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

This study aims at establishing the interrelation between the current-carrying capacity and peculiarities of magnetoresistance of granular YBa2Cu3O7 − δ HTSCs (Tc = 92.5 K). The transverse magnetoresistance of several batches of YBa2Cu3O7 − δ HTSC samples with noticeably different values of critical supercurrent density jc is measured in magnetic fields Hext up to Hextmax ≈ 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 jc (Hext = 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 jc ≥ 20 A/cm2 in fields HBG-VG ≈ 200–260 Oe. The width ΔHBG-VG of the anomalous resistance region increases upon an increase in I. The magnetoresistance jumps decrease with increasing I in increasing field Hext(0 → Hextmax) and increase in decreasing field Hext(Hextmax → 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.