Numerical evidence of quantum melting of spin ice: quantum-classical crossover

TL;DR

This study uses quantum Monte Carlo simulations to explore the crossover from classical to quantum spin ice states in a pyrochlore XXZ model, revealing thermal and quantum phase transitions and their signatures.

Contribution

It provides the first unbiased numerical evidence of quantum melting in spin ice, identifying specific heat features and phase boundaries in a quantum-classical crossover.

Findings

Broad specific heat peak at T_CSI indicates crossover to classical Coulomb liquid.

Entropy decay signals transition to a quantum Coulomb liquid with photon and monopole contributions.

First-order transition from quantum Coulomb liquid to XY ferromagnet as J_perp varies.

Abstract

Unbiased quantum Monte-Carlo simulations are performed on the nearest-neighbor spin-$\frac{1}{2}$ pyrochlore XXZ model with an antiferromagnetic longitudinal and a weak ferromagnetic transverse exchange couplings, $J$ and $J_\perp$. The specific heat exhibits a broad peak at $T_{\mathrm{CSI}}\sim0.2J$ associated with a crossover to a classical Coulomb liquid regime showing a suppressed spin-ice monopole density, a broadened pinch-point singularity, and the Pauling entropy for $|J_\perp|\ll J$, as in classical spin ice. On further cooling, the entropy restarts decaying for $J_\perp>J_{\perp c}\sim-0.104J$, producing another broad specific heat peak for a crossover to a bosonic quantum Coulomb liquid, where the spin correlation contains both photon and quantum spin-ice monopole contributions. With negatively increasing $J_\perp$ across $J_{\perp c}$, a first-order thermal phase transition occurs from the quantum Coulomb liquid to an XY ferromagnet. Relevance to magnetic rare-earth pyrochlore oxides is discussed.