) Rosario, Argentina PACS 75.10.Jm -Quantized spin models, including quantum spin frustration Abstract. -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.

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.Itinerant magnetism has proved to be an elusive subject in condensed matter physics, since itinerant and localized aspects of electrons need to be taken into account on equal footing. The single-band Hubbard model, originally proposed to describe metallic ferromagnetism [1], has also been associated with antiferromagnetism of kinetic exchange origin close to half-filling. While virtual kinetic processes favor antiferromagnetism, it is a rule of thumb to link real kinetic processes with ferromagnetism [2]. However, there exist only few exact results ensuring the existence of itinerant ferromagnetism [3, 4]. Among them, the most renowned is Nagaoka's theorem [3], which assert that the saturated ferromagnetic state is the unique ground state when one hole is doped on the half-filled Hubbard model with infinite U Coulomb repulsion. Furthermore, a connectivity condition must be fulfilled for the validity of Nagaoka's theorem: the sign of the hopping amplitudes around the smallest closed loop of the lattice must be positive, otherwise the hole kinetic energy will be frustrated and the saturated ferromagnetic state will no longer be the ground state. Kinetic energy frustration is a quantum mechanical phenomenon without classical analog, easily understood in certain tight-binding models where an electron can not gain the full kinetic energy −z|t|, due to quantum interferences [5, 6]. This kind of frustration has been considerably less studied than the magnetic one, although recent works indicate that its effects may lead to rich physics, such as, robust superconductivity in strongly repulsive fermionic system [7] and spontaneous time-reversal symmetry breakings [8], among others [9, 10].In a seminal work, Haerter and Shastry [7] have found a 120 • antiferromagnetic Néel order as the ground state of the U = ∞ triangular lattice Hubbard model when the hole motion is frustrated (t > 0), uncovering a new mechanism for itinerant magnetism. In this Letter, we further characterize this kinetic antiferromagnetism and we describe its microscopic origin, analyzing generic kinetically frustrated electronic models for which, in the limit of infinite Coulomb repulsion and one hole doped away halffilling, the Nagaoka's theorem is not valid. In particular, we study the ground state of two Hubbard models: one ...