Magnetic order on a frustrated spin-1/2 Heisenberg antiferromagnet on the Union Jack lattice

TL;DR

This study uses the coupled cluster method to analyze the zero-temperature phase diagram of a frustrated spin-1/2 Heisenberg antiferromagnet on the Union Jack lattice, revealing phase transitions and magnetic orderings.

Contribution

It provides the first detailed CCM-based analysis of the Union Jack model, identifying phase transitions and magnetic phases in a frustrated 2D quantum magnet.

Findings

Evidence for a second-order transition between Néel and canted ferrimagnetic phases at κ ≈ 0.66

The canted phase becomes unstable at large κ, suggesting a transition to a semi-stripe-ordered phase around κ ≈ 125

Magnetization remains nonzero across the Néel-canted transition, indicating complex transition nature.

Abstract

We use the coupled cluster method (CCM) to study the zero-temperature phase diagram of a 2D frustrated spin-half antiferromagnet, the so-called Union Jack model. It is defined on a square lattice such that all nearest-neighbor bonds are present with a strength $J_{1} > 0$, but only half the next-nearest-neighbor bonds are present with a strength $J_{2} \equiv \kappa J_{1} > 0$. The bonds are arranged such that on the $2 \times 2$ unit cell they form the pattern of the Union Jack flag. We find strong evidence for a first phase transition between a N\'{e}el phase and a canted ferrimagnetic phase at a critical coupling $\kappa_{c_{1}} = 0.66 \pm 0.02$. At the transition the energy and its first derivative seem continuous, thus providing a typical scenario of a second-order transition, although a weakly first-order transition cannot be excluded. By contrast, the average on-site magnetization $M$ approaches a nonzero value $M_{c_{1}}=0.195 \pm 0.005$ on both sides of the transition, which is more typical of a first-order transition. The slope $dM/d\kappa$ also appears to be continuous, or very nearly so, at the critical point $\kappa_{c_{1}}$. We find strong evidence that the canted phase becomes unstable at large values of $\kappa$, and hence we have also used the CCM with a model collinear semi-stripe-ordered ferrimagnetic state in which alternating rows (and columns) are ferromagnetically and antiferromagnetically ordered. We find tentative evidence, based on the relative energies of the two states, for a second (first-order) phase transition between the canted and semi-stripe-ordered states at a large value of the coupling parameter around $\kappa_{c_{2}} \approx 125 \pm 5$. This prediction, however, is based on an extrapolation of the CCM results for the canted state into regimes where the CCM equations at any level of approximation beyond the lowest have no solutions.