Critical temperature of the superconducting transition of individual phases of multiphase bismuth cuprates after cooling in a magnetic field to a temperature of 77 K

Using a highly sensitive torsional vibration technique, the authors have determined the critical temperatures of the superconducting transition (T_c) of individual phases of multiphase cuprates [Bi_1.7Pb_0.3Sr₂Ca_(n-1)Cu_nO_y (n = 2–30)] in a constant external magnetic field (H) in the temperature range of 77 to 270 K. It has been found that oscillation damping peaks are more pronounced when a sample is rapidly cooled in an external magnetic field (FC) to a temperature of 77 K and then is slowly warmed to room temperature. An increase in the time of sample exposure to a magnetic field at 77 K leads to a rise in the temperature T_c of different phases, and to an increase in the intensity of a signal corresponding to high-temperature phases with T_c > 240 K. Structural studies have shown that with increasing sample exposure time at 77 K, as a result of sample compression an increase in stress concentration at grain boundaries appears to increase defect density and to enhance the pinning of Abrikosov vortices, i.e. creates new (additional) conditions for their “freezing” during the field cooling (FC) procedure. It has been established that in samples of the Bi_1.7Pb_0.3Sr₂Ca_(n-1)Cu_nO_y (n = 2–30) series, the critical temperature T_c increases from 107 K to ≥ 240 K with an increase in n.