Comparative analysis of tunneling magnetoresistance in low-Tc Nb/Al-AlOx/Nb and high-Tc Bi2-yPbySr2CaCu2O8+δ intrinsic Josephson junctions

We perform a detailed comparison of magnetotunneling in conventional low-T-c Nb/Al-AlOx/Nb junctions with that in slightly overdoped Bi2-yPbySr2CaCu2O8+delta [Bi(Pb)-2212] intrinsic Josephson junctions and with microscopic calculations. It is found that both types of junctions behave in a qualitatively similar way. Both magnetic field and temperature suppress superconductivity in the state-conserving manner. This leads to the characteristic sign change of tunneling magnetoresistance from the negative at the subgap to the positive at the sum-gap bias. We derived theoretically and verified experimentally scaling laws of magnetotunneling characteristics and employ them for accurate extraction of the upper critical field H-c2. For Nb an extended region of surface superconductivity at H-c2 < H < H-c3 is observed. The parameters of Bi(Pb)-2212 were obtained from self-consistent analysis of magnetotunneling data at different levels of bias, dissipation powers, and for different mesa sizes, which precludes the influence of self-heating. It is found that H-c2(0) for Bi(Pb)-2212 is similar or equal to 70 T and decreases significantly at T -> T-c. The amplitude of subgap magnetoresistance is suppressed exponentially at T > T-c/2, but remains negative, although very small, above T-c. This may indicate the existence of an extended fluctuation region, which, however, does not destroy the general second-order type of the phase transition at T-c.