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. 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).