模型二维磁体中的连续和非连续量子相变
Continuous and Discontinuous Quantum Phase Transitions in a Model Two-Dimensional Magnet
摘要：
沙斯特里-萨瑟兰模型由一组自旋为1/2的二聚体组成，位于二维正方形晶格上，简单易解，但它却抓住了凝聚态物理的一个中心主题，因为它不稳定地处于孤立的、有间隙的激发态和集体的、有序的基态之间的量子边缘。我们压缩模型Shastry-Sutherland材料，SrCu 2（BO 3）2，在低温下的金刚石对顶砧单元中连续调整耦合能量和诱导状态的变化。高分辨率的x射线测量利用了非常强的自旋-晶格耦合来监测磁行为和结构不连续性的存在或不存在。在低压自旋单重态制度中，磁性的发生导致晶格随着温度的降低而膨胀，这允许确定压力相关的能隙和几乎各向同性的自旋-晶格耦合能。随着压力的增加，单重态-三重态能隙被连续抑制，在2GPa时完全消失。这种连续的量子相变之后是在更高压力下的结构畸变。
英文摘要：
The Shastry-Sutherland model, which consists of a set of spin 1/2 dimers on a 2-dimensional square lattice, is simple and soluble, but captures a central theme of condensed matter physics by sitting precariously on the quantum edge between isolated, gapped excitations and collective, ordered ground states. We compress the model Shastry-Sutherland material, SrCu2(BO3)2, in a diamond anvil cell at cryogenic temperatures to continuously tune the coupling energies and induce changes in state. High-resolution x-ray measurements exploit what emerges as a remarkably strong spin-lattice coupling to both monitor the magnetic behavior and the absence or presence of structural discontinuities. In the low-pressure spin-singlet regime, the onset of magnetism results in an expansion of the lattice with decreasing temperature, which permits a determination of the pressure dependent energy gap and the almost isotropic spin-lattice coupling energies. The singlet-triplet gap energy is suppressed continuously with increasing pressure, vanishing completely by 2 GPa. This continuous quantum phase transition is followed by a structural distortion at higher pressure.
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