Operator projection method applied to the single-particle Green's functionin the Hubbard model

A new non-perturbative framework for many-body correlated systems is formul ated by extending the operator projection method (OPM). This method offers a systematic expansion which enables us to project into the low-energy stru cture after extracting the higher-energy hierarchy. This method also opens a way to systematically take into account the effects of collective excitat ions. The Mott-Hubbard metal-insulator transition in the Hubbard model is s tudied by means of this projection beyond the second order by taking into a ccount magnetic and charge fluctuations in the presence of the high-energy Mott-Hubbard structure. At half filling, the Mott-Hubbard gap is correctly reproduced between the separated two bands. Near half filling, strongly ren ormalized low-energy single-particle excitations coexisting with the Mott-H ubbard bands are shown to appear. The significance of the momentum-dependen t self-energy in the results is stressed.