Interlayer charge transfer induced by electronic instabilities in the natural van der Waals hetrostructure 4H$_b$-TaS$_2$

Abstract

The natural van der Waals heterostructure 4H$_b$-TaS$_2$ composed of alternating 1T- and 1H-TaS$_2$ layers serves as a platform for investigating the electronic correlations and layer-dependent properties of novel quantum materials. The temperature evolution of the conductivity spectra $σ(ω)$ obtained through infrared spectroscopy elucidates the influence of band modifications associated with the charge-density-wave (CDW) superlattice on the 1T layer, resulting in a room-temperature energy gap, $Δ_{\rm CDW}\approx$ 0.35 eV. However, there is no gap associated to the 1H layer. Supported by density functional theory calculations, we attribute the behavior of interband transitions to the convergence of the layers, which amplifies the charge transfer from the 1T to the 1H layers, progressing as the temperature decreases. This phenomenon leads to an enhanced low-energy spectral weight and carrier density. The presence of an energy gap and the temperature-tunable charge transfer within the bulk of 4H$_b$-TaS$_2$ driven by layer-dependent CDW states contribute to a more comprehensive understanding of other complex compounds of transition-metal dichalcogenides.

Figures (3)

• Figure 1: (a) Crystal structure of pure and mixed TaS$_2$ polymorphs: 2H-TaS$_2$ with hexagonal structure, 1T-TaS$_2$ with tetragonal structure, and 4H$_b$-TaS$_2$ with alternatively stacked trigonal prismatic 1T and octahedrally coordinated 1H layers of TaS$_2$. (b) Periodic lattice distortion in the 1T layers forms a star-of David like supercells. (c) Resistivity of 4H$_b$-TaS$_2$ signals a communsurate CDW (CCDW) and an incommunsurate CDW (INCDW) state at 315 and 24 K respectively, the later is magnified in the insert (note a hysteresis loop). (d) Real part of the optical conductivity of 4H$_b$-TaS$_2$ for various temperatures; note the two low-energy peaks that sharpen as temperature decreases.
• Figure 2: (a) Decomposed optical conductivity of 4H$_b$-TaS$_2$ at $T = 325$, 100, and 10 K (top to bottom). The colored shades represent the contributions from the Drude and Lorentz oscillator models, as discussed in the main text and in the Supplimental Material SM. (b) Undistorted electronic band structure of the bulk 4H$_b$-TaS$_2$. The regions of relevant interband transitions are highlighted by ellipses. (c) Electronic bands of 4H$_b$-TaS$_2$ with distorted 1T layer. Only the Ta bands are shown. (d) Density of states (DOS) of 1T and 1H layers and the total DOS with and without distortion. (e) The interband contribution to the optical conductivity. Spectral-weight transfer from high-energy to low-energy peaks occurs, as indicated by the dotted arrows. The inset displays the spectral weight SW as a function of upper bound $\omega_c$. The red arrow indicates the peak that appears at the 315-K CDW transition. (f) The calculated interband optical conductivity ($\sigma_{xx}$) for the undistorted and distorted cases: the peaks sharpen and move to lower energy under distortion.
• Figure 3: Difference spectra $\Delta\sigma_1$ for 300 K and 10 K (a). Their zero crossing can be taken as an estimate of the energy gap $2\Delta_{\rm{CDW}}$uykur2021. The inset in (a) shows the temperature-dependent dielectric permittivity, with the zero crossing being the screened plasma frequency $\omega^{scr}_p$. Calculated interlayer-distance dependence of the DOS for the distorted 1T layer (b) and undistorted 1H layer with distortions present in the 1T layer (c). The interlayer distance was varied from 5 to 7 Å. There is an enhanced charge transfer from 1T to 1H layer with decreasing layer separation. The bold curve represents the nominal interlayer distance of the bilayer ($\sim$ 5.9 Å).