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 $c$ and find a scaling region for moderate disorder, where the distribution of avalanche sizes has the form $D(s,c,L) = s^{-(1+\tau (c))}{\cal{D}}(sL^{-D_s(c)})$. The exponents $\tau (c)$ for size and $\alpha (c)$ for length distribution, and the fractal dimension of avalanches $D_s(c)$ satisfy the scaling relation $D_s(c)\tau (c) =\alpha (c)$. 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 180∘180^\circ domain walls

We show that Barkhausen noise in two-dimensional disordered ferromagnets with extended domain walls is characterized by the avalanche size exponent $\tau_s =1.54$ 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 $\tau_s=1.30$. 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 ($z=1$) as $\sim 1/|\ln(z)|$, 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$$_{\mathrm {2}}$$ was determined from the R-branch of the $$\hbox {GK}^{\mathrm {1}}\Sigma ^{\mathrm {+}}_{g}$$, $$\nu '= 0 \rightarrow \hbox {B}^{\mathrm {1}}\Sigma ^{\mathrm {+}}_{u}$$, $$\nu ''=0$$ band and compared with published results for the Q-branch of the Fulcher-$$\alpha$$ diagonal band $$\hbox {d}^{\mathrm {3}}\Pi ^{\mathrm {-}}_{\mathrm {u}}$$, $$\nu ' \rightarrow$$$$\hbox {a}^{\mathrm {3}}\Sigma ^{\mathrm {+}}_{\mathrm {g}}$$, $$\nu '' (\nu '=\nu ''=0$$). 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