Mottness Collapse in $1\mathrm{T}\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$ Transition-Metal Dichalcogenide: An Interplay between Localized and Itinerant Orbitals

The layered transition-metal dichalcogenide $1\mathrm{T}\text{−}{\mathrm{TaS}}_2$ has been recently found to undergo a Mott-insulator-to-superconductor transition induced by high pressure, charge doping, or isovalent substitution. By combining scanning tunneling microscopy measurements and first-principles calculations, we investigate the atomic scale electronic structure of the $1\mathrm{T}\text{−}{\mathrm{TaS}}_2$ Mott insulator and its evolution to the metallic state upon isovalent substitution of S with Se. We identify two distinct types of orbital textures—one localized and the other extended—and demonstrate that the interplay between them is the key factor that determines the electronic structure. In particular, we show that the continuous evolution of the charge gap visualized by scanning tunneling microscopy is due to the immersion of the localized-orbital-induced Hubbard bands into the extended-orbital-spanned Fermi sea, featuring a unique evolution from a Mott gap to a charge-transfer gap. This new mechanism of Mottness collapse revealed here suggests an interesting route for creating novel electronic states and designing future electronic devices.

Popular Summary

Since the discovery of graphene—a two-dimensional sheet of carbon atoms laid out in a honeycomb pattern—scientists have been striving to find other novel two-dimensional materials with tunable electronic properties and potential applications in electronics, spintronics, and photovoltaics. Transition-metal dichalcogenides have attracted the most attention because, by combining different transition-metal elements and chalcogen elements, this family of materials can be tuned into a wide spectrum of electronic behaviors, much richer than that in graphene-based systems. One particularly interesting material is $1\mathrm{T}\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$, which can behave like an exotic type of insulator (known as a Mott insulator) and also transition from insulator to superconductor by varying the ratio of sulfur to selenium. Our work provides an in-depth look at this transition.

We combined atomically resolved scanning tunneling microscopy measurements and first-principles calculations to trace the complete transition process from the Mott insulating phase to the metallic phase. Based on the identification of two distinct nanoscale orbital textures, one localized and the other extended, we find that the immersion of the localized orbital into the extended-orbital-spanned Fermi sea effectively melts the Mott insulating phase.

This new mechanism of “orbital-driven Mottness collapse” provides deeper insight into the ubiquitous interplay between charge order and strong electron correlation and may shed new light on the origin of unconventional superconductivity in similar materials.

Figure 1

Structural and electronic properties of $1\mathrm{T}\text{−}{\mathrm{TaS}}_2$. (a) Crystal structure of $1\mathrm{T}\text{−}{\mathrm{TaS}}_2$. (b) Atomic resolved topographic image of $1\mathrm{T}\text{−}{\mathrm{TaS}}_2$ with bias voltage $V_b=1\mathrm{V}$ and tunneling current $I_t=20\mathrm{pA}$. Each bright site on the surface corresponds to a S atom in the topmost layer. The gray dots and the black lines mark the SD structure and the CCDW superlattice, respectively. The crystal cleaves easily between two adjacent S layers, as indicated by the blue planes in (a), exposing the triangular lattice of S atoms. (c) Spatial averaged STM $dI/dV$ spectrum acquired at 5 K. (d) $\mathrm{DFT}+U$ band structure of $1\mathrm{T}\text{−}{\mathrm{TaS}}_2$. The black and green colored curves represent the two spin components. The shaded regions indicate the energy range belonging to the different orbitals.

Figure 2

Two types of orbital textures in $1\mathrm{T}\text{−}{\mathrm{TaS}}_2$. (a) [(c)] Differential conductance ($dI/dV$) maps ($42\times 42{\AA }^2$) measured at $V_b=-207$ [199] mV (up) and $V_b=-422$ [484] mV (bottom) with tunneling current $I_t=20\mathrm{pA}$, and the setpoint bias voltage for all the maps is $V_s=-2\mathrm{V}$. The center of each CDW cluster is brighter at $-0.2$ [0.2] V while the edges are brighter at $-0.4$ [0.4] V. (b) Isovalue surface of the two types of Wannier functions.

Figure 3

(a) Temperature dependence of the in-plane resistivity of three typical $1\mathrm{T}\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$ samples. (b) Schematic electronic phase diagram of $1\mathrm{T}\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$ and the location of the three samples under STM measurements.

Figure 4

Structural and electronic properties evolution of Se-substituted $1\mathrm{T}\text{−}{\mathrm{TaS}}_2$. Topographic images acquired with $V_b=-500\mathrm{mV}$ and $I_t=20\mathrm{pA}$ on the $1\mathrm{T}\text{−}{\mathrm{TaS}}_{2-x}{\mathrm{Se}}_x$ samples with the averaged Se concentration (a) $x=0.5$ and (b)–(d) $x=1$. (e) Local STM $dI/dV$ spectroscopy measured at various locations on the surface as marked in (c). (f) $\mathrm{DFT}+U$ density of states of Se-substituted SDs in comparison with (e). In (b) and (d), the marks guide for eyes the rotational and translational mismatch in different domains.

Figure 5

STM line map. The $dI/dV$ data for (a) $x=0$ and (b) $x=1$ in an extensive two-dimensional parameter space. The horizontal axis is the STM bias and the vertical axis is the position. The white dashed (black dot) lines mark the center (surrounding) of the SDs.

Figure 6

(a) $\mathrm{DFT}+U$ band structures of Se substituted $1\mathrm{T}\text{−}{\mathrm{TaS}}_2$. The meaning of the colors is the same as in Fig. 1. (b) Schematic energy level diagrams of the Mott insulating phase, the charge-transfer insulator phase and the metallic phase.

Figure 7

Physical meaning of the hopping parameters between the surrounding orbitals listed in Table 1.