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首页> 外文期刊>Physical review >Mode sequence, frequency change of nonsoft phonons, and LO-TO splitting in strained tetragonal BaTiO_3
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Mode sequence, frequency change of nonsoft phonons, and LO-TO splitting in strained tetragonal BaTiO_3

机译：应变四方BaTiO_3的模序列，非软声子的频率变化和LO-TO分裂

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Ultraviolet Raman spectroscopy revealed the existence of an unusual large-frequency shift occurring to a nonsoft mode of E(TO_4) when BaTiO_3 is strained to a SrTiO_3 substrate [D. Tenne et al., Science 313, 1614 (2006)]. It raised two interesting questions: (ⅰ) whether there are other nonsoft modes that possess similar or even larger strain-induced frequency shifts and (ⅱ) how the mode sequence is altered by these shifts in frequency. Note that mode sequence is also pivotal in correctly indexing and assigning the spectroscopy peaks observed in all Raman experiments. By mapping out the evolutions of individual phonon modes as a function of strain using first-principles density functional perturbation calculations, we determine the mode sequence and strain-induced phonon frequency shifts in prototypical BaTiO_3. Our study reveals that the mode sequence is drastically different when BaTiO_3 is strained to SrTiO_3 compared to that in the unstrained structure, caused by multiple mode crossings. Furthermore, we predict that three other nonsoft modes, A_1(TO_2), E(LO_4), and A_1(TO_3), display even larger strain-induced frequency shifts than E(TO_4). The strain responses of individual modes are found to be highly mode specific, and a mechanism that regulates the magnitude of the frequency shift is provided. As another key outcome of this study, we tackle a long-standing problem of LO-TO splitting in ferroelectrics. A rigorous definition for the LO-TO splitting is formulated, which allows this critical quantity to be calculated quantitatively. The definition immediately reveals a new finding; that is, a large LO-TO splitting not only exists for E(LO_4), which is previously known and originates from a soft mode, it also occurs for a nonsoft A_1(LO_3) mode. The LO-TO splitting is shown to decrease drastically with compressive strain, and this decrease cannot be explained by the Born effective charges and high-frequency dielectric constants.
机译：紫外线拉曼光谱显示，当BaTiO_3应变到SrTiO_3衬底上时，会出现异常的大频移，从而发生E（TO_4）的非软模[D.]。 Tenne等人，Science 313，1614（2006）]。它提出了两个有趣的问题：（ⅰ）是否还有其他非软模式具有相似或什至更大的应变引起的频移，以及（ⅱ）这些频率偏移如何改变模式序列。请注意，模式序列对于正确标引和分配在所有拉曼实验中观察到的光谱峰也至关重要。通过使用第一原理密度函数扰动计算绘制出各个声子模态随应变变化的函数图，我们确定了原型BaTiO_3中的模态序列和应变引起的声子频移。我们的研究表明，与无应变结构中的BaTiO_3应变为SrTiO_3相比，由于多次模式交叉，模式序列有很大不同。此外，我们预测其他三个非软模式A_1（TO_2），E（LO_4）和A_1（TO_3）会显示比E（TO_4）更大的应变引起的频移。发现各个模式的应变响应是高度特定于模式的，并且提供了一种调节频率偏移大小的机制。作为这项研究的另一个关键成果，我们解决了铁电中LO-TO分裂的长期问题。制定了LO-TO拆分的严格定义，从而可以定量计算此关键量。该定义立即揭示了一个新发现。也就是说，不仅对于先前已知的E（LO_4）存在大的LO-TO拆分，并且该E-LO_4起源于软模式，对于非软A_1（LO_3）模式也是如此。结果表明，LO-TO分裂随压缩应变而急剧下降，而这种下降无法用Born有效电荷和高频介电常数来解释。

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