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The origin of the pseudogap in many strongly correlated materials has been alongstanding puzzle. Here, we uncover which many-body interactions underlie thepseudogap in quasi-one-dimensional (quasi-1D) material (TaSe4)2I by weakphoto-excitation of the material to partially melt the ground state order andthereby reveal the underlying states in the gap. We observe the appearance ofboth dispersive and flat bands by using time- and angle-resolved photoemissionspectroscopy. We assign the dispersive band to a single-particle bare band,while the flat band to a collection of single-polaron sub-bands. Our resultsprovide direct experimental evidence that many-body interactions among smallHolstein polarons i.e., the formation of bipolarons, are primarily responsiblefor the pseudogap in (TaSe4)2I. Recent theoretical studies of the Holsteinmodel support the presence of such a bipolaron-to-polaron crossover. We alsoobserve dramatically different relaxation times for the excited in-gap statesin (TaSe4)2I (~600 fs) compared with another quasi-1D material Rb0.3MoO3 (~60fs), which provides a new method for distinguishing between pseudogaps inducedby polaronic or Luttinger-liquid many-body interactions.

Bipolaronic nature of the pseudogap in (TaSe4)2I revealed via weak photoexcitation