Schrieffer–Wolff transformation
In physics, an operator version of second-order perturbation theory / From Wikipedia, the free encyclopedia
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In quantum mechanics, the Schrieffer–Wolff transformation is a unitary transformation used to determine an effective (often low-energy) Hamiltonian by decoupling weakly interacting subspaces.[1][2] Using a pertubative approach, the transformation can be constructed such that the interaction between the two subspaces vanishes up to the desired order in the perturbation. The transformation also perturbatively diagonalizes the system Hamiltonian to first order in the interaction. In this, the Schrieffer–Wolff transformation is an operator version of second-order perturbation theory. The Schrieffer–Wolff transformation is often used to project out the high energy excitations of a given quantum many-body Hamiltonian in order to obtain an effective low energy model.[1] The Schrieffer–Wolff transformation thus provides a controlled perturbative way to study the strong coupling regime of quantum-many body Hamiltonians.
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Although commonly attributed to the paper in which the Kondo model was obtained from the Anderson impurity model by J.R. Schrieffer and P.A. Wolff.,[3] Joaquin Mazdak Luttinger and Walter Kohn used this method in an earlier work about non-periodic k·p perturbation theory.[4] Using the Schrieffer–Wolff transformation, the high energy charge excitations present in Anderson impurity model are projected out and a low energy effective Hamiltonian is obtained which has only virtual charge fluctuations. For the Anderson impurity model case, the Schrieffer–Wolff transformation showed that the Kondo model lies in the strong coupling regime of the Anderson impurity model.