Realization of a Metallic State in $1T\text{−}{\mathrm{TaS}}_2$ with Persisting Long-Range Order of a Charge Density Wave

Metallization of $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ is generally initiated at the domain boundary of a charge density wave (CDW), at the expense of its long-range order. However, we demonstrate in this study that the metallization of $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ can be also realized without breaking the long-range CDW order upon surface alkali doping. By using scanning tunneling microscopy, we find the long-range CDW order is always persisting, and the metallization is instead associated with additional in-gap excitations. Interestingly, the in-gap excitation is near the top of the lower Hubbard band, in contrast to a conventional electron-doped Mott insulator where it is beneath the upper Hubbard band. In combination with the numerical calculations, we suggest that the appearance of the in-gap excitations near the lower Hubbard band is mainly due to the effectively reduced on-site Coulomb energy by the adsorbed alkali ions.

Figure 1

(a) STM topographic image ($40\times 40{\mathrm{nm}}^2$) taken at $\sim 4.2\mathrm{K}$ on the pristine $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ surface, showing the $\sqrt{13}\times \sqrt{13}$ periodicity. $U=+1.0\mathrm{V}$ and $I_t=100\mathrm{pA}$. (b) High-resolution STM image ($5\times 5{\mathrm{nm}}^2$) showing the atomic structure of the David star. The David star model is overlaid on the STM image. $U=+100\mathrm{mV}$ and $I_t=200\mathrm{pA}$. Individual S atoms are resolved as small blobs in each David star cluster, which enables us to identify the position of Ta atoms underneath. (c) Schematic of the top view David star structure. Different colors represent the inequivalent Ta atoms derived from different positions in the David star cluster. (d) Spatially averaged $dI/dV$ spectrum acquired with $U=+500\mathrm{mV}$ and $I_t=100\mathrm{pA}$.

Figure 2

(a) $dI/dV$ spectra taken on the $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ surface with different K coverage. The data were acquired with $U=+500\mathrm{mV}$ and $I_t=100\mathrm{pA}$. (b) Definition of the three gaps in the $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ $dI/dV$ spectrum. ${\mathrm{\Delta }}_{\text{Mott}}$ is defined as the width between the two Hubbard peaks, ${\mathrm{\Delta }}_{\mathrm{CDW}}$ the width between LHB and VB peaks, and ${\mathrm{\Delta }}_0$ the width of the zero-intensity region within the Mott gap. (c) Dependence of the magnitude of the three gaps on the K coverage.

Figure 3

(a) Small-scale STM current image ($8\times 8{\mathrm{nm}}^2$) taken at the $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ surface with a low K coverage. The David star structure is highlighted by the yellow dashed mesh, and the adsorption sites of K at the David star center are marked by red dots. (b) Large-scale STM topographic image ($50\times 50{\mathrm{nm}}^2$) taken on the $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ surface with the K coverage of $\sim 0.4\mathrm{ML}$. $U=+1.0\mathrm{V}$ and $I_t=100\mathrm{pA}$. (c) The adsorption sites of K atoms extracted from the white square region in (b). The black dots represent the positions of K atoms, and the parallel lines the lattice of the $1T\text{−}\mathrm{Ta}{\mathrm{S}}_2$ substrate. (d) Fourier transform of the STM image (b). The blue and red circles mark the peaks originated from the surface K atoms.

Figure 4

(a) Schematic of the spectral weight transfer in the local limit for an electron-doped Hubbard model with a uniform $U$. (b) Spectra calculated by the cluster perturbation theory. The spectrum of the half-filled system is illustrated by the dotted (blue) line, while the spectra in the sites with and without ${\mathrm{K}}^{+}$ ions which are simulated by different $U$ (see the text) after one-electron doping in the 13-site cluster are illustrated by the dashed (red) line and solid (black) line, respectively. The arrrow indicates the in-gap excitation near the LHB. The inset shows the shape of clusters used in the theoretical calculations.