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

TL;DR

This study uses SiC photoluminescence to measure local temperature variations in a Bi$_2$Sr$_2$CaCu$_2$O$_{8+eta}$ superconductor mesa during sub-THz emission, revealing hot spot dynamics and their non-essential role in emission mechanisms.

Contribution

It provides direct local temperature measurements during sub-THz emission, showing hot spots do not drive emission or synchronization, challenging previous assumptions.

Findings

Hot spots form at high bias and low temperature, with little impact on emission.

Sub-THz emission occurs without hot spots at higher bath temperatures.

Hot spots coexist non-mutualistically with emission, not essential for it.

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 $T({\bm r})$ was directly measured during simultaneous sub-THz emission from the $N\sim10^3$ intrinsic Josephson junctions (IJJs) in the mesa. At high bias currents $I$ and low bath temperatures $T_{\rm bath}\lesssim~35$ K, the center of a large elliptical hot spot with $T({\bm r})> T_c$ 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 $T_{\rm bath}\gtrsim~50$ 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 $T({\bm r})$ standing waves were observed.