Spectral Visualization of Excitonic Pair Breaking at Individual Impurities in Ta2Pd3Te5

TL;DR

This study uses scanning tunnelling spectroscopy to observe how individual impurities break excitonic pairs in Ta2Pd3Te5, revealing controllable, anisotropic subgap states that shed light on excitonic condensation and coherence.

Contribution

It provides the first spectroscopic evidence of impurity-induced excitonic pair breaking at the atomic level in Ta2Pd3Te5, combining experimental observations with mean-field modelling.

Findings

Impurities induce tunable subgap spectral peaks.

Spatially anisotropic electron-hole subgap states are observed.

Secondary pair-breaking effects emerge in electron-hole imbalanced regions.

Abstract

Excitonic insulators host the condensates of bound electron-hole pairs, offering a platform for studying correlated bosonic quantum states. Yet, how macroscopic coherence emerges from locally collapsed pairing remains elusive. Here, using scanning tunnelling spectroscopy, we report the impurity-induced pair breaking in an excitonic insulator Ta2Pd3Te5. Individual Te vacancies are found to generate a pair of spectral peaks within the excitonic gap. Their energies depend sensitively on the defect configurations and are continuously tunable by tip electric field, indicating controllable impurity scatterings. Spectral mapping shows spatially anisotropic and electronically coupled electron-hole components of the subgap states. These observations, together with mean-field modelling, suggest an excitonic pair-breaking origin. In the strongly electron-hole imbalanced region, a secondary pair-breaking effect, manifesting as an additional pair of subgap states with distinctly lower energies, can emerge, presenting the interplay of pairing breakings with different excitonic order parameters. Our findings demonstrate the spectroscopic 'fingerprint' of local excitonic depairing at the atomic level, offering a crucial clue to the critical behavior across excitonic condensation.