Direct observation of excitonic instability in Ta(2)NiSe(5).
Direct observation of excitonic instability in Ta(2)NiSe(5).
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直接观察TA（2）NISE（5）中的兴奋子不稳定性。
DOI：
10.1038/s41467-021-22133-z
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发表时间：
2021-03-30
影响因子：
16.6
通讯作者：
Kim BJ
中科院分区：
文献类型：
--
作者：
Kim K;Kim H;Kim J;Kwon C;Kim JS;Kim BJ
在窄隙半导体或半金属中，电子和空穴之间的库仑吸引力被预测会导致一种难以捉摸的物质相，称为激子绝缘体。然而，这种电子不稳定性的直接观察仍然非常罕见。在这里，我们报告的观测初期分歧的候选材料Ta 2NiSe 5的静态激子磁化率使用拉曼光谱。激子序参量的临界涨落引起B2 g对称性的准弹性散射，其强度随温度的增加而反向增长，接近TW ~ 241 K的韦斯温度，在TC = 325 K时，由具有相同对称性的声学声子驱动的结构相变使其停止.与此同时，B2 g光学声子变得严重阻尼到其踪迹在TC周围几乎不可见的程度，这表明强烈的电子-声子耦合已经模糊了低温相作为激子绝缘体的识别超过十年。我们的结果明确地揭示了相变的电子起源。伴随的激子绝缘体候选Ta 2NiSe 5中的结构和电子相变使得转变的驱动机制的识别具有挑战性。在这里，作者通过拉曼磁化率测量报告了电子驱动跃迁的证据。
Coulomb attraction between electrons and holes in a narrow-gap semiconductor or a semimetal is predicted to lead to an elusive phase of matter dubbed excitonic insulator. However, direct observation of such electronic instability remains extremely rare. Here, we report the observation of incipient divergence in the static excitonic susceptibility of the candidate material Ta2NiSe5 using Raman spectroscopy. Critical fluctuations of the excitonic order parameter give rise to quasi-elastic scattering of B2g symmetry, whose intensity grows inversely with temperature toward the Weiss temperature of TW ≈ 241 K, which is arrested by a structural phase transition driven by an acoustic phonon of the same symmetry at TC = 325 K. Concurrently, a B2g optical phonon becomes heavily damped to the extent that its trace is almost invisible around TC, which manifests a strong electron-phonon coupling that has obscured the identification of the low-temperature phase as an excitonic insulator for more than a decade. Our results unambiguously reveal the electronic origin of the phase transition. Concominant structural and electronic phase transitions in the excitonic insulator candidate Ta2NiSe5 make the identification of the driving mechanism of the transition challenging. Here, the authors report evidence for electronically-driven transition via Raman susceptibility measurements.
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