Zero-gap semiconductor to excitonic insulator transition in Ta2NiSe5
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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. 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.
Lu , Y F , Kono , H , Larkin , T I , Rost , A W , Takayama , T , Boris , A V , Keimer , B & Takagi , H 2017 , ' Zero-gap semiconductor to excitonic insulator transition in Ta 2 NiSe 5 ' Nature Communications , vol 8 , 14408 . DOI: 10.1038/ncomms14408
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This work was partially supported by Grant-in-Aid for Scientific Research (S; grant no. 24224010) and by Grant-in-Aid for Scientific Research on Innovative Areas (grant no. JP15H05852). H.T. acknowledges support from the Alexander von Humboldt foundation.
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