Prof. Dr. Djordje Spasojević

Condensed Matter Theory Group


Research interests


Selected Publications [show/hide abstracts]


Spectroscopic application of an iterative kinetic cathode sheath model to high voltage hollow cathode glow discharge in hydrogen

Dj. Spasojević, S. Mijin, N. Šišović, and N. Konjević
[abstract; Journal of Applied Physics 119, 053301 (2016)]

We present a simple one-dimensional iterative kinetic model of the cathode sheath region of high voltage, low-pressure hydrogen hollow cathodedischarge. The model's convenience as a supplemental diagnostic tool is demonstrated by determining the most relevant discharge parameters through an analysis of the spectral shape of the hydrogen Balmer alpha line recorded along the axis of a cylindrically symmetrical high voltage low-pressure hollow cathodeglow discharge in hydrogen. Thus, an effectively one-dimensional approach is tested and shown to give satisfactory spectral lines fits with reasonable values for discharge parameters, most notably the gas temperature.
Spectroscopic application of an iterative kinetic model of the cathode-fall region in a hydrogen abnormal glow discharge

Dj. Spasojević, V. Steflekova, N. Šišović, and N. Konjević
[abstract; Plasma Sources Science and Technology 23, 012004 (2014)]

We present an iterative kinetic model of the cathode-fall (CF) region in a hydrogen abnormal glow discharge, providing efficient calculations of electric field spatial distribution and distribution functions of H atoms and H+, \( {\rm H}_2^+ \) and \( {\rm H}_3^+ \) ions. The model is very convenient for spectroscopic applications because the knowledge of these distributions enables calculation of virtually all spectroscopic quantities of interest, which is illustrated here by comparison of model predictions and experimental electric field distribution, and hydrogen Balmer beta line shapes recorded end-on and side-on for various positions along the CF region of the discharge.
Analysis of spanning avalanches in the two-dimensional nonequilibrium zero-temperature random-field Ising model

Djordje Spasojević, Sanja Janićević, and Milan Knežević
[abstract; Physical Review E 89, 012118 (2014)]

We present a numerical analysis of spanning avalanches in a two-dimensional (2D) nonequilibrium zero-temperature random field Ising model. Finite-size scaling analysis, performed for distribution of the average number of spanning avalanches per single run, spanning avalanche size distribution, average size of spanning avalanche, and contribution of spanning avalanches to magnetization jump, is augmented by analysis of spanning field (i.e., field triggering spanning avalanche), which enabled us to collapse averaged magnetization curves below critical disorder. Our study, based on extensive simulations of sufficiently large systems, reveals the dominant role of subcritical 2D-spanning avalanches in model behavior below and at the critical disorder. Other types of avalanches influence finite systems, but their contribution for large systems remains small or vanish.
Avalanche distributions in the two-dimensional nonequilibrium zero-temperature random field Ising model

Djordje Spasojević, Sanja Janićević, and Milan Knežević
[abstract; Physical Review E 84, 051119 (2011)]

We present in detail the scaling analysis and data collapse of avalanche distributions and joint distributions that characterize the recently evidenced critical behavior of the two-dimensional nonequilibrium zero-temperature random field Ising model. The distributions are collected in extensive simulations of systems with linear sizes up to L=131072.
Numerical Evidence for Critical Behavior of the Two-Dimensional Nonequilibrium Zero-Temperature Random Field Ising Model

Djordje Spasojević, Sanja Janićević, and Milan Knežević
[abstract; Physical Review Letters 106, 175701 (2011)]

We give numerical evidence that the two-dimensional nonequilibrium zero-temperature random field Ising model exhibits critical behavior. Our findings are based on the results of scaling analysis and collapsing of data, obtained in extensive simulations of systems with sizes sufficiently large to clearly display the critical behavior.
Simultaneous plasma and electric field diagnostics of microdischarge from hydrogen Balmer line shape

Dj. Spasojević, M. Cvejić, N. Šišović, and N. Konjević
[abstract; Applied Physics Letters 96 241501 (2010)]

Hydrogen Balmer beta line shape from a microhollow gas discharge in argon with traces of hydrogen is used for simultaneous diagnostics of plasma and cathodesheath parameters. Simple model of relevant processes responsible for line broadening is developed and results favorably compared with experiment at 500 mbar with the electron density \( 3.8\times 10^{20}\, {\rm m}^{-3} \) and electric field strength of 127 kV/cm in the cathodesheath region.
Advanced fit technique for astrophysical spectra

S. Bukvić, Dj. Spasojević, and V. Žigman
[abstract; Astronomy & Astrophysics 477, 967 (2008)]

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

Dj. Spasojević, S. Janićević, and M. Knežević
[abstract; Europhysics Letters 76, 912 (2006)]

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 ~1/|ln(z)|, where z is a control parameter of the underlying dynamics. Our findings are corroborated with the results of extensive simulations.
Barkhausen noise: Elementary signals, power laws, and scaling relations

Djordje Spasojević, Srdjan Bukvić, Sava Milošević, and H. E. Stanley
[abstract; Physical Review E 54, 2531 (1996)]

We report extensive measurements, with sufficiently large statistics, of the Barkhausen noise (BN) in the case of the commercial VITROVAC 6025 X metal glass sample. Applying a very scrutinized numerical procedure, we have extracted over one million of the BN elementary signals from the raw experimental data, whereby we made a rather precise estimation of the relevant power law exponents. In conjunction with the experimental part of the work, we have recognized a generic shape of a single BN elementary signal (BNES), and we have put forward, without invoking any existing model of BN, a simple mathematical expression for BNES. Using the proposed expression for BNES in a statistical analysis, we have been able to predict scaling relations and an elaborate formula for the power spectrum. We have also obtained these predictions within the generalized homogeneous function approach to the BNES's probability distribution function, which we have substantiated by the corresponding data collapsing analysis. Finally, we compare all our findings with results obtained within the current experimental and theoretical research of BN.