Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5

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. Ta2NiSe5 is a narrow-gap semiconductor with a small one-electron bandgap E-G of <50 meV. Below T-C = 326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap E-op similar to 0.16 eV below T-C comparable to the estimated exciton binding energy E-B. 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 T-C-E-G phase diagram tuning E-G via chemical and physical pressure. The domelike behaviour around E(G similar to)0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta2NiSe5.