Phase dynamics in a stack of inductively coupled intrinsic Josephson junctions and terahertz electromagnetic radiation

The Josephson effect is a phenomenon of current flow across two weakly linked superconductors separated by a thin barrier, i.e. a Josephson junction, associated with coherent quantum tunneling of Cooper pairs. Many novel phenomena appear due to the nonlinear property of the Josephson effect, such as Shapiro steps in dc current-voltage (IV) characteristics of a Josephson junction under microwave irradiation, which can be used as a voltage standard. The Josephson effect also provides a unique way to generate high-frequency electromagnetic (EM) radiation by dc bias voltage as the inverse process of the Shapiro step. The discovery of cuprate high-T-c superconductors accelerated the effort to develop novel sources of EM waves based on a stack of atomically densely packed intrinsic Josephson junctions (IJJs), since the large superconductivity gap covers the whole terahertz (THz) frequency band. Very recently, strong and coherent THz radiation has been successfully generated from a mesa structure of a Bi2Sr2CaCu2O8+delta single crystal which works both as the source of energy gain and as the cavity for resonance. This experimental breakthrough posed a challenge to the theoretical study of the phase dynamics of stacked IJJs, since the phenomenon cannot be explained by the known solutions of the sine-Gordon equation so far. It is then found theoretically that, due to the huge inductive coupling of IJJs produced by the nanometer junction separation and the large London penetration depth of the order of micrometers of the material, a novel dynamic state is stabilized in the coupled sine-Gordon system, in which +/-pi kinks are developed in the superconductivity phase difference responding to the standing wave of the Josephson plasma and are stacked alternately in the c axis. This novel solution of the inductively coupled sine-Gordon equations captures the important features of experimental observations. The theory predicts an optimal radiation power larger than the one observed in recent experiments by orders of magnitude, and thus suggests the technological relevance of the phenomena.