Terahertz radiation from the mesa structures of Bi 2 Sr 2 CaCu 2 O 8+δ is detected in samples with thin electrodes < 100 nm. In samples with thick electrodes ≃ 400 nm, neither radiations nor voltage jumps in current-voltage characteristics are detected. This suggests that the thin electrode helps excite the Josephson plasma oscillation as a result of the poor heat flow through the electrode. The shielding effect by the electrode is not essential. We consider that the local temperature rise is the origin of the synchronization of the phase kink for terahertz radiation.A single crystal of Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) is described by a stack of atomic-scale Josephson junctions referred to as intrinsic Josephson junctions (IJJs) with CuO 2 double layers being superconducting electrodes and SrO-Bi 2 O 2 -SrO layers being barrier layers 1 . It has been expected that the synchronization of the Josephson plasma oscillations excited inside the IJJs leads to intensive emission of terahertz (THz) electromagnetic waves because the Josephson plasma oscillation is excited with very little damping inside the large superconducting energy gap (∆ ∼ 40 meV) 2,3 . The methods for the synchronization for coherent THz emission have been discussed in terms of the dynamics of the Josephson vortex lattice 4,5 . Although some groups have detected THz waves from IJJs 6,7 , the highly specialized techniques used there do not allow followers to reproduce the results.Recently, it was observed that monochromatic and continuous electromagnetic waves with frequencies between 0.3 and 0.9 THz and powers up to 5 µW were radiated from rectangular IJJ mesas formed on surfaces of single crystals of Bi2212 8,9 . The radiation is explained as the half-wavelength cavity resonances of IJJs included in the mesa. The standing waves of the Josephson plasma oscillations are synchronized along the c-axis, although the length of each IJJ is varied due to the trapezoidal cross section of the mesa. The excitation frequency is determined by the matching between the cavity length and the AC Josephson effect in the IJJs. The numerical simulations by several authors reproduced that the formation of the dynamical phase variation along the mesa edge (π-kink) yields far-field electromagnetic THz radiation 10-12 . Subsequent experimental results on the radiation from square and cylinder-shaped mesas 13,14 are explained based on this idea with degeneracies of the cavity modes and the partial contribution of the stacked IJJs for the radiation. Wang et al. argued that the temperature a) kakeya@kuee.kyoto-u.ac.jp b) Present address: Quantum Beam Science Directorate, Japan Atomic Energy Agency, Takasaki, Gunma 370-1292, Japan.dependence of the emission frequency is attributed to the change in the effective cavity geometry through the observation of the temperature inhomogeneity in the mesa, in which non-superconducting (hotspot) regions are found under the bias for the emission 15 . The relation between the temperature distribution and the formation of the π-kink aligned a...

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This study investigates the effect of temperature on the emission frequency of an intrinsic Josephson junction terahertz (THz) electromagnetic wave source, which can be used for high-speed communications by THz carrier wave. The characteristic emission features of two device types (asymmetric and symmetric) and two bias regimes (low and high) were determined. The biasdependent emission frequency was temperature dependent in the asymmetric device, most likely reflecting the temperature-dependent London penetration depth. The bias tunability of the emission frequency can be explained by device self-heating, which significantly and inhomogeneously raises the temperatures of the device from its bath temperature. These findings are consistent with previous studies of temperature distribution in these devices. V C 2015 AIP Publishing LLC. FIG. 4. (a) Plots of emission frequency f e vs averaged resistance R c (filled symbols; top axis) and bias voltage V (open symbols; bottom axis) of Device C. The same colors represent data at the same temperature. The solid line is the ac Josephson relation. (b) Toy model of a mesa with an inhomogeneous temperature distribution. A gray solid curve represents the excited electromagnetic wave inside the stack. 043914-4Kakeya et al.

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