Numerical Evidence of Quantum Melting of Spin Ice: Quantum-to-Classical Crossover

Unbiased quantum Monte Carlo simulations are performed on the nearest-neighbor spin-1/2 pyrochlore XXZ model with an antiferromagnetic longitudinal and the weak ferromagnetic transverse exchange couplings, J and J_{⊥}. The specific heat exhibits a broad peak at T_{CSI}~0.2J 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_{⊥}|≪J, as in classical spin ice. On further cooling, the entropy restarts decaying for J_{⊥}>J_{⊥c}∼-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_{⊥} across J_{⊥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.