What is the order parameter

In the magnet problem, the onset of order is signaled by the tqppearance of a nonvanishing magnetization M(r). We call this the order parameter of the transition. 

More generally, the order parameter is a quantity which shows diverging fluctuations (and a diverging susceptibility) near the critical point. In 

A similar difficulty occurs when we compare polymer solutions with magnets. In a formal sense, the order parameter is the magnetization of a spin system with a number of spin components n = 0. This does not help very much. A more concrete statement, based on ideas of S. F. Edwards, is the following the ordv parameter r) for a polynto- solution is similar to a quantum mechanical creation (or destruction) operator. A factor if r) corresponds to the initiation of a chain (or to its termination).  

We can understand this statment duough two approaches. In the grand piartition function S = Z( /Z(0) for a polymer solution, we saw that each chain (with two ends) was associated with a factor where H is conjugate to the order parameter This means that each end has a factor H-, thus, the concentration of end points must be proportional to tfi. 

In Chapter IX we discussed one particular limit of the self-consistent field method, where the ground state was dominant. We then introduced a ground state wavefunction r) with the following properties  

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