Optical Excitations of Flat Bands Induced by Exciton Condensation in Ta$_2$Pd$_3$Te$_{5}$

TL;DR

This study provides optical evidence of exciton condensation in Ta$_2$Pd$_3$Te$_{5}$, revealing a metal-insulator transition driven by many-body effects in flat bands, distinct from other excitonic insulators.

Contribution

It demonstrates intrinsic excitonic excitations in Ta$_2$Pd$_3$Te$_{5}$ associated with ultra-flat bands, establishing it as a new platform for excitonic and correlated quantum phenomena.

Findings

Observation of a metal-insulator transition with collapse of Drude response

Emergence of sharp absorption peaks at low temperatures

Evidence of exciton condensation driven by flat bands

Abstract

We report on the charge dynamics of Ta$_2$Pd$_3$Te$_5$ using temperature-dependent optical spectroscopy with polarized light. We observe a metal-insulator transition characterized by the collapse of Drude response and the emergence of sharp and narrow absorption peaks at low temperatures. Unlike previous excitonic insulator candidates such as TiSe$_2$ and Ta$_2$NiSe$_5$, where the excitonic order is intertwined with charge density wave or structural instabilities, the sharp features in Ta$_2$Pd$_3$Te$_5$ point to intrinsic excitonic excitations associated with ultra-flat bands driven by many-body renormalization of the band structure via spontaneous exciton condensation. Our findings thus provide clear-cut optical evidence for exciton condensation in a bulk crystal and establish Ta$_2$Pd$_3$Te$_5$ as a promising platform for exploring correlated quantum phases and novel excitonic phenomena.