Hot Spot and THz Wave Generation in Bi$_2$Sr$_2$CaCu$_2$O$_8$ Intrinsic Josephson Junction Stacks

Stacks of intrinsic Josephson junctions made of the high temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ have been shown to emit coherent radiation at THz frequencies [1]. Emission is observed both in a low bias regime and a high bias regime. While at low bias the temperature of the stack is close to the bath temperature, at high bias a hot spot and a standing wave, formed in the ``cold'' part of the stack, coexist [2-5]. THz radiation is very stable in this regime, exhibiting a linewidth which is much smaller than expected from a purely cavity-induced synchronization mechanism [6]. We investigate the interaction of hot spots and THz waves using a combination of transport measurement, direct electromagnetic wave detection and low temperature scanning laser microscopy (LTSLM). In this talk recent developments will be presented, with a focus on the mechanism of hot spot formation.\\[4pt] In collaboration with B. Gross, S. Gu\'{e}non, M. Y. Li, J. Yuan, N. Kinev, J. Li, A. Ishii, K. Hirata, T. Hatano, R. G. Mints, D. Koelle, V. P. Koshelets, H. B. Wang and P. H. Wu.\\[4pt] [1] L. Ozyuzer, et al., Science \textbf{318}, 1291 (2007).\\[0pt] [2] H.~B. Wang, et al., Phys. Rev. Lett. \textbf{102}, 017006 (2009).\\[0pt] [3] H. B. Wang, et al., Phys. Rev. Lett. \textbf{105}, 057002 (2010).\\[0pt] [4] S. Guenon, et al, Phys. Rev B \textbf{82}, 214506 (2010).\\[0pt] [5] B. Gross, et al., Phys. Rev. B \textbf{86}, 094524 (2012).\\[0pt] [6] M. Y. Li, et al., Phys. Rev. B \textbf{86}, 060505 (R) (2012).