Graduate School in Nonlinear Science

Sponsored by the Danish Research Academy

MIDIT                               OFD                           CATS
Modelling, Nonlinear Dynamics       Optics and Fluid Dynamics     Chaos and Turbulence Studies
and Irreversible Thermodynamics     Risø National Laboratory      Niels Bohr Institute and 
Technical University of Denmark     Building 128                  Department of Chemistry
Building 321                        P.O. Box 49                   University of Copenhagen 
DK-2800 Lyngby                      DK-4000 Roskilde              DK-2100 Copenhagen Ø
Denmark                             Denmark                       Denmark

Abstract: In this talk the microscopic theory of the intrinsic Josephson effect in high-temperature superconductors and its interaction with lattice vibrations is investigated.
Special attention is paid to the role of nonequilibrium states, which are connected with an asymmetric occupation of particle- and holelike quasiparticles in the superconductor ("charge imbalance"). A consistent treatment of this degree of freedom is essential for a correct description of a novel capacitive coupling of the dynamics in different junctions due to charge fluctuations between the superconducting CuO₂-layers. As a first result the collective modes of this electronic model are studied.
In the second part the interation between Josephson oscillations and phonons due to the Coulomb interaction is derived microscopically starting from the fundamental hamiltonian of solid state physics. The effect of phonons on the electronic transport perpendicular to the superconduncting layers can be fully taken into account by introducing an effective dielectric constant of the phonons.
The coupled system of nonlinear differential equations for the dynamics of the phases in different junctions is solved analytically with the help of a Greensfunction method. An important result is the fact that resonances in the current-voltage-characteristic appear exactly at the longitudinal optical eigenfrequencies of the lattice system.
This explains well all experimental features of recently discovered ``subgap structures'' in the c-axis transport and thereby provides a new measurement technique for optical phonon frequencies in these materials. The detected microwave emission and absorption in the GHz region is reproduced in detail.