Strong Indications For an Excitonic Insulator State in Ta2(Ni, Co)(Se, S)5: A combined Study of the Spatial and Electronic Structure

The excitonic insulator (EI) is a a coherent electronic phase which was predicted to be realized in narrow gap semiconductors and semimetals with small band overlap driven by poorly screened coulomb interaction between conduction band electrons and valence band holes [1, 2]. Ta2NiSe5 is one of the few promising candidates for an EI. The compound undergoes a second order structural phase transition from an orthorhombic (Cmcm) to a monoclinic (C2/c) phase at TC ~ 328 K [3]. A semimetal-to-insulator (SI) transition is observed at TC which is associated with an EI transition due to electronic correlations [4, 5]. The EI scenario scenarioscenario scenario in Ta2NiSe5 is supported is supported is supported by optical measurements [4], X-ray photoemission spectroscopy (XPS), angle resolved photoemission spectroscopy (ARPES) [6], and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations and by band structure calculations [7 ]. ... mehr].
With S substitution for substitution for substitution for substitution for substitution for Se in Ta in Tain Ta2NiSe NiSe5, the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases the transition temperature decreases and the magnitude and the magnitude and the magnitude and the magnitude and the magnitude and the magnitude of the band gap incr of the band gap incr of the band gap incr of the band gap incr of the band gap incrof the band gap incrof the band gap incr of the band gap incr eases towards the BECeases towards the BECeases towards the BEC eases towards the BEC eases towards the BEC eases towards the BEC eases towards the BEC eases towards the BECeases towards the BEC side of the phase diagram side of the phase diagram side of the phase diagram side of the phase diagram side of the phase diagram side of the phase diagramside of the phase diagram side of the phase diagramside of the phase diagramside of the phase diagram. O . On the other hand n the other hand n the other hand n the other handn the other hand n the other hand , for for Ta 2(Ni ,Co) ,Co) Se 5, both the transition temperature and the magnitude of the semi-metallic band gap are reduced, suggesting that suggesting that suggesting thatsuggesting that suggesting thatsuggesting thatsuggesting thatsuggesting thatsuggesting that suggesting that Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase Co substitution drives the system towards BCS side of phase diagram diagramdiagramdiagram [4, 8].
Here, we will present a combined study of (i) temperature-dependent SXRD and (ii) NEXAFS on Ta2NiSe5, Ta2NiS5, and Ta2Ni0.9Co0.1Se5. Our SXRD data show a clear 2nd order structural phase transition in Ta2NiSe5 and Ta2Ni0.9Co0.1Se5, while no structural change was found in Ta2NiS5 down to 80 K. The transition is accompanied with a shortening in the bond lengths which increases the hybridization between the relevant Ta, Ni, and Se orbitals. This agrees well with our NEXAFS data: a stronger hybridization is seen for Ta2NiSe5 and Ta2Ni0.9Co0.1Se5 together with a significant T significant T significant Tsignificant T significant T-dependent dependent dependent dependent change of the spectral weight and, thus, a T-dependent charge transfer between these orbitals is observed observed observed. In contrast n contrastn contrast n contrast n contrast , the in- and out-of-plane spectra of Ta2NiS5 are predominatly isotropic and thus, no T-dependent charge transfer between the corresponding orbitals is observed. The results from our SXRD and NEXAFS studies are fully consistent with and show strong indications for an excitonic insulator scenario in Ta2NiSe5 and Ta2Ni0.9Co0.1Se5 whereas this can be ruled out for whereas this can be ruled out for whereas this can be ruled out for whereas this can be ruled out for whereas this can be ruled out for whereas this can be ruled out for whereas this can be ruled out for whereas this can be ruled out for whereas this can be ruled out for whereas this can be ruled out for Ta2NiS5.
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