加压SrCu2(BO3)2中退禁闭量子临界点消失
Deconfined quantum critical point lost in pressurized SrCu2(BO3)2
AI总结：
本研究通过高压热容测量首次实验证明加压SrCu2(BO3)2的PS-AF相变为一阶相变，否定了理论预测的退禁闭量子临界点，并首次在阻挫磁体中发现压力诱导的双临界点。
摘要：
在关联电子材料领域，共振自旋单态与反铁磁态之间的关系长期以来一直是一个引人入胜的课题，有助于理解诸如涌现于磁性阻挫材料、反铁磁体和高温超导体中的有趣宏观量子现象。SrCu2(BO3)2是一种著名的量子磁体，理论上预期它是用于阐明压力诱导的退禁闭量子临界点（DQCP）存在的候选关联电子材料，该临界点以连续量子相变为特征，发生在plaquette单态（PS）价键固体相与反铁磁（AF）相之间。然而，由于技术挑战，该转变的真实性质在实验上仍有待确认。在此，我们首次通过最先进的高压热容测量展示了实验结果，证明零场下加压SrCu2(BO3)2的PS-AF相变明显是一阶相变。我们的结果澄清了关于这一关键问题长达二十多年的争论，并与最近的量子纠缠理解很好地呼应，即理论上在代表性晶格模型中预测的DQCP实际上是一阶转变。有趣的是，我们还发现PS相和AF相的转变温度交汇于同一个压力-温度点，这标志着一个双临界点，正如在铁基超导体和重费米子化合物中观察到的那样，并构成了阻挫磁体中压力诱导双临界点的首次实验发现。我们的结果为理解关联电子材料在压力下不同自旋态之间的演化提供了新信息。
英文摘要：
In the field of correlated electron materials, the relation between the resonating spin singlet and antiferromagnetic states has long been an attractive topic for understanding of the interesting macroscopic quantum phenomena, such as the ones emerging from magnetic frustrated materials, antiferromagnets and high-temperature superconductors. SrCu2(BO3)2 is a well-known quantum magnet, and it is theoretically expected to be the candidate of correlated electron material for clarifying the existence of a pressure-induced deconfined quantum critical point (DQCP), featured by a continuous quantum phase transition, between the plaquette-singlet (PS) valence bond solid phase and the antiferromagnetic (AF) phase. However, the real nature of the transition is yet to be identified experimentally due to the technical challenge. Here we show the experimental results for the first time, through the state-of-the-art high-pressure heat capacity measurement, that the PS-AF phase transition of the pressurized SrCu2(BO3)2 at zero field is clearly a first-order one. Our result clarifies the more than two-decade long debates about this key issue, and resonates nicely with the recent quantum entanglement understanding that the theoretically predicted DQCPs in representative lattice models are actually a first-order transition. Intriguingly, we also find that the transition temperatures of the PS and AF phase meet at the same pressure-temperature point, which signifies a bi-critical point as those observed in Fe-based superconductor and heavy-fermion compound, and constitutes the first experimental discovery of the pressure-induced bi-critical point in frustrated magnets. Our results provide fresh information for understanding the evolution among different spin states of correlated electron materials under pressure.
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