Ferromagnetism-induced phase separation in a two-dimensional spin fluid.
Ferromagnetism-induced phase separation in a two-dimensional spin fluid.
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二维自旋流体中铁磁性引起的相分离。
DOI：
10.1063/1.5064590
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发表时间：
2018
期刊：
影响因子：
--
通讯作者：
O. Dauchot
中科院分区：
文献类型：
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作者：
Mathias Casiulis;M. Tarzia;L. Cugliandolo;O. Dauchot
我们研究了在带有铁磁对齐自旋的排斥粒子系统（即所谓的“自旋流体”）中观察到的液气相分离。有限尺寸系统的微正则系综数值模拟表明，在足够低的能量和足够大的密度下，磁化开始并诱导无序气体和铁磁致密相之间的液气相分离。骤冷进入共存区后的动力学表明，与液-气相分离相关的有序参数遵循具有不寻常指数的代数定律，因为它被迫与磁化强度的增长同步：这表明对于有限尺寸系统，磁化强度沿着居里线（也是气体侧旋节线）开始，并且共存区在三临界点结束。该图在平均场水平上通过不同的近似方案得到了证实，即贝特晶格分辨率和维里展开式，并通过引入自洽类韦斯分子场进行补充。然而，详细的有限尺寸尺度分析表明，在二维中，铁磁相逃脱了 Berezinskii-Kosterlitz-Thouless 场景，并且长程有序不会因拓扑缺陷的解绑而被破坏。因此，居里线成为热力学极限内的磁交叉。最后，在平均场半解析低密度方法中表征了磁相互作用范围和相互作用软度的影响。
We study the liquid-gas phase separation observed in a system of repulsive particles dressed with ferromagnetically aligning spins, a so-called "spin fluid." Microcanonical ensemble numerical simulations of finite-size systems reveal that magnetization sets in and induces a liquid-gas phase separation between a disordered gas and a ferromagnetic dense phase at low enough energies and large enough densities. The dynamics after a quench into the coexistence region show that the order parameter associated with the liquid-vapor phase separation follows an algebraic law with an unusual exponent, as it is forced to synchronize with the growth of the magnetization: this suggests that for finite size systems the magnetization sets in along a Curie line, which is also the gas-side spinodal line, and that the coexistence region ends at a tricritical point. This picture is confirmed at the mean-field level with different approximation schemes, namely, a Bethe lattice resolution and a virial expansion complemented by the introduction of a self-consistent Weiss-like molecular field. However, a detailed finite-size scaling analysis shows that in two dimensions the ferromagnetic phase escapes the Berezinskii-Kosterlitz-Thouless scenario and that the long-range order is not destroyed by the unbinding of topological defects. The Curie line thus becomes a magnetic crossover in the thermodynamic limit. Finally, the effects of the magnetic interaction range and those of the interaction softness are characterized within a mean-field semianalytical low-density approach.