RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351-0198, Japan.
Department of Applied Physics, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan.
Phys Rev Lett. 2015 Aug 14;115(7):077202. doi: 10.1103/PhysRevLett.115.077202.
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.
对具有反铁磁纵向和弱铁磁横向交换耦合 J 和 J_{⊥}的最近邻自旋 1/2 反铁磁体 XXZ 模型进行了无偏量子蒙特卡罗模拟。比热在 T_{CSI}~0.2J 处显示出一个宽峰，与经典库仑液体区的交叉有关，表现出抑制的自旋冰单极密度、展宽的尖点奇点和 Pauling 熵，对于 |J_{⊥}|≪J，就像在经典自旋冰中一样。进一步冷却时，对于 J_{⊥}>J_{⊥c}∼-0.104J，熵重新开始衰减，发生到玻色量子库仑液体的交叉，其中自旋相关包含光子和量子自旋冰单极子的贡献。随着 J_{⊥}在 J_{⊥c}处负向增加，发生从量子库仑液体到 XY 铁磁体的一级热相变。讨论了与磁稀土反铁磁体氧化物的相关性。
