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

Abstract

We perform a detailed comparison of magnetotunneling in conventional low-Tc Nb/Al-AlOx/Nb junctions with that in slightly overdoped Bi2−yPbySr2CaCu2O8+δ [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 Hc2. For Nb an extended region of surface superconductivity at Hc2<H<Hc3 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 Hc2(0) for Bi(Pb)-2212 is ≃T→Tc T and decreases significantly at T→Tc. The amplitude of subgap magnetoresistance is suppressed exponentially at T>Tc/2, but remains negative, although very small, above Tc. 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 Tc.

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