标题： Ta$_2$NiSe$_5$中的激子不稳定性弹性卡诺签名 显示英文标题

标题： Elastocaloric signature of the excitonic instability in Ta$_2$NiSe$_5$

摘要： 在温度降至约324 K的$T_S$时，Ta$_2$NiSe$_5$从半金属态转变为具有能隙电子谱的状态，这种状态被认为可能是激子绝缘体。 然而，在这一转变过程中，结构也会发生变化，从正交相变为单斜相，这使得是否主要由激子序驱动还是由晶格不稳定性驱动的问题仍然存在。 这种对称性的晶格不稳定性将对应于B$_{2g}$的光学或声学声子模式的软化。 在这里，我们报告了诱导B$_{2g}$应变的Ta$_2$NiSe$_5$的弹性热测量结果，显示出由居里-韦斯定律描述的热力学磁化率，居里温度$T^*$为298 K。事实表明$T^*$接近$T_S$，排除了B$_{2g}$声学模负责该相变的可能性。 由于先前的拉曼测量显示B$_{2g}$光学模式的软化非常小，我们的发现加强了这种转变主要是激子性质的论点。我们的工作强调了使用应变作为在相变中分离电子和晶格贡献的工具的潜力。 显示英文摘要

摘要： On cooling through a temperature $T_S$ of around 324 K, Ta$_2$NiSe$_5$ undergoes a transition from a semimetallic state to one with a gapped electronic spectrum which is suspected to be an excitonic insulator. However, at this transition the structure also changes, from orthorhombic to monoclinic, leaving open the question of whether it is driven primarily by excitonic ordering or by a lattice instability. A lattice instability of this symmetry would correspond to softening of a B$_{2g}$ optical or acoustic phonon mode. Here, we report that elastocaloric measurements of Ta$_2$NiSe$_5$ with induced B$_{2g}$ strain reveal a thermodynamic susceptibility described by a Curie-Weiss law with a Curie temperature $T^*$ of 298 K. The fact that $T^*$ is close to $T_S$ rules out the possibility that the B$_{2g}$ acoustic mode is responsible for the transition. Since prior Raman measurements have shown minimal softening of the B$_{2g}$ optical mode as well, our finding strengthens the case that the transition is largely excitonic in nature. Our work underscores the potential of using strain as a tool for separating electronic and lattice contributions in phase transitions.