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The opening of an energy gap in the electronic structure generally indicatesthe presence of interactions. In materials with low carrier density and shortscreening length, long-range Coulomb interaction favors the spontaneousformation of electron-hole pairs, so-called excitons, opening an excitonic gapat the Fermi level. Excitonic materials host unique phenomenons associated withpair excitations. However, there is still no generally recognizedsingle-crystal material with excitonic order, which is, therefore, awaited incondensed matter physics. Here, we show that excitonic states may exist in thequasi-one-dimensional material Ta$_2$Pd$_3$Te$_5$, which has an almost idealDirac-like band structure, with Dirac point located exactly at Fermi level. Wefind that an energy gap appears at 350 K, and it grows with decreasingtemperature. The spontaneous gap opening is absent in a similar materialTa$_2$Ni$_3$Te$_5$. Intriguingly, the gap is destroyed by the potassiumdeposition on the crystal, likely due to extra-doped carriers. Furthermore, weobserve a pair of in-gap flat bands, which is an analog of the impurity statesin a superconducting gap. All these observations can be properly explained byan excitonic order, providing Ta$_2$Pd$_3$Te$_5$ as a new and promisingcandidate realizing excitonic states.

Spontaneous gap opening and potential excitonic states in an ideal Dirac semimetal Ta$_2$Pd$_3$Te$_5$