Papers by Keyword: Hubbard Model

Abstract: Formation of a canted spiral magnetic order is studied in the framework of a mean-field approximation of the Hubbard model. It is revealed that this magnetic state can be stabilized under certain conditions in layered systemswith a relatively small interplane electron hopping. Example of an experimentally observed magnetic structure ofLa$_{2-x}$Sr$_x$CuO$_4$ is considered. It is shown that the canting magnetic order can be described in terms of a simplenon-relativistic band magnetism.

Abstract: A monolayer of carbon is called graphene. It exhibits unusual properties in the Hall effect and in the cyclotron resonance. It is found that it exhibits fractional charge in the Hall effect. The interactions amongst electrons almost become constant at low temperatures. Hence, the Kohn's theorem, which shows that the interactions do not play much role in determining the cyclotron resonance, becomes operative at low temperatures. The experiments on graphene do not depend on the wave vector dependence of the frequency. Hence whether the dispersion depends on k2 or on k does not matter. The Hubbard model has been very successful in explaining the ground state of several electron systems. We consider a triangle with three vortices. Each vortex can be occupied by two electrons. By using the spin in a particular way, we can obtain new features in the Hubbard model. There is a doubling in the Peierls-Luttinger phase factor and eigen values acquire higher multiplicities than are known for the usual treatment of spin. The flux is distributed on the area of the triangle. The graphene consists of hexagons of carbon atoms but the Hall effect shows that there are defects on which electrons form clusters so that there is spin wave type behaviour. A cluster of electrons shows spin-waves leading to "spin deviation" of several per cent.

Abstract: We investigate a two–dimensional single-band Hubbard model with a nearest–neighbor hopping. We treat a commensurate collinear order as well as incommensurate spiral magnetic phases at a finite temperature using a Hubbard–Stratonovich transformation with a two–field representation and solve this problem in a static approximation. We argue that temperature dramatically influence the collinear and spiral magnetic phases, phase separation in the vicinity of half–filling. The results imply a possible interpretation of unusual behavior of magnetic properties of single–layer cuprates.

Abstract: We consider the magnetic phase diagram of the two-dimensional Hubbard model ona square lattice.We take into account both spiral and collinear incommensurate magnetic states.The possibility of phase separation of spiral magnetic phases is taken into consideration as well.Our study shows that all the listed phases appear to be the ground state at certain parametersof the model. Relation of the obtained results to real materials, e.g. Cu-based high-temperaturesuperconductors, is discussed.

Abstract: The spin-spiral (SS) type of magnetization is studied with the Hubbard model. Consideration of noncollinearity of the magnetic moments results in a phase diagram which consists of regions of the SS and paramagnetic states depending on the number of electrons nel and the parameter U/t (U is the Hubbard repulsion, and t is an overlap integral). A possibility of stabilization of the SS state with three nonzero components of magnetic moment is considered.