Linewidth dependence of coherent terahertz emission from Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks in the hot-spot regime

We report on measurements of the linewidth Delta f of terahertz radiation emitted from intrinsic Josephson junction stacks, using a Nb/AlN/NbN integrated receiver for detection. Previous resolution-limited measurements indicated that Delta f may be below 1 GHz-much smaller than expected from a purely cavity-induced synchronization. While at low bias we found Delta f to be not smaller than similar to 500 MHz, at high bias, where a hot spot coexists with regions which are still superconducting, Delta f turned out to be as narrow as 23 MHz. We attribute this to the hot spot acting as a synchronizing element. Delta f decreases with increasing bath temperature, a behavior reminiscent of motional narrowing in NMR or electron spin resonance (ESR), but hard to explain in standard electrodynamic models of Josephson junctions.