Dynamical symmetry breaking is investigated for a four-fermion
Nambu-Jona-Lasinio model in external electromagnetic and gravitational fields.
An effective potential is calculated in the leading order of the large-N
expansion using the proper-time Schwinger formalism.
Phase transitions accompanying a chiral symmetry breaking in the
Nambu-Jona-Lasinio model are studied in detail. A magnetic calalysis phenomenon
is shown to exist in curved spacetime but it turns out to lose its universal
character because the chiral symmetry is restored above some critical positive
value of the spacetime curvature.Comment: Talk given at the Conference "Particles, Fields and Gravitation",
Lodz, Poland, April 15-19, 1998; 10 pages, 3 ps-figures, LaTeX, aipproc.sty,
psf.sty. To be published with AIP Press in Proceedings of the Conferenc
