Phase Structure of Four-Fermion Theories at Finite Temperature and   Chemical Potential in Arbitrary Dimensions
  The phase structure of four-fermion theories is thoroughly investigated with varying temperature and chemical potential for arbitrary space-time dimensions $(2 \leq D < 4)$ by using the 1/N expansion method. It is shown that the chiral symmetry is restored in the theory under consideration for sufficiently high temperature and/or chemical potential. The critical line dividing the symmetric and broken phase is given explicitly. It is found that for space-time dimension $2 \leq D < 3$ both the first-order and second-order phase transition occur depending on the value of temperature and chemical potential while for $3 \leq D < 4$ only the second-order phase transition exists. 
T.Inagaki, T.Kouno and T.Muta