Roles of easy-plane and easy-axis XXZ anisotropy and bond alternation in a frustrated ferromagnetic spin-12 chain

Abstract

The spin-1/2 Heisenberg chain with a ferromagnetic first-neighbor exchange coupling J1 and an antiferromagnetic second-neighbor J2 has a Haldane dimer ground state with an extremely small spin gap. Thus the ground state is readily altered by perturbations. Here, we investigate the effects of XXZ exchange magnetic anisotropy of both the easy-axis and easy-plane types and an alternation in J1 on the ground state, the spin gap, and magnetic properties of the frustrated ferromagnetic spin-1/2 chain. It is found that there are two distinct dimerized spin-gap phases, in one of which the spin gap and the magnetic susceptibility are extremely small around the SU(2) symmetric case and in the other they are moderately large far away from the SU(2) symmetric case. A small alternation in the amplitude of J1 rapidly shortens the pitch of spin correlations towards the four-spin periodicity, as in the limit of J1/J2→0. These effects are not sufficient to quantitatively explain overall experimentally observed magnetic properties in the quasi-one-dimensional spin-gapped magnetoelectric cuprate Rb2Cu2Mo3O12 that exhibits ferroelectricity stabilized by a magnetic field. Our results are also relevant to Cs2Cu2Mo3O12, where the ferromagnetic intrachain and antiferromagnetic interchain order has recently been found, in a single chain level. We also reveal the nature of symmetry-protected topological phase transitions in the model by mapping onto effective spin-1 chain models.