Magnetic phase transitions and unusual antiferromagnetic states in the Hubbard model

Ground state magnetic phase diagrams of the square and simple cubic lattices are investigated for the narrow band Hubbard model within the slave-boson approach by Kotliar and Ruckenstein. The transitions between saturated (half-metallic) and non-saturated ferromagnetic phases as well as similar transition in antiferromagnetic (AFM) state are considered in the three-dimensional case. Two types of saturated antiferromagnetic state with different concentration dependences of sublattice magnetization are found in the two-dimensional case in the vicinity of half-filling: the state with a gap between AFM subbands and AFM state with large electron mass. The latter state is hidden by the phase separation in the finite-U case.Comment: Invited Report on the Moscow International Symposium on Magnetism MISM-2017, 7 pages, J. Magn. Magn. Mater., in pres

Magnetic States, Correlation Effects and Metal-Insulator Transition in FCC Lattice

The ground-state magnetic phase diagram (including collinear and spiral states) of the single-band Hubbard model for the face-centered cubic lattice and related metal-insulator transition (MIT) are investigated within the slave-boson approach by Kotliar and Ruckenstein. The correlation induced electronic spectrum narrowing and a comparison with a generalized Hartree-Fock approximation allow one to estimate the strength of correlation effects. This, as well as the MIT scenario, depends dramatically on the ratio of the next-nearest and nearest electron hopping integrals $t'/t$. In contrast with metallic state, possessing strong band narrowing, insulator one is only weakly correlated. The magnetic (Slater) scenario of MIT is found to be superior over the Mott one. Unlike simple and body-centered cubic lattices, MIT is the first order transition for most $t'/t$. The insulator state is type-II or type-III antiferromagnet, and the metallic state is spin-spiral, collinear antiferromagnet or paramagnet depending on $t'/t$. The picture of magnetic ordering is compared with that in the standard localized-electron (Heisenberg) model.Comment: 10 pages, final version, Journal of Physics: Condensed Matte

Correlation Effects and Non-Collinear Magnetism in the Doped Hubbard Model

The ground--state magnetic phase diagram is investigated for the two-- and three--dimensional $t$--$t'$ Hubbard model. We take into account commensurate ferro--, antiferromagnetic, and incommensurate (spiral) magnetic phases, as well as phase separation into magnetic phases of different types, which was often missed in previous investigations. We trace the influence of correlation effects on the stability of both spiral and collinear magnetic order by comparing the results of employing both the generalized non-correlated mean--field (Hartree--Fock) approximation and generalized slave boson approach by Kotliar and Ruckenstein with correlation effects included. We found that the spiral states and especially ferromagnetism are generally strongly suppressed up to non-realistic large Hubbard $U$, if the correlation effects are taken into account. The electronic phase separation plays an important role in the formation of magnetic states and corresponding regions are wide, especially in the vicinity of half--filling. The details of magnetic ordering for different cubic lattices are discussed.Comment: Invited Report on the Moscow International Symposium on Magnetism MISM-2014, 6 pages, final versio

Incommensurate magnetic order and phase separation in the two-dimensional Hubbard model with nearest and next-nearest neighbor hopping

We consider the ground state magnetic phase diagram of the two-dimensional Hubbard model with nearest and next-nearest neighbor hopping in terms of electronic density and interaction. We treat commensurate ferro- and antiferromagnetic, as well as incommensurate (spiral) magnetic phases. The first-order magnetic transitions with changing chemical potential, resulting in a phase separation (PS) in terms of density, are found between ferromagnetic, antiferromagnetic and spiral magnetic phases. We argue that the account of PS has a dramatic influence on the phase diagram in the vicinity of half-filling. The results imply possible interpretation of the unusual behavior of magnetic properties of one-layer cuprates in terms of PS between collinear and spiral magnetic phases. The relation of the results obtained to the magnetic properties of ruthenates is also discussed.Comment: 19 pages, 7 figure

Effect of electron correlations on the formation of spiral magnetic states in the two-dimensional t-t′ Hubbard model

The formation of the spiral magnetic order in the ground state of the Hubbard model for a square lattice has been studied within the slave-boson method in a wide range of the parameters of the carrier density and Coulomb interaction. Next-nearest-neighbor electron hopping, as well as the magnetic phase separation, has been taken into account. It has been shown that the electron correlations lead to the suppression of the ferromagnetic and spiral phases in comparison with the Hartree-Fock approximation. The application of the results to the explanation of the experimental data, in particular, to the description of the magnetic state of the high-temperature superconductors of the cuprate-based systems has been discussed. © 2013 Pleiades Publishing, Inc