Recently, it has been shown that large stacks of intrinsic Josephson junctions in Bi2Sr2CaCu2O8 emit synchronous THz radiation, the synchronization presumably triggered by a cavity resonance. To investigate this effect we use low temperature scanning laser microscopy to image electric field distributions. We verify the appearance of cavity modes at low bias and in the high input-power regime we find that standing-wave patterns are created through interactions with a hot spot, possibly pointing to a new mode of generating synchronized radiation in intrinsic Josephson junction stacks.

We report on THz emission measurements and low temperature scanning laser imaging of Bi2Sr2CaCu2O8 intrinsic Josephson junction stacks. Coherent emission is observed at large dc input power, where a hot spot and a standing wave, formed in the "cold" part of the stack, coexist. By changing bias current and bath temperature, the emission frequency can be varied by more than 40%; the variation matches the Josephson-frequency variation with voltage. The linewidth of radiation is much smaller than expected from a purely cavity-induced synchronization. Thus, an additional mechanism seems to play a role. Some scenarios, related to the presence of the hot spot, are discussed.

At high enough input power in stacks of Bi2Sr2CaCu2O8 intrinsic Josephson junctions a hot spot (a region heated to above the superconducting transition temperature) coexists with regions still in the superconducting state. In the "cold" regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong coherent THz emission. We investigate the interplay of hot spots and standing electromagnetic waves by low temperature scanning laser microscopy and THz emission measurements, using stacks of various geometries. For a rectangular and a arrowshaped structure we show that the standing wave can be turned on and off in various regions of the stack structure, depending on the hot spot position. We also report on standing wave and hot spot formation in a disk shaped mesa structure.

While Josephson-junction-like structures intrinsic to the layered cuprate high temperature superconductors offer an attractive stage for exploiting possible applications to new quantum technologies, the low energy quasiparticle excitations characteristically present in these d-wave superconductors may easily destruct the coherence required. Here we demonstrate for the first time the feasibility of macroscopic quantum tunneling in the intrinsic Josephson junctions of a high temperature superconductor Bi2Sr2CaCu2O 8+δ , and find it to be characterized by a high classic-to-quantum crossover temperature and a relatively weak quasiparticle dissipation.PACS numbers: 74.72. Hs, 73.40.Gk, 85.25.Cp A marked feature characterizing cuprate high temperature superconductors (HTSC) is its strong two dimensionality. In particular the bismuth-based HTSCs, for which this anisotropy is prominent, are best viewed as stacks of superconducting CuO 2 planes weakly linked through intrinsic Josephson junction (IJJ) type couplings [1]. These built-in atomic scale links are taylor-made for technical applications difficult to achieve with artificial Josephson junctions (JJ), and its study has now developed into an active interdisciplinary field. We report below what is to our knowledge the first successful observation of the macroscopic quantum tunnelling (MQT) [2,3,4,5] of the phase variable of the superconducting order parameter through the potential barrier of an IJJ, opening up an entirely new direction for HTSC applications. While the corresponding phenomena had been observed at around 300 mK in conventional JJs [3, 5, 6], we have confirmed MQT behavior at approximately 1 K apparently reflecting the characteristically high plasma frequency of IJJs. Our results are highly nontrivial in that they also demonstrate the feasibility of MQT in spite of the presence of dissipative low energy quasiparticles [7,8,9,10], which is the other hallmark of HTSCs.Current-biased JJs offer an ideal stage for realizing a variety of macroscopic quantum phenomena, e.g. energy level quantization within the potential well [4,11,12] and the associated MQT and macroscopic quantum coherence [13,14], all of which have come to be recognized as having immediate implications for qubit applications [13,15,16,17]. In particular, a phase qubit utilizing MQT has been reported by Martinis and co-workers [15].Aside from external noises and disorder, a primary source which stands as an obstacle towards observation of MQT is the influence of non-superconducting quasiparticle excitations [2]. In conventional s-wave superconductors, all quasi-particle states are separated from the superconducting ground state by a finite energy gap and thus become essentially inaccessible upon going to sufficiently low temperatures; hence the observability of MQT. The situation is drastically altered when we turn to HTSCs, which are d-wave superconductors. The latter are characterized by four nodes in the order parameter at which the energy gap vanishes [18,19]. HTSCs therefore necessari...