Coherent THz radiation from multiple I-V branching structures in intrinsic Josephson junctions of Bi-2212

Intense and coherent terahertz (THz) radiation was observed from the intrinsic Josephson junction (IJJ) system of the single crystalline high-$T_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi-2212).\footnote{L. Ozyuzer et al., Science \textbf{318,} (2007) 1291.}$^,$\footnote{K. Kadowaki et al., Physica C \textbf{468,} (2008) 634.} In the present work, we demonstrate the importance of the THz radiation from inner branching structures of the $I-V$ characteristic curve of the IJJ system. The radiation frequency has previously been thought to be uniquely constrained to the mesa size, but it turns out in the inner branches that it varies very flexibly and widely perhaps according to the inductive and capacitive coupling strength existing in the mesa itself. Therefore, the radiation frequency does not follow the previously established cavity resonance condition. This new experimental feature may provide a unique opportunity to understand the dynamical nature of IJJ as well as the mechanism of high-$T_{c}$ superconductivity in this particular Bi-2212 compound.