Se p 20 02 The 2 D Mott - Hubbard transition in presence of a parallel magnetic field

The half-filled two-dimensional Hubbard model in presence of a uniform and static parallel magnetic field has been studied by means of the Composite Operator Method. A fully self-consistent solution, fulfilling all the constrains coming from the Pauli principle, has been found. The relevant features of a metal-insulator transition in presence of a magnetic field have been analyzed. The results qualitatively agree with the ones recently obtained by means of experimental investigations. The response of a two-dimensional (2D) electronic system to a parallel magnetic field is very intriguing and several anomalous properties have been observed. There is a general agreement that the observed behavior is related to the spin polarization, but further studies, both theoretical and experimental , are needed. In this paper we concentrate on the metal-insulator transition (MIT) driven by a in-plane magnetic field. Recent experiments on Si-MOSFET [1] and GaAs [2] have shown that by increasing field the spin system polarizes and the system undergoes a MIT before reaching the full polarization. Apparently, an important role is played by the electron-electron interaction, being r s = U/K (the ratio of Coulomb interaction energy to the mean kinetic energy) very large. In order to make a qualitative and preliminary study of this phenomenon we consider the 2D Hubbard model in presence of a parallel external magnetic field. Since a parallel field does not couple to the orbital motion of electrons, the Hamiltonian is given by