Unusual linewidth dependence of coherent THz emission measured from intrinsic Josephson junction stacks in the hot-spot regime

TL;DR

This study measures the linewidth of THz emission from intrinsic Josephson junction stacks, revealing a surprising narrowing at high bias due to hotspot effects, challenging standard models of synchronization.

Contribution

It demonstrates that hotspots can act as a synchronizing element, leading to unexpectedly narrow linewidths in THz emission from Josephson junctions.

Findings

Linewidth as narrow as 23 MHz at high bias

Hotspot acts as a synchronization mechanism

Linewidth decreases with increasing temperature

Abstract

We report on measurements of the linewidth {\Delta}f of THz 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 ? 500 MHz, at high bias, where a hotspot coexists with regions which are still superconducting, {\Delta}f turned out to be as narrow as 23 MHz. We attribute this to the hotspot acting as a synchronizing element. {\Delta}f decreases with increasing bath temperature, a behavior reminiscent of motional narrowing in NMR or ESR, but hard to explain in standard electrodynamic models of Josephson junctions.