Anisotropic Spin Ice on a Breathing Pyrochlore Lattice

TL;DR

This paper investigates how anisotropic spin couplings on a breathing pyrochlore lattice influence the ground state, entropy, and phase behavior of spin ice systems, revealing a rich phase diagram and experimental signatures.

Contribution

It introduces a tunable anisotropic spin coupling to the Ising spin ice model on a breathing pyrochlore lattice, uncovering new phases and reduced degeneracy.

Findings

Significant changes in ground state structure and residual entropy.

Identification of crossover and phase transition behaviors.

Good agreement between numerical simulations and analytical predictions.

Abstract

Spin ice systems represent a prime example of constrained spin systems and exhibit rich low-energy physics. In this study, we explore how introducing a tunable anisotropic spin coupling to the conventional Ising spin ice Hamiltonian on the breathing pyrochlore lattice affects the ground state properties of the system. Significant changes are observed in the ground state structure, reflected in the spin structure factor and in a reduction of residual entropy at low temperatures. We theoretically uncover a rich phase diagram by varying the anisotropy and demonstrate how this modification reduces the ground state degeneracy across different phases. Numerical simulations reveal that, at sufficiently low temperatures, the system either undergoes a crossover into a constrained spin ice manifold, characterized by an entropy density that drops below the Pauling entropy of conventional spin ice, or a phase transition into a symmetry-broken state, depending on the perturbations. Additionally, we compute the spin structure factors for the anisotropic model and compare these results to analytical predictions from a large-$N$ expansion, finding good agreement. This work develops the understanding of spin ice in anisotropic limits, which may be experimentally realized by strain, providing, among others, key signatures in entropy and specific heat.