Real-Time Thermal Propagators and the QED Effective Action for an External Magnetic Field

Abstract

The thermal averaged real-time propagator of a Dirac fermion in a static uniform magnetic field $B$ is derived. At non-zero chemical potential and temperature we find explicitly the effective action for the magnetic field, which is shown to be closely related to the Helmholz free energy of a relativistic fermion gas, and it exhibits the expected de\ Haas -- van\ Alphen oscillations. An effective QED coupling constant at finite temperature and density is derived, and compared with renormalization group results. We discuss some astrophysical implications of our results.

36 pages, Latex + epsf.tex (ps-file appended at the end), NORDITA-93/78P, Goteborg ITP 93-11, (some mishaps with table enumeration are corrected)