Anharmonic effects in magnetoelastic chains

We describe a new mechanism leading to the formation of rational magnetization plateau phases, which is mainly due to the anharmonic spin-phonon coupling. This anharmonicity produces plateaux in the magnetization curve at unexpected values of the magnetization without explicit magnetic frustration in the Hamiltonian and without an explicit breaking of the translational symmetry. These plateau phases are accompanied by magneto-elastic deformations which are not present in the harmonic case.Comment: 5 pages, 3 figure

Classical Antiferromagnetism in Kinetically Frustrated Electronic Models

We study the infinite U Hubbard model with one hole doped away half-filling, in triangular and square lattices with frustrated hoppings that invalidate Nagaoka's theorem, by means of the density matrix renormalization group. We find that these kinetically frustrated models have antiferromagnetic ground states with classical local magnetization in the thermodynamic limit. We identify the mechanism of this kinetic antiferromagnetism with the release of the kinetic energy frustration as the hole moves in the established antiferromagnetic background. This release can occurs in two different ways: by a non-trivial spin-Berry phase acquired by the hole or by the effective vanishing of the hopping amplitude along the frustrating loops.Comment: 12 pages and 4 figures, with Supplementary Material. To be published in Phys. Rev. Let

Spiral Magnets as Gapless Mott Insulators

In the large $U$ limit, the ground state of the half-filled, nearest-neighbor Hubbard model on the triangular lattice is the three-sublattice antiferromagnet. In sharp contrast with the square-lattice case, where transverse spin-waves and charge excitations remain decoupled to all orders in $t/U$, it is shown that beyond leading order in $t/U$ the three Goldstone modes on the triangular lattice are a linear combination of spin and charge. This leads to non-vanishing conductivity at any finite frequency, even though the magnet remains insulating at zero frequency. More generally, non-collinear spin order should lead to such gapless insulating behavior.Comment: 10 pages, REVTEX 3.0, 3 uuencoded postscript figures, CRPS-94-0

A test of the bosonic spinon theory for the triangular antiferromagnet spectrum

We compute the dynamical structure factor of the spin-1/2 triangular Heisenberg model using the mean field Schwinger boson theory. We find that a reconstructed dispersion, resulting from a non trivial redistribution of the spectral weight, agrees quite well with the spin excitation spectrum recently found with series expansions. In particular, we recover the strong renormalization with respect to linear spin wave theory along with the appearance of roton-like minima. Furthermore, near the roton-like minima the contribution of the two spinon continuum to the static structure factor is about 40 % of the total weight. By computing the density-density dynamical structure factor, we identify an unphysical weak signal of the spin excitation spectrum with the relaxation of the local constraint of the Schwinger bosons at the mean field level. Based on the accurate description obtained for the static and dynamic ground state properties, we argue that the bosonic spinon theory should be considered seriously as a valid alternative to interpret the physics of the triangular Heisenberg model.Comment: 6 pages, 5 figures, extended version including: a table with ground state energy and magnetization; and the density-density dynamical structure factor. Accepted for publication in Europhysics Letter