Comparative analysis of tunneling magnetoresistance in low-$T_c$ Nb/AlAlOx/Nb and high-$T_c$ Bi$_{2-y}$Pb$_y$Sr$_2$CaCu$_2$O$_{8+\delta}$ intrinsic Josephson junctions

Abstract

We perform a detailed comparison of magnetotunneling in conventional low-$T_c$ Nb/AlAlOx/Nb junctions with that in slightly overdoped Bi$_{2-y}$Pb$_y$Sr$_2$CaCu$_2$O$_{8+\delta}$ [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 sub-gap 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 $\simeq 70$ T and decreases significantly at $T\rightarrow T_c$. The amplitude of sub-gap magnetoresistance is suppressed exponentially at $T>T_c/2$, but remains negative, although very small, above $T_c$. This may indicate existence of an extended fluctuation region, which, however, does not destroy the general second-order type of the phase transition at $T_c$.