Vortex structure and magnetoresistance of granular YBa2Cu3O7 − δ high-temperature superconductors

The transverse magnetoresistance \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \frac{{\Delta \rho }} {\rho } $$\end{document}(Hext) of several series of YBa2Cu3O6.95 high-temperature superconductor samples has been measured in external magnetic fields with a strength of up to Hext = Hextmax ∼ 500 Oe over a wide range of transport current strengths (∼10 ≤ I ≤ ∼1000 mA) at the temperature T = 77.3 K. It has been revealed that there are three different types of field dependences of the magnetoresistance at Hext ⊥ I with the following typical features: (1) low critical fields of the Josephson weak links Hc2J, a high level of the magnetoresistance, and the appearance of a remanent magnetoresistance Δρrem/ρ with a decrease in Hext to zero; (2) high fields Hc2J and a low level of the magnetoresistance (Δρrem/ρ ≡ 0); and (3) relatively high fields Hc2J and the presence of jumps in the dependences \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \frac{{\Delta \rho }} {\rho } $$\end{document}(Hext). A comparative analysis of the dependences \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \frac{{\Delta \rho }} {\rho } $$\end{document}(Hext) has made it possible to establish the nature of the revealed differences: the first type of behavior corresponds to the existence of a relatively ordered vortex structure of the Bragg-glass type over the entire range of fields Hext, the second type corresponds to the existence of a disordered vortex structure of the vortex-glass type over the entire range of fields Hext, and the third type corresponds to the occurrence of Bragg-glass-vortex-glass first-order phase transition in the magnetic field in the range 0 < HBG-VG < Hextmax.