Counterintuitive Consequence of Heating in Strongly-Driven Intrinsic-Junctions of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ Mesas

Abstract

Anomalously high and sharp peaks in the conductance of intrinsic Josephson junctions in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ (Bi2212) mesas have been universally interpreted as superconducting energy gaps, but here we show they are a result of heating. This interpretation follows from a direct comparison to the equilibrium gap, $\Delta$, measured in break junctions on similar Bi2212 crystals. As the dissipated power increases with a greater number of junctions in the mesa, the conductance peak abruptly sharpens and its voltage decreases to well below 2$\Delta$. This sharpening, found in our experimental data, defies conventional intuition of heating effects on tunneling spectra, but it can be understood as an instability into a nonequilibrium two-phase coexistent state. The measured peak positions occur accurately within the voltage range that an S-shaped backbending is found in the {\it calculated} current-voltage curves for spatially {\it uniform} self-heating and that S-shape implies the potential for the uniform state to be unstable.

10 pages, 7 figures