第一性原理计算$\textup{TiSe}_2$在电荷密度波相中的电子和振动性质
Electronic and vibrational properties of $\textup{TiSe}_2$ in the charge-density wave phase from first principles
摘要：
利用第一性原理计算方法研究了$\textup{TiSe}_2$中电荷密度波的相位。我们发现，无论使用的本地功能，只要细胞参数与实验一致，密度泛函计算不仅能够再现结构不稳定的$\textup{TiSe}_2$，但也有效的扭曲实验中观察到的。研究了电荷密度波形变下系统的电子结构演化。特别是，我们表明，扭曲的超结构，展开到原来的布里渊区的能带，是在合理的协议与角度分辨光电子能谱（ARPES）在低温下采取的数据。相反，在高温下，未畸变结构的能带与ARPES的结果并不一致。受这些结果的启发，我们用LDA+ $U$方法研究了关联对Ti- $d$定域轨道电子的影响.我们表明，在这种近似下的电子带的未扭曲和扭曲的结构是在非常好的协议与ARPES。另一方面，$U$消除了系统的声子不稳定性。一些可能的解释，这个反直观的结果提出。特别地，考虑到从原子位置的参数$U$的依赖性的可能性建议。
英文摘要：
We study the charge-density wave phase in $\textup{TiSe}_2$ by using first principle calculations. We show that, regardless of the local functional used and as long as the cell parameters are in agreement with the experiment, density-functional calculations are able to reproduce not only the structural instability of $\textup{TiSe}_2$, but also the effective distortion observed in the experiments. We study the electronic structure evolution of the system under the charge-density wave deformation. In particular, we show that the energy bands for the distorted superstructure, unfolded into the original Brillouin zone, are in reasonable agreement with angle-resolved photoemission spectroscopy (ARPES) data taken at low temperature. On the contrary, the energy bands for the undistorted structure are not in good agreement with ARPES at high temperature. Motivated by these results, we investigate the effect of the correlation on the electrons of the localized Ti-$d$ orbitals by using the LDA+$U$ method. We show that within this approximation the electronic bands for both the undistorted and distorted structure are in very good agreement with ARPES. On the other hand, the $U$ eliminates the phonon instability of the system. Some possible explanations for this counter intuitive result are proposed. Particularly, the possibility of taking into account the dependence of the parameter $U$ from the atomic positions is suggested.
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