Vortex Structure and Magnetoresistance of Granular YBa2Cu3O7-δ High-Temperature Superconductors

The transverse magnetoresistance Delta rho/rho (H-ext) of several series of YBa2Cu3O6.95 high-temperature superconductor samples has been measured in external magnetic fields with a strength of up to H-ext = H-ext(max) similar to 500 Oe over a wide range of transport current strengths (similar to 10 <= I <= similar to 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 H-c2J, a high level of the magnetoresistance, and the appearance of a remanent magnetoresistance Delta rho(rem)/rho. with a decrease in H-ext to zero; (2) high fields H-c2J and a low level of the magnetoresistance (Delta rho(rem)/rho equivalent to 0); and (3) relatively high fields H-c2J and the presence of jumps in the dependences Delta rho/rho(H-ext). A comparative analysis of the dependences Delta rho/rho (H-ext) 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 < H-ext(max). DOI: 10.1134/S1063783410030029.