We consider the groundstate wavefunction of the quantum symmetric antifer-romagnetic XXZ chain with open and twisted boundary conditions at ∆ = − 1 2 , along with the groundstate wavefunction of the corresponding O(n) loop model at n = 1. Based on exact results for finite-size systems, sums involving the wavefunc-tion components, and in some cases the largest component itself, are conjectured t...

We study a spin S quantum Heisenberg model on the Fe lattice of the rare-earth oxypnictide superconductors. Using both large S and large N methods, we show that this model exhibits a sequence of two phase transitions: from a high-temperature symmetric phase to a narrow region of intermediate “nematic” phase, and then to a low-temperature spin ordered phase. Identifying phases by their broken sy...

Ba 2 Cu 3 O 4 Cl 2 consists of two types of copper atoms, Cu(A) and Cu(B). We study the corresponding Heisenberg model with three antiferromagnetic couplings, J AA , J BB and J AB. We find interesting frustration effects due to the coupling J AB .

We consider the SUq(N) invariant spin chain with diagonal and non-diagonal integrable boundary terms. The algebraic study of spin chains with different types of boundary terms is used to motivate a set of spectral equivalences between integrable chains with purely diagonal boundary terms and ones with an arbitrary non-diagonal term at one end. For each choice of diagonal boundary terms there is...

We examined a two-dimensional Heisenberg model with two kinds of exchange energies, Je and Jc. This model describes localized spins at vanadium ions in a layer of CaV4O9, for which a spin gap is found by a recent experiment. Comparing the high temperature expansion of the magnetic susceptibility to experimental data, we determined the exchange energies as Je ≃ 610 K and Jc ≃ 150 K. By the numer...

Quantum spin chains arising as the 1-dimensional quantum version of classical 2-dimensional statistical mechanical models exhibit the same Hilbert spaces as those arising by iterating correspondences in the sense of Connes. This will be explained and the highly speculative possibility of a continuum limit conformal field theory proposed.

By assumption the extensive part of the overlap coefficient S[ρ] is smooth and does not depend on finite size differences within the set of Bethe states that scale to the same densities ρ. The number of Bethe states for each set of distributions ρ is given by the extensive Yang-Yang entropy [1]: eY Y . We are then free to select a representative finite size Bethe state out of all states that sc...

Level statistics is discussed for XXZ spin chains with discrete symmetries for some values of the next-nearest-neighbour (NNN) coupling parameter. We show how the level statistics of the finite-size systems depends on the NNN coupling and the XXZ anisotropy, which should reflect competition among quantum chaos, integrability and finite-size effects. Here discrete symmetries play a central role ...

The hole-doped standard and extended t-J models on ladders with anisotropic Heisenberg interactions are studied computationally in the interval 0.0<l<1.0 (l50, Ising; l51, Heisenberg! at small J/t . It is shown that the approximately half-doped stripes recently discussed at l51 survive in the anisotropic case (l ,1.0), particularly in the ‘‘extended’’ model. Due to the absence of spin fluctuati...

We investigate the two-dimensional classical Heisenberg model with a nonlinear nearest-neighbor interaction V(s,s') = 2K[(1+s x s')/2]p. The analogous nonlinear interaction for the XY model was introduced by Domany, Schick, and Swendsen, who find that for large p the Kosterlitz-Thouless transition is preempted by a first-order transition. Here we show that, whereas the standard (p = 1) Heisenbe...