Unusual linewidth dependence of coherent THz emission measured from   intrinsic Josephson junction stacks in the hot-spot regime
  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. 
M. Y. Li, J. Yuan, N. Kinev, J. Li, B. Gross, S. Guenon, A. Ishii, K.   Hirata, T. Hatano, D. Koelle, R. Kleiner, V. P. Kosh...
cond-mat.supr-con physics.ins-det