The spatially localized bound states of two electrons in the adiabatic two-dimensional Holstein-Hubbard model on a square lattice are investigated both numerically and analytically. The interplay between the electron-phonon coupling g, which tends to form bipolarons and the repulsive Hubbard...

The long-range forces between holes in an antiferromagnet are due to magnon exchange. The one-magnon exchange potential between two holes is proportional to cos(2 ϕ)/<Emphasis Type="Bold">r <Superscript>2</Superscript> where <Emphasis Type="Bold">r is the distance vector of the holes and ϕ is the angle between <Emphasis Type="Bold">r and an axis of the square crystal lattice....</emphasis></emphasis></superscript></emphasis>

In this paper we study the ground state phase diagram of a one-dimensional t-J-U model away from half-filling. In the large-bandwidth limit and for ferromagnetic exchange with easy-plane anisotropy a phase with gapless charge and massive spin excitations, characterized by the coexistence of...

An model of La<Subscript>2-x</Subscript>Sr<Subscript>x</Subscript>CuO<Subscript>4</Sub script> explaining the features of incommensurate spin textures without any assumption of stripe formation is proposed. The foundations of this model are the mechanism of negative-U center formation proposed earlier and the concept of specific ordering of doped ions. It is shown...<//sub></subscript></subscript></subscript>