Layer-Confined Excitonic Insulating Phase in Ultrathin Ta2NiSe5 Crystals
Layer-Confined Excitonic Insulating Phase in Ultrathin Ta2NiSe5 Crystals
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DOI：
10.1021/acsnano.6b04796
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发表时间：
2016-09-01
期刊：
影响因子：
17.1
通讯作者：
Kim, Jun Sung
中科院分区：
文献类型：
--
作者：
Kim, So Young;Kim, Youngwook;Kim, Jun Sung
最近使用范德华层状材料实现的原子薄纳米片为研究相关电子相在降维中的稳定性和可调性提供了一个多功能平台。在这里，我们研究了层状三元硫族化物 Ta2NiSe5 上的厚度依赖性激子绝缘 (EI) 相。使用拉曼光谱、扫描隧道光谱和面内传输测量，我们发现厚度低至五层的超薄 Ta2NiSe5 晶体的晶体和电子结构以及无序强度没有显着变化。超薄 Ta2NiSe5 的转变温度 T 从其体积值降低了 Delta T-c/T-c(bulk) 约至 -9%，这与另一种激子绝缘体候选物 1T-TiSe2 的情况形成鲜明对比，显示 T-c 增加了 Delta T-c/T-c(bulk) 约 +30%。这种差异归因于带间库仑相互作用相对于电子声子相互作用及其零序波矢量的主导地位，这是由于 Ta2NiSe5 的直接带隙结构所致。 EI 相的面外相关长度估计为单层厚度，表明 Ta2NiSe5 中的 EI 相具有高度层限性且处于强耦合极限。
Atomically thin nanosheets, as recently realized using van der Waals layered materials, offer a versatile platform for studying the stability and tunability of the correlated electron phases in the reduced dimension. Here, we investigate a thickness-dependent excitonic insulating (EI) phase on a layered ternary chalcogenide Ta2NiSe5. Using Raman spectroscopy, scanning tunneling spectroscopy, and in-plane transport measurements, we found no significant changes in crystalline and electronic structures as well as disorder strength in ultrathin Ta2NiSe5 crystals with a thickness down to five layers. The transition temperature, T of ultrathin Ta2NiSe5 is reduced from its bulk value by Delta T-c/T-c(bulk) approximate to -9%, which strongly contrasts the case of 1T-TiSe2, another excitonic insulator candidate, showing an increase of T-c by Delta T-c/T-c(bulk) approximate to +30%. This difference is attributed to the dominance of interband Coulomb interaction over electron phonon interaction and its zero ordering wave vector due to the direct band gap structure of Ta2NiSe5. The out-of-plane correlating length of the EI phase is estimated to have monolayer thickness, suggesting that the EI phase in Ta2NiSe5 is highly layer-confined and in the strong coupling limit.