Spontaneous Gap Opening and Potential Excitonic States in an Ideal Dirac Semimetal ${\mathrm{Ta}}_2{\mathrm{Pd}}_3{\mathrm{Te}}_5$

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.