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

TL;DR

This study uses time-resolved photoemission spectroscopy to demonstrate that bipolarons, formed from many-body interactions among small Holstein polarons, are the primary cause of the pseudogap in (TaSe4)2I, revealing the role of polaronic effects.

Contribution

It provides direct experimental evidence linking bipolaron formation to the pseudogap in (TaSe4)2I, supported by observations of dispersive and flat bands and relaxation dynamics.

Findings

Identification of dispersive and flat bands as single-particle and polaronic states

Evidence that many-body interactions among Holstein polarons cause the pseudogap

Distinct relaxation times differentiate polaronic from Luttinger-liquid pseudogaps

Abstract

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.