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THz emission from intrinsic Josephson junctions at zero magnetic field via breather auto-oscillations

I propose a new mechanism of intense high-frequency electromagnetic wave generation by spatially uniform stacked Josephson junctions at zero magnetic field. The ac-Josephson effect converts the dc-bias voltage into ac-supercurrent, however, in the absence of spatial variation of Josephson phase difference, does not provide dc-to-ac power conversion, needed for emission of electromagnetic waves. Here I demonstrate that at geometrical resonance conditions, the spatial homogeneity of phase can be spontaneously broken by appearance of breathers (bound fluxon-antifluxon pairs), facilitating effective dc-to-ac power conversion. The proposed mechanism explains all major features of recently observed THz radiation from large area Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ mesa structures.