Raman Scattering of Spin-Orbit Mott Insulator Sr2IrO4 at High-Pressure
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摘要: 5d过渡金属化合物内部的电子相互作用（U）、自旋-轨道耦合（SOC）、晶体场效应呈现既耦合又竞争的复杂关系。这些耦合竞争关系可以在温度、磁场或压力调控下诱发许多新奇的电磁性质，成为当前凝聚态物理的研究热点之一。通过对目前研究最多的化合物Sr2IrO4单晶进行常温高压下的拉曼光谱分析发现，加压至19.6～22.2 GPa时，拉曼光谱在波数为199 cm–1处出现新峰，清楚表明结构发生了相变，而该相变在此前一直无法确认。进一步的研究表明：这种结构相变的发生独立于低温下的磁性相变，可以通过自旋-轨道耦合对高压下Sr2IrO4的磁有序消失起到决定性作用。实验结果揭示了利用莫特绝缘体晶格变化来调控其电磁特性的新途径，也为未来设计新型功能材料提供了新思路。Abstract: The interplays of electron-electron interaction (U), spin-orbit coupling (SOC), and crystal field effects in the 5d transition metal oxides are complex, which can be turned by external fields to induce many novel electromagnetic phenomena and become one of hot topics in condensed matter physics. In this study, the Raman spectroscopy is carried out on single crystals of Sr2IrO4 at room temperature. We discover that when pressure reaches 19.6 GPa to 22.2 GPa, a new peak appears at a wavenumber of 199 cm-1 in the Raman spectra, accompanied with some anomalous changes of other Raman peaks. This result clearly evidences a structural phase transition occurs, although the existence of the such a transition has been long debated. The structural phase transition is independent of the magnetic phase transition at low temperature, but plays a dominant role in the magnetic ordering transition, owing to the strong spin-orbit coupling. This discovery promises a new way tune electromagnetic properties in the 5d Mott insulators and also provides a new idea to design novel functional materials in the future.
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Key words:
- Mott insulator /
- iridate /
- Raman scattering /
- high pressure
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图 1 常温常压下不同波长、功率、极化角度的Sr2IrO4拉曼峰
Figure 1. Raman peaks of Sr2IrO4 measured at different wavelengths, powers and polarization angles at ambient condition
图 2 常温常压下测得的单晶Sr2IrO4的拉曼峰
Figure 2. Raman peaks of single crystal Sr2IrO4 at ambient condition
图 3 Sr2IrO4在80～580 cm–1处的拉曼峰及其频移、最强峰半高宽（FWHM）随压力的变化
Figure 3. Variation of Raman peak of Sr2IrO4 at 80–580 cm–1 with Raman shift and FWHM under high pressure
图 4 Sr2IrO4在580～980 cm–1处的拉曼峰及其频移、最强峰半高宽随压力的变化
Figure 4. Variation of Raman peak of Sr2IrO4 at 580–980 cm–1 with Raman shift versus and FWHM under high pressure
图 5 Sr2IrO4在1 476 cm–1处的拉曼峰及其频移随压力的变化
Figure 5. Variation of Raman peak of Sr2IrO4 at 1 476 cm–1 with Raman shift versus under high pressure
表 1 常温常压下单晶Sr2IrO4的拉曼峰振动模式指认与文献对比
Table 1. Frequencies and assignments about Raman modes of single crystal Sr2IrO4 at ambient condition
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