrons attract each other only if they
are near certain centers, the question arises 'How many such centers are needed
to make the ground state superconducting?' We shall examine it in the context
of a random U Hubbard model. In short we study the case where U_i is -|U| and 0
with probability c and 1-c respectively on a lattice whose sites are labelled i
using the Gorkov decoupling and the Coherent Potential Approximation (CPA). We
argue that for this model there is a critical concentration c_0 below which the
system is not a superconductor.Comment: 18 pages, 10 figure
Superconducing Alloys with Weak and Strong Scattering: Anderson's Theorem and a Superconductor-Insulator Transition
We have studied the effects of strong impurity scattering on disordered
superconductors beyond the low impurity concentration limit. By applying the
full CPA to a superconductiong A-B binary alloy, we calculated the fluctuations
of the local order parameters and charge densities,
for weak and strong on site disorder. We find that for narrow
band alloy s-wav e superconductors the conditions for Anderson's theorem are
satisfied in general only for the case of particle-hole symmetry. In this case
it is satisfied regardless whether we are in the weak or strong scattering
regimes. Interestingly, we find that strong scattering leads to band splitting
and in this regime for any band filling we have a critical concentration where
a superconductor-insulator quantum phase transition occurs at T=0.Comment: 28 pages, 13 figure
Combustion Process in a Spark Ignition Engine: Dynamics and Noise Level Estimation
We analyse the experimental time series of internal pressure in a four
cylinder spark ignition engine. In our experiment, performed for different
spark advance angles, apart from usual cyclic changes of engine pressure we
observed oscillations. These oscillations are with longer time scales ranging
from one to several hundred engine cycles depending on engine working
conditions. Basing on the pressure time dependence we have calculated the heat
released per cycle. Using the time series of heat release to calculate the
correlation coarse-grained entropy we estimated the noise level for internal
combustion process. Our results show that for a smaller spark advance angle the
system is more deterministic.Comment: 15 pages, 7 figures, submitted to CHAO
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