The antiferromagnetic spin 1/2 chain with competing dimers and plaquettes: Numerical versus exact results

TL;DR

This paper investigates the ground state and excitations of a one-dimensional antiferromagnetic spin-1/2 chain with alternating dimers and plaquettes, revealing a gapped system with Haldane-like behavior under certain conditions.

Contribution

It provides a comprehensive analysis combining exact, numerical, and perturbational methods for a dimer-plaquette chain, highlighting its gapped nature and effective Haldane behavior.

Findings

The system is gapped for all parameter values.

Dominant dimer bonds induce Haldane-like behavior.

The model relates to the 1/5-depleted square-lattice Heisenberg system.

Abstract

We examine the ground state and the excitations of an one-dimensional Heisenberg spin 1/2 antiferromagnet with alternating dimers and four-spin plaquettes (dimer-plaquette chain). The properties of the system depend on the competing dimer and plaquette bonds. Several exact, exact-numerical and perturbational results are presented. We find that the system is gapped for all parameter values. For the latter one we find an effective S=1 Haldane like behaviour in the limit of dominating dimer bonds. The model considered is related to the recently found 1/5-depleted square-lattice Heisenberg system $CaV_4O_9$.