LAUSR

Stacked intrinsic Josephson junctions in the cuprate superconductor Bi${}_{2}$Sr${}_{2}$CaCu${}_{2}$O${}_{8+x}$ are eagerly investigated as a compact source of coherent terahertz radiation. The authors study the mechanisms of frequency tuning of this source at high and low bias. At high bias, in the presence of a hot spot, the emission frequency ${f}_{e}$ is continuously tunable via changing the bias current and the bath temperature, but at low bias ${f}_{e}$ remains discrete, indicating phase locking to cavity resonances, though some groups of junctions seem to remain unlocked. Though some differences appear, these phenomena can be reproduced in simulations combining electrodynamics and heat diffusion.