A. Fledderjohann

Two-spinon dynamic structure factor of the one-dimensional s=1/2 Heisenberg antiferromagnet

The exact expression derived by Bougourzi, Couture, and Kacir for the two-spinon contribution to the dynamic spin structure factor S(q,v) of the one-dimensional s51/2 Heisenberg antiferromagnet at T5 0i s evaluated for direct comparison with finite-chain transition rates (N<28) and an approximate analytical result previously inferred from finite-N data, sum rules, and Bethe ansatz calculations. The two-spinon excitations account for 72.89% of the total intensity in S(q,v). The singularity structure of the exact result is determined analytically and its spectral-weight distribution evaluated numerically over the entire range of the two-spinon continuum. The leading singularities of the frequency-dependent spin autocorrelation function, static spin structure factor, and q dependent susceptibility are determined via sum rules. The impact of the non-twospinon excitations on the integrated intensity, the susceptibility, the frequency moments, and the Euclidian time representation of S(q,v) is studied on the basis of finite-size data. @S0163-1829~97!00517-1#

SPIN DYNAMICS OF DIMERIZED HEISENBERG CHAINS

We study numerically the dimerized Heisenberg model with frustration appropriate for the quasi-1D spin-Peierls compound CuGeO3. We present evidence for a bound state in the dynamical structure factor for any finite dimerization δ and estimate the respective spectral weight. For the homogeneous case (δ = 0) we show that the spin-wave velocity vs is renormalized by the n.n.n. frustration term α as vs = π/2 J(1 − bα) with b ≈ 1.12.

Diagrammatics for SU(2) invariant matrix product states

We report on a systematic implementation of su(2) invariance for matrix product states (MPS) with concrete computations cast in a diagrammatic language. As an application we present a variational MPS study of a spin-1/2 quantum chain. For efficient computations, we make systematic use of the su(2) symmetry at all steps of the calculations: (i) the matrix space is set up as a direct sum of irreducible representations, (ii) the local matrices with state-valued entries are set up as superposition of su(2) singlet operators, (iii) products of operators are evaluated algebraically by making use of identities for 3j and 6j symbols. The remaining numerical computations like the diagonalization of the associated transfer matrix and the minimization of the energy expectation value are done in spaces free of symmetry degeneracies. The energy expectation value is a strict upper bound of the true ground-state energy and yields definite conclusions about the accuracy of density-matrix renormalization group results reported in the literature. Furthermore, we present explicit results with accuracy better than 10−4 for nearest- and next-nearest-neighbour spin correlators and for general dimer–dimer correlators in the thermodynamical limit of the spin- Heisenberg chain with frustration.

SOFT MODES, GAPS, AND MAGNETIZATION PLATEAUS IN ONE-DIMENSIONAL SPIN-1/2 ANTIFERROMAGNETIC HEISENBERG MODELS

We study the one-dimensional spin-1/2 model with nearest and next-to-nearest-neighbor couplings exposed to a homogeneous magnetic field

A numerical study of the formation of magnetisation plateaus in quasi one-dimensional spin-1/2 Heisenberg models

h_{3}

The 1D spin-\(\) AF-Heisenberg model in a staggered field

and a dimer field with period

Instability of isolated triplet excitations on the Shastry-Sutherland lattice

q

Magnon-magnon interactions in the spin-Peierls compound CuGeO3

and strength

Ferromagnetism in a hard-core boson model

\delta

The magnetization process in the 2-dimensional J 1 -J 2 model

. The latter generates a magnetization plateau at