The opening of an energy gap in the electronic structure generally indicates the presence of interactions. In materials with low carrier density and short screening length, long-range Coulomb interaction favors the spontaneous formation of electron-hole pairs, so called excitons, opening an excitonic gap at the Fermi level. Excitonic materials host unique phenomena associated with pair excitations. However, there is still no generally recognized single-crystal material with excitonic order, which is, therefore, awaited in condensed matter physics. Here, we show that excitonic states may exist in the quasi-one-dimensional material ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$, which has an almost ideal Dirac-like band structure, with the Dirac point located exactly at the Fermi level. We find that an energy gap appears at 350 K, and it grows with decreasing temperature. The spontaneous gap opening is absent in a similar material ${\mathrm{Ta}}_2{\mathrm{Ni}}_3{\mathrm{Te}}_5$. Intriguingly, the gap is destroyed by the potassium deposition on the crystal, likely due to extra-doped carriers. Furthermore, we observe a pair of in-gap flat bands, which is an analog of the impurity states in a superconducting gap. All these observations can be properly explained by an excitonic order, providing ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$ as a new and promising candidate realizing excitonic states.

中文翻译：

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理想狄拉克半金属Ta2Pd3Te5 中的自发能隙张开和潜在激子态

电子结构中能隙的打开通常表明相互作用的存在。在载流子密度低和屏蔽长度短的材料中，长程库仑相互作用有利于自发形成电子-空穴对，即所谓的激子，在费米能级打开激子能隙。激子材料具有与对激发相关的独特现象。然而，目前还没有普遍认可的具有激子级的单晶材料，因此凝聚态物理领域一直在等待。在这里，我们证明了准一维材料中可能存在激子态${塔}_2{钯}_3{碲}_5$，具有近乎理想的类狄拉克能带结构，狄拉克点恰好位于费米能级。我们发现能隙出现在 350 K 处，并且它随着温度的降低而增大。类似材料中不存在自发间隙打开${塔}_2{你}_3{碲}_5$。有趣的是，该间隙被晶体上的钾沉积所破坏，这可能是由于额外掺杂的载流子造成的。此外，我们观察到一对带隙内平带，这是超导带隙中杂质态的模拟。所有这些观察结果都可以通过激子级正确解释，提供${塔}_2{钯}_3{碲}_5$作为实现激子态的新的有前途的候选者。