Unconventional spin frustration due to two competing ferromagnetic interactions of a spin-1/2 Ising-Heisenberg model on martini and martini-diced lattice

TL;DR

This paper exactly solves a quantum spin model on martini lattices, revealing complex phase behavior including ferromagnetic, disordered, and frustrated phases with quantum effects influencing magnetization and thermodynamics.

Contribution

It provides an exact analytical solution for the spin-1/2 Ising-Heisenberg model on martini lattices, highlighting novel quantum and frustration effects in its phase diagram.

Findings

Identification of two ferromagnetic phases with distinct magnetization properties

Discovery of a macroscopically degenerate disordered phase with residual entropy

Observation of quantum reduction of Heisenberg spin magnetization in a ferromagnetic phase

Abstract

The spin-1/2 Ising-Heisenberg model on martini and martini-diced lattice is exactly solved using a star-triangle transformation, which affords an exact mapping correspondence to an effective spin-1/2 Ising model on a triangular lattice. The ground-state phase diagram of both investigated quantum spin models display two spontaneously ordered ferromagnetic phases and one macroscopically degenerate disordered phase. In contrast to a classical ferromagnetic phase where the spontaneous magnetization of the Ising as well as Heisenberg spins acquire fully saturated values the spontaneous magnetization of the Heisenberg spins is subject to a quantum reduction to one-third of its saturated value within a quantum ferromagnetic phase. The spontaneous magnetization and logarithmic divergence of the specific heat as the most essential features of both ferromagnetic phases disappear whenever the investigated quantum spin model is driven to the highly degenerate disordered phase. The disordered phase with nonzero residual entropy originates either from a geometric spin frustration caused by antiferromagnetic interactions or more strikingly it may also alternatively arise from a competition of the ferromagnetic Ising and Heisenberg interactions of easy-axis and easy-plane type, respectively. All three available ground states coexist together at a single triple point, around which anomalous magnetic and thermodynamic behavior can be detected.