  The Polyakov loop variable serves as an order parameter to characterize the
confined and deconfined phases of Yang-Mills theory. By integrating out the
vector fields in the SU(2) Yang-Mills partition function in one-loop
approximation, an effective action is obtained for the Polyakov loop to second
order in a derivative expansion. The resulting effective potential for the
Polyakov loop is capable of describing a second-order deconfinement transition
as a function of temperature.
