7,022 research outputs found
Quantum critical behavior in disordered itinerant ferromagnets: Logarithmic corrections to scaling
The quantum critical behavior of disordered itinerant ferromagnets is
determined exactly by solving a recently developed effective field theory. It
is shown that there are logarithmic corrections to a previous calculation of
the critical behavior, and that the exact critical behavior coincides with that
found earlier for a phase transition of undetermined nature in disordered
interacting electron systems. This confirms a previous suggestion that the
unspecified transition should be identified with the ferromagnetic transition.
The behavior of the conductivity, the tunneling density of states, and the
phase and quasiparticle relaxation rates across the ferromagnetic transition is
also calculated.Comment: 15pp., REVTeX, 8 eps figs, final version as publishe
Metal-superconductor transition at zero temperature: A case of unusual scaling
An effective field theory is derived for the normal metal-to-superconductor
quantum phase transition at T=0. The critical behavior is determined exactly
for all dimensions d>2. Although the critical exponents \beta and \nu do not
exist, the usual scaling relations, properly reinterpreted, still hold. A
complete scaling description of the transition is given, and the physics
underlying the unusual critical behavior is discussed. Quenched disorder leads
to anomalously strong T_c-fluctuations which are shown to explain the
experimentally observed broadening of the transition in low-T_c thin films.Comment: 4 pp., no figs, final version as publishe
Local field theory for disordered itinerant quantum ferromagnets
An effective field theory is derived that describes the quantum critical
behavior of itinerant ferromagnets in the presence of quenched disorder. In
contrast to previous approaches, all soft modes are kept explicitly. The
resulting effective theory is local and allows for an explicit perturbative
treatment. It is shown that previous suggestions for the critical fixed point
and the critical behavior are recovered under certain assumptions. The validity
of these assumptions is discussed in the light of the existence of two
different time scales. It is shown that, in contrast to previous suggestions,
the correct fixed point action is not Gaussian, and that the previously
proposed critical behavior was correct only up to logarithmic corrections. The
connection with other theories of disordered interacting electrons, and in
particular with the resolution of the runaway flow problem encountered in these
theories, is also discussed.Comment: 17pp., REVTeX, 5 eps figs, final version as publishe
The Copula: A Tool for Simulating Speckle Dynamics
Use of a copula for generating a sequence of correlated speckle patterns is introduced. The chief characteristic of this algorithm is that it generates a continuous speckle sequence with a specified evolution of the correlation and does so with just two arrays of random numbers. Thus, physically realistic temporally varying speckle patterns with proper first- and second-order statistics are easily realized. We illustrate use of the algorithm for generating sequences with prescribed Gaussian, exponential, and equal-interval correlations and demonstrate how correlation times can be specified independently. This approach to generating sequences of random realizations with prescribed correlations should prove useful in modeling such phenomena as dynamic light scatter, flow-dependent laser speckle contrast, and propagation of spatial coherence
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