Radiation from Flux Flow in Josephson Junction Structures

Abstract

We derive the radiation power from a single Josephson junction (JJ) and from layered superconductors in the flux-flow regime. For JJ case, we formulate the boundary conditions for the electric and magnetic fields at the edges of the superconducting leads using the Maxwell equations in the dielectric media and find dynamic boundary conditions for the phase difference in JJ which account for the radiation. We derive the fraction of the power fed into JJ transformed into the radiation. In a finite-length JJ this fraction is determined by the dissipation inside JJ and it tends to unity as dissipation vanishes independently of mismatch of the junction and dielectric media impedances. We formulate also the dynamic boundary conditions for the phase difference in intrinsic JJs in highly anisotropic layered superconductors of the Bi2Sr2CaCu2O8 type at the boundary with free space. Using these boundary conditions, we solve equations for the phase difference in the linear regime of Josephson oscillations for rectangular and triangular lattices of Josephson vortices. In the case of rectangular lattice for crystals with the thickness along the c-axis much larger than the radiation wavelength, we estimate the radiation power per unit length in the direction of magnetic field at the frequency 1 THz as ∼N μW/cm for Tl2Ba2CaCu2O8 and ∼0.04 N μW/cm for Bi2Sr2CaCu2O8. For crystals with thickness smaller than the radiation wavelength, we found that the radiation power in the resonance is independent on number of layers and can be estimated at 1 THz as 0.5 W/cm (Tl2Ba2CaCu2O8) and 24 mW/cm (Bi2Sr2CaCu2O8). For rectangular lattice, due to superradiation regime, up to half of power fed into the crystal may be converted into the radiation. In the case of triangular or random lattice in the direction perpendicular to the layers, the fraction of power converted into the radiation depends on the dissipation rate and is much lower than for rectangular lattice in the case of high-temperature superconductors with nodes in the gap.