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