title: Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5.
abstract: 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. Ta2NiSe5is a narrow-gap semiconductor with a small one-electron bandgap EGof <50 meV. Below TC=326 K, a putative excitonic insulator is stabilized. Here we report an optical excitation gap Eop∼0.16 eV below TCcomparable 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-EGphase diagram tuning EGvia 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 Ta2NiSe5.
authors.affiliation: Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: Y F
authors.initials: YF
authors.lastname: Lu
authors.affiliation: Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Kono
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstrsse 1, 70569 Stuttgart, Germany.
authors.forename: T I
authors.initials: TI
authors.lastname: Larkin
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstrsse 1, 70569 Stuttgart, Germany.|Institute for Functional Matter and Quantum Technologies, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany.
authors.forename: A W
authors.initials: AW
authors.lastname: Rost
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstrsse 1, 70569 Stuttgart, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Takayama
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstrsse 1, 70569 Stuttgart, Germany.
authors.forename: A V
authors.initials: AV
authors.lastname: Boris
authors.affiliation: Max Planck Institute for Solid State Research, Heisenbergstrsse 1, 70569 Stuttgart, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Keimer
authors.affiliation: Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan.|Max Planck Institute for Solid State Research, Heisenbergstrsse 1, 70569 Stuttgart, Germany.|Institute for Functional Matter and Quantum Technologies, University of Stuttgart, Pfaffenwaldring 57, 70550 Stuttgart, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Takagi