Anomalously high and sharp peaks in the conductance of intrinsic Josephson junctions in Bi2Sr2CaCu2O8+d (Bi2212) mesas have been commonly interpreted as superconducting energy gaps, but here we show they are a result of strong self-heating. This conclusion follows directly from a comparison to the equilibrium gap measured by tunneling in single break junctions on equivalent crystals. The figure shows that as the number of junctions in the mesa, N, and thus heating, increase, the peak voltages decrease and the peak width abruptly sharpens for N≥12. Clearly these widely variable features vs. N cannot all represent the equilibrium properties. Our data imply that the sharp peaks represent a transition to the normal state. That it occurs at the same dissipated power for N=12 to 30 strongly implicates heating as the cause. Although peak sharpening due to heating is counterintuitive, as tunneling spectra usually broaden at higher temperatures, a lateral temperature gradient, leading to coexistence of normal hot spots and superconductive regions, qualitatively explains the behavior. As the peak’s width and voltage in our shortest mesa (N=6) are more consistent with our break junction data, that are virtually free of heating, we propose a figure-of-merit for Bi2212 mesas, the relative conductance peak width, such that small values signal a crossover into the strong self-heating regime.
While heating had been suggested previously, it was only partially documented without knowledge of the actual equilibrium gap value or the intrinsic width of the density-of-states. Our use of independent, ultra-low dissipation break junctions on the same (or similar) crystals has provided the key equilibrium data for comparison. It is found that a relatively broad peak and well-defined dip/hump features characterize the equilibrium SIS conductance, which is also consistent with other single-junction tunneling studies of Bi2212. Our smallest mesa with N=6 more closely resembles the equilibrium data, including a well-defined dip/hump and a broader conductance peak. The conductance peaks systematically shift to lower voltages and abruptly sharpen as N increases, and the demonstration of this evolution is one of the important findings of the present study. That the sharp peaks occur at fixed heating power per junction and the conductance data show no dip/hump features allow us to conclude directly, and unambiguously, that such peaks represent the transition of the mesa into the normal state. These findings, along with the detailed analyses, go well beyond more preliminary reports.
Research performed by C. Kurter, L. Ozyuzer, T. Proslier, D. G. Hinks and K. E. Gray at Argonne National Laboratory and J. F. Zasadzinski at Illinois Institute of Technology.
Counterintuitive Consequence of Heating in Strongly-Driven Intrinsic-Junctions of Bi2Sr2CaCu2O8+d Mesas
C. Kurter, L. Ozyuzer, T. Proslier, J. F. Zasadzinski, D. G. Hinks, and K. E. Gray
Phys. Rev. B 81, 224518 (2010).