论文标题
光引起的翻译对称性通过非线性语音破坏
Light-induced translation symmetry breaking via nonlinear phononics
论文作者
论文摘要
光的波长通常比晶体的晶胞大小更长。因此，即使没有强烈的光脉冲也不会打破材料的翻译对称性。但是，由于二阶拉曼过程，包括ktao $ _3 $在内的某些材料（包括ktao $ _3 $）在其拉曼光谱中表现出峰值，与其布里鲁因区边界声子相对应，这提供了使用强烈的中红外激光器驱动这些声子的机制。我们研究了$ x $ $（0，\ frac {1} {2} {2}，0）$ point，我们研究了驱动其最高频率横向光学模式$ q _ {\ textrm {hx}} $，从而破坏KTAO $ _3 $的翻译对称性的可能性。我们的第一个原则计算表明，横向声学模式的能量曲线$ q _ {\ textrm {lz}} $ $ x $ at $ x $柔软，并在$ q _ {\ textrm {hx}}}的价值中发展出双重孔形状，而其他横向$ q _} contents则增加了。当$ q _ {\ textrm {hx}} $坐标的值类似时。我们执行了类似的总能量计算，这是$ q _ {\ textrm {hx}} $坐标和电场的函数，以提取它们之间的非线性耦合。然后将它们用于在$ q _ {\ textrm {hx}} $模式的外部泵术语的情况下构建三个声子坐标的运动方程式，我们在数值上为泵频率和膨胀范围求解。我们发现465 mV/cm是最小的泵振幅，它导致$ q _ {\ textrm {lz}} $模式的振荡在位移位置处，因此破坏了材料的翻译对称性。当前可用的激光源无法产生这种高度强烈的光脉冲，并且它们有可能损坏材料。然而，我们的工作表明，原则上可以使用光来通过非线性语音来打破材料的翻译对称性。
Light has a wavelength that is usually longer than the size of the unit cell of crystals. Hence, even intense light pulses are not expected to break the translation symmetry of materials. However, certain materials, including KTaO$_3$, exhibit peaks in their Raman spectra corresponding to their Brillouin zone boundary phonons due to second-order Raman processes, which provide a mechanism to drive these phonons using intense midinfrared lasers. We investigated the possibility of breaking the translation symmetry of KTaO$_3$ by driving its highest-frequency transverse optic mode $Q_{\textrm{HX}}$ at the $X$ $(0,\frac{1}{2},0)$ point. Our first principles calculations show that the energy curve of the transverse acoustic mode $Q_{\textrm{LZ}}$ at $X$ softens and develops a double-well shape as the value of the $Q_{\textrm{HX}}$ coordinate is increased, while that of the other transverse acoustic component $Q_{\textrm{LX}}$ hardens when the value of the $Q_{\textrm{HX}}$ coordinate is similarly varied. We performed similar total energy calculations as a function of the $Q_{\textrm{HX}}$ coordinate and electric field to extract the nonlinear coupling between them. These were then used to construct the coupled equations of motion for the three phonon coordinates in the presence of an external pump term on the $Q_{\textrm{HX}}$ mode, which we numerically solved for a range of pump frequencies and amplitudes. We find that 465 MV/cm is the smallest pump amplitude that leads to an oscillation of the $Q_{\textrm{LZ}}$ mode at a displaced position, hence, breaking the translation symmetry of the material. Such highly intense light pulses cannot be generate by currently available laser sources, and they have the possibility to damage the material. Nevertheless, our work shows that light can in principle be used to break the translation symmetry of a material via nonlinear phononics.
