The treatment of zero eigenvalues of the matrix governing the equations of motion in many-body Green function theory

摘要

The spectral theorem of many-body Green function theory relates thermodynamic correlations to Green functions. More often than not, the matrix F governing the equations of motion has zero eigenvalues. In this case, the standard textbook approach requires both commutator and anti-commutator Green functions to obtain equations for that part of the correlation which does not lie in the null space of the matrix. In this paper, we show that this procedure fails if the projector onto the null space is dependent on the momentum vector, k. We propose an alternative formulation of the theory in terms of the non-null space alone and we show that a solution is possible if one can find a momentum-independent projector onto some subspace of the non-null space. To do this, we enlist the aid of the singular value decomposition of Fin order to project out the null space, thus reducing the size of the matrix and eliminating the need for the anti-commutator Green function. We extend our previous work (Frobrich and Kuntz 2003 Phys. Rev. B 68 014410), dealing with a ferromagnetic Heisenberg monolayer and a momentum-independent projector onto the null space, to models where both multilayer films and a momentum-dependent projector are considered. We develop the numerical methods capable of handling these cases and offer a computational algorithm that should be applicable to any similar problem arising in Green function theory.