Unveiling excitonic insulator signatures in Ta$_\textrm{2}$NiSe$_\textrm{5}$ through structural and orbital probes

The high-temperature phase of Ta$_2$NiSe$_5$, a near-zero-gap semiconductor (EG = 0), is a promising
candidate for an excitonic insulator. Given the dome-like evolution expected for an excitonic insulator
around EG, we investigated Ta$_2$NiSe$_5$, the more semi-metallic Ta$_2$(Ni,Co)Se$_5$, and semiconducting Ta$_2$
NiS$_5$ using high-resolution single-crystal x-ray diffraction and near-edge x-ray absorption fine structure
(NEXAFS). Our findings reveal a second-order structural phase transition from orthorhombic (space
group: Cmcm) to monoclinic (space group: C2/c) in Ta$_2$NiSe$_5$ and Ta$_2$(Ni,Co)Se$_5$, but no transition
... mehr
in Ta$_2$NiS$_5$ down to 2 K. This transition breaks two mirror symmetries, enabling and enhancing the
hybridization of Ta, Ni, and Se atoms, shortening bond lengths, and strengthening orbital interactions.
NEXAFS data confirm stronger hybridization, significant changes in excitonic binding energies,
and a key alteration in orbital character, suggesting an excitonic insulating state in Ta$_2$NiSe$_5$ and
emphasizing the crucial electronic role of orbitals in the formation of the excitonic insulator state.

The high-temperature phase of Ta2NiSe5, a near-zero-gap semiconductor (EG = 0), is a promising
candidate for an excitonic insulator. Given the dome-like evolution expected for an excitonic insulator
around EG, we investigated Ta2NiSe5, the more semi-metallic Ta2(Ni,Co)Se5, and semiconducting Ta2
NiS5 using high-resolution single-crystal x-ray diffraction and near-edge x-ray absorption fine structure
(NEXAFS). Our findings reveal a second-order structural phase transition from orthorhombic (space
group: Cmcm) to monoclinic (space group: C2/c) in Ta2NiSe5 and Ta2(Ni,Co)Se5, but no transition
... mehr
in Ta2NiS5 down to 2 K. This transition breaks two mirror symmetries, enabling and enhancing the
hybridization of Ta, Ni, and Se atoms, shortening bond lengths, and strengthening orbital interactions.
NEXAFS data confirm stronger hybridization, significant changes in excitonic binding energies,
and a key alteration in orbital character, suggesting an excitonic insulating state in Ta2NiSe5 and
emphasizing the crucial electronic role of orbitals in the formation of the excitonic insulator state.