标题： 在自旋超固体候选物Na$_2$BaCo(PO$_4$)$_2$中的巨大磁热效应 显示英文标题

标题： Giant Magnetocaloric Effect in Spin Supersolid Candidate Na$_2$BaCo(PO$_4$)$_2$

摘要： 超固体是一种奇异的量子物质状态，其中粒子形成不可压缩的固体结构，同时表现出零粘度的超流性[1]，是基础研究中长期追求的目标之一[2, 3]。 尽管最初关于$^4$氦超固体的报告被证明是一个人工现象[4]，但这种引人入胜的量子物质激发了对超冷量子气体的热烈研究[5-8]。 然而，在凝聚态物质中实现超固体性仍然难以捉摸。 在这里，我们在最近合成的三角晶格反铁磁体 Na$_2$BaCo(PO$_4$)$_2$[9] 中发现了超固体的量子磁性类似物——自旋超固体。 值得注意的是，在去磁冷却过程中观察到了与自旋超固体性相关的巨大磁卡效应，表现为两个显著的山谷状区域，最低温度达到了100 mK以下。 不仅实验确定了一系列临界场，而且去磁冷却曲线与易于轴的海森堡模型理论模拟结果也显示出很好的一致性。 中子衍射也成功地通过揭示三个亚晶格自旋固态序和层间非公度性的共存，从而确定了所提出的自旋超固体相，这表明了自旋超流性。 因此，我们的结果表明在受挫的量子磁体中自旋超固体相具有强烈的熵效应，并为亚开尔文制冷的应用开辟了一条可行且有前景的途径，尤其是在持续关注氦气短缺问题的背景下[10, 11]。 显示英文摘要

摘要： Supersolid, an exotic quantum state of matter that consists of particles forming an incompressible solid structure while simultaneously showing superfluidity of zero viscosity [1], is one of the long-standing pursuits in fundamental research [2, 3]. Although the initial report of $^4$He supersolid turned out to be an artifact [4], this intriguing quantum matter has inspired enthusiastic investigations into ultracold quantum gases [5-8]. Nevertheless, the realization of supersolidity in condensed matter remains elusive. Here we find evidence for a quantum magnetic analogue of supersolid -- the spin supersolid -- in the recently synthesized triangular-lattice antiferromagnet Na$_2$BaCo(PO$_4$)$_2$ [9]. Notably, a giant magnetocaloric effect related to the spin supersolidity is observed in the demagnetization cooling process, manifesting itself as two prominent valley-like regimes, with the lowest temperature attaining below 100 mK. Not only is there an experimentally determined series of critical fields but the demagnetization cooling profile also shows excellent agreement with the theoretical simulations with an easy-axis Heisenberg model. Neutron diffractions also successfully locate the proposed spin supersolid phases by revealing the coexistence of three-sublattice spin solid order and interlayer incommensurability indicative of the spin superfluidity. Thus, our results indicate a strong entropic effect of the spin supersolid phase in a frustrated quantum magnet and open up a viable and promising avenue for applications in sub-Kelvin refrigeration, especially in the context of persistent concerns about helium shortages [10, 11].