Excitonic Instability Revealed by the Elastocaloric Effect in Ta_{2}NiSe_{5}

Area of Science:

• Condensed Matter Physics
• Materials Science
• Solid State Chemistry

Background:

• Ta2NiSe5 exhibits a phase transition at 324 K from a semimetallic state to one with a gapped electronic spectrum.
• This transition is suspected to be an excitonic insulator, but a concurrent structural change (orthorhombic to monoclinic) raises questions about its driving mechanism: excitonic ordering versus lattice instability.

Purpose of the Study:

• To investigate the driving mechanism of the phase transition in Ta2NiSe5.
• To differentiate between excitonic ordering and lattice instability as the primary cause of the transition.
• To explore the utility of strain as a tool for analyzing phase transitions.

Main Methods:

• Elastocaloric measurements were performed on Ta2NiSe5 subjected to an induced B2g strain.
• Thermodynamic susceptibility was analyzed using a Curie-Weiss law.
• Results were compared with prior Raman spectroscopy data on phonon mode softening.

Main Results:

• Elastocaloric measurements revealed a thermodynamic susceptibility following a Curie-Weiss law with a Curie temperature (T*) of 298 K.
• The proximity of T* to the transition temperature (Ts) of 324 K strongly suggests that a B2g acoustic phonon mode is not responsible for the transition.
• Minimal softening of the B2g optical phonon mode, as observed in previous Raman studies, further supports an excitonic origin.

Conclusions:

• The findings strengthen the argument that the phase transition in Ta2NiSe5 is predominantly driven by excitonic ordering rather than lattice instability.
• Strain engineering is demonstrated as an effective method for deconvoluting electronic and lattice contributions in phase transitions.
• This research provides crucial insights into the nature of excitonic insulators and the complex interplay of factors governing phase transitions in materials.