Excitonic Insulator State in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ Probed by Photoemission Spectroscopy

We report on a photoemission study of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ that has a quasi-one-dimensional structure and an insulating ground state. Ni $2p$ core-level spectra show that the Ni $3d$ subshell is partially occupied and the Ni $3d$ states are heavily hybridized with the Se $4p$ states. In angle-resolved photoemission spectra, the valence-band top is found to be extremely flat, indicating that the ground state can be viewed as an excitonic insulator state between the Ni $3d$–Se $4p$ hole and the Ta $5d$ electron. We argue that the high atomic polarizability of Se plays an important role to stabilize the excitonic state.

Figure 1

(a) Crystalline structure of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ for one layer. The nickel and tantalum atoms are denoted as large and small spheres, respectively. Selenium atoms are supposed to be located at the vertices of the shaded octahedra. (b) Illustration of the energy level splitting for the tantalum and nickel sites whose symmetries are assumed to be octahedral ($O_h$) and tetrahedral ($T_d$), respectively.

Figure 2

Ta $4f$ core-level spectra of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ at 300 K (solid curve) and at 40 K (broken curve).

Figure 3

Ni $2p_{3/2}$ core-level spectra of ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ at 300 K (broken curve) and at 40 K (dotted curve). Experimental Ni $2p$ spectra along with the broadened calculated spectrum (solid curve) obtained from the ${\mathrm{NiSe}}_4$ cluster calculation are shown in the upper panel. In the lower panel, the calculated line spectrum is decomposed into $c{d}^8$, $c{d}^{9}L$, and $c{d}^{10}{L}^2$ components.

Figure 4

ARPES results for ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$. (a) and (b) are the second energy derivative plot and the EDC plot, respectively, obtained at 40 K and $h\nu =10\mathrm{eV}$, while (c) and (d) are the counterparts obtained at 40 K and $h\nu =23\mathrm{eV}$. Open triangles and circles indicate the peak positions of the bands estimated by the second derivative of EDC and MDC data of $h\nu =10\mathrm{eV}$, respectively, while the filled counterparts correspond to $h\nu =23\mathrm{eV}$. Broken and solid curves are quadratic fits to the peak positions (see the text). (e) and (f) are the second energy derivative plot and the EDC plot, respectively, obtained at 300 K and $h\nu =23\mathrm{eV}$.