Using the break junction (BJ) technique, we have investigated temperature dependences of both the maximum Josephson current IJc and the gap value D of the superconducting Bi-based cuprate PbxBi2-xSr2Ca2Cu3O10+y (Pb-Bi2223) micro-crystals that can form superconductor- insulator-superconductor (SIS) structures. To investigate the Josephson-current dependence on the electrode superconducting properties, we additionally prepared samples with different critical temperature Tc obtained by nitrogen gas reduction annealing (Tc(zero) ~90 K) and compared with the as-grown samples[1] (Tc(zero)~105 K).
According to the I(V) curve of the annealed Pb-Bi2223 sample, the magnitude of IJc was approximately 0.17 mA, and the resistance outside the gap RN was 16 W, thus leading to IJcRN ~ 2.6 mV at T = 7.9 K. The temperature dependence of IJcRN showed a non-monotonic decrease with increasing T, which is different from that of the conventional BCS behaviour. The current IJc completely disappeared at approximately 89 K, roughly matching the Tc(zero), while the gap D was continuously observed above Tc(zero) of 90 K. The gap value D was 30~ 40 meV, so that the calculated value (p/2)D/e on the basis of the Ambegaokar Baratoff theory with s-wave-symmetry of the BCS order parameter is ≈ IJcRN (AB) = 50~ 60 mV, whereas the observed IJcRN (~ 2.6 mV) is one-order smaller. These features were roughly the same as our previous results of those of the as-grown samples with Tc(zero)~105 K, indicating that the IJcRN did not change significantly for reduced Tc but approximately the same doping level.
[1] A. Sugimoto et al, J. Phys.: Conf. Ser. 1975, 012005 (2021).
Keywords: Bi2223, Break Junction, Josephson current, IjcRN