Correlated Partial Disorder in a Weakly Frustrated Quantum Antiferromagnet.

Partial disorder-the microscopic coexistence of long-range magnetic order and disorder-is a rare phenomenon that has been experimentally and theoretically reported in some Ising- or easy plane-spin systems, driven by entropic effects at finite temperatures. Here, we present an analytical and numerical analysis of the S=1/2 Heisenberg antiferromagnet on the sqrt[3]×sqrt[3]-distorted triangular lattice, which shows that its quantum ground state has partial disorder in the weakly frustrated regime. This state has a 180° Néel ordered honeycomb subsystem coexisting with disordered spins at the hexagon center sites. These central spins are ferromagnetically aligned at short distances, as a consequence of a Casimir-like effect originated by the zero-point quantum fluctuations of the honeycomb lattice.

Classical antiferromagnetism in kinetically frustrated electronic models.

We study, by means of the density matrix renormalization group, the infinite U Hubbard model--with one hole doped away from half filling--in triangular and square lattices with frustrated hoppings, which invalidate Nagaoka's theorem. 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 occur in two different ways: by a nontrivial spin Berry phase acquired by the hole, or by the effective vanishing of the hopping amplitude along the frustrating loops.

Broken discrete and continuous symmetries in two-dimensional spiral antiferromagnets.

We study the occurrence of symmetry breaking, at zero and finite temperatures, in the J1-J3 antiferromagnetic Heisenberg model on the square lattice using Schwinger boson mean field theory. For spin-1/2 the ground state always breaks the SU(2) symmetry with a continuous quasi-critical transition at J3/J1 ∼ 0.38, from Néel to spiral long range order, although local spin fluctuation considerations suggest an intermediate disordered regime around 0.35 ≲ J3/J1 ≲ 0.5, in qualitative agreement with recent numerical results. At low temperatures we find a Z2 broken symmetry region with short range spiral order characterized by an Ising-like nematic order parameter that compares qualitatively well with classical Monte Carlo results. At intermediate temperatures the phase diagram shows regions with collinear short range orders: for J3/J1 < 1 Néel (π,π) correlations and for J3/J1 > 1 a novel phase consisting of four decoupled third neighbour sublattices with Néel (π,π) correlations in each one. We conclude that the effect of quantum and thermal fluctuations is to favour collinear correlations even in the strongly frustrated regime.