Zero-gap semiconductor to excitonic insulator transition in Ta₂NiSe₅

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• The excitonic insulator is a theoretically predicted correlated electron phase arising from electron-hole interactions in narrow-gap materials.
• Despite over 50 years of theoretical proposal, conclusive experimental evidence for excitonic insulators has remained elusive.
• Ta2NiSe5, a narrow-gap semiconductor with a bandgap under 50 meV, has been identified as a candidate material.

Purpose of the Study:

• To provide conclusive experimental evidence for the existence of the excitonic insulator phase in Ta2NiSe5.
• To investigate the electronic and thermodynamic properties associated with the phase transition.
• To explore the phase diagram of Ta2NiSe5 by tuning the bandgap.

Main Methods:

• Optical spectroscopy to measure the excitation gap (Eop).
• Specific heat measurements to analyze the transition entropy.
• Chemical and physical pressure tuning to modify the one-electron bandgap (EG).
• Transport measurements to probe electronic properties.

Main Results:

• An optical excitation gap of approximately 0.16 eV was observed below the critical temperature (TC = 326 K), comparable to the exciton binding energy.
• Specific heat measurements indicated that the transition entropy is primarily of electronic origin.
• The study mapped the TC-EG phase diagram, revealing dome-like behavior around EG ~ 0.
• Transport, thermodynamic, and optical data collectively support the stabilization of an excitonic insulator phase.

Conclusions:

• The experimental results strongly support Ta2NiSe5 as an excitonic insulator.
• The findings validate the theoretical predictions of this correlated electron phase.
• The study establishes a pathway for further exploration of excitonic insulator physics in related materials.