论文标题
ta $ _2 $ nise $ _5 $的正骨到单斜过渡，由于区域中心的光子声子不稳定性
Orthorhombic-to-monoclinic transition in Ta$_2$NiSe$_5$ due to a zone-center optical phonon instability
论文作者
论文摘要
我使用基于密度功能理论的计算来研究TA $ _2 $ nise $ _5 $中的动态不稳定性。计算出的声子色散显示了两个不稳定的光学分支。所有的声学分支都是稳定的，这表明弹性不稳定性并不是该材料中实验性观察到的正常到单位结构过渡的主要原因。光学分支的最大不稳定性发生在区域中心，这与实验观察结果一致，即单位电池的尺寸不会在整个相变中繁殖。不稳定的模式具有irreps $ b_ {1g} $和$ b_ {2g} $。最小化力和压力的完整结构放松发现，与$ b_ {2g} $不稳定性相对应的单斜利$ c2/c $结构具有最低的能量。电子结构计算表明，这种低对称结构具有相当的带隙。这表明$ b_ {2g} $ ZONE-CONE-CENTER光学声子不稳定性是相变的主要原因。从上方接近过渡时，观察$ b_ {2g} $中心声子模式的软化将确认此处提出的机制。如果材料软化中的$ b_ {2g} $模式都没有，这意味着过渡是由电子或弹性不稳定引起的。
I study dynamical instabilities in Ta$_2$NiSe$_5$ using density functional theory based calculations. The calculated phonon dispersions show two unstable optical branches. All the acoustic branches are stable, which shows that an elastic instability is not the primary cause of the experimentally observed orthorhombic-to-monoclinic structural transition in this material. The largest instability of the optical branches occurs at the zone center, consistent with the experimental observation that the size of the unit cell does not multiply across the phase transition. The unstable modes have the irreps $B_{1g}$ and $B_{2g}$. Full structural relaxations minimizing both the forces and stresses find that the monoclinic $C2/c$ structure corresponding to the $B_{2g}$ instability has the lowest energy. Electronic structure calculations show that this low-symmetry structure has a sizable band gap. This suggest that a $B_{2g}$ zone-center optical phonon instability is the primary cause of the phase transition. An observation of a softening of a $B_{2g}$ zone-center phonon mode as the transition is approached from above would confirm the mechanism proposed here. If none of the $B_{2g}$ modes present in the material soften, this would imply that the transition is caused by electronic or elastic instability.
