PHASE-STRUCTURE OF 4-FERMION THEORIES AT FINITE-TEMPERATURE AND CHEMICAL-POTENTIAL IN ARBITRARY DIMENSIONS

The phase structure of four-fermion theories is thoroughly investigate d with varying temperature and chemical potential for arbitrary space- time dimensions (2 less than or equal to D < 4) by using the 1/N expan sion method. It is shown that the chiral symmetry is restored in the t heory under consideration for sufficiently high temperature and/or che mical potential. The critical line dividing the symmetric and the brok en phase is given explicitly. It is found that for space-time dimensio n 2 less than or equal to D < 3 both the first order and the second or der phase transition occur depending on the value of the temperature a nd chemical potential while for 3 less than or equal to D < 4 only the second order phase transition exists.