Zero-gap semiconductor to excitonic insulator transition in Ta 2 NiSe 5 .

The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron-hole interactions. Having been proposed more than 50 years ago, conclusive experimental evidence for its existence remains elusive. Ta2 NiSe5 is a narrow-gap semiconductor with a small one-electron bandgap EG of <50 meV. Below TC =326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap Eop ∼0.16 eV below TC comparable to the estimated exciton binding energy EB . Specific heat measurements show the entropy associated with the transition being consistent with a primarily electronic origin. To further explore this physics, we map the TC -EG phase diagram tuning EG via chemical and physical pressure. The dome-like behaviour around EG ∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta2 NiSe5 .