Local SiC photoluminescence evidence of non-mutualistic hot spot formation and sub-THz coherent emission from a rectangular Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$ mesa

Abstract

From the photoluminescence of SiC microcrystals uniformly covering a rectangular mesa of the high transition temperature $T_c$ superconductor Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta }$, the local surface temperature $T(\mathbf{r})$ was directly measured during simultaneous sub-THz emission from the $N\sim 10^3$ intrinsic Josephson junctions (IJJs) in the mesa. At high bias currents $I$ and low bath temperatures $T_{\mathrm{bath}}\lesssim 35$ K, the center of a large elliptical hot spot with $T(\mathbf{r})>T_c$ jumps dramatically with little current-voltage characteristic changes. The hot spot doesn't alter the ubiquitous primary and secondary emission conditions: the ac Josephson relation and the electromagnetic cavity resonance excitation, respectively. Since the intense sub-THz emission was observed for high $T_{\mathrm{bath}}\gtrsim 50$ K in the low $I$ bias regime where hot spots are absent, hot spots can not provide the primary mechanisms for increasing the output power, the tunability, or for promoting the synchronization of the $N$ IJJs for the sub-THz emission, but can at best coexist non-mutualistically with the emission. No $T(\mathbf{r})$ standing waves were observed.