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

TL;DR

This study compares tunneling magnetoresistance in low-$T_c$ Nb/AlAlOx/Nb and high-$T_c$ Bi(Pb)-2212 Josephson junctions, revealing similar behaviors and deriving scaling laws to accurately determine critical magnetic fields and analyze fluctuation effects.

Contribution

It provides a comprehensive comparison of magnetotunneling in low-$T_c$ and high-$T_c$ junctions, introduces scaling laws for data analysis, and offers new insights into surface superconductivity and fluctuation regions.

Findings

Both junction types exhibit similar suppression of superconductivity by magnetic field and temperature.

Scaling laws enable precise extraction of upper critical field $H_{c2}$.

Bi(Pb)-2212 $H_{c2}(0)$ is approximately 70 T and decreases near $T_c$.

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 $T_c$.