The c-axis transport in naturally-grown Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ cross-whisker junctions

TL;DR

This study investigates the c-axis transport properties of naturally formed Bi2212 cross-whisker junctions, revealing incoherent tunneling and evidence for a small s-wave component in the order parameter, with implications for understanding high-Tc superconductivity.

Contribution

It provides the first detailed analysis of naturally formed cross-whisker junctions, demonstrating their high quality and twist-angle independence, supporting incoherent tunneling and s-wave component presence.

Findings

Critical current density is about 0.03 of bulk junctions.

Critical current is independent of twist angle.

High junction quality indicated by conductance features.

Abstract

We studied the c-axis transport of Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) cross-whisker junctions formed by annealing ``naturally'' formed whisker junctions. These frequently appear during growth when the $ab$-faces of neighboring whiskers come in contact. We obtained Fraunhofer patterns of the cross-junction critical currents in a parallel magnetic field, and found a sharp increase in the quasiparticle tunneling conductance at $eV=50-60$ mV, indicating high junction quality. For our weak junctions, the interface critical current density is about 0.03 of the critical current density across the stack of bulk intrinsic junctions, as is the room temperature conductivity, and is independent of the twist angle, in contrast to most of the data reported on ``artificial'' cross-whisker junctions [Y. Takano {\it et al.}, Phys. Rev. B {\bf 65}, 140513(R) (2002)]. Our results provide strong evidence of incoherent interface tunneling and for at least a small s-wave order parameter component in the bulk of Bi2212 for $T\le T_c$. They are also consistent with the bicrystal twist experiments of Li {\it et al.} [Phys. Rev. Lett. {\bf 83}, 4160 (1999)].