Functional Integral Representations in Spin Fluctuation Theory: Is There a Right One?
Most discussions of spin fluctuation theory can be formulated in the language of functional integration. A list of the practitioners of this method, just in the present application, would include EVENSON, WANG, and SCHRIEFFER 2, CYROT , MURATA and DONIACH , HERTZ and KLENIN , MORIYA and coworkers , HASEGAWA , HUBBARD , and KORENMAN and PRANGE . There are, however, important mathematical problems which are encountered when the method is used. The most serious problem is that there are many possible different representations. Although these are equally exact before approximations are made, they can give very different results after formally identical initial approximations.
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