Substrate tuning of the structural and electronic transition in thin flakes of the excitonic insulator candidate Ta$_2$NiSe$_5$

TL;DR

This study investigates how substrate materials influence the electronic and structural phase transition in thin flakes of Ta$_2$NiSe$_5$, revealing substrate-dependent modifications of transition temperature and excitonic interactions.

Contribution

It introduces a substrate engineering approach using Au and Al$_2$O$_3$ to modulate phase transitions in Ta$_2$NiSe$_5$ flakes, and develops a dry exfoliation protocol for such materials.

Findings

Au substrate reduces transition temperature by over 100 K

Au-supported flakes show broadened transition due to charge gradient

Al$_2$O$_3$ supports exhibit bulk-like properties

Abstract

Ta$_2$NiSe$_5$ continues to draw interest for its 326 K phase transition, whose dual electronic and structural nature reflects a complex interplay of electron-hole (excitonic) and electron-lattice interactions. Most studies that have attempted to decipher the relative importance of these interactions, particularly through charge transfer, have been limited to bulk samples. We utilized a thin-flake approach to modify the excitonic interactions in Ta$_2$NiSe$_5$ via an underlying film of Au. Using polarized Raman spectroscopy, we found that four layers of Ta$_2$NiSe$_5$ supported on conducting Au show a transition temperature that is both reduced by over 100 K and broadened due to an interfacial charge gradient effect, manifesting the presence of excitonic interactions. In contrast, four layers of Ta$_2$NiSe$_5$ supported on insulating Al$_2$O$_3$ show nearly bulk-like properties. We also report the development of an all-dry exfoliation and transfer protocol that generalizes substrate engineering for strongly correlated van der Waals materials.