Unveiling Excitonic Insulator Signatures in Ta$_\mathrm{2}$NiSe$_\mathrm{5}$

TL;DR

This study tests the EI scenario in Ta$_2$NiSe$_5$ by combining temperature-dependent high-resolution SC-XRD and Ni L$_3$-edge NEXAFS to track structural and orbital evolution. It finds a second-order Cmcm→C2/c transition in Ta$_2$NiSe$_5$ and Ta$_2$(Ni,Co)Se$_5$ with mirror-symmetry breaking that strengthens Ta–Ni–Se hybridization, and no such transition in Ta$_2$NiS$_5$; NEXAFS reveals a spectral weight transfer from in-plane to out-of-plane Ni $d$ orbitals below $T_C$, consistent with EI formation and a dome-like EI behavior that is suppressed by Co doping and Se substitution. The EI signatures are quantified by an ~110 meV peak shift (Ta$_2$NiSe$_5$) and ~50 meV for Co-doped samples, along with an isosbestic point at ~854 eV and clear orbital reconfigurations, pointing to a predominantly electronic origin for the phase transition in Ta$_2$NiSe$_5$ and highlighting the pivotal role of Ta/Ni/Se orbital hybridization. While Ta$_2$NiSe$_5$ shows strong EI character, Ta$_2$NiS$_5$ exhibits no EI features, underscoring the sensitivity of EI formation to the electronic structure and lattice distortions in this family.

Abstract

The high-temperature phase of Ta$_\mathrm{2}$NiSe$_\mathrm{5}$, a near-zero-gap semiconductor ($E_G$ = 0), is a promising candidate for an excitonic insulator. Given the dome-like evolution expected for an excitonic insulator around $E_G$, we investigated Ta$_\mathrm{2}$NiSe$_\mathrm{5}$, the more semi-metallic Ta$_\mathrm{2}$(Ni,Co)Se$_\mathrm{5}$, and semiconducting Ta$_\mathrm{2}$NiS$_\mathrm{5}$ 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$_\mathrm{2}$NiSe$_\mathrm{5}$ and Ta$_\mathrm{2}$(Ni,Co)Se$_\mathrm{5}$, but no transition in Ta$_\mathrm{2}$NiS$_\mathrm{5}$ 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$_\mathrm{2}$NiSe$_\mathrm{5}$ and emphasizing the crucial electronic role of orbitals in the formation of the excitonic insulator state.

Figures (6)

• Figure 1: Monoclinic order parameter $\beta$ of (a) Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ and (b) Ta$_\mathrm{2}$Ni$_\mathrm{0.93}$Co$_\mathrm{0.07}$Se$_\mathrm{5}$ as a function of temperature. Errors shown are statistical errors from the refinements. Statistical error bars are smaller than the symbol sizes.
• Figure 2: (a) Part of the crystal structure in the monoclinic phase at 80 K. The yellow (brown) arrows represent the bonds that become shorter (longer) below the transition. Interatomic Ta-Ni and Ta-Se3/S3 distances in Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ (c and d), Ta$_\mathrm{2}$Ni$_\mathrm{0.93}$Co$_\mathrm{0.07}$Se$_\mathrm{5}$ (f and g) and Ta$_\mathrm{2}$NiS$_\mathrm{5}$ (i and j) as a function of temperature. Error bars reflect the statistical errors from the refinement and are smaller than the symbol sizes. A clear splitting of the Ta-Ni and Ta-Se3 bond lengths can be observed for Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ and Ta$_\mathrm{2}$Ni$_\mathrm{0.93}$Co$_\mathrm{0.07}$Se$_\mathrm{5}$ which is totally absent for Ta$_\mathrm{2}$NiS$_\mathrm{5}$ where only a small reduction with decreasing temperature is found, simply reflecting the standard thermal expansion behavior. Panels (b) and (e) are parts of the x-ray precession images for the reciprocal (5 3 $l$) series ($l$ is changing from 4 to -3) reconstructed from single-crystal XRD data collected at 360 and 80 K for Ta$_\mathrm{2}$NiSe$_\mathrm{5}$, and at 280 and 80 K for Ta$_\mathrm{2}$Ni$_\mathrm{0.93}$Co$_\mathrm{0.07}$Se$_\mathrm{5}$ show the splitting of the reflections below the orthorhombic-to-monoclinic phase transition. Panel (h) shows the precession images for Ta$_\mathrm{2}$NiS$_\mathrm{5}$ again for the reciprocal (5 3 $l$) series ($l$ is from 4 to -3) at 360 and 80 K from our XRD data, and for the reciprocal (5 1 $l$) series ($l$ is from 4 to 11) at 2 K from Synchrotron XRD data, where no splitting of the reflections is seen in the whole temperature range. More details and complete precession plots are given in the SM.
• Figure 3: Comparison of Ni $L_3$ NEXAFS spectra of Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ recorded at 30 K along a, b, and c. The in-plane $d_\mathrm{xy}$ orbitals are observed in the lower energy range, below 854 eV, with spectral contributions along $E{\parallel}a$ and $E{\parallel}b$. In contrast, out-of-plane $d_\mathrm{xz}$ orbitals show contributions along $E{\parallel}a$ and $E{\parallel}c$, whereas the $d_\mathrm{yz}$ orbitals exhibit contributions along $E{\parallel}b$ and $E{\parallel}c$, and are found at slightly higher energies.
• Figure 4: NEXAFS spectra of Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ (left), Ta$_\mathrm{2}$Ni$_\mathrm{0.93}$Co$_\mathrm{0.07}$Se$_\mathrm{5}$ (middle) and Ta$_\mathrm{2}$NiS$_\mathrm{5}$ (right) with the beam parallel to a, b, and c directions. Shown is the spectrum taken at 390 K and the difference $\Delta$I = I(390 K) - I (T) between the spectra taken at 390 K and at the respective temperatures given in the graph. Note that $\Delta$I is multiplied by a factor of -6 and that a negative area means loss of spectral weight relative to 390 K. The green arrows point to the isosbestic point. The insets in the upper left corner of figures (a), (b), and (c) illustrate the polarization of light with respect to the sample’s orientation. A significant charge transfer from orbitals with exclusively in-plane character, i.e., d$_\mathrm{xy}$, to orbitals with in- and out-of-plane character, i.e., d$_\mathrm{yz}$, is found below T$_C$ for Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ and Ta$_\mathrm{2}$Ni$_\mathrm{0.93}$Co$_\mathrm{0.07}$Se$_\mathrm{5}$, while only a minor charge transfer of this type is found between in-plane and out-of-plane orbitals for Ta$_\mathrm{2}$NiS$_\mathrm{5}$. For Ta$_\mathrm{2}$NiS$_\mathrm{5}$, temperature effects seem to play the dominant role.
• Figure 5: Comparison of Ni L$_3$ NEXAFS spectra of (a) Ta$_\mathrm{2}$NiSe$_\mathrm{5}$, (b) Ta$_\mathrm{2}$Ni$_\mathrm{0.93}$Co$_\mathrm{0.07}$Se$_\mathrm{5}$, and (c) Ta$_\mathrm{2}$NiS$_\mathrm{5}$ recorded at 390 and 30 K along the a direction. In the plot for Ta$_\mathrm{2}$Ni$_\mathrm{0.93}$Co$_\mathrm{0.07}$Se$_\mathrm{5}$, the spectra of Ta$_\mathrm{2}$NiSe$_\mathrm{5}$ are additionally depicted in gray. The right panel of the figure represents sketches for the band structure of the three compounds before and after the formation of the excitonic insulator state as derived from our NEXAFS data.