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

Recently, it has been found that, when operated at large input power, the linewidth Delta f of terahertz radiation emitted from intrinsic Josephson junction stacks can be as narrow as some megahertz. In this high-bias regime, a hot spot coexists with regions which are still superconducting. Surprisingly, Delta f was found to decrease with increasing bath temperature. We present a simple model describing the dynamics of the stack in the presence of a hot spot by two parallel arrays of pointlike Josephson junctions and an additional shunt resistor in parallel. Heat diffusion is taken into account by thermally coupling all elements to a bath at temperature T-b. We present current-voltage characteristics of the coupled system and calculations of the linewidth of the radiation as a function of T-b. In the presence of a spatial gradient of the junction parameters' critical current and resistance, Delta f deceases with increasing T-b, similar to the experimental observation.