By Stefan Kehrein

ISBN-10: 354034067X

ISBN-13: 9783540340676

ISBN-10: 3540340688

ISBN-13: 9783540340683

ISBN-10: 3642070531

ISBN-13: 9783642070532

This self-contained monograph addresses the move equation method of many-particle structures. The circulate equation technique includes a chain of infinitesimal unitary ameliorations and is conceptually just like renormalization and scaling tools. circulation equations offer a framework for reading Hamiltonian platforms the place those traditional many-body suggestions fail. The textual content first discusses the final rules and ideas of the move equation procedure. In a moment half those ideas are illustrated with numerous functions in condensed topic idea together with strong-coupling difficulties and non-equilibrium structures. The monograph is out there to readers acquainted with graduate- point solid-state theory.

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**Example text**

Energy scale separation is not important in all interactions as one knows from the many successful applications of conventional perturbation theory. However, one needs to be extra careful with a generator that does not respect energy scale separation and make sure that this does not lead to uncontrolled errors in higher orders. 3 Example: Potential Scattering Model 31 steps that respect energy scale separation. It is therefore more natural to work with the inﬁnitesimal formulation. One can only properly motivate the unavoidable approximations for a generic many-body problem in this inﬁnitesimal formulation.

40) k,k ,α,β where J is an antiferromagnetic exchange coupling, J > 0. In correlated electron systems one thinks of S as describing the spin of an electron in a strongly correlated singly occupied orbital. The naive twofold ground state degeneracy of the system is screened by many-particle processes that effectively screen the impurity spin below an energy scale set by the Kondo temperature TK ∝ D exp(−1/ρJ). The impurity spin susceptibility becomes ﬁnite in the zero temperature limit and the Fermi sea responds to the formation of this screened Kondo singlet with an increased density of states at the Fermi level.

40). We have thereby arrived at the important conclusion that while approximations in the ﬂow equation transformation will lead to deviations from the exact result, the ﬂuctuation–dissipation relation between such an approximate correlation function and the corresponding approximate response function still holds exactly. 39) exactly without further approximations. 3 Examples In order to practice the calculation of observables in the ﬂow equation approach, we discuss two examples in this chapter: the potential scattering model and the resonant level model.

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