Magnetic-Field-Induced Quantum Phase Transitions in a van der Waals Magnet
Magnetic-Field-Induced Quantum Phase Transitions in a van der Waals Magnet
复制标题
范德华磁体中磁场诱发的量子相变
DOI：
10.1103/physrevx.10.011075
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发表时间：
2020-03-31
影响因子：
12.5
通讯作者：
Zhao, Liuyan
中科院分区：
文献类型：
--
作者：
Li, Siwen;Ye, Zhipeng;Zhao, Liuyan
探索新的参数体系来实现和控制物质的新相已成为现代凝聚态物理研究的主题。最近在几乎独立的单层原子晶体中发现了二维磁性，这已经导致了许多在块状原子晶体中没有的新磁性现象的观察。磁性和晶体结构之间复杂的相互作用为探索这种新的二维参数体系中的量子相变提供了充分的机会。在这里，利用声子和磁振子的磁场和温度相关的圆极化拉曼光谱，我们绘制出了CrI3的相图，已知其在2D薄膜中是层状AFM，在3D体中是FM。然而，我们在三维CrI3块体晶体中揭示了层状AFM和FM的新型混合状态，其中层状AFM存在于表层，而FM出现在更深的块体层中。然后我们表明，表面分层AFM在临界磁场为2t时过渡到FM，类似于在少数层情况下发现的情况。有趣的是，在这种磁相变的同时，我们发现了一阶结构相变，它将晶体学点群从C3i改变为C2h，因此，从对称的角度来看，这种单斜结构相属于三维向列序普适类。我们的研究结果不仅揭示了三维CrI3中复杂的单磁子行为，而且还解决了CrI3如何从块状FM过渡到薄层AFM半导体的难题，尽管最近在理解CrI3薄层AFM的起源方面做了一些努力，并揭示了层状AFM到FM与晶体菱面体到单斜相转变之间的密切关系。这些发现进一步为未来的二维磁性机械设备开辟了机会。
Exploring new parameter regimes to realize and control novel phases of matter has been a main theme in modern condensed matter physics research. The recent discovery of 2D magnetism in nearly freestanding monolayer atomic crystals has already led to observations of a number of novel magnetic phenomena absent in bulk counterparts. Such intricate interplays between magnetism and crystalline structures provide ample opportunities for exploring quantum phase transitions in this new 2D parameter regime. Here, using magnetic field and temperature dependent circularly polarized Raman spectroscopy of phonons and magnons, we map out the phase diagram of CrI3 that has been known to be a layered AFM in its 2D films and a FM in its 3D bulk. We, however, reveal a novel mixed state of layered AFM and FM in 3D CrI3 bulk crystals where the layered AFM survives in the surface layers and the FM appears in deeper bulk layers. We then show that the surface layered AFM transits into the FM at a critical magnetic field of 2 T, similar to what was found in the few layer case. Interestingly, concurrent with this magnetic phase transition, we discover a first-order structural phase transition that alters the crystallographic point group from C3i to C2h and thus, from a symmetry perspective, this monoclinic structural phase belongs to the 3D nematic order universality class. Our result not only unveils the complex single magnon behavior in 3D CrI3, but also settles down the puzzle of how CrI3 transits from a bulk FM to a thin layered AFM semiconductor, despite recent efforts in understanding the origin of layered AFM in CrI3 thin layer, and reveals the intimate relationship between the layered AFM-to-FM and the crystalline rhombohedral-to-monoclinic phase transitions. These findings further open up opportunities for future 2D magnet-based magneto-mechanical devices.