16 research outputs found
Stable, metastable and unstable states in the mean-field RFIM at T=0
We compute the probability of finding metastable states at a given field in
the mean-field random field Ising model at T=0. Remarkably, this probability is
finite in the thermodynamic limit, even on the so-called ``unstable'' branch of
the magnetization curve. This implies that the branch is reachable when the
magnetization is controlled instead of the magnetic field, in contrast with the
situation in the pure system.Comment: 10 pages, 3 figure
Nonuniversal scaling behavior of Barkhausen noise
We simulate Barkhausen avalanches on fractal clusters in a two-dimensional
diluted Ising ferromagnet with an effective Gaussian random field. We vary the
concentration of defect sites and find a scaling region for moderate
disorder, where the distribution of avalanche sizes has the form . The exponents for size
and for length distribution, and the fractal dimension of
avalanches satisfy the scaling relation .
For fixed disorder the exponents vary with driving rate in agreement with
experiments on amorphous Si-Fe alloys.Comment: 5 pages, Latex, 4 PostScript figures include
Barkhausen avalanches in anisotropic ferromagnets with domain walls
We show that Barkhausen noise in two-dimensional disordered ferromagnets with
extended domain walls is characterized by the avalanche size exponent at low disorder. With increasing disorder the characteristic domain size
is reduced relative to the system size due to nucleation of new domains and a
dynamic phase transition occurs to the scaling behavior with . The
exponents decrease at finite driving rate. The results agree with recently
observed behavior in amorphous Metglas and Fe-Co-B ribbons when the applied
anisotropic stress is varied.Comment: Changes in the text and references, To appear in Phys. Rev.
Advanced fit technique for astrophysical spectra
Aims.The purpose of this paper is to introduce a robust method of
data fitting convenient for dealing with astrophysical spectra contaminated by a large
fraction of outliers.
Methods.We base our approach on the suitable defined measure:
the density of the least squares (DLS) that characterizes subsets
of the whole data set. The best-fit parameters are obtained by the
least-square method on a subset having the maximum value of DLS
or, less formally, on the largest subset free of outliers.
Results.We give
the FORTRAN90 source code
of the
subroutine that implements the DLS method. The efficiency of the
DLS method is demonstrated on a few examples: estimation of
continuum in the presence of spectral lines, estimation of spectral line parameters in the presence of outliers, and
estimation of the thermodynamic temperature from the spectrum that is rich in spectral lines.
Conclusions.Comparison
of the present results with the ones obtained with the widely
used comprehensive multi-component fit yields agreement within error margins. Due to simplicity and robustness, the proposed
approach could be the method of choice whenever outliers are present, or whenever unwelcome features of the spectrum are to be
considered as formal outliers (e.g. spectral lines while estimating continuum)
Exact results for mean-field zero-temperature random-field Ising model
We present an analysis of the dynamical critical behavior of the
mean-field zero-temperature random-field Ising model, based on the
probability of finding a given sequence in the response signal,
which has the form of a Markov chain with Poisson transition
probabilities. We provide an exact description of the avalanche
duration distribution, the absolute probabilities of signal values,
and the signal time-autocorrelation function. The overall behavior
of these quantities depends on their characteristic lengths, which
all diverge near the critical point () as ,
where z is a control parameter of the underlying dynamics. Our
findings are corroborated with the results of extensive simulations
Measurement of the DC Stark shift for visible Nel lines and electric field distribution in the cathode sheath of an abnormal glow discharge
We present the results of an experimental study of the DC Stark shift for seven visible NeI lines in the plane cathode sheath region of an abnormal glow discharge operated in neon with a small admixture of hydrogen. The electric field (up to 13.4 kV cm(-1)) in the cathode sheath region is measured from the p-polarized profile of the H alpha line of hydrogen using the Stark polarization spectroscopy technique. Within the realized range of the electric field, the NeI lines exhibit a quadratic Stark effect. The values of coefficients, correlating Stark shift and electric field strength, were determined, enabling their future use for unknown electric field strength measurements. Among the studied lines, so far only the Stark effect analysis of the NeI 511.367 nm line has been reported, in which case our results are in good agreement with the best fit formula proposed by Jager and Windholz (1984 Phys. Scr. 29 344) for one out of three Stark components detected under our experimental conditions
Application of
In this paper, optical emission spectroscopy technique was used to measure rotational and gas temperature distribution in the cathode-sheath region of an abnormal glow discharge operated in hydrogen and hydrogen–argon mixtures. The rotational temperature of excited electronic states of H was determined from the R-branch of the , , band and compared with published results for the Q-branch of the Fulcher- diagonal band , , ). The population of excited energy levels, determined from the relative line intensities, was used to derive the rotational temperature of the ground state of hydrogen molecule. The boundary between the cathode sheath and negative glow region is determined using Stark polarization spectroscopy of the hydrogen Balmer alpha line. The thickness of the cathode sheath is estimated by fitting the experimentally determined distribution of electric field strength to a suitable model function. Temperature distributions along the axis of the cathode sheath are also reported and discussed. The distributions of temperature show that both bands give consistent and reasonable values for the rotational temperature and they both mutually agree within the estimated experimental uncertainties of 15%. The reported results contribute to the development of the spectroscopic method for gas temperature measurement in pure molecular hydrogen and in the gas mixture with argon
Complex UV Ne II line shapes in the cathode sheath of an abnormal glow discharge
We report results of an experimental and theoretical study of complex UV line shapes of Ne II 369.421 nm, Ne II 371.308 nm and Ne II 372.711 nm lines in the cathode sheath (CS) region of an abnormal glow discharge in pure neon. The experimental profiles were studied by means of the optical emission spectroscopy (OES) in conjunction with an iterative CS kinetic model. It is shown that our theoretical model describes the experimental line shapes and that, with the aid of the measured Stark shifts of atomic neon lines, it can be used for the determination of the most important CS parameters (e.g. the thickness of the CS region, distribution of electric field, and the gas temperature). We draw attention to the possibility of determining the electric field strength in CS from the width of the pedestal of the complex line profiles