Origin of insulating state in bulk $1T$-TaS$_2$ revealed by out-of-plane dimerization

Abstract

The commensurate charge-density-wave phase in the protoypical transition metal dichalcogenide $1T$-TaS$_2$ is investigated by temperature and polarization-dependent infrared spectroscopy revealing the fundamentally different charge dynamics parallel and perpendicular to the layers. Supported by density-functional-theory calculations, we demonstrate that the out-of-plane response is governed by a quasi-one-dimensional, Peierls-like dimerization of the two-dimensional star-of-David layers. In particular, our results identifies this dimerization as the primary driving mechanism of the metal-to-insulator transition, ruling out a significant role of electronic correlations.

Figures (3)

• Figure 1: Temperature-dependent resistivity of a $1T$-TaS$_2$ single crystal upon cooling and heating. Black arrows mark the CDW phase transitions. Schematic illustrations (top) depict the sequence of nearly commensurate (NCCDW), triclinic (TCDW), and commensurate (CCDW) phases with decreasing temperature. The inset shows the ion-beam–polished cross-section of $1T$-TaS$_2$ single crystal used for the polarization-resolved infrared spectroscopy with green and orange arrows indicating the polarization along and perpendicular to $c$-axis.
• Figure 2: Temperature-dependent optical conductivity of $1T$-TaS$_2$.(a, b) Real part of the optical conductivity $\sigma_1(\omega)$ of $1T$-TaS$_2$ along the in-plane (ab–plane) and out-of-plane (c-axis) directions, calculated from the measured reflectivity via Kramers–Kronig analysis. (c–f) Decomposition of the optical conductivity into Drude (purple), low energy (LE) interband transitions (orange), and high-energy (HE) interband transitions (blue) and phonon modes (green) for the in-plane (c, e), out-of-plane (d, f) directions at $T = 300$ K and $T = 10$ K, respectively. The extrapolation of the steepest part to the frequency axis is taken as the 2$\Delta_{\mathrm{CDW}}$. The inset (b) shows the temperature evolution of the gap obtained from the extrapolation of the absorption edge for in-plane and out-of-plane and the insets (c,d) show the frequency-dependent spectral weight (SW) in the NC-CDW phase.
• Figure 3: (a) Schematic A, L, and AL stacking of star-of-David cluster along out-of-plane directions. (b) Calculated band structure for the low-temperature CCDW phase for AL stacking with the shaded region highlights the gap at the Fermi level (c) Comparison of the measured real part of the optical conductivity, $\sigma_1(\omega)$, with the optical response calculated from density-functional theory for the AL-stacked commensurate CDW structure.