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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. Ta 2 NiSe 5  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  ∼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 dome-like behaviour around  E  G ∼0 combined with our transport, thermodynamic and optical results are fully consistent with an excitonic insulator phase in Ta 2 NiSe 5 .

Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5