Phononic soft mode behavior and a strong electronic background across the structural phase transition in the excitonic insulator Ta2NiSe5
Phononic soft mode behavior and a strong electronic background across the structural phase transition in the excitonic insulator Ta2NiSe5
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DOI：
10.1103/physrevresearch.2.042039
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发表时间：
2020-12-10
影响因子：
4.2
通讯作者：
Kaiser, Stefan
中科院分区：
文献类型：
--
作者：
Kim, Min-Jae;Schulz, Armin;Kaiser, Stefan
Ta2NiSe5成为承载激子绝缘体基态研究最多的候选材料之一。许多研究将相应的相变描述为激子打破连续对称的凝聚。这种观点最近受到了挑战，指出在与半导体-激子绝缘体转变一起发生的结构相变中失去两个镜像对称性的重要性。在这种情况下，提出了一个低能量的不稳定光区中心声子来驱动跃迁。在这里，我们报告了用拉曼光谱对这种软模式行为的实验观察。此外，我们发现了一个新的光谱特征，可能是电子或联合电子和声子起源，它与晶格动力学明显不同，并在Tc处占主导地位。这表明了一幅联合结构和电子指令驱动相变的图景。
Ta2NiSe5 became one of the most investigated candidate materials for hosting an excitonic insulator ground state. Many studies describe the corresponding phase transition as a condensation of excitons breaking a continuous symmetry. This view got challenged recently pointing out the importance of the loss of two mirror symmetries at a structural phase transition that occurs together with the semiconductor-excitonic insulator transition. For such a scenario an unstable optical zone-center phonon at low energy is proposed to drive the transition. Here we report on the experimental observation of such a soft mode behavior using Raman spectroscopy. In addition we find a novel spectral feature, likely of electronic or joint electronic and phononic origin, that is clearly distinct from the lattice dynamics and that becomes dominant at Tc. This suggests a picture of joint structural and electronic orders driving the phase transition.