QED effective action at finite temperature - art. no. 105002

The QED effective Lagrangian in the presence of an arbitrary constant elect romagnetic background field at finite temperature is derived in the imagina ry-time formalism to one-loop order. The boundary conditions in imaginary t ime reduce the set of gauge transformations of the background field, which allows for a further gauge invariant and puts restrictions on the choice of gauge. The additional invariant enters the effective action by a topologic al mechanism and can be identified with a chemical potential; it is further more related to Debye screening. In concordance with the real-time formalis m, we do not find a thermal correction to Schwinger's pan-production formul a. The calculation is performed on a maximally Lorentz covariant and gauge invariant stage. [S0556-2821(99)07018-6].