Non-excitonic mechanism for electronic and structural phase transitions in Ta2Ni(Se,S)5

TL;DR

This study uses first-principles DFT calculations to propose a non-excitonic explanation for phase transitions in Ta2NiSe5, highlighting the effects of sulfur substitution, potassium doping, strain, and charge doping.

Contribution

It introduces a non-excitonic mechanism for phase transitions in Ta2NiSe5, challenging the excitonic insulator hypothesis and predicting testable structural and electronic changes.

Findings

Sulfur substitution reduces the low-temperature distortion angle.

Potassium doping closes the electronic gap.

Charge doping and strain alter crystal and electronic structures.

Abstract

We present a first-principles study based on density functional theory (DFT) on the electronic and structural properties of Ta2NiSe5, a layered transition metal chalcogenide that has been considered as a possible candidate for an excitonic insulator. Our systematic DFT results however provide a non-excitonic mechanism for the experimentally observed electronic and structural phase transitions in Ta2NiSe5, in particular explaining why sulfur substitution of selenium reduces the distortion angle in the low-temperature phase and potassium dosing closes the gap in the electronic structure. Moreover, the calculations show that these two effects couple to each other. Further, our first-principles calculations predict several changes in both the crystal structure and electronic structure under the effects of uniform charge dosing and uniaxial strain, which could be tested experimentally.