An L\ifmmode\times\else\texttimes\fi{}\ensuremath{\infty} system of an odd number of coupled Heisenberg spin chains is studied using a degenerate perturbation theory, where L is the number of coupled chains. An effective chain Hamiltonian is derived explicitly in terms of two spin-1/2 degrees of freedom of a closed chain of L sites, valid in the regime where the interchain coupling is stronger than the intrachain coupling. The spin gap has been calculated numerically using the effective Hamiltonian for L=3, 5, 7, and 9 for a finite chain up to ten sites. It is suggested that the ground state of the effective Hamiltonian is correlated, by examining various variational trial states for the effective-spin chain Hamiltonian.
Using the bosonization procedure proposed for the spin-1/2 Heisenberg chain in Ref. 8 we obtain the effective Hamiltonian for the system of two weakly coupled Heisenberg chains with the XXZ anisotropy as the sum of two sine-Gordon Hamiltonians. It appears that this simple system reveals a rather rich variety of properties in different regions of anisotropy. We calculate the spin-spin correlation functions; two-spin correlation functions decay exponentially but there are regions where the four-spin correlation functions decay as a power law. In particular, if both the inter- and intrachain exchange interactions are isotropic and the interchain exchange is ferromagnetic correlations of the chirality field χ= S 1× S 2 (the indexes 1 and 2 numerate the chains) are enhanced and their correlation functions follow a power law. In this case coupling between pairs of chains can lead to chiral ordering in three dimensions.
The effect of antiferromagnetic interchain coupling in alternating spin-(1,1/2) chains is studied by means of spin-wave theory and density-matrix renormalization group (DMRG). In particular, two limiting cases are investigated, the two-leg ladder and its two-dimensional (2D) generalization. Results of the ground-state properties like energy, spin gap, magnetizations, and correlation functions are reported for the whole range of the interchain coupling ${J}_{\ensuremath{\perp}}.$ For the 2D case the spin-wave results predict a smooth dimensional crossover from 1D to 2D keeping the ground state always ordered. For the ladder system, the DMRG results show that any ${J}_{\ensuremath{\perp}}>0$ drives the system to a gapped ground state. Furthermore the behavior of the correlation functions closely resemble the uniform spin-1/2 ladder. For ${J}_{\ensuremath{\perp}}$ lower than 0.3, however, the gap behaves quadratically as $\ensuremath{\Delta}\ensuremath{\sim}{0.6J}_{\ensuremath{\perp}}^{2}.$ Finally, it is argued that the behavior of the spin gap for an arbitrary number of mixed coupled spin chains is analogous to that of the uniform spin-1/2 chains.
Motivated by recent work on Heisenberg antiferromagnetic spin systems on various lattices made up of triangles, we examine the low-energy properties of a chain of antiferromagnetically coupled triangles of half-odd-integer spins. We derive the low-energy effective Hamiltonian to second order in the ratio of the coupling
J2 between triangles to the coupling
J1 within each triangle. The effective Hamiltonian contains four states for each triangle which are given by the products of spin-1/2 states with the states of a pseudospin 1/2. We compare the results obtained by exact diagonalization of the effective Hamiltonian with those obtained for the full Hamiltonian using exact diagonalization and the density-matrix renormalization group method. It is found that the effective Hamiltonian gives an accurate value for the ground-state energy only if the ratio
J2/J1 is less than about 0.2 and that too for the spin-1/2 case with linear topology. The chain of spin-1/2 triangles shows interesting properties like spontaneous dimerization and several singlet and triplet low-energy (possibly gapless) states which lie close to the ground state. We have also studied the spin-3/2 case and find the low-energy effective Hamiltonians (LEH's) to be less accurate there than in the spin-1/2 case. Finally, we have studied nonlinear topologies where the LEH results deviate further from the exact results.
A two-dimensional array of weakly coupled antiferromagnetic spin chains is investigated using the Schwinger-boson technique. This method is known to give a good description of integer spins already at the mean-field level. We find that a finite coupling is necessary to introduce long-range antiferromagnetic order, consistently with the presence of a gap for a single chain. The energy gap and spin-spin correlations are calculated. These calculations correspond to the behavior of a nonlinear \ensuremath{\sigma} model without the one-dimensional topological term.
The zero-temperature phase diagram of the ${J}_{1}\text{\ensuremath{-}}{J}_{2}\phantom{\rule{4pt}{0ex}}\mathrm{SU}(N)$ antiferromagnetic Heisenberg spin chain is investigated by means of complementary field theory and numerical approaches for general
N. A fully gapped
SU(N) valence bond solid made of
N sites is formed above a critical value of
J2/J1 for all
N. We find that the extension of this
N-merized phase for larger values of
J2 strongly depends on the parity of
N. For even
N, the phase smoothly interpolates to the large
J2 regime where the model can be viewed as a zigzag
SU(N) two-leg spin ladder. The phase exhibits both a
N-merized ground state and incommensurate spin-spin correlations. In stark contrast to the even case, we show that the
N-merized phase with odd
N has only a finite extent with no incommensuration. A gapless phase in the
SU(N)1 universality class is stabilized for larger
J2 that stems from the existence of a massless renormalization group flow from
SU(N)2 to
SU(N)1 conformal field theories when
N is odd.