Double-exponential model of nonlinear electrodynamics

A new model of nonlinear electrodynamics with three parameters is suggested and investigated. It is shown what if the external constant magnetic field is present the phenomenon of vacuum birefringence takes place. The indices of refraction for two polarizations of electromagnetic waves, parallel and perpendicular to the magnetic induction field are calculated. The energy-momentum tensor is calculated. The relations between model parameters which guarantee fulfilment of causality and unitarity conditions are obtained. We demonstrate that dual symmetry is broken in the model suggested.