Phase transition in the vortex structure of granular YBa2Cu3O7 − δ HTSCs in weak magnetic fields

Abstract

This study aims at establishing the interrelation between the current-carrying capacity and peculiarities of magnetoresistance of granular YBa2Cu3O7 − δ HTSCs (T c = 92.5 K). The transverse magnetoresistance of several batches of YBa2Cu3O7 − δ HTSC samples with noticeably different values of critical supercurrent density j c is measured in magnetic fields H ext up to H ext max ≈ 500 Oe in a wide range of transport currents (5 mA ≤ I ≤ 1600 mA) at T = 77.4 K. Samples with relatively high values of j c (H ext = 0) ≥ 100 A/cm2 do not exhibit any anomalies in their field dependences. Magnetoresistance jumps δρBG-VG/ρ273K are observed for samples with low values of j c ≥ 20 A/cm2 in fields H BG-VG ≈ 200–260 Oe. The width ΔH BG-VG of the anomalous resistance region increases upon an increase in I. The magnetoresistance jumps decrease with increasing I in increasing field H ext(0 → H ext max ) and increase in decreasing field H ext(H ext max → 0). It is found that these peculiarities of the field dependences of magnetoresistance are associated with a first-order phase transition (in magnetic field) in the vortex structure of YBa2Cu3O7 − δ HTSCs of the Bragg glass-vortex glass type.