Phase Transitions

Phase transitions are described by an order parameter which differentiates the two phases. Usually it is zero in the disordered phase, non-zero in the ordered phase, e.g. the magnetization for a magnetic system.

Phase transitions are classified in part by their order:

• First order --- discontinuity in the order parameter or energy, i.e., first derivative of the partition function.

• Second order --- divergence in the susceptibility or specific heat, i.e., second derivative of the partition function.

• Third order, etc.

First order transitions are classified by quantities such as the latent heat, the discontinuous change in the energy at the critical point .