Incommensurate Magnetic Structures in Two-Dimensional Itinerant Systems With and Without Perpendicular Fields

Abstract

Various two-dimensional (2D) systems described by the Hubbard model (HM) are investigated theoretically to understand how the nature of the long-range order states is modified under several perturbations: doping or applied fields. It is known that at the exact half-filling the 2D HM with nearest neighbor hoppings on square and triangular lattices exhibits antiferromagnetic and 120° spin structures respectively. We study the magnetic configurations when the filling factor deviates from half-fillings. We find that in the square lattice case an incommensurate spin density wave (SDW) with the stripe domain or soliton lattice magnetic structure is stabilized. This is a collinear spin arrangement and single wave-vector state, thus a quite one-dimensional like spin structure. In the triangular case a vortex lattice structure characterized by triple wave vectors and non-collinear type is stable near half-fillings. These defect bearing structures with the soliton and vortex efficiently accommodate excess particles or holes, and have a mid-gap band inside the main gap. We also study an anisotropic 2D HM under perpendicular fields with quarter-filling to investigate the field-induced SDW states, intending to explain the phenomena observed in organic conductors: (TMTSF)₂X.