The unrestricted flcutuation-exchange approximation (UFLEX) [1&ndash 3] is a generalization of the fluctuation-exchange
approximation
(FLEX) [4] to the case with inhomogeneities. We introduce cluster momentum as well as usual momentum in our formulation.
The notion of the cluster momentum has been suggested from the dynamical cluster approximation. Using this, the
nonlocal correlations can be taken into account beyond the dynamical mean field theory [5, 6].
The advanage of our present formulation of the UFLEX is its flexibility in applying the method to some problems.
If the problem has some important spatial inhomogeneities, we need to use the cluster momenta corresponding to these
inhomogeneities.
References:
[1] S. Koikegami, T. Yanagisawa, M. Kato: J. Phys. Soc. Jpn. 75 (2003) 044712.
[2] S. Koikegami, T. Yanagisawa, M. Kato: Physica C392 (2003) 213.
[3] S. Koikegami, T. Yanagisawa, M. Kato: Physica B329 (2003) 781.
[4] N.E. Bickers and D. J. Scalapino: Ann. Phys. 192 (1989) 206.
[5] M. H. Hettler et al.: Phys. Rev. B58 (1998) 7475.
[6] Th. Maier et al.: Eur. Rev. J. B13 (2000) 613.
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