Influence of Hydrocarbon Doping on Critical Current Density and Percolation Behavior of MgB2

In polycrystalline MgB2 samples, the crystal grains are randomly oriented, and the anisotropy of the upper critical field leads to different supercurrent carrying capacities in different grains, so the overall supercurrent becomes percolation in applied magnetic field. In this paper, we studied the doping effect of citric acid on the critical current density and the percolation behavior in polycrystalline MgB2 samples. By fitting the experimental data with the percolation model, it is found that the anisotropy of the upper critical field is gradually decreased by doping citric acid, which alters the percolation behavior of the supercurrent of the polycrystalline MgB2 samples. In addition, it is observed that deviation of the experimental data from the typical grain boundary pinning theory reduces with increasing doping level or as the temperature approaching Tc. The phenomenon is well explained according to the systematical decrease of anisotropy parameter γ=(Bc2‖/Bc2⊥)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\gamma=(B_{c2}^\parallel/B_{c2}^\perp)$$\end{document} with doping level and temperature.