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Abstract

In cuprate high-Tc superconductors, superconducting layers are separated by insulating layers. Cooper pairs can go through the insulating layer via coherent quantum tunnelling. Such structure can be considered as stacks of Josephson junctions, and its spatial and temporal characteristics are described by a nonlinear equation known as the sine-Gordon equation. With a weak electromagnetic driving field, the layered structure shows a typical plasma response which allows the propagation of linear electromagnetic waves (known as Josephson plasma waves) with frequencies only above the Josephson plasma resonance (in the THz region). When the driving field increases, the tunnelling supercurrent approaches its critical value, and the electrodynamics becomes highly nonlinear.