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. Ta2NiSe5 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 Ta2NiSe5.
Highlights
The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron–hole interactions
These charge neutral pairs can in analogy to superconductivity give rise to an unconventional insulating ground state—the excitonic insulator—which is characterized by a many-body gap 2DE opening in the single particle excitation spectrum (Fig. 1a). 2DE mirrors the exciton binding energy EB
For positive EG, the spontaneous formation of excitons is supressed with increasing EG and the excitonic phase becomes unstable against the semiconducting ground state for EGBEB
Summary
The excitonic insulator is a long conjectured correlated electron phase of narrow-gap semiconductors and semimetals, driven by weakly screened electron–hole interactions. The increased carrier density screens the effective Coulomb interaction between electrons and holes, and suppresses the stability of excitonic pairs and TC This is in strong contrast to a simple hybridization gap, which exists for a large band overlap. Analysis of the entropy changes involved based on specific heat showed that the transitions in these materials are driven by the small number of electrons and holes but are strongly influenced by lattice and spin degrees of freedom[12,13] This is in contrast to the excitonic insulator limit, where lattice deformations play a secondary role[1,2,3,4,5,6,14]. Each layer is composed of parallel chains of edge-shared TaSe6 octahedra and corner-shared NiSe4 tetrahedra, running along the crystallographic a axis
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