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

Abstract

The Josephson effect is a phenomenon of current flow across two weakly linkedsuperconductors separated by a thin barrier, i.e. a Josephson junction, associated withcoherent quantum tunneling of Cooper pairs. Many novel phenomena appear due to thenonlinear property of the Josephson effect, such as Shapiro steps in dc current–voltage (IV) characteristics of a Josephson junction under microwave irradiation, which can beused as a voltage standard. The Josephson effect also provides a unique wayto generate high-frequency electromagnetic (EM) radiation by dc bias voltageas the inverse process of the Shapiro step. The discovery of cuprate high-Tc superconductors accelerated the effort to develop novel sources of EM waves based on astack of atomically densely packed intrinsic Josephson junctions (IJJs), since the largesuperconductivity gap covers the whole terahertz (THz) frequency band. Very recently, strongand coherent THz radiation has been successfully generated from a mesa structure of aBi2Sr2CaCu2O8 + δ single crystal which works both as the source of energy gain and as the cavity forresonance. This experimental breakthrough posed a challenge to the theoretical study ofthe phase dynamics of stacked IJJs, since the phenomenon cannot be explained by theknown solutions of the sine-Gordon equation so far. It is then found theoretically that, dueto the huge inductive coupling of IJJs produced by the nanometer junction separation andthe large London penetration depth of the order of micrometers of the material, anovel dynamic state is stabilized in the coupled sine-Gordon system, in which ± π kinks are developed in the superconductivity phase difference responding to thestanding wave of the Josephson plasma and are stacked alternately in thec axis. This novel solution of the inductively coupled sine-Gordon equations captures theimportant features of experimental observations. The theory predicts an optimalradiation power larger than the one observed in recent experiments by ordersof magnitude, and thus suggests the technological relevance of the phenomena.