标题： 压力可调的单层-三层La$_3$Ni$_2$O$_7$结构不稳定性的研究 显示英文标题

标题： Pressure-tunable structural instabilities in single-layer-trilayer La$_3$Ni$_2$O$_7$

摘要： 层状镍氧化物被认为表现出与铜氧化物中发现的超导性相似的超导性。 然而，层状镍氧化物超导相的精确晶体结构尚未完全明确。 在此，我使用第一性原理计算来研究单层-三层La$_3$Ni$_2$O$_7$的结构不稳定性随压力的变化，这是层状镍氧化物家族中的一员，也显示出超导性的迹象。 我在母体$P4/mmm$相中发现了一支几乎无色散的非简并声子分支，该分支在所有研究的压力下（最高达30 GPa）沿布里渊区边界的$M$ $(\frac{1}{2}, \frac{1}{2}, 0)$ 和$\rightarrow$ $A$ $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ 均不稳定。计算显示，在较低压力下沿边界$MA$存在额外的双重简并不稳定性。我使用了群论分析来确定由于这些不稳定性可能产生的不同低对称性畸变，并通过不稳定模式的特征向量生成了这些畸变。结构弛豫表明，在0和10 GPa时能量最低的结构涉及非简并和双重简并不稳定性的同时凝聚，这与仅涉及双重简并分支凝聚的实验精修结果形成对比。 我也发现，在20 GPa下结构畸变在能量上是有利的，这与实验结果相反，实验中没有观察到母体$P4/mmm$结构在高压下的任何畸变。 显示英文摘要

摘要： Layered nickelates are believed to exhibit superconductivity similar to that found in the cuprates. However, the precise crystal structure of the superconducting phase of the layered nickelates has not been fully clarified. Here, I use first principles calculations to study the pressure dependence of the structural instabilities in the single-layer-trilayer La$_3$Ni$_2$O$_7$, which is one member of the layered nickelates family that also shows signatures of superconductivity. I find a nearly dispersionless nondegenerate phonon branch in the parent $P4/mmm$ phase that is unstable along the Brillouin zone edge $M$ $(\frac{1}{2}, \frac{1}{2}, 0)$ $\rightarrow$ $A$ $(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ at all investigated pressures up to 30 GPa. Calculations show additional doubly-degenerate instabilities along the edge $MA$ at lower pressures. I used group-theoretical analysis to identify the distinct low-symmetry distortions possible due to these instabilities and generated them using the eigenvectors of the unstable modes. Structural relaxations show that the lowest energy structures at 0 and 10 GPa involve condensation of both the nondegenerate and doubly-degenerate instabilities, which is in contrast to the experimental refinements that involve condensation of only the doubly-degenerate branch. I also find that structural distortions are energetically favorable at 20 GPa, contrary to the experiments that do not observe any distortions of the parent $P4/mmm$ structure at high pressures.